SPRAYED SEALING PRACTICE IN AUSTRALIA
Walter Holtrop, Australian Asphalt Pavement Association,
Australia
ABSTRACT
This paper provides an overview of current sealing practice in Australia, including priming and
primersealing, types of sprayed seals and their selection, the current national seal design
procedure, and plant and field procedures commonly used.
Asphalt is the preferred treatment in urban areas, on heavily trafficked urban freeways and
arterial roads, and areas of high traffic stresses. Sprayed sealing is the surfacing treatment
commonly used in rural areas, and is the most economic type of surfacing for the rural road
network. It is also used for specific applications, such as strain alleviating membranes to
minimise crack reflection, on all classes of roads.
The main type of sprayed seal is a single layer of binder covered with a single layer of
aggregate (single/single seal) used on both new and resurfacing work. On new work the
pavement material is usually locally available gravel, often of marginal quality, with better quality
crushed rock material used on the more heavily trafficked roads.
Sprayed seal design as referred to in this paper is the design of rates of application of binder
and aggregate spread rates. The continued success of sprayed seals as a surfacing requires
care in choosing an appropriate treatment for the conditions, a high standard of preparation of
pavements and attention to detail. To successfully select and design a sprayed seal requires a
mix of engineering and ‘practical know how’.
INTRODUCTION
The technique of sprayed sealing was adopted in Australia because of its relatively low cost and
speed of construction compared to other forms of pavement surfacing, and has been the
mainstay of road authorities in Australia to provide a safe, all-weather rural road network. Major
factors that contributed to improvements in the performance of seals in the early days were:

Adoption of the design procedure developed by F M Hanson, and national continuous
improvement thereof over many years by state road authorities.

Development of major plant items and practical, proven, field procedures specifically for
sprayed sealing works in Australian conditions.

Introduction of hot bulk bitumen, and the use of cutter oil to field produce cutback
bitumen mixtures, on site at the time of use and most appropriate for the prevailing
weather conditions.

In 2005, ARRB Group, at the direction of Austroads (the national body which represents
the interests of the state road authorities) arranged a national Sprayed Sealing
Workshop for practitioners. Attendees were from road authorities and sealing
contractors, including senior management and technical specialists. The following are
considered most important of the many issues that were discussed.
—
Loss of skilled practitioners, and the difficulty in replacing them in the short term.
—
Rapid increase in the number of “large heavy vehicles” and their effect on seal
performance.
—
Potential embedment of aggregate into new granular bases, and asphalt patches
on existing roads to be resealed, and how to determine the allowance required to
be made for this in the seal design procedure.
—
Selecting the “cheapest” type of treatment available.
—
Unrealistic performance/service life expectations when resealing
cracked/distressed pavements.
—
Developing a formal design method for primerseals.
AUSTRALIA
Climate
There are major climatic variations throughout Australia from semi-tropical to extreme hot
temperature conditions, and some areas of alpine conditions (see Figure 1). Rainfall patterns
range from high rainfall in the north to extremely low rainfall in the central desert regions.
Figure 1: Variations in Maximum Temperature
Traffic
Traffic volumes used in seal design are generally provided as Average Annual Daily Traffic
(AADT), which is the total traffic carried by the road. Traffic can vary from very low, < 100 AADT
for local roads in rural areas to as high as 40 000 AADT on freeways and major routes.
Commercial vehicles in general make up about between 5 and 10% of the total traffic, but this
can vary from almost nil to as high as 35% on the major freight carrying roads, and over 50% on
quarry and mining access roads. The rapid increase in large heavy vehicles over the last few
years (classed as having seven or more axles) is placing increased demand on pavement
materials, sprayed seals and the design and construction processes. This increase in large
heavy vehicles is expected to continue.
Some facts about Australia
The following information is collected from various sources such as Austroads Road Facts,
Australian Census, AAPA industry statistics. This is provided for background information only,
and not as accurate statistics:
(i) Population
Approximately 21 million, the majority of whom live within 100 km of the coast
(ii) Area
7.7 million square km (roughly 3000km x 2800km)
(iii) Road network

approx 800 000 km total length

approx 500 000 km gravel surface, earth or unformed

approx 307 000 km surfaced with seals or asphalt, and minor lengths of slurry and
concrete.
(iv) Sprayed seals
Estimated 270 000km (approximately 90% of the surfaced length)
(v) Bitumen
Average annual use varies from 750 - 800 000 tons, with approximately half of this used on
sealing works
(vi) Network value
In excess of $100 billion (Australian)
(vii) Annual expenditure on sealing
Approx $450 million (Australian)
(viii) Average seal life
Based on typical intervention levels, for single/single seals from about seven years for small
aggregate to 12 – 15 years for larger aggregate.
(ix) Sprayers operating
An average of 200 calibrated and operating sprayers are listed on the AAPA web page. Sealing
contractors own most of these, but a number of road authorities and councils also own sprayers.
MATERIALS
Bituminous materials
Bituminous based materials used are covered in various Australian Standards and Austroads
specifications. Common materials used in sprayed sealing are:
(i) Bitumen
Class 170 (approximately equivalent to 85/100 penetration).
(ii) Cutback bitumen
Cutback bitumens are used for:

