BEST PRACTICE GUIDELINES FOR
SPECIFICATION OF MODERN NEGATIVE
TEXTURE SURFACES (NTS) ON LOCAL
AUTHORITY HIGHWAYS
Contents
Page No
1
EXECUTIVE SUMMARY
1
2
INTRODUCTION
3
2.1.
2.2.
2.3.
2.4.
2.5.
2.6.
3
Overview and Objectives
Code of Practice for Highway Maintenance Management
Scope of Application and Use
Terms and Definitions
References
Summary
PROCESS AND MATERIALS DEVELOPMENT
3.1.
3.2.
3.3.
3.4.
3.5.
4
Development History
Generic Systems
Proprietary Systems
References
Summary
3
4
6
6
7
7
8
8
11
13
14
15
SITE CONSIDERATIONS AND INSTALLATION 16
4.1.
4.2.
4.3.
4.4.
4.5.
4.6.
4.7.
4.8.
4.9.
4.10.
4.11.
5
General
Serviceability
Substrate – Existing Pavement
Bond Coat
Early Life Surface Characteristics
Skid Resistance Characteristics
Horses on the Highway
Installation
Installers and Inspection
References
Summary
THE RIGHT MATERIAL FOR THE RIGHT SITE
5.1.
5.2.
5.3.
5.4.
5.5.
5.6.
6
Site Evaluation
Chart A
Chart B
Chart C
Chart D
Chart E
7
Introduction
Failure Groups
Types of Deterioration
Deterioration Model
References
Summary
39
39
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40
43
52
52
LOCAL AREA REINSTATEMENT
7.1.
28
28
30
30
30
30
36
EXPIRED LIFE ENGINEERING
6.1.
6.2.
6.3.
6.4.
6.5.
6.6.
16
17
17
19
22
23
24
24
26
26
26
Performance Characteristics
53
53
i
7.2.
7.3.
7.4.
7.5.
8
Reinstatement Size and Position
Installation and Compaction
Street Furniture and Ironwork
Summary
ASSET MANAGEMENT
8.1.
8.2.
8.3.
8.4.
8.5.
8.6.
Asset Management Plan
Whole Life Costing
Winter Maintenance
Distress Mechanism and Forecast
References
Summary
55
55
57
58
59
59
59
61
61
62
62
Figures
Figure 1 - Structure and relationship of Best Practice Guidelines
Figure 2 - Schematic diagram of positive and negative textures.
Figure 3 - Chart A
Figure 4 - Chart B
Figure 5 - Chart C (1 of 2)
Figure 6 - Chart C (2 of 2)
Figure 7 - Chart D
Figure 8 - Chart E (1 of 2)
Figure 9 – Chart E (2 of 2)
Figure 10 – Fattting-up
Figure 11 - Fretting
Figure 12 - Mini paver machine
Figure 13 -Whole Life Cost
5
6
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32
33
34
35
37
38
41
45
53
60
Tables
Table 1 - British Standard surfacing mixtures for thin layers
Table 2 - Classification of Bond Coats by Vialit Test Value
Table 3 - Classification of bond coats by torque bond strength
Table 4 - Selection of Bond Coat
Table 5 - Site evaluation checklist
Table 6 – Elements of a Deterioration Model
9
20
21
21
29
44
Appendices
Appendix A - Functional Carriageway Hierarchy
Appendix B - Glossary of Terms and Definitions
Appendix C - Questionnaire
Appendix D – Summary of Responses
Appendix E - Regional Workshops
Appendix F - BBA Scheme Guidance
Appendix G - Investigation Protocols
Appendix H – Specifying Proprietary Materials
Appendix I – Installers Observations
Acknowledgements
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68
71
72
73
77
81
84
ii
1
EXECUTIVE SUMMARY
These Best Practice Guidelines for Negative Texture Surfaces (NTS)
provide a methodology for site evaluation and material selection, to
ensure that the right material is installed in the right site, together with
a structured approach to the factors which may have a bearing on
distress mechanisms.
• Enquiry responses have indicated generally that NTS is performing
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
equally well or better than traditional materials in most
circumstances on local authority highways.
Substrate condition evaluation is essential in the selection of NTS
on evolved local authority highways.
The surface layer on thin local highways can constitute a significant
proportion of the sound bituminous material in the pavement
structure.
The wide variety of proprietary system surfaces which are available
broadens the scope of sites that are potentially suitable for their
application. However, there are many situations where generic
types of surface material may provide serviceability at best value.
A rational approach to the engineering selection of NTS is provided
as a series of flow diagrams.
A checklist together with a series of decision charts enables a
consistent approach for site evaluation by consideration of five
distress modes for the substrate.
An investigation of the existing carriageway is particularly important
for thin flexible local pavements to ensure optimum maintenance
designs. Investigation protocols have been developed to support
this.
A surface material decision chart considers key engineering
performance requirements to ensure strain compatibility between
surface layer material and support substrate.
A process of bond coat selection is developed to ensure good inservice adhesion between the surface and the substrate.
All sites for surfacing should be inspected and evaluated by
experienced
and
knowledgeable
personnel
with
joint
client/contractor inspection as appropriate.
An assessment should be undertaken of the period of time required
for removal of binder film in early life through vehicle wheelovers.
Attention to detail in the location and formation of joints, especially
in highly stressed areas will avoid premature distress in an
otherwise serviceable surface.
Fretting and cracking were found to be the most significant
mechanisms of deterioration in NTS.
Five factors of influence for causes of distress of NTS have been
determined and related to a mechanistic deterioration model. The
influencing factors do not always behave in isolation and are
frequently inter-dependent.
For local area repair the care and preparation of the area to be
reinstated is at least as important as the installation of the surface
material itself.
Failure of NTS, when it occurs, can be a swift process. A procedure
to predict the onset of distress by forecasting has not been
1
identified. This is a significant area for future study, since the
absence of an appropriate deterioration model could result in lack
of certainty in asset management planning.
2
2
INTRODUCTION
2.1.
Overview and Objectives
2.1.1.
Negative Texture Surfaces (NTS) comprise a family of modern asphalt
materials which have been developed to provide safe, durable and
quieter highway surfaces. The common feature of NTS is that surface
texture, an essential component of skid resistance, is provided in a
downward (negative) direction beneath the vehicle tyres. The NTS
family comprises a suite of proprietary surfaces, known collectively as
thin surfacings and generic stone mastic asphalt. Thin surfacings were
developed to provide safety through the adequate provision and
retention of negative texture over time. Generic stone mastic asphalts
used in the UK have been modified from the original European
materials to provide a sufficient installed surface texture.
2.1.2.
The Roads Liaison Group (RLG) was established in 2001 in order to
advise local authorities and Central Government on highway
maintenance issues. Members of RLG include the Department for
Transport (DfT), the Local Government Association (LGA), and
representatives of national and local highway authorities in England,
Scotland, Wales and Northern Ireland. The RLG is supported by four
boards - Roads, Bridges, Lighting and Traffic Management. This
project was commissioned by the DfT on behalf of the UK Roads
Board and these Best Practice Guidelines have been developed for
application on the local authority highway network.
2.1.3.
A major element of local road maintenance relates to the surface
course of the highway. Hot rolled asphalt (HRA) surface course was
the standard road surface material throughout the country for very
many years but this traditional, virtually impermeable, positive surface
texture material has swiftly given way to NTS.
2.1.4.
The terms “thin surfacing” (TS) and “stone mastic asphalt” (SMA) have
been in common use for about a decade and consequently they have
been retained in this document but all such materials belong to the
highway surface performance family of NTS materials.
2.1.5.
The impact of NTS has been so great that some authorities have
virtually ceased to use traditional rolled asphalt surfaces. The rapid
introduction, however, has meant that there has been little time for
longer term performance lessons to be learnt on the network.
2.1.6.
Local authority enquiry responses undertaken during development of
these Guidelines have indicated, overall, that NTS is performing as
well or better than, traditional materials in most circumstances where
the right material has been installed in the right site. Development of
innovative surface materials will continue in the future with advancing
technology and therefore these Guidelines are not timeless. These
Guidelines should be reviewed within an elapsed period of five years
to accommodate any new developments, incorporate performance
feedback, consider the implications of further research and address
the introduction and embedment of European Standards for asphalt
surface materials.
3
2.1.7.
2.2.
The core objective of these Best Practice Guidelines is to define
distress conditions of NTS to aid both engineering understanding and
asset management planning for the network. This will assist in a
managed change from reactive to programmed maintenance as
recommended in the Code of Practice for Highway Maintenance
Management. These Guidelines have been developed to enable a
rational decision making and selection process for NTS materials.
Code of Practice for Highway
Maintenance Management
2.2.1.
The Code of Practice for Highway Maintenance Management, “Well
Maintained Highways”, describes good practice and sets out a series
of recommendations for highway authorities to implement.
www.roadscodes.org
2.2.2.
Figure 1 illustrates the relationship of these Best Practice Guidelines
for NTS with the Code of Practice, asset management and local
transport planning.
2.2.3.
These Guidelines should be read in the context of these documents,
as they provide more detailed guidance for the application of NTS in
the delivery of the core objectives of the Code of Practice:
•
•
•
•
Safety;
Serviceability;
Sustainability;
Customer service.
4
Government Transport Policy
Local Transport Planning
Guidance
CSS Framework for Highway
Asset Mnaagment
Codes of Practice
Highway
Maintenance
Management
Management Of
Highway
Structures
Highway
Lighting
Management
Best Practice
Guidelines
Negative Texture
Surfaces (NTS)
Figure 1 - Structure and relationship of Best Practice Guidelines
2.2.4.
The Code defines carriageway hierarchy functionality which is
reproduced in Appendix A. It is recommended that this hierarchy
should be developed by an authority to take into account current and
expected traffic characteristics. It is this relationship between traffic
and hierarchy that is developed later in these Guidelines as the basis
for selection of surfacing. This approach is consistent with the Code
and local transport planning. The hierarchy and subsequent design
approach adopted for surfacing should also be consistent with
adjoining authorities, to meet road users’ reasonable expectations.
2.2.5.
In the selection of surfacing, authorities should consider both the
future maintenance implications of the surfacing together with any
environmental implications. The Code provides designers with check
lists to assist with the development of designs for each of these
aspects. These checklists should be used by surfacing specifiers in
the context of these Guidelines. Local Transport Plan guidance
identifies that additional maintenance costs arising from all new and
improved infrastructures should be explicitly identified and taken into
account in evaluating the whole life cost of the scheme. This will
include the surfacing aspects of the maintenance or improvement
works.
5
2.3.
Scope of Application and Use
2.3.1.
Distress which has occurred in NTS has generally been attributed to
inappropriate site conditions, selection of material or installation
practices. These factors are much more significant on the local
network than on the strategic network. This is due to the evolved
nature of the local network, its more variable construction, geometry,
junctions, layout and the frequency and scale of highway maintenance
works.
2.3.2.
The development of these Guidelines was undertaken through
dialogue with a number of stakeholders, including clients, specifiers,
installers and highway maintenance engineers, together with
examination of the surfacing failure mechanisms. This was conducted
with the objective of producing robust guidance of value to the
profession.
2.3.3.
The early life surface frictional characteristics of negatively textured
surfaces are not within the scope of these Guidelines and are covered
in detail elsewhere. However, the overall mechanisms are reviewed
insofar as they inform a distress condition.
2.3.4.
The focus of the Guidelines is “the right material for the right site” to
ensure value for money and maximum longevity in service. The
guidelines are articulated through engineering performance and
materials science factors. Decision charts provide a process for site
evaluation and surface materials selection. No attempt is made to
define which material should be used in particular circumstances but
the Guidelines provide a broad mechanism for structured evaluation
and choice of surfacing type in a highway maintenance context.
2.4.
2.4.1.
Terms and Definitions
The provision of surface texture on a pavement is a key difference
between traditional surfacing, such as HRA, and the family of NTS
materials. Positive texture as provided by HRA with chippings and
negative texture as provided by NTS is illustrated in Figure 2.
Figure 2 - Schematic diagram of positive and negative textures.
6
2.4.2.
2.5.
There is no universally agreed categorisation for TS, even the limits of
the term thin surfacing are ill-defined as the materials have developed
and evolved. Terms and definitions are described in Appendix B and
are based upon those published previously (Nicholls, Carswell and
Langdale, 2002) but adapted and extended to address the wider
scope of surfacing solutions that are the subject of these Guidelines.
References
1. Well Maintained Highways. Code of Practice for Highway
Maintenance Management (2005), The Stationery Office, London.
ISBN 0-11-552643-9
2. Walsh I.D. (2000), “Out of the skid pan”. Surveyor, 9th November
2000, pp12-15
3. Nicholls J.C, Carswell I. and Langdale P.C. (2002), “Durability of
thin asphalt surfacing systems. Part 1 Initial findings” TRL Report
557, TRL Limited.
2.6.
Summary
• These Best Practice Guidelines provide underpinning support for
•
•
•
•
•
•
•
“Well Maintained Highways”.
Surfacing selection should be based upon highway functionality.
Future maintenance implications should be considered in surfacing
selection.
Surfacing distress is related to inappropriate choice of site,
installation and materials selection.
Local highway networks have evolved with variable construction,
geometry, junctions and layout.
The scope of the Guidelines includes both generic and proprietary
surfacing.
The Guidelines were developed through extensive consultation with
stakeholders.
Focus is “the right material for the right site”.
7
3
PROCESS AND
DEVELOPMENT
3.1.
MATERIALS
Development History
3.1.1.
Before the availability of NTS the traditional surface course materials
for pavements in the UK were HRA with high polish resistant precoated chippings, dense or close graded bitumen macadam (DBM)
and surface dressing (SD). High stone content asphalt (HSCA), which
has no surface applied pre-coated chippings, was also a frequently
used surface material.
3.1.2.
Selection of the surfacing material was dependent on:
•
•
•
•
Traffic levels;
Availability and cost of suitable crushed rock aggregate;
Condition of substrate;
Intended laying season.
3.1.3.
The service life of HRA surfaces was generally between ten and
twenty years and in some specific cases significantly longer. After this
period it could be overlain by a new HRA surface course if levels
permitted and this added significantly to the structural strength and
flexibility of the pavement as a whole.
3.1.4.
DBM (or close graded macadam) surfacing was a common choice on
minor highways. It was generally expected to have a typical service
life of between eight and fifteen years although during this period it
may require surface dressing (SD) treatment. As with HRA it could be
overlaid at the end of its service life and would continue to contribute
to the structural strength of the pavement.
3.1.5.
Many contractors developed variations on the British Standard BS
4987 close graded mixtures with more exacting control limits on the
grading and special binder types.
Such materials were used
successfully in urban areas where thin overlays were required to
address difficulties with access levels. Although many proprietary
systems and local specifications provided for relatively durable
surfacing down to 25mm nominal thickness, as did BS 4987, materials
which could confidently be laid on more heavily trafficked or highly
stressed sites were not available. Table 1 illustrates the BS surfacing
mixtures which can be installed at thicknesses of 35mm or less.
8
Table 1 - British Standard surfacing mixtures for thin layers
British
Standard
BS 5941
Description
BS 5941
BS
4987-1
30%
Stone
content
Fine
graded
surface course
0/10
35
0/4
15 - 25
Table 6, Col
6/3
Clause 7.7
BS
4987-1
Medium
graded surface
course
0/6
20 - 25
Clause 7.6
BS
4987-1
Dense surface
course
0/6
20 - 30
Clause 7.5
BS
4987-1
Porous asphalt
surface course
2/10
30 - 35
Clause 8.2
BS
4987-1
Close graded
surface course
0/10
30 - 40
Clause 7.4
15%
Stone
content
Aggregate
size (mm)
0/10
Thickness
(mm)
30
Reference
Table 6, Col
6/2
Note: The above table is a means of illustration only and should not be taken as implying suitability of a particular
surface.
3.1.6.
A revision of Chapter 8 of the Traffic Signs Manual, in the early 1990s
introduced a requirement for a safety barrier zone between operatives,
plant and active traffic. For most rural and many urban highways this
safety zone requirement meant that the machine application of
chippings to the surface of HRA could not be used without a complete
road closure.
3.1.7.
At about the same time as the introduction of the increased
construction site safety measures, some asphalt manufacturers were
developing SMA and derivative materials. These products were based
on continental practice, particularly German splittmastixasphalt. Other
manufacturers chose to take out licences for the production of similar
surface products which had already been developed overseas. The
adoption and use of these materials reduced the contractual risk
associated with chipping on HRA and also needed less resources for
installation. Thus, the use of HRA, which was the favoured surface
course material for many situations, became problematic and fell into
reduced use.
3.1.8.
Early NTS products included the Jean Lefebvre ULM thin asphaltic
concrete, originally licensed to A. McAlpine Ltd, and Safepave, a
paver-laid surface dressing licensed to Associated Asphalt Ltd. As
these products were already fully developed they were capable of
immediate larger scale application and were quickly adopted on a trial
basis by many authorities.
3.1.9.
The development, and client acceptance, of mixtures based on the
German splittmastixasphalt, known in English speaking countries as
SMA, lagged slightly behind the acceptance of the licensed systems.
9
In Northern Continental Europe the requirement was for durable dense
surfacing mixtures for heavy traffic conditions. The delivery of
optimum skid resistance, whilst desirable, did not assume the same
priority as in the UK. The German practice was to use a relatively stiff
bitumen binder with high bitumen content and high stone content. The
lack of fine aggregate and filler in the mixture could lead to bleeding of
bitumen during transport and laying. To guard against this the
splittmastixasphalt mixture normally included cellulose fibres to
partially immobilise the bitumen before laying.
3.1.10. The UK skid resistance requirements necessitated the provision of
significant macro-texture depth. The specifications were relatively
easily met by HRA with pre-coated chippings but with
splittmastixasphalt formulations the stone matrix was virtually fully
filled with bitumen. In developing UK equivalents, suppliers needed to
be able to reduce the relative volume of binder-filler matrix. This was
to allow adequate texture to remain on the surface after compaction.
To ensure low permeability and therefore good durability, the bulk of
the layer was fully filled with bitumen mastic. This compromise was
satisfactorily achieved at bitumen contents in the order of 6%-7% by
mass. The mixtures were particularly stable with aggregate interlock
providing good resistance to wheel track rutting. Previous experience
of bitumen rich SMA in Northern Europe had shown rutting under
heavy traffic conditions.
3.1.11. Early versions of SMA in the UK suffered with development problems
when the ratio of layer thickness to nominal stone size was less than
two. Most of the currently available SMA and Thin Asphalt Concrete
(TAC) variants now have a minimum layer depth close to, or in excess
of, twice their nominal stone sizes.
3.1.12. Based upon trials and the use of SMA as a high stability surface
course material (Nunn, 1994), many authorities developed generic
specifications for the material and its installation.
3.1.13. Owing to the many proprietary surface systems which had entered the
market, a product type approval evaluation system was developed by
the Highways Agency. This process required the system proprietor to
lay a trial site, usually on a trunk road. The manufacture and
installation was monitored together with laboratory testing and upon
completion of the trial a panel of experts inspected the site. The
inspection process was repeated at regular intervals for at least two
years.
3.1.14. Requirements for bituminous surface material and techniques to be
used on the strategic network are encapsulated in the Design Manual
for Roads and Bridges (DMRB) HD 37/99. Site trials confirmed the
German experience that generic SMA mixtures are sensitive to small
variations in aggregate grading and binder content. This can result in a
reduction in surface texture as evidenced by a localised patchy
appearance and fatting of the surface. Potential inconsistency in terms
of texture retention, which is essential for high speed skid resistance
means that SMA produced to a generic specification must not be used
on the Highways Agency network. However, the constraints of traffic
management and congestion on the local network are not so great and
generic SMA has continued to be used. Additionally, there are differing
10
speed limits which are not so texture dependant enabling the
installation of mixtures, which may be less exacting in this
performance parameter than is the case for trunk roads.
3.1.15. The type approval scheme, although used by authorities as a
specification requirement for NTS was not entirely appropriate for nontrunk road application. In 1995 the CSS and Highway Agency
established the Highway Authorities Product Approval Scheme
(HAPAS) to enable independent certification of the performance of
proprietary surfacing products and systems. The British Board of
Agrément (BBA) was appointed to administer the scheme. A specialist
advisory group (SG3) was established in 1996 to broaden the scope of
the product approval scheme. SG3 developed “Guidelines for the
Assessment and Certification of Thin Surfacing Systems”. Various
proprietary thin surfacing systems were assessed with the first
certificate issued in 2000.
By November 2005 twenty seven
proprietary systems had been certified. Throughout these Guidelines
the term product is taken as referring to the manufactured material and
the term system to the product and its installation protocol.
3.1.16. The introduction of BBA certification did not make the use of generic
specifications redundant. Over half of all authorities specify SMA
surfacing by generic means for their network applications. These
generic SMA specifications have generally been evolved in
collaboration with local asphalt manufacturers to suit the local
availability of mineral resources and local network needs. Some
authorities use both generic specification and BBA certification
selectively as the site requirements dictate.
3.2.
Generic Systems
3.2.1.
British Standards, BS 594 for HRA and BS 4987 for Coated Macadam
(asphalt concrete), provide a limited spectrum of surfaces suitable for
layer thicknesses of less than 40mm.
3.2.2.
Tables 3 and 4 of BS 594-1 provide only a 0/2mm nominal size
aggregate designed asphalt mixture for laying at 25mm thickness.
The sand carpet mixture is unsuitable for carriageway surfacing
application due to low surface macro-texture and potential mix
instability. Table 6 of BS594-1 provides recipe specifications for 15%
stone content 0/10mm nominal size mixtures for laying at a thickness
of 30mm and a 30 % stone content, 0/10 nominal size mixture for
35mm thickness. The former material is generally only regarded as
suitable for footway and very light vehicle use. The 30% (0/10)
mixture can be used for light traffic application but the non-availability
of a design mix equivalent inhibits its wider acceptance, particularly in
areas where the native sand tends to produce unsuitable mixtures.
However, with critical formulation and the use of modern polymer bond
coats, the 30% (0/10) mixture may well be capable of meeting many of
the requirements of a thin surfacing system, if designed in accordance
with the principles of Specification for Highway Works (SHW) Clause
943. Since Clause 943 does not include for modification to the 30%
(0/10) mixture the resultant material would need to be specified as a
special material or manufactured as a proprietary product.
11
3.2.3.
BS 4987 has been the traditional source for the specification of thin
surfacing layers. Many of these British Standard formulations perform
well and when specified and installed in appropriate situations in the
network provide good value for money. BS 4987-2, Table 6 specifies
nominal and minimum layer thicknesses for coated macadam
indicating seven mixtures capable of being laid at depths of 35mm or
less. The 0/10 close graded surface course is suitable for general
highway use but the 0/6 dense surface course material cannot be
relied upon to provide adequate surface texture depth for higher speed
applications. The open graded mixtures can provide adequate surface
texture but can fret in high lateral stress areas. The 0/10 close graded
mixture is unlikely to produce a texture depth suitable for areas with a
speed limit above 30mph. As with the 0/10 HRA mixture, the use of a
premium (high performance) bond coat could make the use of
conventional material more reliable under relatively heavy trafficking.
3.2.4.
The generic SMA specifications used by authorities include the
following essential requirements and are based upon the style of
specification in PR 65 (Nunn, 1994). However many authorities
adapted their specifications locally with bespoke modification of
composition and other aspects to reflect serviceability, resource
constraints and installation requirements, which have been developed
over time in collaboration with local asphalt manufacturers. The core
features comprise:
• A permitted range for the target grading envelope;
• A range of potential tolerances to be applied to the suppliers
chosen grading curve;
• Maximum and minimum binder content;
• An air void requirement in the designed mix.
3.2.5.
Forthcoming European Specifications for asphalt will impact on current
British Standards specifications for surface materials. The relevant
parts of the emergent technical standards are:
• EN 13108-1Bituminous mixtures – Part 1 Asphalt Concrete;
• EN 13108-2Bituminous mixtures – Part 2 Very Thin Layer Asphalt
Concrete;
• EN 13108-5Bituminous mixtures – Part 5 Stone Mastic Asphalt;
• EN 13108-7Bituminous mixtures – Part 7 Porous Asphalt.
3.2.6.
All of the above four parts of EN 13108 are progressing through the
European Standards body, CEN, and are likely to be implemented in
the UK by 2008.
3.2.7.
EN 13108-2 makes reference to an intended installed layer thickness
of between 20mm and 30mm. Asphalt concrete surfacing greater than
30mm thick will be specified to EN 13108-1. Layers less than 20mm
in thickness will also be covered by EN 13108. Asphalt concrete
products 10mm to 20mm in thickness may be capable of submission
for product approval certification.
3.2.8.
Porous Asphalt is specified in the Specification for Highway Works,
(Nov 2004 amendment), and a similar material, but less definitively
described, is included in BS 4987-1. Both materials have 6/20 mm
12
nominal size aggregate and are placed at 45mm to 50mm in thickness.
BS 4987 also includes a 2/10mm nominal size mixture, which may be
laid at thicknesses of 30 to 35mm. Porous asphalt has advantages
over close graded mixtures in respect of noise and spray reduction.
However, it is little used on local roads because of limited structural
contribution, winter maintenance management and relatively poor
durability.
3.3.
Proprietary Systems
3.3.1.
Thin surfacing systems may be certified by BBA through the HAPAS
mechanism. All thin surfacing systems for use on trunk roads must be
BBA certified. Some authorities also make certification mandatory on
their network whilst others require BBA certification only for selected
parts of their network.
3.3.2.
To achieve certification a system proprietor must satisfy the process
requirements of the BBA “Assessment and Certification of Thin
Surfacing Systems for Highways”. The certification is described in full
in Section 3 of the BBA protocol and is summarised as follows:
3.3.3.
Stage 1: Assessment of applicant submitted data
• Company details;
• System details and history;
• A quality plan covering;
• binder type(s);
• aggregate source(s), characteristics etc;
• ancillary products detail;
• final products, thickness, composition etc;
• Details of total quality structure and system;
• Installation method(s).
3.3.4.
Stage 2: Assessment of factory production control:
• Subsequent site and plant inspections by BBA.
3.3.5.
Stage 3 Laboratory testing (Mandatory):
•
•
•
•
PSV and AAV test on proposed aggregates;
Wheel tracking;
Sensitivity to water;
Torque bond test for assessing the adhesion at the layer to the
substrate;
• Visual assessment;
• Texture depth.
3.3.6.
Stage 3 Laboratory testing (Optional):
Optional tests carried out by the BBA, based upon in-house protocols,
to evaluate properties within a menu which an applicant system
proprietor may elect to have included on their certificate. The
properties that may be covered include:
13
•
•
•
•
•
•
•
•
3.3.7.
Stiffness;
Retained stiffness after fuel immersion;
Ageing;
Resistance to binder stripping;
Noise reduction relative to HRA;
Regulating ability;
Hydraulic conductivity;
In service skid resistance.
Stage 4 – Installation Trial:
• Construction of a section of surfacing witnessed by the BBA;
• The installation is subject to laboratory testing as required by BBA.
3.3.8.
Stage 5 – Performance Trial:
• Each trial section is monitored and tested for a period of two years
and any loss of texture recorded;
• Expert panel inspections are carried out at the end of the two year
evaluation period.
3.3.9.
Stage 6 – Certification
• Certificates verify the system compliance with BBA requirements,
•
•
•
•
define the systems assessed, the conditions of application, likely
performance and the results of laboratory and site tests;
Once certified a system may not be changed without the approval
of BBA;
The acceptability of any proposed changes to a system will be
evaluated by further testing as BBA require;
Audits of each system are conducted at least annually;
Full review of each system is undertaken by BBA every five years.
3.3.10. BBA certificates which have been issued for proprietary thin surfacing
systems are available at www.bbacerts.co.uk .
3.4.
References
1. British Standard Institution, (2005). BS 594-1 “Hot rolled asphalt for
roads and other paved areas. Specification for constituent materials
and asphalt mixtures” BSI, London.
2. British Standard Institution, (2003). BS 594-2 “Hot rolled asphalt for
roads and other paved areas. Specification for transport, laying and
compaction of rolled asphalt” BSI, London.
3. British Standard Institution, (2005) BS 4987-1 “Coated macadam
(asphalt concrete) for roads and other paved areas. Specification
for constituent materials and mixtures” BSI, London.
4. British Standard Institution, (2005) BS 4987-2 “Coated macadam
(asphalt concrete) for roads and other paved areas. Specification
for transport, laying and compaction” BSI, London.
5. Nunn M.E. (1994) “Evaluation of Stone Mastic Asphalt (SMA): A
high stability wearing course material” TRL Project Report 65, TRL
Limited. ISSN 0968-4093.
14
6. Highways Agency. Manual of Contract Documents for Highway
Works, Volume 1: Specification for Highway Works, The Stationery
Office, London.
7. Highways Agency. Design Manual for Roads and Bridges. Volume
7, Section 5 “Bituminous surfacing materials and techniques”. HD
37/99 Amd No1, The Stationery Office, London.
3.5.
Summary
• Introduction of improved safety standards and reduction of risk
associated with installation of NTS, led to the decline of traditional
HRA as the preferred surface material.
• NTS materials, originally based upon continental European practice,
have been modified to provide adequate surface texture for use in
the UK.
• Proprietary thin surfacing in the UK has evolved and is now certified
by the BBA through the HAPAS protocol. A summary of the
certification process is provided for ease of reference and
understanding.
15
4
SITE CONSIDERATIONS AND
INSTALLATION
4.1.
General
4.1.1.
A thorough understanding of the engineering performance of the
existing pavement substrate upon which the surface is to be placed is
a key factor for maintaining serviceability.
4.1.2.
Key elements comprise:
• A suitably resilient substrate on which to place the surface material;
• Selection of an appropriate surface material or system.
4.1.3.
An enquiry questionnaire presented in Appendix C was issued to all
highway authorities to establish local aspects for surfacing related to:
•
•
•
•
4.1.4.
The summary findings from the questionnaires are presented in
Appendix D and illustrate:
•
•
•
•
•
4.1.5.
Site situation;
Materials;
Performance prediction;
Serviceability and performance.
Cracking and fretting to be the most common failure mechanisms;
The key role of the substrate;
The importance of bond with the substrate;
Approximately equal use of generic and proprietary specifications;
Surface serviceability, in general, was good or better than
traditional materials, but performance in high stress situations was
indicated not to be quite so good.
Analysis of the enquiry questionnaire response led to regional
interactive participatory workshops in Birmingham, London and
Huddersfield. The format for these workshops is summarised in
Appendix E and provided the opportunity for practising maintenance
engineers to address the issues of:
• Site evaluation;
• Materials selection.
4.1.6.
The views of a cross section of installers with experience in NTS were
sought in parallel with authority enquiries. These observations are
summarised in Appendix I and contributed to the subsequent
development of decision charts for substrate evaluation and material
selection which followed the participatory workshops.
16
4.2.
Serviceability
4.2.1.
The local highway has generally evolved over time. For a typical
length of carriageway various structural strengthening, surfacing or
repair treatments have been undertaken in the past. These measures
have resulted in a network which is variable both in terms of thickness
and types of construction materials. Consequently, these pavements
are significantly different from designed highways such as motorways
which perform in a stable and long life manner. In comparison, the
local highways are much thinner and more susceptible to structural
changes resulting from increased traffic loading and subgrade
moisture conditions.
4.2.2.
Over time the local highway has come into equilibrium with its
surroundings including underlying soil conditions, traffic loading and
drainage. Many of the materials used in previous construction and
repair are not now in common use and as a result of just in time
maintenance practices may have become distressed through cracking
and embrittlement. The relative proximity of the subgrade results in the
thinner pavement being intrinsically more flexible in its structural
behaviour under traffic loading. At the surface there are many traffic
braking, accelerating and turning manoeuvres and consequently any
surfacing material has to be capable of absorbing these forces in
addition to working in harmony with the underlying pavement.
4.2.3.
Compatibility of strain is important if the surfacing is to perform well in
service. The inclusion of a stiff brittle layer in an otherwise relatively
flexible structure is unlikely to yield longevity of performance. The
selection of surfacing therefore, needs to consider the ability to provide
strain compatibility, as well as the polishing resistance and texture
characteristics, which are necessary for safety. The nature of the
evolved pavement and the condition and type of pre-existing materials
requires that careful consideration should also be given to the porosity
of the new surface material. The introduction of water through a more
porous surface can lead to serviceability distress, especially when the
pavement is in an existing state of limiting equilibrium. This can be
ameliorated with the use of bond coat and an impermeable binder
course, providing there is an effective drainage system for the
carriageway.
4.3.
Substrate – Existing Pavement
4.3.1.
The condition of the existing pavement which will form the support
platform for the surface is a key factor in the overall performance.
4.3.2.
Five distress features form the basis of condition evaluation for the
existing road:
•
•
•
•
•
Oxidisation;
Fretting;
Cracking;
Texture;
Rutting.
17
4.3.3.
Oxidisation of the surface results in a dull or burnt appearance due to
the degradation of the exposed hydrocarbon binder. This can result in
wrinkle cracks and the road surface has a tired appearance since the
binder component has reached the end if its ductile life. The depth of
distress is probably not great unless the oxidisation is well advanced
or other deeper seated distress factors are also present.
4.3.4.
Fretting of the aggregate and/or matrix from the pavement surface
occurs when the micromechanical bond between binder and
aggregate reaches a critical point. The mechanisms which cause this
to occur are complex and frequently interactive but are likely to be
triggered by environmental factors. Rapid failure can occur with water
pressure and suction effects on the surface resulting from the passage
of vehicle tyres. In a matrix dominated material, such as HRA, this
process occurs slowly as these materials are generally impermeable
and environmental intrusion is very limited, but in aggregate
dominated materials, such as NTS, once the lateral support of one
particle is lost, fretting can occur swiftly and progressively.
4.3.5.
Cracking of an existing surface is frequently the most difficult to define.
From a surface material selection perspective it is important to
establish if the cracking is surface initiated and is propagating
downward or has been initiated at depth and propagating upwards. If
the root cause of the cracking incorrectly diagnosed then a
replacement surface may simply crack again after a relatively short
period of service life. Three principal types of cracking have been
established:
• Longitudinal;
• Map;
• Transverse.
4.3.6.
Dependent upon severity and crack propagation direction, a single
layer, two layers or multi layer solution may be required and in some
cases the inclusion of a Stress Absorbing Membrane Interface (SAMI)
may provide additional safeguard against crack propagation.
4.3.7.
Surface texture is a key safety feature for high speed skid resistance.
Any texture loss or gain will only occur within the existing surface layer
and is a defect which can be corrected relatively easily with a single
layer solution.
4.3.8.
Rutting can owe its origin either in the surface course (non-structural
rutting) or from a deeper seated origin, and it is important to identify
the root cause. Non-structural rutting, which is caused by deformation
within the surface, is generally accompanied by horizontal pushing of
material at the margins of the longitudinal rut. Deeper seated rutting
has a more general depressed area beneath the overall surface level.
Both surface material rutting and deeper seated rutting can yield the
same numerical measure of rut depth but cause must be understood
in order to ensure an effective surfacing solution.
4.3.9.
The addition of an overlay, wherever practicable, will add to the overall
structural capacity of the pavement.
There is little published
information on the extended structural life for overlays which are less
than 40mm in thickness. However, it is considered that overlays of
18
30mm thickness do offer some structural contribution to thin evolved
pavements The structural contribution effect of an overlay 40mm or
greater in thickness can be considerable (Kennedy and Lister, 1978).
In situations where the pavement can be overlain with a new surface
then this should be the first choice option.
4.3.10. Where access levels or other physical features prevent the addition of
an overlay, then an inlay may be the only practical solution. This is
frequently the case in urban areas where the existing defective depth
of pavement may have to be removed and new surfacing material
installed in its place. Inlay construction generates aged material from
the existing pavement which may be recycled in-situ or elsewhere.
However, the inlay itself is unlikely to provide much direct
improvement in structural terms, other than degraded material being
replaced with new material, resulting in the exclusion of surface water
by a newer and less permeable surfacing.
4.3.11. In some urban kerbed areas it may be possible to channel plane close
to the kerb edge to enable the new surfacing to be installed to the
previous road level at the kerb. The remaining central portion of the
road is not milled and in effect there is an inlay at the edge and overlay
in the centre. The key area is the transition from inlay to overlay at the
milled step in the existing pavement. When a new surfacing is installed
in this fashion the thickness can be at its thinnest at this point and this
may be directly in alignment with the nearside wheel track.
4.3.12. Where planing operations in the existing pavement are required to
enable inlay surfacing, care should be taken to ensure that the planed
depth is such that each old layer which is disturbed is removed in its
entirety. The thickness of the existing layers and their condition will be
known from investigation or local knowledge. Remnants of old layers
which are not removed can have a detrimental effect on the in-service
performance of the inlay.
4.4.
Bond Coat
4.4.1.
All pavement layers need to be bonded to the layers above and/or
below to assure the maximum structural efficiency of the whole
pavement structure under vertical loading.
4.4.2.
An effective bond between layers enables horizontal shear stress
between layers to be transmitted. Greatest efficiency is obtained
where the horizontal shear strain in a bond coat is small and
approaches that in the adjacent asphalt layers. Horizontal shear
stresses are small by comparison with the tensile stress in the overall
pavement but nonetheless need to be fully transmitted.