sealing, generally C170 mixed with cutter as required

priming/primersealing, Australian Standard grades, proprietary grades and field
produced equivalent to the standard grades of cutback.
(iii) Cutter oil
A light solvent such as lighting kerosene or aviation turbine fuel
(iv) Bitumen emulsion
Generally Australian Standard grades of cationic emulsions, with specialty grades developed for
priming.
(v) Aggregate precoating materials
Oil or bitumen based, specialty grade of bitumen emulsion.
(vi) Adhesion agents
Amines to promote wetting and adhesion in damp conditions, and for aggregates with poor
affinity to bitumen.
(vii) Polymer modified binders (PMB)
Manufactured based on SBS, PBD and crumbed rubber polymers, generally used as hot
binders but also available as emulsion.
(viii) Multigrade bitumen
C500/170 for sprayed sealing, designed to behave like C500 in hot conditions and C170 in
cooler conditions.
Aggregates
Single sized aggregates are preferred because this provides maximum tyre contact and macro
texture for surface drainage. Common aggregate sizes used are 7, 10 and 14 mm, with some
16 and 20 mm.
Australia is fortunate in that there are sufficient different aggregates available able to meet the
specified requirements for the general range of traffic and climatic conditions. The aim is to
select an appropriate and economic aggregate for the conditions. As deposits diminish, and it
becomes more difficult to open new quarries due to environmental restrictions, it is expected
that greater value will be placed on aggregates of better quality required for heavily trafficked
roads and to meet skid resistance requirements.
Aggregates are commonly manufactured by crushing and screening of suitable rock deposits.
Aggregate specifications are reasonably uniform across Australia and appropriate properties are
specified to cover the range of traffic conditions, loads and weather conditions, including specific
properties, for example, such as may be required for wearing qualities and polished stone value
for skid resistance.
Quarries are required to separate production into lot sizes, and test and report on grading,
median size, flakiness index and Average Least Dimension (ALD).
TYPES OF SPRAYED TREATMENTS
Sprayed treatments are broadly separated into two main types:
(i) Initial treatment on new pavements:

prime and seal

primerseal (small aggregate), followed by a final seal (usually larger aggregate) one or two
years later

prime and small aggregate seal, followed by a final seal one or two years later

a ‘final seal’ is a part of the selection and design process for the overall initial treatment.
New works are generally constructed using unbound granular materials such as local gravels on
low traffic roads, with crushed rock on the more heavily trafficked roads. When rehabilitating
these roads they may be overlaid with another layer of granular material, or they may be
stabilised using cement. Bitumen stabilisation is more expensive and therefore not often used.
Where practical, pavements are primed and sealed, but to minimise disruption to traffic the
predominant treatment is now a primerseal, followed say 12 to 18 months later with a final
surfacing.
The prolonged drought conditions are affecting road construction, particularly in rural areas, and
this aspect has to be considered when selecting a suitable initial sprayed seal surfacing
treatment.
Cheaper local marginal materials used in rural locations are no longer able to adequately
support heavy loads as is evident in the loss of texture early in the seal life. On roads carrying
heavy vehicles it may be necessary to import and use more expensive crushed rock material as
the base material.
Concrete pavements, if required to have a bituminous surfacing, are primed prior to placing
asphalt or a sprayed seal.
(ii) Reseals over existing bituminous surface:
Existing surfacing may be seals, asphalt or slurry.
Prime
A prime is the application of a suitable viscosity primer to a properly constructed and prepared
pavement, as a preliminary treatment to the application of a seal or asphalt surfacing.
Primers perform best when applied in warm/dry conditions. Primers are usually light grades of
cutback bitumen but special emulsion primer is becoming more popular and has the advantage
that it can be used in cooler conditions, dries quicker, contains little cutter and thus may have
the surfacing applied sooner. Emulsion is a lower risk environmentally than cutback bitumen in
cooler and/or damp conditions.
The pavement must be allowed to dry back, to about 70% of optimum moisture content, and the
surface dry/damp only to enable the primer to penetrate into the pavement surface.
Primerseal
A primerseal is the application of a primerbinder and aggregate cover to provide a temporary
surfacing treatment.
Sprayed Seals
Standard, commonly used types of seal are generally identified by the number of layers of
bitumen and aggregate. The common seal types are:
(i) Single/single
Single layer of aggregate covered with a single layer of aggregate as shown in Figure 2. This is
the most used and economic type of seal on low to medium traffic roads with 10 mm aggregate.
Figure 2: Single/single seal
(ii) Double/double
A combination of two layers of binder and aggregate as shown in Figure 3. The first application
seal uses the larger aggregate and the second application aggregate is ideally half the size of
the first. This is a more robust seal for areas of high traffic loading or stress, or using an
economical aggregate that has a low affinity for bitumen.
Figure 3: Double/double seal
(iii) Inverted seal
Similar to a double/double but the first application is the smaller aggregate seal as shown in
Figure 4. Used to correct existing non-uniform texture prior to resealing, resealing existing fatty
seals or to minimise potential aggregate embedment on initial treatments when sealing soft
pavement bases, (limestone and sandstone).
Time interval between the two seals varies from a few weeks to several years depending on the
locations, reason for selection.
Figure 4: Inverted seal
(iv) Regulating or correction seal
A single/single type seal used to correct existing surface texture prior to a reseal if the existing
surface texture is considered to be too coarse for a 10 mm or larger reseal. Generally a size 5
or 7 mm aggregate is used to fill and reduce the existing voids.
Special purpose seals
(i) Strain Alleviating Membrane (SAM)
A single/single seal (typically using 10 or 14 mm aggregate) as shown in Figure 5, with a PMB
to minimise reflection cracking and/or provide a more waterproof surfacing on roads constructed
with moisture sensitive pavement material or subgrade, or sealing minor surface cracks in
concrete bridge decks or pavements.
Cracks
Figure 5: Strain alleviating membrane (SAM)
(ii) Strain Alleviating Membrane Interlayer (SAMI)
A single/single seal (typically using 10 or 14 mm aggregate) similar to a SAM as shown in
Figure 5, with a high modification PMB to minimise reflection cracking into an asphalt overlay, or
to provide a very waterproof membrane under open graded asphalt. Design is similar to a
single/single SAM but usually with higher rates of application of PMB.
(iii) High Stress Seal (HSS)
May be a single/single or double/double seal, as shown in Figures 2 and 3, (can use all
aggregate sizes) but with a PMB to improve seal performance in areas of high traffic loading
and stress.
(iv) Other types of specific purpose seals using a PMB
The following types of seal have been developed in practice over a number of years, with the
main aim to provide improved performance over a normal seal with C170 bitumen for specific
applications. The PMB used in these applications generally do not meet the Austroads PMB
specification (Austroads 2006b) for treatments such as SAM, SAMI and HSS described above.
The treatments have been included based on their proven performance and appropriate PMB
factors have been developed to use in the Austroads design procedure. Four main applications
are:

aggregate retention in heavy/high speed traffic conditions for both single/single and
double/double seals. Generally the seals are subject to less stress than a HSS and use a
lower modification PMB

waterproofing weak pavements, particularly pavements constructed over moisture
sensitive subgrade, which may have a high potential to crack. The seal is generally a
single/single but the application is less demanding than for a SAM.

as holding treatments (short term only) for sealing distressed pavements until they can be
reconstructed. Generally a single/single seal.

for safety of the public allow early removal of loose aggregate on high speed roads,
without causing damage to the seal by sweeping with a rotary road broom or a vacuum
broom.
Geotextile Reinforced Seal (GRS)
A single/single or double/double seal reinforced with a geotextile membrane as shown in Figure
6, used as an alternative to a SAM or SAMI. The common membrane used is a polyester fabric
of about 140 g/m². Also used as initial treatment where reasonable quality pavement materials
are not available locally and/or often too expensive to haul long distances.
Figure 6: Geotextile reinforced seal
Dry matting seal technique
A repair technique developed to restore the properties of a new seal that early in its life lost
considerable amount of aggregate, from the wheel paths, mainly due to cool/damp weather
conditions. This technique may also be used to reseal an existing fatty seal to minimise
potential bleeding, but this requires some experience, care and attention to detail to do
successfully. Loose dry aggregate is carefully spread in the stripped areas, and then a full width
small aggregate applied.
Figure 7: Dry matting technique
SELECTION OF SPRAYED SEAL TREATMENTS
PRIMES
A prime and seal is the preferred treatment for all new work as it improves the bond, provides
flexibility and reduces the risk of early seal failure. Priming is recommended when sealing or
placing asphalt over a concrete pavement.
Selection and design of primes
There is no formal design method, and selection of a suitable grade of cutback bitumen primer
is based on experience within the local area, construction practices, type and compaction of
pavement material and the seal or hot mix asphalt treatment to follow. The primer is expected to
be allowed to dry and cure for a minimum of three days before being sealed.
Table 1: Selection and rate of application of cutback bitumen primer
Grade of
primer
Viscosity
Pa.s at 60° C
Pavement condition
Very light
0.010 – 0.020
Tight, hard surface, or stabilised
Light
0.025 – 0.050
Tight, not as hard, more fines
Medium
0.050 – 0.080
Some porosity
Heavy
0.080 – 0.200
Porous surface
Very Heavy
0.200 – 0.400
Limestone/sandstone/hill gravels
with high fines
Typical rates of
application at 15° C
In the range of 0.8 to
1.2 L/m² depending on
type and compaction of
pavement material and
moisture content
Special bitumen emulsion primers provide alternatives to the very light and light grade of
cutback bitumen and are applied at about the same rate of application – refer to Figures 8 and 9
to compare emulsion and cutback primers. Typical life of a prime may vary from several weeks
for a light primer, up to 6 – 8 weeks for a heavy primer on a low traffic road.
Figure 8: Freshly primed surface, primed with special emulsion primer
Figure 9: Primed surface several weeks old, primed with light grade of cutback bitumen primer
PRIMERSEALS
Primerseals were originally developed as an initial treatment alternative to a prime and seal
when it was not practical to prime, such as in cool/damp weather conditions. It is now the
preferred initial treatment on roads with an AADT of 200 or more, for porous pavements, and in
weather conditions where normal sealing practice is considered to be a high risk.
On new work, pavement construction and preparation requirements are the same as for priming
except the surface must be kept damp to prevent balling up of the primerbinder.
Primerseals are mostly single/single, but multiple applications may be used in high stress areas.
Selection of aggregate size
Aggregates may range in size from coarse sand to 10 mm. The aggregate size and spread rate
is selected taking into account traffic and prevailing weather conditions, as shown in Table 2.
Aggregate spread rates are based on normal seal design plus an additional 10% aggregate.
Table 2: Primer seals - selection of size of aggregate and spread rates
Design traffic (v/l/d)
Recommended size of
aggregate (mm)
Recommended aggregate
spread rate (m²/m³)
< 1200
5 and 7
5 and 7: 130 – 150
>1200
7 or 10
10: 110 – 120
Note: Design traffic is determined separately for each traffic lane (based on AADT)
Selection of primerbinder
The selection of the type and grade of primerbinder is based primarily on the prevailing weather
conditions, type and condition of the pavement, and timing of the next treatment, as shown in
Table 3.
A primerseal constructed with a cutback bitumen primerbinder, should not have the final seal
applied within twelve months to minimise potential bleeding of the final seal. If bitumen emulsion
is used as the primerbinder the time may be reduced to six months.
The base bitumen is usually C170, but harder grades of base bitumen and/or faster curing
cutter may be used on heavily trafficked roads, areas of high stress, or in cooler/damper
prevailing weather conditions.
Table 3 provides a guide to the selection of a suitable primerbinder for prevailing weather
conditions.
Table 3: Selection of type of primerbinder
Type of primerbinder
Recommended use