4.4.3.
Adhesive bond becomes critical in near surface layers. At points
remote from an applied wheel load normal stresses can be tensile and
thus no frictional transmission of horizontal shear is possible and total
reliance must be placed on the bond coat. Additionally, at shallow
depths the horizontal shear forces due to braking, turning, traction on
gradients, and differential thermal movements are far more
concentrated than at greater depths.
19
4.4.4.
The efficient transmission of horizontal shear stress is critical to the
function of the pavement as a whole. The bond coat is essential to
ensure the survival of the surface course. Fretting, cracking and
delamination have been recorded to be the most serious and frequent
forms of deterioration of NTS and since fretting and delamination are
frequently initiated by cracking it follows that a good bond coat is also
beneficial in slowing failures due to cracking.
4.4.5.
Virtually all thin surfacing systems are more permeable than the HRA
material they have replaced. The ingress of water particularly in undesigned pavements is the major cause of deterioration. Thick bond
coats can act as a water barrier and assist in extending the life of a
pavement. However, the existence of a horizontal water barrier puts
particular importance on the performance of the surface layer when
saturated.
4.4.6.
BS594 and BS4987 define tack coat as K1-40 or K1-60 bitumen
emulsion conforming to BS434-1 and bond coat as proprietary
material having performance characteristics certified by BBA. When
certifying a complete thin surfacing system, rather than a bond coat,
BBA also differentiate between tack coat and bond coat. The certified
torque bond values on a BBA certificate only usually relate to a single
tack/bond coat even when the certificate covers a number of bond/tack
alternatives.
4.4.7.
The Notes for Guidance on the “SHW Clause 920” Bond Coats, Tack
Coats and other Bituminous Sprays suggests a classification into three
categories, based upon the Vialit Pendulum Energy Loss Test as
illustrated in Table 2.
Table 2 - Classification of Bond Coats by Vialit Test Value
Classification
Vialit pendulum, peak cohesion
value (J/cm2)
>0.5*
>1.0
>1.2
Conventional (UV)
Intermediate (IV)
Premium (PV)
*Inferred from Table NG913
4.4.8.
Alternatively, bond coats may be classified by the torque bond test.
The torque bond strengths in Table 3 are derived for the BBA
certificated values. Test variability combined with substrate variability
make the application of this approach difficult for site specific
application and therefore, the use of the values in specifications is not
recommended until further research is completed. The ranges in Table
3 are based upon best available knowledge and may benefit from
revision in the future but illustrate three categories of classification for
bond coat. If site measured values of torque bond strength are less
than forty percent of the values shown in Table 3 then further
investigation should be carried out.
20
Table 3 - Classification of bond coats by torque bond strength
Classification
Conventional (UT)
Intermediate (IT)
Premium (PT)
Torque bond strength (kPa)
>250
>500
>1000
Note: The tabulated values are for guidance only - no correlation with the values in Table 2 is implied nor established.
Selection
4.4.9.
Selection is dependent primarily on:
•
•
•
•
Substrate condition;
Surface thickness;
Surface type;
Site stress conditions.
4.4.10. Suggested grades of bond coat which are likely to be appropriate are
presented in Table 4. These are grouped into three categories;
conventional (U), intermediate (I) and premium (P).
4.4.11. For those surfaces which are covered by generic specifications and
the site condition criterion indicates differing requirements for minimum
bond coat, the highest overall grade should be selected.
4.4.12. For those surface systems which are proprietary in nature the BBA
certification should take precedence but the authority should still
evaluate the site against the condition criteria in Table 4. If this results
in an anomaly with the proprietary proposals then it should be brought
to the attention of the system proprietor.
Table 4 - Selection of Bond Coat
Recommended
coat
Condition
Substrate sound and
rugous
Substrate smooth /
polished
Substrate
lacking
bitumen
Substrate
slight
cracking
Surface thickness
30-40mm
20-30mm
<20mm
Site
stress
level
(NG942)
U
I
P
P
U
I
P
U
I
P
P
1
2
3
4
21
bond
4.4.13. Bond coats should be spread at the rates in the BBA certification for
proprietary systems or at the rates in BS 594 or BS 4987 for generic
requirements as appropriate. There should be a continuous even
spread without puddles of coating and the rate of spread should be
regularly checked.
4.4.14. Spraying should not be too far in advance of the paving process and if
there is any sign of picking up by delivery trucks the bond coat should
be lightly gritted. Self-propelled sprayers or integral paver mounted
spray bars should always be used where appropriate. When integral
spray bars are used the bond coat formulation adopted should ensure
virtual instant breaking of the emulsion. For the best quality of work
and least traffic disruption integral spray bars applying intermediate or
premium grade bond coat are preferred to other systems.
4.5.
Early Life Surface Characteristics
4.5.1.
Previously much of the surfacing on the local network was of matrix
dominated HRA type. This surfacing displays positive texture as
chippings are embedded in the material to provide the skid resistant
surface.
4.5.2.
The film of bitumen which coated the chippings in the surface of HRA
was very thin (circa 2 microns) and was generally unmodified residual
petroleum bitumen. The passage of vehicle tyres removed this thin film
rapidly due to the scrubbing effect of the constantly flexing tyres, the
relative roughness of the surface and the positive texture encountered
in resistance to rolling. The aggregate mineralogy of the chipping was
exposed swiftly to deliver the in-service skid resistance.
4.5.3.
Negative texture surfaces as provided by NTS are significantly
different in their early life characteristics. There is less resistance to
rolling due to the negative texture and the tyres do not impart the
same scrubbing effect as the wheel travels forward. The tyres pass
over the surface with only the vertical component of load having a
wear effect.
4.5.4.
The binder film is much thicker (circa 10 microns) for SMA negatively
textured surface material. This results from the need to formulate the
thin surfacing material as a whole to produce a stable mastic structure
to support the mineral skeleton of aggregate. To enhance the binder
stability and durability and to prevent binder drainage during transport,
additives such as cellulose fibres or polymers are often used. These
binder modifications result in a greater tenacity and adhesion between
the binder and the mineral aggregate. With the combination of a thick
binder film and the inclusion of modifiers, a substantially greater
number of wheelovers are required to wear the binder film and expose
the aggregate as the skid resistant surface.
4.5.5.
Under the action of vehicle braking in an emergency on NTS, the thick
binder film may be mobilised, either by melting or shearing. This plays
an active role in effecting the maximum level of friction that can be
generated between a locked and sliding tyre and pavement surface
during the braking manoeuvre.
22
4.5.6.
4.6.
The early life dry skid resistance of asphalt surfaces (Roe and
Lagarde-Forest, 2005) has confirmed that the phenomena can be
attributed to the presence of a film of binder, that can adhere to the
surface of aggregate particles for a significant period of time.
Skid Resistance Characteristics
4.6.1.
Until recently, almost all consideration of skid resistance in the
maintenance of highways has been directed towards the investigation
and routine monitoring, of frictional properties of the surfaces of wet,
rather than dry, roads.
4.6.2.
Skidding resistance survey requirements are described in the Code of
Practice, (Section 9.8), which recommends that authorities should
publish their skid resistance strategy as part of their Highway Asset
Management Plan. The mechanisms and physics of skid resistance
are complex. Vehicle tyre interaction with the surface is one of the key
characteristics and beyond the scope of these Guidelines. Useful
information and references are contained in the Code of Practice and
also in the joint AA Motoring Trust / CSS publication on tyres, road
surfaces and traffic accidents (Get a Grip, 2005 and Bullas, 2004).
4.6.3.
For the purposes of erecting warning signs to alert road users to early
life skidding, the Code of Practice recommends that authorities should
state in their skid resistance strategy that they should either:
• Follow the requirements of Highways Agency Interim Advice Note
49/03; or
• Produce an early life strategy that is generally based upon the
requirements of IAN 49/03.
4.6.4.
For NTS, once the early life stage has been passed and the binder film
removed from the aggregate surface it would be anticipated that the
material would provide a good period of service life before wear and
polishing required future corrective maintenance. NTS materials
generally contain a high proportion of coarse premium aggregate but
the polishing resistance of the fine aggregate also plays a part in the
skid resistance properties of the surface. Careful aggregate choice will
ensure a long service life. Rather than remove and replace a worn or
polished surface, consideration could be given to surface restorative
techniques, to ensure that value for money is delivered from these
premium mineral assets.
4.6.5.
The removal of the relatively thick binder film from aggregate at the
surface in NTS types of material will take time to occur. The two
agencies which have an active role in this process are:
• Vehicle tyres;
• Environment.
4.6.6.
Environmental ageing of the binder film will commence as soon as it is
installed but this is a slow process of gradual embrittlement and is
anticipated to have only a secondary influence in the removal of binder
film in the shorter term.
23
4.6.7.
The primary removal method is a physical one caused by the passage
of vehicle tyres. If a particular type of NTS is used in a heavily
trafficked situation, the binder film will be removed in a much shorter
time than if the same material was used in a lightly trafficked situation.
This has prospective implications for risk evaluation which must be
undertaken in the selection of surface type on the network.
4.6.8.
The early life period of trafficking could be much longer than on the
strategic network because of the potentially lower density of traffic on
the local network. The number of wheelovers necessary to remove the
binder film remains the same irrespective of location and for local
highways with lower traffic densities could take an extended time. The
nature and thickness of the binder film introduces further complexity
since differing types of NTS would be expected to display differing
binder film thicknesses and also different binder types, such as
unmodified bitumen, cellulose modified bitumen, polymer modified
bitumen or cellulose / polymer modified bitumen. The relative wear
rates and number of wheelovers to remove the binder film requires
definition, in order that specifiers are better able to predict the duration
of early life characteristics in various situations on their network.
4.6.9.
In situations where a high friction surface is to be placed over NTS but
the surface will be trafficked prior to the application of the high friction
surface, then 3mm grit should be applied and rolled in to provide
enhanced short-term skid resistance (HD37/99). The use of surface
applied grit on NTS on the local network may also be appropriate to
accelerate the removal of thick binder film in early life in those
situations which are required to exceed a minimum skid resistance
intervention level.
4.7.
Horses on the Highway
4.7.1.
The particular friction characteristics of steel shod horses on thick
binder film TS and SMA is described in guidance produced jointly by
CSS and the British Horse Society entitled “Horses and Highway
Surfacing: A Guidance Note for Highway Authorities”. (CSS, 2005).
4.7.2.
The guidance includes recommendations for the application of grit
during the construction process at locally determined sites, which may
include gradients, normal routes used by horse riders and roads
adjacent to racecourses and riding stables.
4.8.
4.8.1.
Installation
The two key elements of successful long-term serviceability for
surfacing comprise:
• A sufficiently resilient substrate on which the surfacing is placed;
and
• A high standard of workmanship in the placement and compaction
of the material.
4.8.2.
Authorities who adopt a generic surfacing have direct control and
responsibility for selection and specification of both bond/tack coat and
also the component elements of the surfacing material. The substrate
24
will also have been evaluated by the authority to ensure that it is
sufficiently competent for the required surface performance.
4.8.3.
Proprietary TS systems, unless defined more closely in site specific
requirements, require much more decision making by the system
proprietor in terms of selection of options, within the scope of the BBA
certificate relating to his system. The requirements and processes of
BBA certification and client confidence expectations are presented in
Appendix F.
4.8.4.
Proprietary system installers have detailed methodologies and
requirements relating to their system encapsulated within their Quality
Plan which is held in confidence between the system proprietor and
BBA. Protocols may be available for substrate evaluation as part of the
system but the scope and extent of these is unlikely to be known.
4.8.5.
Substrate resilience is a significant factor in terms of strain
compatibility and integral performance with the characteristics of the
installed surface. Substrate evaluation should be undertaken through
objectively defined limits and properties, together with identification of
any flaws which require remedial works prior to surfacing installation.
4.8.6.
Once an appropriate test method has been determined to characterise
flexibility of NTS, and other traditional surface materials, the
compatibility of this with the substrate flexibility, perhaps determined
by deflection devices, will enable more refined selection. At present,
however, these processes are not developed and may not be so for
some time in the future.
4.8.7.
To provide clarity of engineering performance, greater transparency of
the decision making process relating to substrate condition would be
beneficial. It is assumed that objective inspection and condition data
already exists within proprietary system installer organisations. This
data is of value both in maximising the long-term performance of
materials and in highlighting likely areas of future distress, both of
which will ultimately provide better prediction of serviceability for risk
and asset management planning.
4.8.8.
The best surface is only as good as its weakest elements and for most
NTS it is the edges, ends of loads, tie-ins, longitudinal joints between
adjacent paver rips and transverse construction day joints. Successful
and durable joints can only be formed whilst the material is hot or
sawn and the vertical face fully coated with bitumen. If the formation of
joints is not undertaken to a high standard then the joint interface can
become a focus for the commencement of fretting under trafficking.
This can lead to premature distress in an otherwise serviceable
surface.
4.8.9.
The location, position and pattern of joints in highly stressed areas
require particular consideration in relation to vehicle wheel paths,
where there may be lateral loading effects. Saw cut slots for induction
loops in NTS at traffic light approaches can also be a focus for
initiation of fretting. The avoidance of dynamic impact loading from
heavy goods vehicles, especially on thinner surfaces, can be
addressed through ensuring evenness of the surface during
installation and the setting of ironwork at finished road surface level.
25
4.9.
Installers and Inspection
4.9.1.
The views and observations of a cross section of installers relating to
substrate conditions, materials selection, client role, placement and
other key issues are presented in Appendix I.
4.9.2.
It is good practice for every site to be inspected by experienced and
knowledgeable personnel. In situations where a generic specification
is used for the surface, any potential adverse interaction of the chosen
surface with the substrate is a risk for the authority. For proprietary
surface systems a joint inspection by the authority and the system
proprietor has merit in highlighting awareness of substrate
characteristics and improving the performance prediction of the
selected proprietary surface system.
4.10. References
1. Kennedy. CK and Lister, NW. (1978). “Prediction of Pavement
Performance and the Design of Overlays”, TRRL Report LR833,
TRL Ltd, Crowthorne. ISSN 0305-1293.
2. Roe, PG and Lagarde-Forest. R (2005). “The early Life Skid
Resistance of Asphalt Surfaces”. Report PPR060, TRL Ltd,
Crowthorne.
3. CSS (2005). “Horses and Highway Surfacing”. Guidance Note,
www.cssnet.org.uk .
4. The AA Motoring Trust/CSS (2005) “Get a Grip”. www.AAtrust.com
or www.cssnet.org.uk .
5. Bullas. JC. (2005), “Tyres, Road Surfaces and Reducing Accidents:
a review”. www.AAtrust.com
4.11. Summary
• NTS performance was found to be as good as traditional materials
except in some specific areas and applications.
• The substrate plays a key role in the overall pavement serviceability.
• Adhesive bond between the surface and the substrate was
•
•
•
•
•
identified as a key performance parameter and a bond coat
selection process has been developed.
The survey indicated that there was almost equal procurement of
surfacing by authorities using generic specifications as there was
with proprietary systems specifications.
The local network has evolved over time and is often maintained in
a state of limiting equilibrium. It is relatively thin and variable in
terms of construction materials. This contrasts with the case of an
engineered, stable, long life pavement.
The influence of water and porosity were identified as key aspects
in terms of performance.
The essence of good performance is attributed to the installation of
a strain compatible surface, which acts in concert with the intrinsic
flexing movements of a thin pavement.
Overlay should be considered as the first choice option for
surfacing owing to potential structural contribution but where inlay is
26
•
•
•
•
necessary the milling processes must be such that there are no
remnants of weak existing materials.
Attention to detail in the location and formation of joints especially in
highly stressed areas, will avoid premature distress in an otherwise
serviceable surface.
Early life characteristics are described in terms of binder film
thickness and removal by vehicle wheelovers. An assessment
should be undertaken to determine the approximate length of time it
will take to remove the binder film by traffic, based upon a forecast
of numbers of wheelovers likely to be applied on a particular
highway.
Guidance has been published elsewhere in respect of TS and SMA
for areas where equestrians may use the network and recommends
the use of grit applied to the freshly laid surface, to enable more
rapid abrasion of the thick binder film.
For generic style specifications the risk for serviceability resides
with the client organisation. For proprietary systems there is benefit
in joint site inspections by the client and system proprietor to ensure
that the most appropriate TS system is selected and installed.
27
5
THE RIGHT MATERIAL FOR
THE RIGHT SITE
5.1.
Site Evaluation
5.1.1.
The evaluation of an existing site comprises the use of a checklist and
four decision charts A to D inclusive.
5.1.2.
The initial step in the evaluation process is to collate all of the
available data relating to the site and to undertake a walk through
inspection by experienced personnel. The walk through inspection will
enable collection of all of the condition information necessary to use
the decision charts. A checklist of details to be collected and
assembled is presented in Table 5 and comprises:
•
•
•
•
•
•
5.1.3.
Geometry and layout;
Traffic;
Road function;
Condition;
Road surface;
Data availability.
Four decision charts compliment the checklist and provide route maps
for evaluation of the existing site conditions.
28
Site Checklist for NTS
Road No:
Location:
Aspect
Details (for completion)
Considerations
Geometry and Layout
Road type
Situation
Junctions or crossings
Exposed site
Gradient
Direction facing
Bend
Roundabout
Road closure possible
Dual, single.
Urban, rural.
Frequency , type, etc
Yes/No
Flat, level, gentle, steep.
North, south, east or west
Yes/No
Yes/No
Yes/No
Traffic
Speed limit
Braking area
Accelerating area
Turning - Bends
Turning - Roundabout
Commercial vehicles
Reduced spray required
Reduced noise required
20, 30, 40, 50, 60, 70 mph
Yes/No
Yes/No
Gentle, acute, etc
Radius: tight/int./gentle.
High, medium, low
Yes/No
Yes(+40mph)/No
Road Functional Hierarchy
CoP function category
1, 2, 3a, 3b, 4a, 4b
Conditions
Surface drainage
Highway drainage
Site prone to flooding
Edge restraint
Restriction on overlay.
Restriction on inlay
Good/fair, falls, gullies, etc
Ditch, filter drain, etc
Yes/No
Good (kerb/strip)/Poor
Thresholds, access, etc
Practicality, etc
Existing Road Surface
Walk through inspection
Surface material
Oxidised
Fretting
Cracking - longitudinal
Cracking – map (craze)
Cracking - transverse
Texture
Rutting
Delamination
Damp or wet areas
Subsidence areas
Yes(date)/No(reason)
HRA, DBM, NTS,etc
Yes/No
Yes (severity)/ No
Yes (severity)/ No
Yes (severity)/ No
Yes (severity)/ No
Good, average, poor
Yes (severity)/ No
Yes/No
Yes/No
Yes/No
Condition Data availability
CVI
DVI
Core records
Exploratory pit records
Deflectograph survey
FWD survey
Ground penetrating radar
Yes/No
Yes/No
Yes/No
Yes/No
Yes/No
Yes/No
Yes/No
Table 5 - Site evaluation checklist
29
5.2.
5.2.1.
5.3.
Chart A
Decision Chart A starts with entry of the site into the works programme
with key decisions based upon the degree of lateral restraint. This
leads to either Chart B or Chart D.
Chart B
5.3.1.
This chart describes the process for sites with poor lateral restraint (i.e.
no specific edge restraint such as kerbs or hardened strip).
5.3.2.
The depth of the distress beneath the surface leads to a number of
pathways based upon superficial depth (Surface selection Chart E),
depth within 100mm (Chart C) or deeper seated where investigation is
required.
5.3.3.
A complimentary investigation protocol is presented in Appendix G.
5.4.
5.4.1.
Chart C
Chart C describes distress features which originate from near surface
horizons as five key types:
•
•
•
•
•
5.4.2.
Each of these pathways leads to a surface solution, compatible with
SCANNER condition data, selected from the following:
•
•
•
•
•
5.4.3.
5.5.
5.5.1.
Oxidised surface;
Fretted surface;
Cracked surface (in three types);
Surface texture;
Rutting.
Maintain observation – do nothing;
Overlay – single layer;
Inlay – single layer;
Overlay – more than one layer;
Inlay – more than one layer.
Overlay should be selected, where practicable, as the first choice
option to enhance pavement structural capacity. Where non-standard
solutions are considered, such as strain absorbing membrane
interfaces to address crack propagation, then the solution selection is
a bespoke process.
Chart D
This chart describes the process for sites with good lateral restraint
which tend to predominate in urban areas and again leads to options
for design solutions.
30
Chart A
Site Evaluation
Project Site Prioritisation
•
•
•
•
•
Condition based prioritisation
Visual condition inspection
Scanner priority schedule
Traffic safety
Local knowledge
Functional Classification
(Well Maintained Highways)
Are access levels adequate to
overlay the road?
YES
NO
Overlay or inlay solutions
Inlay solutions
Does the site have good
lateral restraint?
(Note 1)
Does the site have good
lateral restraint?
(Note 1)
YES
Chart D
NO
YES
Chart B
Chart D
NO
Chart B
Note 1 The road surface drainage should be evaluated also as this may have a
bearing on the effectiveness of edge restraint
Figure 3 - Chart A
31
Chart B
Site Evaluation
Unkerbed or Poor Lateral
Restraint Site
Nature of Site Distress
Deep
(binder/base/foundation)
NO
Superficial
(within 30mm)
YES
NO
Surface Material Selection
(Chart E)
YES
Surface Material Selection
(Chart E)
Foundation or Base
Unclear
Surface or Near Surface
Layer (within 100mm)
Engineering Analysis
Investigation
(Note 1)
Established with confidence
that there is no deeper
seated failure
Unlikely that NTS alone will
rectify this type of deeper
seated defect
Engineering Analysis
Near Surface Defect
Evaluation (Chart C)
Design bespoke solution
which could include a
Negative Texture Surface
Unlikely that NTS alone will
rectify this type of deeper
seated defect
Design bespoke solution
which could include a
Negative Texture Surface
Note 1 Investigation protocol - Appendix G
Note 2 If the road pavement is structually sound then a single layer solution may be practicable but if
the pavement is not structually sound a two (or more) layer solution will be necessary.
Figure 4 - Chart B
32
Oxidised
Fretting
Fretting
Less
Solution
C
Fretting
More
Is surface sufficiently
intact?
Solution
A, B or C
Site Evaluation
Chart C
Sheet 1 of 2
Developing/
unacceptable
Surface of Near Surface Layer
Defect (within 100mm of road
surface)
Stable/
acceptable
Solution
C
Rutting
Too smooth/
too rugous
Solution
A
Texture
Good/
acceptable
Solution
B or C
33
Nature of Distress
Cracking
or
crazing
Chart C
Sheet 2 of 2
Solution
A
Solution A - Do nothing (maintain observation)
Solution B - Single layer Overlay (40mm or less)
Solution C - Single layer Inlay (40mm or less)
Solution D - Thicker -two layer - Overlay (More than 40mm)
Solution E - Thicker - two layer - Inlay (More than 40mm)
Figure 5 - Chart C (1 of 2)
Chart C
Sheet 2 of 2
Establish Crack Type
Longitudinal
Deeper seated
and combined
with rutting/
subsidence?
YES
Map
Lack of
lateral
restraint?
Parallel
NO
Transverse
Stable
Deteriorating
intergrity
Solution
C
Solution
E
Reflection pattern?
YES
NO
Investigation
(Note 1)
Investigation
(Note 1)
YES
NO
Design as
Chart D
Engineering
Analysis
Engineering
Analysis
Solution
C
Solution D or E
dependant
upon crack
depth
Unlikely that NTS
alone will rectify
this defect
Design bespoke
solution which
could include a
negative texture
surface
Assess crack
severity and
frequency
Investigation
(Note 1)
If stable and
minor
cracking
Engineering
anaylsis
Solution D
or E
(Elastomeric
binder)
Solution A - Do nothing (maintain observation)
Solution B - Single layer Overlay (40mmor less)
Solution C - Single layer Inlay (40mm or less)
Solution D - Thicker -two layer - Overlay (More than 40mm)
Solution E - Thicker - two layer - Inlay (More than 40mm)
Note 1
Investigation protocol - Appendix G
Figure 6 - Chart C (2 of 2)
34
Unlikely that NTS
alone will rectify
this defect
Design bespoke
solution which
could include a
negative texture
surface
Chart D
Site Evaluation
Good Lateral Restraint Site Kerbed or Hardened Strip
Nature of Distress
Deep
(in excess of 100mm)
The development of
deeper seated distress in
an established, good
lateral restraint road is
generally unlikely. If there
is any doubt about the
cause of distress then this
must be evaluated by
investigation and an
appropriate solution
designed which could
include a negative texture
surface.
Near Surface
(within 100mm)
Unclear
Investigation
(Note 1)
The five distress
mechanisms in Chart C all
apply. The likely treatment
will be inlay where overlay
is impractical
Engineering Analysis
Design bespoke solution
which could include a
negative texture surface.
Solutions A, C
or E as
appropriate
Solution A - Do nothing (maintain observation)
Solution B - Single layer Overlay (40mm or less)
Solution C - Single layer Inlay (40mm or less)
Solution D - Thicker -two layer - Overlay (More than 40mm)
Solution E - Thicker - two layer - Inlay (More than 40mm)
Note 1
Note 2
Investigation protocol - Appendix G
Local knowledge of kerbed/good lateral restraint sites - usually in urban
areas - can be a key issue especially the maintenance history in service
and the nature, characteristics and variability of road foundations and
drainage.
Figure 7 - Chart D
35
5.6.
Chart E
5.6.1.
The pathway for selection of the surface is presented in Chart E.
5.6.2.
The key input parameter is Road Function Category in accordance
with the Code of Practice. This sets the traffic speed which leads to
surface texture requirements, the aggregate size to deliver the
required texture and thereafter to layer thickness. Selection can be
from one of five categories of surface texture. On higher stress sites,
such as roundabouts, it is recommended that the smaller sizes of
aggregate should be selected as appropriate from the range of
optional aggregate sizes.
5.6.3.
Key performance factors are described in four areas:
•
•
•
•
Site;
Strain compatibility;
Durability;
Installation.
5.6.4.
For selection of the most appropriate surface, the relevant key
performance factors for the particular site applications must be
reviewed against the properties of the surface material to ensure
compatibility.
5.6.5.
The key performance factors and key attributes for surfaces should be
considered together in a technical review evaluation. The purpose is to
establish the engineering requirements to take forward into
specification for site specific application. There are many variables to
be examined in this process, since each site is unique in terms of
characteristics and there is a broad array of surface materials which
could be considered. The purpose of Chart E is to present a process
mechanism for consideration of all of the various facets and interplay
between surface and substrate rather than to lead to a specific
definitive solution.
5.6.6.
Based upon the engineering requirements the information and
guidance in Appendix H should be used to compile a job specific
specification Appendix or Engineering Schedule for proprietary system
surfaces. For generic specifications the information presented in
Appendix H provides assistance in compilation and includes the facets
which require consideration.
5.6.7.
The process will provide confidence that the right material will be
installed in the right site and that the surface characteristics for each of
the key performance factors are identified and translated into
specification requirements.
36
Chart E
Sheet 1 of 2
Surface Material Selection
Road Function Category
Primary
Selection
Factor
Traffic Speed greater
than 40mph?
Yes
No
Surface
Texture
Category 3
Texture
Category 2
Texture
Category 1
Texture
Category 0
Aggregate
Size (mm)
20/14/10
14/10/6
10/6
10/6
Thickness/
Design Life
Min 2.5 x aggregate size or proprietary system requirements
Continued on
Chart E
Sheet 2 of 2
Texture Category 0
Texture Category 1
Texture Category 2
Texture Category 3
Figure 8 - Chart E (1 of 2)
37
-1.0mm
+1.0mm
+1.2mm
+1.5mm
Figure 9 – Chart E (2 of 2)
38
Site specific
requirements
Definition
Engineering
requirements
Key attributes
for surfacing
Performance
Factors
Site
Flexibility
Substrate movements
Waterproofing
• Characteristics to match
substrate
• Substrate residual cracking
• Substrate porosity
• Cyclical subgrade movement
Strain compatability
Durability
Bond
Air voids
Aggregate gradation
Binder content
• Drainage
• Surface porosity
• Surface thickness
• Freeze and thaw
Bond
Thickness
Temperature
Weather conditions
• Substrate condition
• Bond or tack coat
• Levels and min thickness
• Winter installation
• Traffic management/access
• Aftercare
Installation
Chart E
Sheet 2 of 2
Proprietary
Include key requirements in Specification Appendix 7/1 or Engineering Schedule for partnership contracts
Generic
Specification
Review each key performance factor in relation to surfacing material characteristics to identify engineering requirements
Bond
Aggregate size
Aggregate gradation
• Traffic braking or turning
• Tight radius curves/bends
• Incline
• Orientation, facing direction
• Degree of exposure
• Traffic stress level
Continued from Chart E
Sheet 1 of 2
Surface Material Selection
6
EXPIRED LIFE ENGINEERING
6.1.
Introduction
6.1.1.
NTS has been adopted relatively recently for the local network and
has been reported to perform at least as well, if not better, than
traditional materials in most situations. When first introduced there
was perhaps some absence of appreciation as to what could be
achieved with NTS and doubtless there were some situations where
the wrong material was used on the wrong site. This is less likely to
occur in the future as more experience is gained and the guidance in
this document adopted.
6.1.2.
At some point, however, all surface materials will reach the end of
their useful service life. The mechanics of distress for NTS are, to a
degree, conjectural but are based upon a broad base of practitioner
experiences, blended with theoretical concepts to produce a first
generation deterioration model. As more in-service data and
experience is accumulated, the model can be further developed and
refined.
6.1.3.
In order to plan and programme surface maintenance works in an
effective way, it is important to understand the mechanics of distress,
so that considered engineering decisions may be made regarding
remediation, as well as to provide a prospective means of forecasting
remaining service life.
6.1.4.
Deterioration mechanisms for NTS are considered in relation to their
expired life condition and the various forces which act upon the
surface throughout its service life. A deterioration model is developed
within a framework of both internal and external influences. Although
focussed upon NTS applications the mechanisms are not unique to
these materials and could have more general application with other
types of asphalt surfaces.
6.2.
Failure Groups
6.2.1.
A comprehensive technical literature review has revealed that there is
little definitive published research into the deterioration mechanisms
that govern the life of NTS. Limited study has been undertaken in
Germany (Private communication, 2005) into some problems with
splizmasticasphalt. NTS surfaces do not always function satisfactorily
to the end of their design life and further study is required to quantify
the engineering variables and values that govern the onset of distress
in service. Mechanisms of failure have been developed based upon
observed performance in-service and upon the views of practicing
engineers gleaned through the development of these Guidelines.
6.2.2.
The deterioration mechanisms are categorised into groups, defined
firstly by the observed mode of deterioration and secondly by internal
and external influences on the system. A series of deterioration types
and mechanisms of deterioration have been identified.
39
6.2.3.
Five groups of influencing factors which act on a surface have been
established. These five influencing factors do not always behave in
isolation. They are frequently inter-dependant and in that way can
produce cumulative effects in terms of performance. Interactive
behaviour is complex to model and for these Guidelines the
deterioration mechanisms are treated individually. However, if two or
more mechanisms occur simultaneously failure will normally be
accelerated.
• Internal
•
•
•
•
6.3.
material based mechanisms, which are largely
independent of external influences.
Traffic loading related forces including direct loading, shear and
tension loading due to braking, acceleration and turning and
compression loading mainly from direct static and dynamic traffic
loads.
Environmental influences largely related to water saturation and to
air exposure leading to the destruction of the aggregate/binder
bond by gradual stripping of bituminous binders.
Substrate strain loading. The substrates onto which NTS are
applied on the local network are frequently not designed pavement
structures and are often subject to vertical and horizontal
movements that impose strains on any overlay or inlay systems.
Interface shear stress. As the surface thickness is reduced there
is an increase in the horizontal shear stress at the base of the
surface layer due to lateral traffic loading. There is also an
increase in the inter-particle stresses within the surface material.
Increase in either type of stress can lead to debonding of the
surface layer from its substrate which may already be in poor
condition as a result of previous strain, thereby providing a reduced
bond with the new surface.
Types of Deterioration
6.3.1.
The deterioration of in-service surfaces has been monitored during the
ten years development period of thin surfacing systems in the UK and
has been classified for visual inspection purposes the BBA specialist
group (SG 3).
6.3.2.
The classes observable on site are described in BBA “Guidelines for
the Assessment and Certification of Thin Surfacing Systems for
Highways”. The same structure is adopted in these Guidelines
although the description is refined to assist in the correlation with
possible causes. Two safety related classes of deterioration, not
included in visual inspections, are also covered namely loss of texture
and surface polishing, as these can be directly measured.
6.3.3.
Deformation under traffic can occur in NTS surfaces but generally
relates to underlying layers. This has not been a significant
performance aspect of NTS in the UK but it has been observed.
Surface deformation is, however, monitored as part of BBA
certification and therefore is included as a possible mode of
deterioration.
40
Variability
6.3.4.
Variability in characteristics or appearance may occur due to a
number of factors and can be considered as:
• Random variability in a mixture which is not related to the location
on the carriageway in respect of traffic loading or geometry.
• Traffic related variability as indicated by transverse differences in
loading between traffic lanes and or across a single lane relative to
wheel paths and paver edges.
Fatting up
6.3.5.
A general or intermittent loss of surface texture without sufficient
exposure of coarse aggregate above the general surface level due to
dominant exposure of the fine matrix of the mixture at the surface,
• Fatting up may occur in patches from the size of a large coin to
several square metres as illustrated in Figure 10. It can occur for a
number of reasons, for example uneven distribution of fibres in a
mix, excess bond coat migrating through the surface or a binder
rich mix which has been over-rolled.
Figure 10- Fatting-up
Loss of chippings from the surface
6.3.6.
The plucking out of chippings in multiple surface dressing systems or
paver laid surface dressing type systems leaving the binder film intact.
Loss of aggregate
6.3.7.
Particles of aggregate dislodged from the surface by the passage of
traffic.
• Loose aggregate observed on and around the carriageway and the
surface shows signs of looseness to the extent that remaining
aggregate can be easily removed from the surface with a pocketknife. This category was introduced for BBA inspection purposes
as a preliminary to the complete loss of aggregate.
41
Loss of mortar
6.3.8.
Fretting of mortar is defined as the loss of mortar (binder, fine
aggregate and filler) from the surface, usually including the adjacent
coarse aggregate.
Growth of vegetation
6.3.9.
Lack of continuity of the mineral skeleton resulting in siltation, seeding
of plants and degradation of more porous mixtures.
Ponding
6.3.10. Local subsidence of the surface to allow the retention of water in the
body of the material, thereby leading to deterioration of the
aggregate/binder bond.
Delamination from the substrate
6.3.11. Delamination is the complete separation of a surface from the
substrate. The failure horizon may be:
• Within the surface mixture, but close to and parallel to the substrate
interface resulting in the virtual complete separation of a section of
surface course.
• Along the plane of the interface resulting in the complete
separation of a section of surface course within the substrate
material.
Stripping
6.3.12. Stripping describes the separation of binder from aggregate.
• Stripping is most visible in the coarse aggregate but also occurs in
the fine aggregate with equally serious consequences.
• The mechanism is largely associated with water ingress in the
mixture.
• Stripping usually results in fretting but can give rise to other forms
of deterioration such as debonding.
Cracking
6.3.13. Crack patterns can be classified as:
•
•
•
•
Longitudinal;
Transverse;
Subsidence;
Map (or crocodile) cracks - A series of interconnected cracks over
an area of carriageway spaced between 100mm to 500mm;
• Fine cracking;
• Very fine cracking with the cracks less than 0.5mm wide or 500mm
long, which can be observed across a surface and passing around
coarse aggregate particles;
• Reflective cracking.
42
Loss of texture
6.3.14. As a surface wears the rugosity of the surface macro-texture may
reduce.
• The maximum permissible rate of reduction of texture is classified
in the BBA Guidelines and measured by the glass spheres patch
test.
Polishing
6.3.15. All surfaces can loose skid resistance due to polishing (loss of microtexture) under traffic loading, particularly the braking, acceleration and
turning of heavy vehicles.
Wheel track deformation
6.3.16. Traditional surfacing materials (notably HRA) have sometimes
exhibited rutting under heavy and slow moving canalised traffic.
• Surface deformation has been less widespread on the local
network than for motorways and trunk roads owing to the differing
traffic spectrum but still occurs in localised situations, such as
approaches to traffic lights and intersections where traffic speed is
reduced.
• Deformation may be surface related or originate within underlying
layers. It has been retained therefore as a potential mode of
deterioration for local roads.
Rationalisation of deterioration modes
6.3.17. The foregoing deterioration modes have been considered as discrete
entities but several can be initiated by common mechanisms. To
reduce this interactive complexity six key deterioration modes have
been identified:
• Fretting mode - typified by a loss of coarse or fine aggregate
•
•
•
•
•
6.4.
6.4.1.
through loss of aggregate or chippings, fretting of mortar or
stripping;
Texture mode – comprising random variability, traffic related
variability, fatting up or change (reduction or increase) of texture;
Cracking mode;
Polishing mode;
Delamination mode;
Deformation (wheel track) mode.
Deterioration Model
For the development of a deterioration model each of the five
influencing factors described previously are considered together with
each of the six key deterioration modes above and illustrated in
Table 6.