Medium cutback bitumen
(12 - 15% cutter by volume)

Cool and/or damp conditions, or on tightlybonded medium porosity pavements

Heavy cutback bitumen
(8 - 10% cutter by volume)

Warm and/or dry conditions, and on porous
pavements

Bitumen emulsion (CRS)
60 - 70% bitumen content
modified emulsion

All year, but more suited to cool and/or damp
conditions, on porous pavements or when
surfacing is to be applied within three months
Design of rates of application of primerbinder
There is no formal design method at this stage, but it is planned this will be addressed next in
the seal design project.
At present, the design is based on the use of ‘base rates’ of primerbinder related to traffic as
shown in Table 4. Allowances for pavement condition and texture must be determined and
added to these base rates to complete the design. Generally, allowances for pavement
condition and texture are in the order of + 0.2 to 0.3 L/m².
Aggregate embedment is a major consideration. Embedment is measured using the ‘ball
embedment’ test as described in the seal design method. If embedment is greater than 4 mm it
is recommended to not proceed and to determine the cause of high embedment. If embedment
is due to moisture content, insufficient time for pavement surface to cure etc. it is recommended
embedment be measured again after the surface has been allowed to dry/cure.
If embedment is due to soft pavement material (such as sandstone), alternatives such as
inverted seal approach, stabilisation or better quality base material should be considered.
Table 4: Recommended base rates of application of primerbinder
Rate of application of primerbinder (total volume in L/m² @ 15 °C)
Design traffic (v/l/d)
Aggregate size
(mm)
Cutback bitumen
primerbinder
Bitumen
emulsion
primerbinder
< 150
5, 7 or 10
1.3 – 1.5
1.6 – 1.7
150 – 1200
5, 7 or 10
1.2 – 1.3
1.5 – 1.6
> 1200
5,7 or 10
1.1 – 1.2
1.4 – 1.5
Note: Design traffic is determined separately for each traffic lane or area being considered
Final seal treatment
Primerseals should be regularly inspected to determine when the final seal should be applied.
A primerseal needs retreatment when the primerbinder is nearly or fully oxidised. This can be
checked by prying out pieces of aggregate and assessing the condition of the primerbinder
adhering to the aggregate. The primerbinder is near the end of its life when it appears dull and
brittle. Another useful further indication of the primerbinder hardening is when small holes and
minor cracking develops in the primerseal.
SPRAYED SEALS
Introduction
The following is an outline of the general philosophy behind selecting a particular type of
sprayed treatment. The choice is influenced by the operating environment, accepted and proven
practice in the local area, availability of materials and whole of life cost of the treatment.
Generally, several treatments will be satisfactory and the designer will have to make the final
decision on choosing the optimum treatment, taking into account the above factors as well as
the risk factor associated with each treatment for the prevailing conditions expected at the time
of application. e.g. a PMB will be a very high risk treatment in cold/damp weather conditions.
The life of a sprayed seal is highly dependent on the quality of the granular base materials and
the standard of surface preparation of pavements prior to resealing.
Main strengths of a seal are:

suitable for both initial and reseals

high flexibility

relatively low cost

high skid resistance (when applied to a uniform surface)

good waterproofing qualities

use of PMB and geotextiles for sealing and waterproofing existing cracked surfaces

matches existing surface level.
Limitations are:

will not strengthen a pavement

will not correct a pavement shape

high tyre noise, particularly with larger aggregates

less satisfactory performance over non-uniform or rutted surfaces

not suitable to treat intersections, roundabouts, heavy vehicle turning areas.
Environmental considerations
The first few hours in the life of a seal are critical with regard to performance and any problems
due to traffic or weather conditions. Therefore in the selection process due consideration must
be given to the risk of failure and likely consequences for safety of the traffic and potential
environmental pollution.
Selection of aggregate size
The nominal size of aggregate chosen should relate to the conditions for the job and should be
the best compromise for the various conditions. The sand patch test and surface texture
allowance table in the seal design method can be used as a guide to selecting aggregate size
for reseals taking into account existing texture. Table 5 sets out the aggregate sizes
recommended based on traffic volumes.
Table 5: Recommended aggregate sizes for single/single seals
Traffic volume
Aggregate size
< 500 AADT
7 or 10 mm, maximum – 10 mm
500 – 2000 AADT
preferred is 10 mm, with a maximum of 14 mm
> 2000 AADT
minimum 10 mm, with a maximum of either 14 or 16 mm
Double/double seals
Preferred combinations are: 10/5 for fine texture as may be required in urban areas; 14/7
common for high speed roads; 20/10 to provide a very robust seal, and 20/7 mm for robust
seals requiring a smoother and finer textured surface.
SAM
Generally as single/single seals using 10 and 14 mm only, because 7 mm generally does not
provide a high enough binder content to be effective and economical. A double/double seal may
be a 14/7, 16/7, 20/10 or 20/7 mm combination.
SAMI
Generally a single/single seal with either 10 or 14 mm depending on the rate of application of
binder required for the conditions and type and size of cracking, and type and thickness of
asphalt overlay.
HSS
Similar to aggregates used in normal single/single or double/double seal.
Spray generation
Larger aggregates, with higher texture depths, generate less water spray than fine aggregate
seals on medium to high speed roads.
Noise considerations
Small aggregates generate less tyre noise than larger aggregates, and often a double/double
seal is used in urban areas to minimise traffic noise.
Surface texture requirements
Minimum surface texture to provide skid resistance for high speed rural roads is considered to
be about 1 – 1.2 mm as measured by the sand patch test. Generally 10 mm or larger aggregate
is required on medium to high traffic volume roads to provide adequate texture. Size 7 mm
aggregate provides adequate texture on low traffic roads, or roads with speed limits of
70 km/hour or less.
Guide to selection of seal treatments
Table 6 has been developed to provide a first step, quick and practical guide to the selection of
a suitable type of seal treatment for the general conditions, and common performance
requirements specified.
Table 6: Quick guide to selection of a suitable seal treatment
Traffic (in v/l/d)
> 2000
 2000
Heavy vehicles
Heavy vehicles
Performance environment
> 15%
 15%
> 15%
 15%
High stress areas (e.g. small radius
roundabout, intersection, turning lane,
etc.)
Sprayed seal
not suitable
S/S + scatter
coat or D/D +
mod 3
Cape seal or
D/D + mod 3
D/D or S/S +
mod 3
D/D + mod 3
D/D or S/S +
mod 3
D/D or S/S +
mod 3
S/S or D/D
Stiff
HSS
S/S
HSS
S/S
Weak
SAM (D/D only)
HSS
SAM (S/S) +
mod 3
HSS
Hot
D/D + mod 2
S/S + mod 2
D/D or
S/S + multi
S/S + multi
Temperate
D/D
S/S
D/D or S/S
S/S
Cold
D/D + mod1
D/D or S/S +
mod1
D/D
S/S
Grades > 5%
Assessment of
pavement
strength
Temperature
(WMAPT)
Legend
S/S:
Single/single, C170/320 or multigrade bitumen
D/D:
Double/double, C170 or 320 or multigrade
bitumen
Cape Seal: Single/single seal filled in with slurry
HSS: High Stress Seal is a S/S or D/D, with 7, 10 or 14 mm
aggregate, medium/high concentration PMB, multigrade
bitumen M500/170
Scatter coat: Light application of small aggregate, 7 or 5 mm,
to temporarily ‘lock in’ a larger aggregate. Suitable for
intersections, driveways, turning slots
Modified 1: Lightly modified PMB, S10E, S35E, 10% crumb
rubber
Modified 2:
Multigrade or lightly modified PMB,
S10E, S35E, 10% crumb rubber
Modified 3:
Higher grade PMB, S20E, S45R
Applications also include small radii roundabouts,
intersections, cul-de-sacs, turning lanes, etc.
Climate: Weighted mean annual pavement
temperatures (WMAPT)
Hot:
WMAPT > 35˚C e.g. Ayr, Cairns,
Townsville, Mt Isa
Temperate:
WMAPT 29˚C < WMAPT  35˚C
eg. Rockhampton, Roma, Mackay, Gympie, Brisbane
Cold:
WMAPT  29˚C e.g. Warwick, Kingaroy,
Toowoomba
Assessment of pavement strength
Stiff:
Rebound deflections
< 0.9 mm
Deflection ratio
> 0.8 - rigid or bound
0.6 to 0.7 - stiff unbound
Residual deflections
< 0.15 mm
Weak: Rebound deflections
> 0.9 mm.
Deflection ratio < 0.6 - potentially weak pavement
Residual deflections
> 0.15 mm
Multigrade: Multigrade bitumen M 500/170
AUSTROADS SEAL DESIGN METHOD
The Austroads (2006a) seal design method (AP-T68/06) is the national method specified by all
state road authorities. The development and use of a practical, national design method is
considered an important factor in the development and sustainability of the extensive national
sealed road network.
Overview
The seal design is still loosely based on the concepts developed by F M Hanson in the 1930s.
Over the years the road authorities amended this design theory to cope with changes in traffic
volume, in particular the increase in heavy vehicles in later years, and in many areas local
factors were developed to achieve the desired outcome.
In 1992, Austroads agreed to fund national road trials to more accurately predict air voids in the
compacted layer of aggregate in seals constructed with modern equipment and subjected to
rolling by heavy vehicles, and in particular large heavy vehicles. Road trials, covering many
different types of roads, traffic volumes and aggregates, were undertaken to more accurately
predict the air voids in a seal constructed with modern equipment and subjected to rolling by
current traffic, in particular by large heavy vehicles.
The design procedure and information provided has been extended and improved over the
previous method; however there are several issues that need further investigation because of
their influence on the outcome and performance of sprayed seals. Monitoring of validation trials
has indicated that there are still several aspects of the seal design method that need to be
further improved, but this will require collecting further data. These aspects include matters
such as:

Potential embedment of aggregate – how to measure this in a practical manner and
determine practical ‘go – no go’ limits and appropriate allowances in the design process
taking into account traffic ranges, type of pavement, size of aggregate etc.

The effect of large heavy vehicles (seven or more axles) on the rolling/packing of
aggregate and voids in the aggregate mat. This will determine the type and/or magnitude
of adjustments that may be required to be made to minimise the potential of flushing and
loss of texture.
Design procedure
Sprayed seals are a system, and sealing trials and subsequent work have shown that the
design of the rates of application of binder and aggregate spread rates are both of major
importance in achieving a satisfactory performance for the service conditions being considered.
The design philosophy adopted applies principally to the design of the most common type of
sprayed seal, the single/single seal using conventional C170 bitumen as the binder and one
sized 10 mm aggregate. Assumptions included in the design of single/single seals are:

Aggregate is single-sized and of appropriate quality. Average least dimension (ALD)
of the aggregate is an important input into the design method and must be
representative of the aggregate being used. The aggregate is expected to have
between 15 and 25% flaky particles (FI). If the FI is outside that range, an adjustment
for aggregate shape has to be made.

Design traffic volume is expressed in vehicles/lane/day (v/l/d) and based on AADT,
with between 5 and 15% heavy vehicles. If the heavy vehicles are outside that
range, an adjustment for heavy vehicles has to be made.

Aggregate spread rate determines the inter-aggregate void space in the seal layer,
and hence the amount of binder required. A single layer of aggregate particles
settles with, typically, 40–60% voids after orientation and packing of the aggregate
by rolling and trafficking.

Binder rise should be a minimum of 35–40% up the height of the aggregate particle
after initial rolling and trafficking, increasing to between 50–65% (i.e.1/2–2/3) about
two years after construction.