43
Table 6 – Elements of a Deterioration Model
Deterioration
mode
Influence
factor
Internal
1
Fretting
Traffic
Environmental
Substrate
Interface
Internal
2
Texture loss
Traffic
Environmental
Substrate
Interface
Internal
3
Cracking
4
Polishing
Traffic
Environmental
Substrate
Interface
Internal
Traffic
Environmental
Substrate
Interface
Internal
Traffic
5
Delamination
Environmental
Substrate
Interface
6
Wheel track
deformation
Internal
Traffic
Environmental
Substrate
Interface
Features which influence the deterioration model
Mixture design
Binder properties
Installation
Aggregate size
Aggregate/binder affinity
Lateral stresses
Fatigue stresses
Water and air permeabilities
Substrate instability causing surface strains
Only influential where aggregate size is large relative to layer thickness
Mixture design
Binder properties
Installation
Aggregate moisture content
Aggregate size
Air voids ratio
Dynamic and static vertical loading
Maximum and average temperature
Hardness, particularly where agg size is large relative to layer thickness
Not influential
Mixture performance parameters e.g. ITS, fatigue properties, ductility
Binder properties
Aggregate/binder affinity
Fatigue stresses
Water and air permeabilities and
Substrate instability causing surface strains
Only influential where aggregate size is large relative to layer thickness
Aggregate polish resistance
Dynamic vertical and lateral loading
Not significant
Not significant
Not significant
Mixture design - stability of mixture under lateral stresses and adhesion to
bond coat
Lateral stresses
Fatigue stresses
Drainage at interface level
Structural stability
Whether prone to settlement, lateral displacement etc.
Surface properties, e.g. porosity, roughness, etc.
Condition - whether pot-holed, cracked, polished etc.
Bond or tack coat used
Mixture design and performance levels particularly wheel tracking rate
Dynamic vertical loading
Maximum and average temperature
Not significant (except where the substrate itself deforms)
Not significant
44
Fretting
6.4.2.
Fretting has been recorded as the most important and frequent single
source of deterioration and an example is illustrated in Figure 11.
Figure 11 - Fretting
6.4.3.
Fretting is usually initiated by the loss of a single particle of coarse
aggregate. The loss of a single particle may not appear too serious
but for NTS once a single stone is loosened an indentation is caused
and water has more ready access to the matrix of the layer. Under
near saturated conditions, pore water pressures can develop and the
remaining material is deprived of some lateral restraint.
6.4.4.
The effect of pore water pressure has been comprehensively studied
for engineering soils and the principles apply to all three phase
physical systems. There is extensive literature on the subject,
(Terzaghi and Peck, 1967). The influence of dynamic pore pressures
has also been studied particularly in relation to earthquake
engineering and oil installations. There is no similar body of literature
for bituminous surfaces but the effect of drainage on road pavement
performance and deterioration has been examined. (Cedergren, 1974)
6.4.5.
Following established three phase soil mechanics theory, any
pressure of water within the material will tend to reduce the normal, at
rest, compressive forces in a layer thereby putting the material into
tension and requiring the binder to absorb greater tensile forces than
in a dry layer. For this mechanism to occur the asphalt must:
• Be sufficiently saturated for a near continuous water path to exist;
• Have a sufficiently low hydraulic conductivity to prevent the release
of pressure;
• Have a source for pressurising the water.
6.4.6.
Many practising highway maintenance engineers who contributed to
the development of these Guidelines have observed that, even in dry
ambient conditions, NTS which has failed is commonly wet. Several
reported that this is the usual condition and contrasted that of failed
HRA where dry conditions normally prevail. Such observations
indicate that a sufficient degree of saturation exists in the surface
material to allow the transmission of pore water pressure.
45
6.4.7.
Under normal conditions, water pressure from surface drainage,
rainfall or even flooding will be low. However, once a surface is
saturated the impact of a wheel load on the surface film will cause
large increases in pore water pressure. The high pressures generated
by the impact of tyres and flexibility of the tyre rubber compound will
enable the development of large sudden transient increases in pore
water pressure and subsequent suction forces. The pressure/suction
reversals forcing water through the upper voids under conditions of
heavy rainfall may be a factor in initiating binder loss and fretting.
6.4.8.
As pore water pressure increases, it will tend to push the aggregate
apart and put the binder film in tension. Immediately below the point
of impact, the direct vertical load will resist the tensile forces.
Adjacent to the loading point, a confining force will not exist and
loosening forces on the aggregate matrix will predominate. If the
tensile stresses exceed the internal tensile strength, surface
aggregate particles will fret from the body of the layer.
6.4.9.
Asphalt is a mixed composite material with all components, other than
the bitumen, being non-continuous. Hence, the sustained performance
of the bitumen film is critical to the overall performance of the material.
Once a single particle is removed from the material by water pressure
or by other forces, the continued serviceability of the surface layer
relies directly on the properties of the binder film. Any second-stage
removal of further particles will put increasing stress on the binder film
and deterioration will progress exponentially. This deterioration might
be mitigated by the selection of ductile binders.
6.4.10. Very little fretting will occur under direct static or dynamic load other
than that caused or triggered by pore water pressure rise. However,
under lateral loading, high shear and tensile forces are generated,
rotation of the coarse aggregate particles is induced and the particle
can become loosened from the material. Thus it would be expected
that fretting would predominate where braking, acceleration and
cornering are present. This has indeed been observed to be the case.
Roundabouts have been generally reported to suffer the worst fretting
problems, although any roundabout fretting problems may also be
associated with installation difficulties.
6.4.11. As each lateral load independently induces strains, some components
of which are residual, it would be expected that:
• Fretting will occur primarily on heavily trafficked sites or situations
with high lateral stresses;
• More fatigue tolerant mixtures such as those incorporating binders
with elastic recovery properties may exhibit better performance
than conventional materials.
6.4.12. Water permeable mixtures will also be air permeable. As airflows are
dispersed through a material more readily than water and as air is
highly compressible it is unlikely that pore air pressure plays a
significant part in deterioration. However, a ready access to a
renewable oxygen source will cause oxidation of the bitumen resulting
in embrittlement leading to a loss of adhesion. Hence, in the more
permeable mixtures, binder film performance becomes a critical factor
in resisting stripping of the binder/aggregate interface, leading to
46
fretting of the mixture. The mechanism for stripping has been widely
reported elsewhere and is elegantly summarised (Read and Whiteoak,
2003) but the step from the stripping of the binder to the fretting of the
surface is highly complex and has not yet been robustly modelled. In
situations where compaction during installation has not been sufficient,
perhaps due to marginal material temperatures or undertaken in
inappropriate weather conditions, poor aggregate interlock can occur
with larger and interconnected air voids leading to air penetration and
a possibly increased rate of oxidisation and embrittlement. This can be
a feature of fretting at construction joints in the surface.
6.4.13. Aggregate size has been recorded as having a significant part in the
fretting process. As aggregate size increases, fretting tends also to
increase. This is not explainable through the internal mixture
geometry but may be a function of the large number of resisting shear
planes in fine graded mixtures. The practice of reducing aggregate
size on laterally loaded sites such as roundabouts to minimise fretting
is widely used.
6.4.14. Where the layer thickness is greater than 2.5 times the nominal
aggregate size substrate condition does not seem influential in the
initial fretting process. For thinner surfaces disaggregation may
propagate upwards from the bottom of the layer, in which case the
substrate stability and bond coat efficiency will influence the
mechanism of fretting.
6.4.15. The processes leading to fretting comprise:
• A surface subject to critical horizontal loading usually acting directly
on coarse aggregate particles;
• Where the bond of the loaded aggregate to the adjacent matrix or
•
•
•
•
•
other coarse aggregate is inadequate, the loaded aggregate is
loosened and eventually displaced. The process is accelerated, or
sub-critical conditions made super-critical, when pore water
pressures are present within the layer;
Once a single aggregate particle is displaced the adjacent
aggregate particles and/or matrix is deprived of lateral support,
progressive deterioration takes place;
During the deterioration process, binder oxidisation rate and
adhesion properties take on greater significance over the initially
important features, such as mixture grading, air voids content,
proportion of voids filled with bitumen;
The risk of fretting has been observed to increase where the
aggregate size/layer thickness ratio is less than 2.5. This appears
to be particularly the case for larger sized aggregates although a
mechanistic model that would explain this has yet to be developed;
Single sized aggregate mixtures such as porous asphalt may be
expected to deteriorate more rapidly once the first particle is
loosened as they depend on lateral support form adjacent coarse
aggregate. This has been observed in-service although the
deterioration rate may be mitigated by the use of a tenacious
binder;
The rate of progress of the fretting is peripherally influenced by
such factors as substrate movements, bond to substrate and
substrate condition.
47
Texture Loss
6.4.16. A low rate of texture change is important for maintained safety and is
a mandatory test property for BBA certification. It is assessed as the
surface texture, measured by the glass sphere texture depth test (BS
EN 13036-1) defined by the initial measurement at the time of laying
and assessment over a two year period.
6.4.17. Insufficient texture may occur due to wet aggregate in the mixture,
compaction of the layer during installation, loss of surface material,
traffic wear or softening of the matrix leading to secondary compaction
and surface flow. Initial texture is often specified but less frequently is
the in-service texture monitored after a period of two years as required
in clause 942 of the SHW.
6.4.18. Texture loss may increase where:
• Air voids are very low, and so give rise to plastic flow or low
•
•
•
•
•
•
compression resistance;
Soft binders are used, particularly in rich binder/filler mixtures;
Aggregate size is reduced;
Segregation during transport and laying occurs, leading to local
areas of excess fines;
Traffic volume is high, speed is low and where tyre pressures on
heavy commercial vehicles are high;
Surface temperature is high, particularly where the mixture is
intrinsically unstable or the binder is inappropriately soft. Stretches
of road sheltered from the north and exposed to the south are
particularly vulnerable;
Soft substrates are present. Substrate condition is most likely to be
critical for mixtures where the coarse aggregate nominal size is
greater than half the layer thickness and so allows the coarse
aggregate to penetrate the substrate and force the matrix of the
mixture to the surface with a consequent loss of texture.
6.4.19. From the foregoing, texture loss is a function of:
•
•
•
•
•
•
•
Mixture design;
Binder hardness;
Aggregate size;
Installation and transport method;
Temperature;
Substrate hardness;
Traffic loading.
6.4.20. Unlike fretting deterioration, texture loss is normally related to one or
more of the above functions acting independently. If each is
considered in the design and installation of the system texture loss will
remain within acceptable limits.
Cracking
6.4.21. Cracking was reported as the second most significant form of
deterioration of NTS. There is no specific evidence that these
48
materials are inherently more or less prone to cracking than HRA. Any
increased incidence of cracking of dense NTS, relative to HRA, may
be largely associated with the reduced surface thickness. Cracking
has been categorised into three types:
• Reflective cracking;
• Map cracking;
• Fine cracking.
6.4.22. The causes for cracking are either material or externally related.
Although mix design deficiencies are not dominant in the crack
initiation, the inability of a material to accommodate the movement
caused by external strains is a common reason for the propagation of
cracks.
6.4.23. Mixes with low flexibility are more prone to cracking as a result of
substrate strains than more flexible mixtures, irrespective of their initial
tensile strength and stability. Development of a suitable test is
underway by BBA to characterise flexibility and assist in the
production of a protocol for flexibility testing and certification.
6.4.24. Cracking resistance and crack propagation both play a key part in the
deterioration model. Thick pavements constructed of stiff materials
contain high residual strain energy. Even small movements along, or
across, an existing joint or crack will require considerable strength in a
surface material if it is to resist crack reflection to the surface. Such
strength is usually beyond the capability of any conventional surface
course. Consequently, reflective cracking is inevitable. Where there
is an angular displacement, as in the case of rocking concrete slabs,
the loading on the surface layer due to the lever effect is even higher
than in the instances of linear movement.
6.4.25. Thin pavements on rural roads and urban streets, possess far less
potential strain energy than thick pavements and any leverage due to
angular displacement is generally less severe.
From these
considerations it might be expected that reflective cracking would be
less of a problem than on thick pavements but observations and
performance do not substantiate this expectation as the local network:
• Does not usually have designed pavement thicknesses;
• Frequently contains buried service utilities, often with numerous
trench reinstatements, some with inadequate compaction, all
increasing the risk of settlement and reflective cracking;
• Is often in a critical, or near critical, structural condition of limiting
equilibrium.
6.4.26. Surface layers on thin pavements can constitute a significant
proportion of the sound bituminous material. It is not uncommon for
the surface to constitute 20% to 50% of the depth of sound bituminous
layer. In these conditions the surface functions, as one of the
structural elements in the pavement, and due to the thinness of layer,
can be subjected to significant strains for which it may not have been
designed. Under these conditions map cracking and longitudinal
cracking can often manifest as a precursor to pavement failure.
6.4.27. Cracking is initiated and propagated by:
49
• Movement of rigid substrate;
• Settlement imposed by traffic on thin pavements where the surface
has a significant structural contribution.
6.4.28. As surface material ages it becomes embrittled due to binder oxidation.
With reduced flexibility, cracks occur more readily and fretting
commences at the cracks, leading to a general pavement surface
distress. For thin pavements therefore, an essential attribute required
to reduce cracking potential is flexibility.
6.4.29. At present flexibility is not a specifiable parameter. NTS are not
necessarily designed to have long-term flexibility and ductility,
although this was an in-built feature of traditional binder rich, matrix
dominated surfaces such as HRA.
Polishing
6.4.30. The general mechanism of aggregate polishing under traffic loading
has been extensively researched (Szatkowski and Hosking, 1972).
The evidence for variations of performance relative to that of HRA is
inconclusive. It was anticipated that NTS would polish more slowly
than HRA, which presents a positive contact with tyres. Road
surfaces with negative texture result in a relatively large proportion of
the gross area beneath a vehicle tyre being in contact with the surface
and the stress therefore approaches an even distribution. By
contrast ,HRA chippings present intrusive contact with the tyre, with
each chipping bearing a high load as it deforms the tyre to a greater
extent than is the case for NTS. The higher contact pressure on HRA
chippings will lead to a scrubbing action and quicker polishing than for
aggregate in a NTS. Surface characterisation (Walsh, 1997) and the
Macrotexture and Road Safety (MARS) project (Walsh, 2003)
examined whether the safety performance of a surface could be
predicted from a knowledge of the form and pattern of surface texture
and studied the characteristics of tyre-road contact.
6.4.31. Medium term inspection results from in-service TS and SMA materials
have been reported (Nicholls and Carswell, 2004) from sites
nominated by both local authorities and contractors. The observations
tend to indicate little polish deterioration with trafficking over a period
of up to eleven years. Some highway maintenance engineers have
reported that the maintained skid resistance of TS and SMA is better
than that of HRA, when the aggregate is from the same source as that
used for the chippings applied to the surface of HRA.
Delamination
6.4.32. Delamination was recorded as the third most significant cause of
deterioration based upon consultation with highway maintenance
engineers and is the complete disassociation of discrete areas of
surface from the substrate. The delaminated areas may vary from a
few square centimetres to several square metres with larger areas
developing through progressive growth from small areas.
6.4.33. Delamination potential is increased with:
• Reduction in the thickness of the surface;
50
•
•
•
•
Smoothness of the substrate;
Ingress of water to the surface/substrate interface;
Lateral loading;
Competence of the bond coat.
6.4.34. Delamination has been found to be a more likely occurrence when
NTS is applied to a smooth substrate rather than to a more rugous
substrate. Furthermore the adhesion is likely to be less when the
surface is applied to a binder deficient substrate.
6.4.35. Early trials applying TS to concrete carriageways revealed difficulties
but these have now been addressed through careful selection of the
mixture and bond coat, to give confidence in the ability of TS to be
used on concrete roads.
6.4.36. The superior performance of elastomeric polymer modified bond coats
relative to that of tack coats has been demonstrated in laboratory tests
and therefore strongly influences in-service performance. The
superiority is likely to be most marked where the layer thickness is
30mm or less. The improved performance of modified bond coats is
as a result of their ability to yield slightly and thus distribute shear
loads over a larger area.
6.4.37. The mechanism of delamination is likely to be:
• Initiated by horizontal bond failure;
• A progressive failure proceeding rapidly where there is little support
offered by the adjacent material;
• Less likely where the bond coat will yield, whilst retaining its
integrity, and thus allow greater restraint to be offered by stronger
areas of surface and perimeter reaction.
Wheel Track Deformation
6.4.38. Wheel track deformation has been an issue associated with the use of
HRA for heavily trafficked roads over the past three decades. This has
been a much less significant aspect on many parts of the local
network. The deformation risk was mitigated by the introduction of the
designed mix procedures in the 1976 revision of BS 594. The
problem recurred in the 1980s with the introduction of more
concentrated heavy goods vehicle wheel loads, greatly increased
volume of traffic and increased summer temperature conditions. To
address these new conditions, Hot Rolled Asphalt Surface Course
(Performance Related Design Mix) is now available in Clause 943 in
the SHW.
6.4.39. TS for use on high speed roads requires a minimum surface texture
depth of 1.5mm. This was not a facet of the earlier European
developed mixtures and to achieve this requirement TS mixtures in
the UK are generally of the stone support type with virtually
continuous aggregate-to-aggregate contact.
6.4.40. Surface texture provision has lead to a high deformation resistant
material.
All the current BBA certified TS materials satisfy
Classification 1 criteria of BS 594, Table D1 and are therefore suitable
for the heaviest trafficked sites and consequently wheel track rutting
51
does not constitute a durability criterion.
With the possible
development of more settlement tolerant mixtures in the future, wheel
track rutting will need to be re-considered.
6.5.
References
1 Szatkowski and Hoskings, (1972) “The effect of Traffic and
Aggregate on the Resistance to Skidding of Bituminous Surfacing”
TRRL, Crowthorne.
2 Terzaghi and Peck, (1967) “Soil Mechanics”
3 Nicholls. C and Carswell. I (2004). “Durability of Thin Asphalt
Surfacing. Part2: findings after three years” TRL Report 606, TRL
Limited. ISSN 0968-4107
4 Cedergren. H (1974). “Drainage of Highway and Airfield Pavements”,
Willey Interscience ISBN 0471 14181 X
5 Read. J and Whiteoak. D. (2003) The Shell Bitumen Handbook”.
Fifth edition. Thomas Telford, London ISBN 07277 3220 X
6 Private Communication (2005). University Karlsruhe on deterioration
mechanisms of SMA in Germany
7 Walsh I.D (1997) “Surface characteristics of thin surfacings” Proc
European Symposium on performance and durability of bituminous
materials, University of Leeds, pp605-616
8 Walsh I.D. (2003) “Macrotexture and road safety project” Abridged
report for CSS. www.cssnet.org.uk
6.6.