The proportion of voids to be filled with binder may be varied to optimise requirements
such as surface texture, maximum seal life, and for specific applications such as nontraffic areas. A minimum texture is generally required for skid resistance.
Design traffic
Determining the ‘design traffic’ is the first step in the design process. The design traffic is
expressed in vehicles/lane/day (v/l/d) and is determined from the AADT and calculated
separately for each lane/area being designed.
Design of rates of application of binder
A general schematic flow chart of the process for determination of binder application rates for
single/single seals is shown in Figure 10.
Traffic Volume
Basic Voids
Factor
Aggregate Shape
Adjustment
Design
Voids Factor
Traffic
Adjustment
ALD
Basic
Binder
Application
Rate
Embedment
Allowance
Existing Surface
Condition
Allowance
Absorption
Allowances
Design
Binder
Application
Rate
Figure 10: Flow chart for design of binder application rates for a single/single seal
Basic Voids Factor (Vf)
The Basic Voids Factor is an estimate of the voids in the aggregate after rolling and trafficking
and is based on normal traffic distribution and typical one-sized aggregates used. The Basic
Voids Factor is determined directly from a graph for the design traffic. Refer to the central black
line in Figure 11 to determine the Basic Voids Factor for design traffic between 50 and 500 v/l/d.
0.30
Target
Stripping Limit
2
Basic Voids Factor, Vf (L/m /mm)
Bleeding Limit
0.25
0.20
0.15
0
100
200
300
400
500
Traffic Volume (v/l/d)
Figure 11: Basic Voids Factor for 50 to 500v/l/d
For example: design traffic is 400 v/l/d, Basic Voids Factor is 0.18 L/m²/mm.
Adjustments to the Basic Voids Factor
There are two adjustments to the Basic Voids Factor to be considered in the design process.
(i)
Adjustment for aggregate shape
The design is based on typical cubical aggregate. If the aggregate varies from the typical shape
an adjustment in binder rate may be required. This is achieved by making an adjustment to the
Basic Voids Factor.
Shown in Figure 12 is a typical cubical piece of 10 mm aggregate. Flakiness is about 20% and
ALD (dimension A) is 6.2 mm. For this aggregate, the Adjustment to be made in the design will
be nil.
Generally the adjustment for aggregate shape can vary the binder rate of application by
between 5 and 8%.
A table, ‘Adjustment to Basic Voids Factor for Aggregate Shape’ is included in the Austroads
design method (AP-T68/06) to easily determine any adjustment required.
C
C
B
B
(ii)
A
A
Figure 12: Cubical piece of sealing aggregate
Adjustment for effect of heavy vehicles
Heavy commercial vehicles have a large influence on aggregate packing and performance of
the seal, and in particular the very large heavy vehicles (LHV), such as B-doubles, which are
trucks with 7 axles or more and gross mass exceeding 45 tonne.
During the development of the design method various options were examined to deal with
heavy vehicles. The first option was to convert heavy vehicles to light vehicles, but it was
decided it was important for the designer to separately consider the effect of heavy vehicles in
the design process. A second option was to use the Equivalent Standard Axles (ESA) concept
as used in pavement design, but it was found this did not correlate well with seal performance.
The concept of basing the design on a truck being the ‘standard vehicle’ is being evaluated, with
the effect of all types and classes of vehicle being expressed to that standard in terms of effect
on a seal performance.
Figure 13: Typical large heavy vehicles (B-doubles)
It is estimated that Large Heavy Vehicles (LHV),as shown in Figure 13, have a damaging effect
about three times larger than the normal type of truck and trailer, or semi-trailer (HV). Using this
concept, the effect of heavy vehicles in the adjustment for heavy vehicles is expressed in
‘Equivalent Heavy Vehicles’ (EHV). The EHV is determined from:
EHV = (HV x 1) + (LHV x 3)
Adjustment for heavy vehicles can vary the binder rate of application by between + 5% and as
much as – 30%.
A table, ‘Adjustment to the Basic Voids Factor for Traffic Effects’ is included in the Austroads
design method (AP-T68/06) to determine the adjustment required for various percentages of
heavy vehicles, expressed as EHV, in the design traffic.
Design Voids Factor (VF) (L/m²/mm)
The Design Voids Factor is determined from adding the Basic Voids Factor and any
adjustments for aggregate shape and traffic effects.
The Basic Binder Rate of Application (Bb) L/m²
This Design Voids Factor is multiplied by the aggregate ALD to calculate the Basic Binder Rate
in L/m².
Bb = VF (L/m²/mm) x ALD (mm) L/m²
Allowances (L/m²)
To complete the design for the binder rates of application it is necessary to assess the condition
and texture of the existing surface, and potential aggregate embedment and/or binder
absorption.
The allowances are in the same unit (L/m²) as the Basic Binder Rate and can be added or
deducted directly. The allowances considered and typical limits are as follows:
Surface texture of the existing surface (most common allowance applied)
This is measured using a standard test method (sand patch) and allowances have been
developed to be applied taking into account the size of aggregate in the existing seal, and the
size of aggregate in the proposed new seal.
Typical allowances are + 0.1 L/m² for a relatively smooth surface to +0.5 L/m² for a coarse
textured surface with say a texture greater than 1.5 mm.
Embedment of aggregate into the pavement surface (initial treatments
only)
A test method was developed for determining potential aggregate embedment. This is based on
a similar method developed in South Africa. A simple and easy to use hammer has been
developed, as shown in Figure 14, and is being tried on a number of new works to collect data
and establish some practical levels of embedment allowance. Risk is related to traffic volume
and composition (in particular large heavy vehicles) and the aim is to initially establish some ‘go
– no go’ levels to minimise risk of bleeding, and an indication of allowances to be applied in the