Summary
• The engineering properties that govern the distress mechanism of
•
•
•
•
•
•
•
NTS have been considered within a structured framework.
The surface on thin pavements can constitute a significant
proportion of the sound bituminous material structural component.
The causes of deterioration have been categorised as five
influencing factors.
The influencing factors are frequently inter-dependant and in that
way can have cumulative effects on serviceability.
The mechanism of deterioration is rationalised through six discrete
deterioration modes and combined with the five influencing factors
into a deterioration model.
The most frequently reported methods of failure for NTS were
found to be, fretting, cracking and delamination.
A model has been developed to address the mechanics of
behaviour which results in fretting and cracking including the
influence of pore water pressure in the process.
In the failed state many highway maintenance engineers reported
that NTS was in a wet condition which contrasts with aged HRA
where normally this is revealed to be dry.
52
7
LOCAL AREA
REINSTATEMENT
7.1.
Performance Characteristics
7.1.1.
Local area reinstatement includes patch repair as well as works
relating to utilities in the highway. Irrespective of the form of surface,
there will almost certainly be a need to undertake small scale
replacements or repairs at some stage in its lifecycle. This could be
due to accidental damage, utility installations, distress of the surface
material or loss of support from other elements of the pavement.
7.1.2.
Small scale works will probably not utilise the same personnel or
equipment as that used in the initial installation and hence different
skills, techniques and potentially different materials are required.
7.1.3.
The operation will involve significantly more manual processes than
full scale modern system installation operations, which are almost fully
mechanised.
During initial installation or at times of major
maintenance, self propelled pavers and suitable heavy compaction
equipment would have been used to ensure rapid placement and that
sufficient density was achieved together with line and level
requirements. The same concept should be followed with subsequent
local reinstatement works and other smaller scale operations
wherever possible.
7.1.4.
It is recommended that whenever practicable, mechanical plant with
integral initial compaction and leveling screeds should be used. An
example of a mechanical footway width paver is illustrated in Figure
12.
Figure 12 - Mini paver machine
7.1.5.
These small scale machines have variable width screeds and the
ability to accommodate full sized delivery vehicles and can be used
with advantage in local installations and repair situations. This
approach leaves only the smallest of works to be carried out with
manual labour and the hand lay approach.
7.1.6.
Specialist works reinstatement vehicles have evolved and progressed
technically to suit efficient installation needs. Use of these bespoke
53
comprehensively equipped reinstatement vehicles provide all of the
necessary plant and equipment for the effective reinstatement of local
areas in the highway.
7.1.7.
This type of purpose designed plant and equipment incorporates all of
the tools and carrying capacity to optimise best practice reinstatement
and patching techniques. Wherever hand lay work is performed and
particularly with high stone content mixes, the final appearance of
hand laid or hand worked material is very different to that of machine
laid material and, because of higher void contents, may be
significantly less durable.
7.1.8.
The limiting factors involved with hand laying operations are those
which can be performed successfully by skilled operatives without
excessive manual handling and other health and safety implications.
7.1.9.
For reinstatement works it is important that all operations are
programmed efficiently. Mechanical plant, working efficiently, reduces
the risk of poor compaction associated with heat loss and ensures a
more homogeneous end product. Hot materials need a period to cool
to near ambient conditions prior to being exposed to traffic, otherwise
deformation may occur.
7.1.10. The majority of local reinstatement work is likely to be based around
the SHW and associated documents. These refer to relevant British
and European technical Standards and also include requirements for
Certification Bodies such as BBA together with other documents from
the DfT. The Specification for the Reinstatement for Openings in
Highways is produced by the working party of the Highways
Authorities and Utilities Committee (HAUC) as the key published
documentation on reinstatement practice.
7.1.11. The existing Regulations and Code of Practice became legislation via
the New Roads and Street Works Act 1991 and the current
specification, including updates, is dated June 2002.
The following
provide links for detailed reference on current documents, changes
and amendments including, in the latter case, Notes for Guidance on
the SHW and
further information on the selection of materials.
www.streetworks.dft.gov.uk/codesofpractice/specifications
and
www.hauc.org
7.1.12. In virtually all cases, surface materials should be selected on the basis
of matching the adjacent pavement structure. This should be for both
material type and technical performance. Having equivalent, or
improved, resistance to water penetration is important as small scale
repairs are potentially more difficult to install than the same material
on a larger scale.
7.1.13. There may be instances where this approach is neither possible nor
practicable due to placement and compaction constraints in confined
and restricted areas. The principle should then be to select a
matching material type but one size down in terms of aggregate size.
In such situations, engineering judgment and expertise should be
used to ensure a safe and durable alternative is achieved.
54
7.2.
Reinstatement Size and Position
7.2.1.
Careful preparation of the substrate and the edges of the excavation
before reinstatement is as important as the installation of the surface
material itself. Where possible, the smallest nominal size aggregate
commensurate with required texture depth should be selected to
minimize any tendency to segregate during hand lay operations.
7.2.2.
Existing surfaces should be cut back to form a square or rectangular
area bounded by undamaged sound materials. All patch repairs
should ensure that unsound material is removed in its entirety,
otherwise further early deterioration can be expected. The edges of
any reinstatement should be delineated by saw cutting or planing in
straight lines for the full depth of the surface course. All edges should
be trimmed and swept clean, painted with either hot grade bitumen or
a cold applied polymer modified intermediate or premium grade
bitumen emulsion, or in the case of proprietary materials, compatible
jointing compounds.
7.2.3.
Where deeper than surface course installations are required due to
pavement deterioration at a lower level, stepped reinstatement
construction should be used.
Traffic running directly along a
construction joint can have deleterious effects and thus surface joints
should avoid wheel track alignments. The SHW defines wheel tracks
as being 0.5 - 1.1m and 2.55 - 3.15m from the centre of any nearside
lane marking and it is recommended that these wheel track
alignments are avoided if at all possible in local patch repairs.
7.3.
Installation and Compaction
7.3.1.
BS 594-2 and BS 4987-2, require that a tack coat or bond coat shall
be applied before the laying of a new surface course, unless this is
laid on a binder course or a base which itself is still in its as laid, clean
condition and which has been laid within the previous seven days.
Rate of application for tack coats is given in terms of kg/m² of residual
bitumen and differs for new and maintenance works applications. The
SHW requires all bituminous surfaces to be treated with either a tack
or bond coat prior to overlay but it is recognised that in small and
inaccessible areas, tack coat may be applied by a hand held sprayer,
as larger mechanical sprayers may be impractical in these situations.
7.3.2.
Bond coats are used in preference to tack coats in the following
situations where:
• Greater confidence in adhesion between layers is required;
• Improvement in impermeability is required;
• The surface may be trafficked a non-stick bond coat should be
used;
• Overlaying concrete with less than 100mm of bituminous materials;
• It forms part of a proprietary system.
7.3.3.
Bond coats form part of proprietary systems and their application is
covered by the associated quality plan and installation method
statement for the system. The same basis for consideration of bond
55
coat as described in Table 4 should also be used for local area
reinstatement situations.
7.3.4.
As the majority of materials currently used in repair treatments are hot
applied, it is of key importance that they are mixed at the correct
temperature and maintained in a condition that ensures they can be
laid and compacted effectively. This means that when material is
discharged from a mixing plant, it should be transported in a suitably
insulated vehicle and then maintained at working temperatures until
finally placed in the repair. Traditional methods of simply protecting
from the elements with a tarpaulin sheet have now been replaced with
specialist hot boxes and sealed insulated systems.
7.3.5.
The discharge of materials from a delivery vehicle on to a hard clean
surface before secondary handling should be avoided in all but
extreme situations. Not only is heat lost, even with sheeting, but the
risk of contamination is highly increased. Temperatures at which
materials are mixed, delivered and when compaction should be
completed are included in relevant technical standards.
7.3.6.
Most manufacturers of aggregate dominated materials recommend a
maximum time period within which installation should have been
completed. All materials should be obtained from suppliers with fully
accredited quality production procedures.
Whilst technical
improvements have been made into the use of temperature reducers
in bituminous materials (in the order of 30oC and commonly called
warm asphalt) by the inclusion of chemical modifiers, it is not a
practice that has been widely adopted in patching and reinstatement
works.
7.3.7.
The use of cold applied materials is restricted to very small areas.
These materials are known as Permanent Cold-lay Surfacing
Materials (PCSM). The overall PCSM formulation, manufacture and
placement are limited only by the need to comply with BBA Guidelines
in permanent situations. Other cold deferred set materials should be
considered only for temporary reinstatements.
7.3.8.
For any material to be effective for the desired time period, it is
essential that it has stability, water insensitivity and resistance to other
external elements (e.g. oxidation, contamination etc). To achieve this
compaction is critical for performance. Suitable plant must be
available, working effectively and under the control of skilled
personnel. The majority of works will utilise steel drum rolling
equipment, but there may be instances where vibro-tampers or
vibrating plates are equally effective or preferable.
7.3.9.
Where specifications call for end product performance testing
procedures will ensure that desired levels are met. However, most
small works are based upon method specification techniques and
practices. Information on plant weight and compaction ability is
available from all manufacturers and is usually readily available from
their websites.
Specifications and standards typically provide
requirements for the number of passes for equipment under each
weight category to achieve the desired compacted layer thicknesses.
An example table is that given in HAUC Appendix A8, Table A8.3. In
56
virtually all cases plant should operate with water forming a bond
breaker between the steel drum face and the bituminous material.
7.3.10. Construction and edge joints and joint positions are critical if long term
performance is to be achieved. All cut faces must be vertical and
formed in sound material. The edges of any reinstatement should be
delineated by saw cutting or planing in straight lines for the full depth
of the surface course. All edges should be trimmed and swept clean,
painted with either hot grade bitumen or a cold applied polymer
modified intermediate or premium grade bitumen emulsion, or in the
case of proprietary materials, compatible jointing compounds.
7.3.11. Replacement material should be installed from the sound edges and
compacted against a solid edge. Over-band sealing may be permitted
but use should be kept to a minimum as a suface resistant to skidding
must be provided. Recent legal decisions, particularly those involving
motor cycle fatalities, have highlighted responsibilities in this practice.
Joints should also be formed in such a way that they do not create
surface irregularities and that they tie in at the same level as the
adjacent materials.
7.3.12. All bituminous surfacing should be at, or approaching, ambient
temperature before opening to traffic. Particularly with local
reinstatement areas trafficking before opening should be limited as
much as practicable. This becomes more important the thinner the
surface layer is. Whilst surface material can be disturbed by moving
traffic it can also be disturbed by wheel and point loads from stationary
traffic. Power steering is now common on vehicles and any surface
course applied where parked vehicles are likely to be situated can
also suffer distress. Therefore, full setting periods should be allowed
in such circumstances.
7.4.
Street Furniture and Ironwork
7.4.1.
In many local repair works, the need for rectification is related to the
presence of street furniture. Local highways contain many different
utilities and services, some of which require access points through the
carriageway. Very often these create a weakness in the surrounding
materials as they do not allow effective compaction of bituminous and
support materials.
7.4.2.
Large items of ironwork can act as a heat sink and thus reduce
mobility of the adjacent hot material during installation by premature
cooling. All existing ironwork should be reset in advance and
stabilised by the use of compatible bedding and surround compounds
prior to being subjected to the stresses imposed by surface installation.
7.4.3.
All vertical surfaces should be painted, not just joints, and this includes
street furniture, etc. All ironwork before painting should be in a sound
condition and free of all rust and detritus. Particular care is necessary
to ensure all level requirements are correct, as plant can create an
irregularity which over-stresses the immediate surface surroundings.
7.4.4.
Where a repair is required immediately adjacent to an item of street
furniture, then a different material may be beneficial to aid installation
and thus longevity. For borders and transitions around street furniture
57
the options include hand applied natural mastic asphalt, HRA with
14mm pre-coated chippings, 0/14mm DBM or proprietary cold
materials.
7.4.5.
7.5.
For localised superficially defective areas alternative rejuvenation or
velocity patching processes could be considered. Rejuvenation
processes comprise gentle indirect heating of the existing surface and
re-profiling prior to the addition of fresh material and compaction. This
approach produces a seamless repair but the fresh material which is
added needs to be carefully selected to ensure that future durability is
not impaired in the repaired area. Velocity patching enables the repair
of cosmetic defects or temporary repair of deeper seated local distress.
The defective area is cleaned, prepared and subsequently sprayed
under pressure with a mixture of bitumen and aggregate resulting in
the bitumen being forced into any cracks in the defect. Both of these
techniques are generally installed as specialist systems using purpose
designed plant, processes and materials.
Summary
• The preparation of the area of local reinstatement is as important
as the selection and installation of the surfacing material.
• Small area reinstatement should be undertaken by mechanised
plant wherever possible and hand-lay reduced to a minimum.
• Matching a reinstatement surface to the surrounding NTS can
present difficulties as proprietary surface materials were not
intended to be laid in small quantities by hand. In these
circumstances the selection of one aggregate size down from the
surrounding surface may ease the situation and provide as close a
match as is practicable.
58
8
ASSET MANAGEMENT
8.1.
Asset Management Plan
8.1.1.
The use of NTS for highway maintenance should be set within the
context of an overall asset management regime. The development of
a Highway Asset Management Plan (HAMP) is fundamental to
demonstrating the value of highway maintenance in delivering the
wider objectives of corporate strategy, transport policy and value for
money.
8.1.2.
The theme of asset management is strengthened by Government
guidance encouraging authorities in England to produce a Transport
Asset Management Plan (TAMP) as part of their Local Transport Plan
(LTP), consistent with the advice contained in the CSS Framework for
Highway Asset Management (CSS, 2004). In particular, it advises that
the compilation of an asset management strategy will provide a tool to
enable the value for money of local highway maintenance to be
considered more effectively against other local transport spending,
and eventually assist local transport strategy and plan development.
8.1.3.
Effective asset management planning will provide the means for
authorities to understand the value and liability of their existing
highway asset. This process will enable the right strategic decisions, to
ensure that its value will be safeguarded for future generations. The
HAMP therefore should detail progress towards; a whole life
maintenance approach for existing assets, the ambition and realism of
LTP asset management targets, the maintenance implications of the
major and other integrated transport schemes, and the implications of
any LTP proposals to delay or bring forward maintenance work.
8.1.4.
“A Guidance document for Highway Infrastructure Asset Valuation”,
www.roadscodes.org has been produced by the Roads Liaison Group
(RLG) as a companion to the CSS Framework for Highway Asset
Management.
8.2.
8.2.1.
Whole Life Costing
Investment in NTS must be considered in the HAMP, particularly in the
context of providing value for money. One method of demonstrating
this is through whole life costing.
59
Negative Texture Surfaces
(NTS)
Deterioration with Time
Performance
Deficiency
Defects
Loss of Service
Volume of Work
Disruption
Renewal Costs
Figure 13 -Whole Life Cost
8.2.2.
The process for the development of a whole life costing approach is
shown in Figure 13.
8.2.3.
Value for money may be demonstrated by balancing performance
deficiency leading to disruption against the cost of renewal arising
from defects. Performance of the surface may be measured against
the four objectives of highway maintenance:
•
•
•
•
8.2.4.
Safety;
Serviceability;
Sustainability;
Customer service.
Condition is measured from SCANNER surveys of the network and
reported from UKPMS. Deterioration with time is a key component in
assessing the future loss of service and volume of work arising from
potential defects. In making the decision to resurface for the
development of future programmes of work as part of the HAMP, the
highway maintenance engineer must balance the risk of loss of
performance against the cost of resurfacing. The fundamental
component in this decision is the deterioration mechanism and the
confidence in the time period over which this will occur. Forecasting
the time at which NTS will reach the end of useful service life is an
60
aspect which would require further consideration since this is a key
factor in highway maintenance planning.
8.3.
Winter Maintenance
8.3.1.
Since the widespread adoption of NTS on the local network winter
weather conditions have been relatively moderate.
8.3.2.
Although some authorities have adjusted their management of Winter
Maintenance service to provide differing rates for salt application on
NTS surfaces many appear not to have altered their salt spreading
regime specifically to address the introduction of NTS. As is the case
with all types of surface low temperatures and the formation of ice can
cause serious damage to its fabric.
8.3.3.
Further guidance on Winter Service is available in Section 13 of The
Code of Practice for Highway Maintenance Management.
8.4.
Distress Mechanism and Forecast
8.4.1.
For many years the traditional surface material was HRA. This is a
matrix dominated material and the dominant distress mechanism is a
gradual accumulation of permanent deformation in wheel paths. The
rutting phenomena, although occurring on some heavily loaded
sections of motorways and trunk roads, was not such a widespread
issue on local roads owing to differing traffic patterns. This was
generally due to development of durable local asphalt designs and
occurred only in areas where slow moving, canalised traffic was
concentrated such as traffic lights, as a result of long loading times or
in specific areas, such as south facing inclines or bridge approaches.
8.4.2.
NTS materials are aggregate dominated and after the early life period
has elapsed with the removal of the thick binder film the material
would be expected to remain stable in service.
8.4.3.
Towards end of service life distress will occur through:
• Gradual loss of surface texture or aggregate polishing resulting in
the need for remediation on safety considerations;
• Increase in surface texture resulting from fretting which, once a
critical point is reached, results in rapid degradation over a
relatively short time span.
8.4.4.
A prediction method to forecast the onset of the fretting mechanism is
required to enable the lifecycle cost model to be developed with
confidence.
8.4.5.
There are many different models and approaches to deterioration of
pavements and surfaces in general. Research into the development
of robust pavement deterioration modelling within UKPMS is under
consideration. However, at present, there is no model available which
has the ability to predict, with confidence, the onset of distress in NTS.
61
8.4.6.
HDM4 provides some prospective indicative approaches as described
in www.HDMGlobal.com
8.4.7.
The HDM4 system contains a model for chip sealed (surface dressed)
and cold-mix surfaced pavements based upon the time to initiation of
ravelling (fretting). Although this approach may hold promise and could
be adopted for NTS it would require calibration to a failure mechanism
for local conditions and this is beyond the scope of these Guidelines.
8.4.8.
If the onset of distress could be determined and predicted rather than
allow the deterioration to continue (resulting in rapid failure and loss of
serviceability) an amelioration strategy could be developed. The
purpose would be to extend the NTS integrity for sufficient time to
enable remediation to be planned and budgeted.
8.4.9.
In developing future maintenance programmes, as part of the HAMP,
the following should be considered:
• A deterministic approach where the past performance of similar
materials in the same circumstance is assessed to provide the life
of the surface and therefore the timing of its replacement;
• Surface condition gleaned through SCANNER surveys, with
particular attention to the texture depth change, as a rapid increase
of texture is an indication of fretting and loss of performance.
8.4.10. The point of intervention should be when loss of service in the NTS is
unacceptable in terms of its performance with regard to the customer,
safety, serviceability and sustainability. These parameters, other than
safety, are ill-defined at present, but should be described as part of the
HAMP development.
8.5.
References
1. CSS (2004) Framework for Highway Asset Management
www.cssnet.org.uk
2. Guidance Document for Highway Infrastructure Asset Valuation
(2005), The Stationary Office, London, London, ISBN. 0-11552695-1.
8.6.
Summary
• The use of NTS for maintenance should be set within a HAMP,
measured against the four core objectives of highway maintenance:
safety, serviceability, sustainability and customer service.
• One method of demonstrating good investment in surfacing is
through whole life costing.
• A key element in maintenance planning is the forecast of onset of
distress. NTS materials are aggregate dominated and once a
critical point is reached in the binder/aggregate bond a process of
rapid degradation can occur over a relatively short time span.
• Traditional surfacing such as HRA approached failure with a slow
accumulation of deformation but for NTS the distress model is
significantly different.
62
• There is no model available which has the ability to predict with
confidence the onset of distress for NTS and will take time to evolve.
On a sound, unyielding, substrate an indeterminate structural life
may be achievable but on a weak, or cracked, substrate or with
material poorly installed or under inappropriate conditions, even a
five year life could be optimistic.
• A forecasting methodology should be developed and calibrated to
local network conditions. A key measure of success would be
ensuring strain compatibility between the surface and the substrate.
• Traffic speed network data collection devices such as SCANNER
may hold the prospect of trend examination of texture since a rapid
increase of texture may be the precursor of fretting and loss of
performance.
• On some parts of the local network safety requirements of inservice skid resistance and aggregate polishing may take
remediation precedence over longevity of the surface before a
distressed condition has been reached.
63
Appendix A - Functional Carriageway Hierarchy
Category
Hierarchy
Description
Type of Road
General Description
Description
1
Motorway
2
Strategic
Route
Limited
access
motorway
regulations
apply
Trunk
and
some
Principal A class roads
between
Primary
Destinations
Routes for fast moving long distance
traffic.
Fully grade separated and
restrictions on use.
Routes for fast moving long distance
traffic with little frontage access or
pedestrian traffic. Speed limits are usually
in excess of 40 mph and there are few
junctions. Pedestrian crossings are either
segregated or controlled and parked
vehicles are generally prohibited.
3a
Main
Distributor
Major Urban Network
and Inter-Primary Links.
Short - medium distance
traffic
3b
Secondary
Distributor
Classified Road (B and
C class) and unclassified
urban
bus
routes
carrying local traffic with
frontage access and
frequent junctions.
4a
Link Road
Roads linking the Main
and
Secondary
Distributor Network with
frontage access and
frequent junctions.
Routes between Strategic Routes and
linking urban centres to the strategic
network with limited frontage access. In
urban areas speed limits are usually 40
mph or less, parking is restricted at peak
times and there are positive measures for
pedestrian safety.
In rural areas these roads link the larger
villages and HGV generators to the
Strategic and Main Distributor Network. In
built up areas these roads have 30 mph
speed limits and very high levels of
pedestrian activity with some crossing
facilities including zebra crossings. On
street parking is generally unrestricted
except for safety reasons
In rural areas these roads link the smaller
villages to the distributor roads. They are
of varying width and not always capable
of carrying two way traffic. In urban areas
they are residential or industrial inter
connecting roads with 30 mph speed
limits random pedestrian movements and
uncontrolled parking
4b
Local
Road
Access Roads serving limited
numbers of properties
carrying only access
traffic
64
In rural areas these roads serve small
settlements and provide access to
individual properties and land. They are
often only single lane width and
unsuitable for HGVs. In urban areas they
are often residential loop roads or cul-desacs.
Appendix B - Glossary of Terms and Definitions
Negative texture
A term used to describe a road surface texture where the voids contributing to
the macro-texture exist below the overall vehicle running surface.
Positive texture
A term used to describe a road surface texture where the texture is provided
above the planar surface by superimposition of chippings.
Generic Stone Mastic Asphalt (SMA)
Stone mastic asphalt manufactured and laid in compliance with a client end
product or recipe specification.
Porous asphalt (PA)
Porous asphalt is an asphalt mixture with inter connected voids designed to
allow the surface water to drain vertically and horizontally. It includes material
previously referred to as friction course and commonly used on airfield
runways.
Thin asphalt concrete (TAC)
Dense mixtures normally, but not essentially, with less than 10% air voids and
polymer modified binder. The grading of TAC includes all stone and sand
sizes.
Thin stone mastic asphalt (TSMA)
Dense mixture (normally with less than 8% air voids) usually with unmodified
binder and fibres added to increase binder stability until compacted. The
mixture is essentially a gap graded mixture of stone and binder filler – sand
matrix.
Hot rolled asphalt (HRA)
A matrix dominated material of coarse and fine gap graded aggregate
components. The engineering properties are dictated by the characteristics of
the binder, filler, and fine aggregate matrix. For surface course a typical
coarse aggregate context would be 30% or 35% and pre-coated chippings are
superimposed on the surface to provide skid resistance characteristics. The
material is virtually impermeable.
Dense bitumen macadam (DBM)
An aggregate dominated material consisting of a continuously graded array of
aggregate sizes. The engineering properties are dictated by the grain packing
and interlocking characteristics of aggregate particles.
65
High stone content asphalt (HSCA)
A mixture of coarse and fine gap graded aggregate with the engineering
properties dictated by the binder, filler and fine aggregate matrix
supplemented by the coarse aggregate. For surface application the coarse
aggregate proportion would be typically in the range 55% to 65%.
Surface dressing (SD)
Surface dressing can vary from a single layer of sprayed binder with a layer of
applied chippings (SSD) to two or more layers of bitumen and chippings of
varying particle size grading (MSD).
66
Appendix C - Questionnaire
BEST PRACTICE GUIDELINES FOR THIN SURFACING – ENQUIRY QUESTIONNAIRE
1. Site situation
1.1
Do you have a documented process for evaluation of sites to
assess their suitability and appropriateness for installation of thin
surfacing?
1.2
What type of distress failures have you observed in the structural
performance of pavements with a thin surfacing and how
extensive is the occurrence of these?
1.3
Can you attribute a particular failure mode to the condition of the
supporting layer(s) beneath the thin surfacing?
1.4
Do you have evidence of enhanced serviceability performance
with particular combinations of nominal size and layer thickness?
1.5
What degree of importance do you assign to the
presence/absence of bond/tack coat and its methodology of
application?
(Do you have any evidence to support?)
1.6
How do you differentiate between the various thin surfacings for
particular sites? (eg historical performance, layer thickness,
nominal size, brand name, etc)
Response
(Please describe)
2. Materials
2.1
Do the demands of thin surfacing feature in your environmental
policy considerations?
(ie premium quality aggregates, transport distances, etc).
2.2
Can you attribute a particular failure mode to a particular thin
surfacing material type?
2.3
Do you use generic SMA specifications or BBA/HAPAS systems?
(If HAPAS are you content with the scheme?)
2.4
Do you have a documented system which covers approvals,
installation and associated practices for thin surfacing?
3. Performance prediction
3.1
Do you have any predictive measurement tools to forecast the
onset of failure? (such that maintenance can be programmed for
asset management purposes)
3.2
Do you have any experience of surface applied rejuvenation
agents or techniques to extend useful service life of thin
surfacing?
4. General
4.1
Please give your view on serviceability performance comparison
between thin surfacing and standard materials such as HRA /
DBM / HSCA / etc (ie. Thin surfacing is "better / equal / worse"
(select as appropriate) for each situation stated).
4.2
What is your view on the public perception of thin surfacing as a
highway maintenance option? (Especially related to traffic `noise`
attenuation properties/durability, etc)
4.3
For reinstatement purposes in a thin surfacing area – do you have
a preferred product or process to match, as closely as possible,
the existing surface?
4.4
Are there any specific, or general, comments you wish to make in
relation to thin surfacing? (eg serviceability improvements, cutting
edge concepts, deterioration forecasting, etc)
4.5
Are you interested in receiving an invitation to attend a one-day
interactive regional workshop in England on the development of
Best Practice Guidelines for Thin Surfacing? Please select as
appropriate.
Urban
Rural
High Speed
Low Speed
High Stress
Low Stress
Workshop Invitation
Preferred region
5. Thanks for your contribution
5.1 Many thanks for completing this enquiry questionnaire.
Please return your completed questionnaire to
[email protected]. All responses will be treated in
confidence and if used in the report will be in an anonymous
format.
Email address
Name
Tel No
Please inset your contact details opposite
67
Appendix D – Summary of Responses
1.1
Do you have a documented process for evaluation of sites to assess their suitability
and appropriateness for installation of thin surfacing?
Majority – No, Occasional – ‘yes but’, others in preparation
1.2
What type of distress failures have you observed in the structural
performance of pavements with a thin surfacing and how extensive is
the occurrence of these?
Reflective cracking the most common initial failure reported, followed by fretting (which
appears in most responses), permeability and debonding.
1.3
Can you attribute a particular failure mode to the condition of the supporting layer(s)
beneath the thin surfacing?
(Please describe)
If substrate is inadequate then rapid failure occurred within the surfacing
1.4
Do you have evidence of enhanced serviceability performance with particular
combinations of nominal size and layer thickness?
Mixed responses but several respondents referred an absolute minimum thickness of 2.5
times nominal aggregate size
1.5
What degree of importance do you assign to the presence/absence of bond/tack
coat and its methodology of application?
(Do you have any evidence to support?)
Great importance assigned to the bond/tack coat for good performance particularly with
thinner layers. Specification enforcement is considered essential, especially for smaller
maintenance schemes.
1.6
How do you differentiate between the various thin surfacings for particular sites?
(e.g. historical performance, layer thickness, nominal size, brand name, etc)
No clear response with a number of differing answers.
2.1
Do the demands of thin surfacing feature in your environmental policy
considerations? (i.e. premium quality aggregates, transport distances, etc).
The majority of responses were ‘yes’ with others under consideration.
2.2
Can you attribute a particular failure mode to a particular thin surfacing material
type?
No particular failure mode related to any individual material.
2.3
Do you use generic SMA specifications or BBA/HAPAS systems?
10 respondents use BBA process, 9 use generic specifications and 6 uses a mixture of both.
No respondent referred to the CSS Guidelines.
68
2.4
Do you have a documented system which covers approvals, installation and
associated practices for thin surfacing?
15 respondents confirmed that they do have a documented procedure, but this did not cover
all facets in all cases. 8 reported not having a procedure with two uncertain responses.
3.1
Do you have any predictive measurement tools to forecast the onset of failure?
(such that maintenance can be programmed for asset management purposes)
General response is ‘no’ but several replies indicated that they were using a variety of tools.
Visual condition examinations seemed to predominate. Recognition that forecast of failure
(for refurbishments) need to be developed for asset management planning.
3.2
Do you have any experience of surface applied rejuvenation agents or
techniques to extend useful service life of thin surfacing?
Respondents generally indicated that they did not have experience of surface rejuvenation
agents although surface dressing and retexturing had been used in this role.
4.1
Please give your view on serviceability performance comparison between thin
surfacing and standard materials such as HRA / DBM / HSCA / etc (i.e. Thin
surfacing is better / equal / worse
Better
Equal
Worse
No response
Urban
32% (8)
32% (8)
16% (4)
20% (5)
Rural
36 % (9)
24% (6)
12% (3)
28% (7)
High Speed
52% (13)
20% (5)
12% (3)
16% (4)
Low Speed
28% (7)
44% (11)
8% (2)
20% (5)
High Stress
28% (7)
20% (5)
28% (7)
24% (6)
Low stress
32% (8)
36% (9)
12% (3)
20% (5)
4.2
What is your view on the public perception of thin surfacing as a highway
maintenance option? (Especially related to traffic noise attenuation
properties/durability, etc)
Public favour due to noise reduction. Issue of equestrian use.
4.3
For reinstatement purposes in a thin surfacing area – do you have a preferred
product or process to match, as closely as possible, the existing surface?
Difficulties of hand laying recognised. HAUC guidance referred to. Appears to be significant
variations in reinstatement material.
4.4
Are there any specific, or general, comments you wish to make in relation to
thin surfacing? (e.g. serviceability improvements, cutting edge concepts,
deterioration forecasting, etc)
19 respondents; Several mentioned early life skid resistance and some, medium to longer
term skid resistance. The relationship between skid resistance and aggregate properties
was queried. Some reported considering returning to Hot Rolled Asphalt as surfacing.
Realism needed in expectation of performance in service.
69
4.5
Are you interested in receiving an invitation to attend a one-day interactive
regional workshop in England on the development of Best Practice Guidelines?
Please select as appropriate.
70
Appendix E - Regional Workshops
Following review of the returned questionnaires from Local Authorities three
regional workshops were held for Local Authority highway engineers in the
south, central and north regions of England.
Delegates from Scotland and Northern Ireland were invited to the central
region workshop owing to the ease of travel via domestic flights to
Birmingham International airport
Each workshop ran from 10.00 to 16.00 and had a common programme
format, as below, and invited delegates attended at each event. All those who
had contributed to the enquiry questionnaire were invited and guaranteed a
place.
A programme comprised a blend of formal and facilitated syndicate sessions
(with about ten delegates in each) focused upon key aspects to address a
series of questions. The presenters/facilitators at each workshop were Steve
Biczysko, Peter Cearns and Harry Potter.
Birmingham, The Axis - Tuesday 12th April - 18 delegates.
London, Euston Tower – Thursday 14th April - 24 delegates.
Huddersfield, Galphram Stadium – 21st April - 26 delegates.
Programme
10.20
10.45
Introduction and objectives of the workshops
Workshop 1 – Site Evaluation for Surfacing
1 Scale and nature of the problem
2 How is the site identified in the maintenance
programme? - a mechanistic approach?
3 What is important in engineering terms?
4 Decision tree
12.05
Feedback
13.45
Workshop 2 – Materials Selection Processes
1 What are the key engineering characteristics?
2 On what basis should selection be made?
3 Decision tree
Feedback
Group sessions followed by plenary discussion.
Summary bringing it all together Right material: right site
14.45
15.35
Group sessions followed by plenary discussion
71
Appendix F - BBA Scheme Guidance
Thin surfacing systems may be specified generically or by stating the performance
levels for independently certified proprietary products.
Highway Authorities Product Approval Scheme (HAPAS) certification applies to an
assessed system and its suitability to be laid and to perform satisfactorily on trial
sites.
The suitability of systems for chosen sites remains the responsibility of the client
unless the contract specification passes the selection or design responsibility to the
contractor.
Although a contractor’s quality plan may require the contractor to inspect each site to
assess its suitability for a proposed system there is no presumption that this will be
done, unless it is required in the works contract or it is known that the inspection is
part of a particular certificate holder’s documented system.
HAPAS was set up by the Highways Agency (HA), CSS (formerly County Surveyors
Society and the British Board of Agrément, with the objective of developing national
approval arrangements for innovative products, materials and systems for use in
highways and related areas, removing the need for individual authorities to carry out
their own assessments and tests. The Scottish, Welsh and Northern Ireland
Departments of State and TAG (Local Government Technical Advisers Group)
support the scheme.
HAPAS Specialist Groups draw up guideline documents giving details of the tests,
assessment criteria and quality assurance requirements, which the BBA then uses in
its evaluations. Specialist Group 3 was established to be responsible for drafting the
guidelines for the assessment of thin surfacing systems. It is constituted from
representatives of the HA, CSS and TAG with the co-opted assistance of
representatives from the Asphalt Industry, including the Quarry Products Association,
the Refined Bitumen Association and selected suppliers and contractors.
The BBA approval process for HAPAS involves laboratory and witness testing, site
inspection and evaluation of the source of production of the material under
assessment. Existing test or performance information is also investigated. Data
generated from the key elements is considered by the BBA technical experts in the
context of the requirements set out in the Guidelines. If the data is acceptable, the
HAPAS certificate is drafted for internal comment. Comments are considered by the
BBA and the certificate is then formally issued. The BBA interest continues after the
award of its certificate. Surveillance visits are normally conducted twice yearly, to
ensure that the specification of the product is being maintained and at the end of
each five-year period a formal review process is undertaken.
The BBA certificates are called up where appropriate in the Specification for
Highways Works. They are intended to provide an independent opinion of the
performance of the products covered by them, enabling highway engineers to specify
these products in the knowledge that they have been thoroughly evaluated.
In the event of a Certified System’s unsatisfactory performance in use, the BBA will
investigate complaints on receipt of written details from the Client. The views of the
Certificate Holder are always sought.
72
Appendix G - Investigation Protocols
The need for investigation in support of scheme design is stated in Appendix E of the
Code of Practice for Highway Maintenance Management. The investigation must
include forensic examination with intrusive exploration in many cases.
The DMRB provides advice in HD30/99 for the systematic and cost effective
investigation of pavements. The advice is as applicable to heavily trafficked local
roads as it is to trunk roads. When time, works programming or traffic management
constraints prevents the use of the HD30/99 protocol there is usually still a need to
carry out a site investigation in order to determine the source of distress in the
existing pavement. Decisions can then be made as to whether the site is suitable for
NTS and what form is appropriate.
Figures A and B are simplified protocols for investigations of pavements for the
purposes of designing surface applications. In the event of a pavement showing
signs of distress it is absolutely essential to identify the cause(s) of the distress
before designing the remedial work.
73
Examine available survey data
e.g. Detailed Visual Condition,
Deflectograph,
Falling Weight Deflectometer (FWD),
Ground Radar (GPR).
Assess the data
Is depth of failure apparent from
assessment?
No
Yes
Return to Chart B or C
Carry Out Coring Survey
Is depth of failure
apparent from
investigation?
Yes
No
Carry out trial trench
investigation
Figure G.1 - Investigation Protocol - Determination of Depth of Failure
The most cost effective investigation methods require an examination of existing
records, particularly pavement assessment survey data and geotechnical survey
reports where they exist. An on site walk through survey should be carried out
particularly noting:
74
•
•
•
•
•
•
•
Crazing or cracking patterns;
Presence and depth of any delamination;
Type and appearance of existing surfacing;
Presence of damp or wet areas;
Presence and depth of spalling;
Local and general areas of subsidence;
Signs of early failure in the existing surfacing.
Only when the desk study and walk through survey fail to produce an answer should
intrusive or non-destructive methods of investigation be necessary.
Coring surveys are frequently the first intrusive surveys. Exploratory cores should be
taken through the full thickness of bound material to sub-base or foundation level.
The sub-base and formation can then be assessed by dynamic probing. Careful
logging of cores and probing data by an experienced technician is essential to the
later interpretation of the data. The logs should note the type and condition of all
materials paying particular attention to any broken or un-recovered material. Where
the surface is cracked the depth of cracking should be determined by coring through
the cracks using 150mm minimum diameter core barrels. To examine the direction
of crack propagation, either upwards or downwards, cores can be taken at the tips of
cracks. To observe the rate of crack growth and when this occurs the crack tip on the
surface may be flash marked with paint and the growth of the crack can be recorded
periodically. It is recommended that, as a minimum, a pair of cores should be taken
for each 100m of carriageway.
If coring fails to reveal the cause of distress exploratory trial trenches may need to be
excavated. Such investigations should always be carried out and logged by
experienced geotechnical or materials engineering staff and a written report
produced giving findings and recommendations.
During examination of specimens from coring or exploratory pits the presence of any
substances such as tar compounds should be noted.
75
Examine available survey data
e.g. Detailed Visual Condition,
Deflectograph,
Falling Weight Deflectometer (FWD),
Ground Radar (GPR).
Assess the data
Is structural weakness apparent?
No
Yes
Carry out geotechnical
appraisal
Carry out coring or trench
excavation survey
Is reason for cracking
apparent?
Is the orgin and depth for
cracking defined?
Yes
No*
No*
Return to Chart C
Yes
Return to Chart C
* If second loop fails tt reveal the reason for distressthe siteis unsuitable for treatment using
thin surfacing or SMA and a full investigation and bespoke pavement design should be undertaken.
FIGURE B INVESTIGATION PROTOCOL - CRACKING AND/OR CRAZING CONDITION
Figure G.2 - Investigation Protocol - Cracking and/or Crazing Condition
76
Appendix H – Specifying Proprietary Materials
Tables A, B and C provide a means of compiling engineering performance
requirements for proprietary system surfacing as a SHW style Appendix 7/1,
Schedule 5.
The following tables are revisions of those originally published in CSS “Advice
Note for the Specification of Thin Surfacing”. (Report Eng/2003.
www.cssnet.org.uk ).
Table A – Notes for completion of Appendix 7/1
Attribute
HA
SHW/NG
Clause No.
Selection of Performance Levels
Traffic level
942.1
Stress level
942.1
NG 942.7
942.7
942.10
State cvd at end of guarantee period (normally 2 years) and at end of
design life (to be established with local knowledge of substrate and likely
system performance).
Refer to Table NG9/27 and NG9/28
Wheel tracking level
and
Polished Stone Value (PSV) of
coarse aggregate
942.5 and NG
942.8
Aggregate
Abrasion
value
(AAV) of coarse aggregate
942.5
Layer thickness
NG942.9
Texture depth
NG942.13
Select from Table NG 9/29 having obtained traffic and stress levels as
above. The traffic for this purpose is the traffic at the end of the guarantee
period. Care should be taken not to over specify. Only at traffic lights and
other areas with frequent stationary traffic has wheel track rutting been
found to be a problem on local roads.
Select using DMRB HD36. At the time of drafting there was no conclusive
evidence that thin surfacing using coarse aggregate of a given PSV
performs significantly better, or worse, than HRA with pre-coated
chippings of the same PSV.
Select using DMRB HD36. It is likely that for dense systems the
aggregate resistance and abrasion is likely to have less influence on the
longevity of the surfacing than that for pre-coated chippings in HRA.
Where favourable local experience exists for a particular source and
environmental and/or economic advantages are available by so doing, the
AAV requirement could be relaxed relative to the HD36 requirement.
For local roads layer thickness is usually an essential requirement. The
nominal thickness should be stated, and where necessary the minimum
and maximum. For nominal thickness less than 30mm, the requirements
of Clause 702.3 of the HA SHW may need to be modified to ensure that
any reduction in surfacing thickness is suitably restricted, say, to less than
15% of the nominal thickness.
Most local authorities will have requirements for various categories of road
surface texture dependant upon functionality. Adequate surface texture
contributes to the provision of a safe road surface and reduces the
potential accidents at all traffic speeds including low speed roads.
However with thin surfacing, increased texture usually implies a more
open surface and there is thus less aggregate restraint to horizontal
loading. Thus, for breaking and/or turning areas such as roundabouts, a
compromise texture is usually desirable
No facility exists within the BBA procedure for specifying maximum
texture. In practice the use of a rational minimum texture has been found
to produce a corresponding reduction in maximum texture. For most highly
stressed sites in speed restricted areas or where the 85 percentile speed
is less than 50mph, a level 2 requirement will be suitable. Where the
speed restriction is 40mph or less level 1 will normally suffice.
Where there are noise constraints the specifier should appreciate that a
requirement for texture could conflict with the noise performance levels.
However, with negatively textured thin surfacing noise can decrease with
increasing texture depth. Before finalising Appendix 7/1 when there are
requirements for both noise and texture the specifier should be satisfied
that sufficient products are available which satisfy both the criteria.
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Road/tyre noise
NG942.10
Road/tyre noise is categorised relative to HRA. Generally thin surfacing
performs significantly better than HRA with the more open materials,
which approach porous asphalt in composition and finer aggregate size
mixtures, likely to perform best. The more porous materials tend to give
lower structural strength contribution than dense materials both because
they are frequently laid thinner and because they are less stiff than dense
surfacings.
The specifier may need to compromise between conflicting requirements.
Road/tyre noise is primarily a problem with heavy, medium and high
speed traffic on urban and sub urban sites. For these and other
environmentally sensitive areas level 2 should be specified which provides
a significantly better standard than HRA. Where very high standards are
required the suppliers should be selected on the basis of their certified
values.
Guarantee period
942.13
Normally 2 years is specified. Should a surfacing fail to perform
satisfactorily after expiry of the guarantee, but within the normally
expected life then the BBA should be informed so that the reason and
responsibility for remediation can be determined.
Table B – Special Attributes Certified by BBA
Note: only to be used where special conditions require
Attribute
Indirect Tensile Stiffness Modulus
(ITSM)
Sensitivity to diesel (or other fluids)
Ageing characteristics using Indirect
Tensile Fatigue Test (ITFT)
Resistance to binder stripping using
the immersed wheel tracking test
(IWTT)
Recommendation
Only for analytically designed pavements is it necessary to specify ITSM. For most
maintenance purposes it is sufficient to use equivalence factors. For resurfacing of
less than 30mm nominal depth, the structural contribution is generally considered
negligible. For thicker overlays the first estimates of structural equivalence given below
are recommended:
HRA wearing course
1.0
Dense thin surfacing (air voids <7.0%)
1.0
Porous surfacing (air voids 7.1% to 20%) 0.6-0.8
Porous asphalt
0.5
For first iteration purposes the structural contribution of surfacing on deep pavements
may be taken as proportional to ITSM x d, where d = depth of layer. For shallow
pavements, the depth element becomes more significant and the proportionality
approaches ITSM x d² for the extreme where the surfacing represents the only sound
material.
Products are under development which are more suitable for parking areas, bus stops
etc. than conventional bituminous material, largely as replacements for the tar bound
mixtures previously used for such sites. At the time of writing none had yet been
certified. It was not therefore possible to make recommendations with regard to
product selection. The BBA website shows current certification and should be
consulted prior to specifying surfacing at risk from fuel spillage.
The certified test parameters when available should only be used for the purpose of
tender list selection. The test and correlation to performance are insufficiently
developed for specification purposes. Unless certified to the contrary ‘porous’ systems
are likely to be sensitive to spillage damage and to protect their substrates less
efficiently than dense materials.
Surfacing applied to carriageways which are subject to elastic deformation and to
settlement will fail earlier than where the same surfacing is laid on an unyielding
substrate. Some mixtures are more tolerant of elastic deformation than others. The
ITFT is designed to differentiate between deformation tolerant and deformation
intolerant mixtures. Insufficient knowledge exists to allow categories to be allocated.
The Specifier should only use the certified parameter for tender list selection.
Water penetration of a bituminous surfacing with the combined effects of traffic loading
can eventually lead to a break down in the bond between the aggregate and the
binder. The IWTT simulates this critical condition. Where data has been certified it
may be used to rank surfacing systems, which are subject to adverse site conditions
such as application to poorly drained carriageways. Insufficient knowledge exists to
allow categories to be allocated. The Specifier should only use the certified parameter
for tender list selection.
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Table C – Special Attributes (not certified by BBA)
Attribute
Mix density
Recommendation
Not all local road sites can tolerate the use of `porous` or ‘open’ surfacing. The use of
dense material can have significant structural benefits and provide a better substrate
for eventual overlay. A lack of suitable drainage, or severe lateral stress conditions,
may also make it desirable to use a low air voids mixture. An upper limit of 7% has
been found to differentiate effectively between the dense mixtures from the more open
mixtures. Where the specifier wishes to limit the tender to only dense surfacing the
following may be included:
‘The in-situ air voids are to be not greater than 7% when measured as the
average of any successive 6 cores with no result greater than 8%. The air voids
shall be defined as 100(A-B)/A.
Where:
A= maximum theoretical density tested in accordance with BS598:Pt104, and
B= bulk density calculated tested in accordance with BS598:Pt104, using wax
sealing where appropriate.’
Declaration of Design
Gritting
In order to allow the Client to be able to audit compliance with the contract, when
necessary, it is recommended that the following be added to the contract.
‘Where required the contractor shall submit to the Overseeing Organisation the
details of the proposed mixture listed below. Should it be necessary to vary the
submitted details during the works changes shall not be made with out the prior
agreement of the Overseeing Organisation.
Details to be submitted:
1. Target grading and tolerances
2. Target binder content and tolerances
3. Sources of coarse and fine aggregate
4. Binder type and details of any polymer modification
5. Binder drainage limits
6. Sector scheme current ‘Q’ values
Bond coat specification, including Vialit cohesion value or BBA certificate.
Some Local Authorities may wish to specify the application of uncoated grit to the road
surface at the time of construction either on `horse routes` or elsewhere on their
network. If gritting is required the following may be used:
‘Where required by the Overseeing Organisation the surface shall be treated with
3mm size uncoated grit as described in Table G1. The grit shall be evenly
distributed on the road surface at a target rate of spread of 1 kg/sq metre using a
calibrated mechanical spreader. The grit shall be applied during the initial
rolling process after at least three passes of the primary roller. Excess grit shall
be removed by mechanical sweeping prior to opening the road to traffic.
BS test sieve (mm)
Percentage passing
6.3
100
4.0
80 - 95
2.8
38 - 68
2.0
10 - 35
1.0
0 - 12
0.5
0-5
0.063
0 - 1,0
(Note. Quartzite grit has been found to be effective with the above grading.
Other locally derived siliceous angular aggregate with similar grading may be
equally effective but demonstration trials should be undertaken to confirm this)
Table G1 Uncoated Grit
Cold Weather Working
SHW 945 covers the laying criteria for most asphalt concrete but, because the laying
conditions are certified for thin surfacing by BBA, thin surfacing is not included in
Clause 945. However the certified limits are considered to be too low in many cases for
overall county road application. To reduce the temperature associated risks it is
recommended that only materials in Category C and with a layer thickness of not less
st
than 30mm should be laid between the 1 December and the 1st April. Where it is
necessary to lay thinner layers in the winter special measures may be required,
particularly if wind chill is likely to be a problem. It is recommended that the contractor
be asked to provide a risk analysis based method statement describing any special
measures for the information of the overseeing authority.
In no case should thin surfacing be laid in heavy rain or over standing or flowing water.
Thin layers of dense mix material have proved particularly susceptible when laid on wet
or saturated porous substrates.
Note to Table C – The text emboldened above should be added to the project specification where appropriate.
79
Appendix 7/1 Schedule 5 – Requirements for Construction Materials
SHW Clause 942 – Thin Surface Course Systems
Traffic at end of guarantee period (cvd)
Traffic at end of design life (cvd)
Site category (A-K)
Stress level (1-4)
Wheel tracking level (0-3)
Polished stone value (min)
Aggregate abrasion value (max)
Layer thickness (nominal) mm
or as type A, B or C*(1)
Texture depth level (0-3)
Road/tyre noise level (0-2)
Guarantee period (years)
Indirect tensile stiffness modulus (min), (MPa) *(3)
Supplementary Options
Declaration of design required (Ref. CSS contract clause)
YES/NO *(2)
Maximum air voids requirement to apply
YES/NO *(2)
Gritting
NOT REQUIRED *(2)
UNCOATED TO BE USED *(2)
Winter (1 December to 1 April) working risk analysis required for
layers less than 30mm depth
YES/NO *(2)
Notes
*(1)
*(2)
*(3)
Layer thickness to be specified by dimension only or by type only.
should not be used together.
Delete as applicable
To be specified only if required by designer.
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Dimension
and
type
Appendix I – Installers Observations
The summarised views and observations of a cross section of installers who were
asked through structured interview in respect of the following;
Installers’ observations relating to substrate
• “If the substrate is not rut-resistant, the Client is wasting his
•
•
•
•
•
•
•
•
•
•
•
money”.
“Historical records appear to be generally poorer for urban
locations”.
“The ideal client has all the substrate details, knows the problems
beneath and carries out any reconstruction required before the
contractor arrives on site. The dream site: an even surface without
any sign of structural failure that is rut resistant”.
“Clients commonly leave the contractor to do site inspections.
Contracts are typically divided into two types of tender: Fixed price
for several sites approx 10% of clients attend site inspections.
Individual sites on a quotation per job basis approx 30% of clients
attend site inspection”.
“Site suitability is down to the Client who is expected to carry out
appropriate surveys to establish this”.
“We expect the client to have cored and understand the substrate
conditions”.
“The onus of assessing substrate suitability appeared to be placed
well and truly on the Client without detailed guidance to this
suitability being readily forthcoming from the supplier”.
“More information about the substrate, such as cracking deflection
and coring, could be brought into the selection process. SCANNER
survey data should both exist and be used in the assessment of all
but estate road schemes”.
“The planing out should be appropriate for the nominal size of the
material used and to a depth that retains some of the existing
surface/wearing course as the substrate. However a lack of
information about the existing base/surface course and/or
irregularities in levels can lead to penetration of the planer into the
underlying materials”.
“A good knowledge of the substrate is essential prior to planing as
the equipment cannot easily accommodate fluctuating boundary
depths between layers as biscuiting can result when the planer
weakens the bond between a thin remnant of, say HRA, and the
underlying layer”.
“Despite the fact that a binder course is recommended other than
for simple resurfacing work (BS 4987/2 Note 1 2003 edition) it
appears not to be widely implemented”.
“Use of binder course is strongly advocated. The cause of the
significantly shorter service life of non-trunk road thin surfacing
schemes was placed on the general failure to use them. Typical
small value of minor works at approximately £10-12k (2000m2) may
mean a binder course is too expensive to use. The 3-5 year service
81
life of SMAs may likewise be attributable to a general lack of use of
binder courses”.
Installers’ observations relating to material choice
• “Thin surfacing could be developed as a universal solution - if the
range of products can accommodate all substrate and traffic
requirements”.
• “Material choices are primarily driven by cost, the addition of BBA
certification and/or the use of polymer modifiers and other additives
increases this cost potentially leading to the use of second division
materials to meet a limited budget to deliver the maximum treated
area - quantity not quality”.
• “The Client usually proposes the material to be used”.
• “At least six or seven lorry loads of material should be on site
before work commences to avoid humps and patchiness resulting
from material being laid cold after a period of delay awaiting
incoming batches”.
Installers’ observations on client role
• “The greatest obstacle to the application of more rigorous
guidelines for installation/selection appeared to be the clients
approach to the delivery of the finished product”.
• “Clients commonly have a desire for the contractor to deliver
materials that will satisfy a short term requirement prior to
reconstruction, i.e. not to provide a product with a service life of 1015 years if reconstruction for realignment etc. is planned in only
three or four”.
• “Clients have the option to instruct for work to proceed without
substrate remediation being carried out. However this departure
would be noted on the Quality Plan notes for future reference, and
if a subsequent failure during the two year maintenance period
cannot be attributed to the problems encountered at the time of
laying the contractor would be liable under the BBA scheme”.
• “Clients are renowned for their ability to ask for a job to be done
regardless of circumstances which may result in a poor job, with the
appropriate disclaimers changing hands accordingly”.
Installers’ observations on placement
• “Experience has shown the use of a hot cut roller made joint along
with bond coat where required is the best procedure to adopt. Aslaid joints tend to be wedge shaped leading to a zone of weakness
in the overlying subsequent surfacing - saw cut joins with the use of
a bond coat to cover any exposed aggregate are a good second”.
• “Despite the fact materials are not supposed to be laid during
periods of adverse weather either due to the time of year or periods
of local bad weather, works do continue in these conditions. Works
proceed regardless where lane closures exist and end-of-year
approaches”.
Installers observations on significant, general issues
82
• “Problems can relate to mixes, adverse temperature and poor
•
•
•
•
•
•
•
weather, but by far the most common causes of premature failure of
their product are substrate failure and the after effects of adverse
working”.
“Foundation quality is the critical factor in terms of site selection,
any potential for this to be within future BBA certification? However
the use of Deflectograph and FWD data was limited by experienced
interpretation”.
“The industry is facing a very real difficulty through the loss of
traditional expertise on all sides”.
“An ideal Best Practice Guide should be stating the obvious - since
it has not been stated already anywhere else”.
“Best practice to date is based primarily on the contractors’
experience”.
“Any guidance should be clear concise and effective. The
specification concerning tack coat selection is a complex area and
difficult one to apply in practice”.
“The greater number of materials may be resulting in the far greater
performance variability seen on County roads when compared with
that of trunk road schemes.”
“The general quality of the non-trunk road network was seen as
poor, with variable foundations as a whole and unsupported edges,
of particular concern”.
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Acknowledgements
Project Sponsor
The UK Roads Board
Project Commissioner
Department for Transport (DfT) – Andrew Oldland
Project Advisor to DfT – John Thorp, Lancashire County Council
Atkins Project Team
Alan Taggart
Mike Kendrick
Steve Biczysko
Peter Cearns
Harry Potter
John Bullas
Victoria Carter
Lila Tachtsi
Steve Finnie
Industry - Client organisations, Producers and Installers
Grateful thanks to all who, knowingly or otherwise, have contributed to the
development of these Best Practice Guidelines with particular thanks to John
Wale, Kent County Council (CSS) and Stephen Child, Surrey County Council
(IHT).
Photographs
Chris Britton Consultancy and UKPMS.com are gratefully acknowledged for
agreement to use selected photographs from the UKPMS inspectors’ manual
to illustrate some of the highway surface features in the decision charts.
Disclaimer
Whilst every care has been taken in the preparation of this Best Practice
document, the authors stress that it is intended for guidance only. The views
expressed are those of the project team and do not necessarily represent
those of the sponsoring or commissioning organisations nor individuals or
organisations consulted during its preparation. No legal liability is accepted for
its contents and the document is not intended as a substitute for legal or
project-specific technical advice.
84