seal design for various embedment values.
Typical values allowed for embedment are – 0.1 to - 0.3 L/m².
Figure 14: Embedment hammer
Note: The embedment hammer is based on the standard Marshall hammer used in asphalt mix
design. The steel ball to assess embedment is 19 mm diameter.
Design Binder Rate (Bd) L/m²
The Design Binder Rate of Application is determined from the Basic Binder Rate and
allowances as follows:
Bd = Bb + Allowances (L/m²)
Aggregate spread rates (ASR) m²/m³
Aggregate spread rates are considerably lighter than in previous design methods. They are
based on ALD, with some adjustments for traffic volumes, aggregate shape, type of binder and
type of treatment. For the most common single/single seals the recommended spread rates are
determined as follows:
ASR = 850/ALD – 900/ALD (m²/m³)
Based on average test results of ALD for 7, 10 and 14 mm, typical aggregate spread rates are
as shown in Table 7.
Table 7: Typical aggregate spread rates
Size of aggregate (mm)
Typical range of ALD (mm)
Aggregate spread rate
(m²/m³)
7
3.8 – 4.2
190 – 240
10
5.8 – 6.5
115 – 135
14
8.4 – 9.0
95 – 110
Design of other types of sprayed seals
The Austroads design method (Austroads 2000a) uses the design process for a single/single
seal as the basic design approach. The design of other types of seal follows the same
procedure to determine he Basic Binder Rate (Bb) as for a normal single/single seal, with C170
binder, and uses factors for specific applications and other types of binder. If the binder is PMB
or emulsions, specific factors for each binder have been developed to determine a modified
basic binder rate.
For example, if the binder is a PMB the Basic Binder Rate (Bb) is multiplied by a Polymer Factor
(PF) to determine the modified Basic Binder Rate (Bbm) as follows:
Bbm = Bb x PF (L/m²)
Polymer factors have been developed for the various treatments and type and grade of PMB
used in the treatment. Table 8 provides an indication of typical polymer factors to use for various
treatments, and grades of PMB as specified in the Austroads (Austroads (2006b) framework
specification for PMB.
Table 8: Typical polymer factors
Type of treatment
Polymer factor (PF)
Comments
HSS
1.1 – 1.2
Lower factor for higher traffic
SAM
1.2 – 1.4
Crumb rubber has higher factor
SAMI
1.5 – 2.0
Higher factor for SAMI under open graded
asphalt, thick layers of DGA
PLANT AND FIELD PROCEDURES
A new design method is only part of improving the overall performance of sprayed seals.
Attention is also being given to improving the plant and field procedures to ensure the binder
and aggregate spread rates are as close as possible to the design rates and aims.
The bitumen sprayer is considered the most important item of plant, closely followed by
aggregate spreaders.
Sprayer calibration
Road authorities used to calibrate all sprayers operating within their jurisdiction. About five years
ago it was agreed to develop a national calibration method acceptable to all road authorities,
accredit the testing facility to monitor their performance to an agreed standard (done by the
Australian National Testing Authority, NATA), and list all calibrated sprayers on the Australian
Asphalt Pavement Association web page for the information of industry and its clients.
Sprayers are required to be calibrated annually. The preferred method is using a fixed test pit
facility where the pump output and transverse distribution are checked in a single operation to
ensure the sprayer can meet the national specification. An alternative method is to use a pit for
checking the pump output, and a field test using a specified type and grade of carpet to check
transverse distribution.
At the same time, a national spraying nozzles specification was adopted.
Figure 15 shows a sprayer on a fixed test pit facility being tested for both output and transverse
distribution of a full width (7.4 m overall) spray bar.
Transverse distribution is assessed over 50 mm wide troughs, and the specification sets specific
limits and allowable tolerances for single troughs, and various combinations of troughs.
The test uses a specified calibration oil, which must have the same viscosity as C170 bitumen
at the accepted normal spraying temperature of 180 °C.
Figure 15: Bitumen sprayer on fixed pit facility
Spraying procedures for sealing
Sprayed widths generally adopted are either traffic lanes (3.7 m wide) or full width of the existing
surfacing, varying from 5.6 to 7.4 m with some large sprayers able to spray up to 8.6 m in a
single pass.
To achieve as long a life as practical from a sprayed seal surfacing, it is essential to apply the
seal over an existing uniform textured surface. If the texture is not uniform, it can be improved
by say applying a slurry seal, asphalt or regulation/correction seal using a small (5 or 7 mm)
aggregate prior to applying the seal treatment.
Alternatively a variable transverse spray rate may be applied across the surface as required.
This may be done:

using a purpose built bitumen sprayer, generally fitted with two spray bars, and able to
vary the spray rate between 10 and 30% in 300 mm widths

in two separate runs by first pre-spraying coarse textured areas, followed by full width
design application using either a single or two standard bitumen sprayers.
Both methods are used with good success.
The seal design procedure involves measuring surface texture in the wheel paths and
between/outside the wheel paths. If the surface texture allowance to be applied varies by 0.3
L/m² or more between the two locations, it is recommended the surface be first regulated, or
variable spray rates applied.
Figure 16 shows an example of applying variable spray rates across the pavement by prespraying the coarse textured areas first at 0.5 L/m², followed by the full width application at the
design rate for the wheel paths, at 1.3 L/m².
Figure 16: Pre-spraying the coarse textured areas
Cutting-back bitumen
In preference to supplying standard grades of cutback bitumen, C170 bitumen is cutback in the
field as required to produce the binder most suitable for the prevailing conditions. Cutter is
generally lighting kerosene or aviation turbine fuel (jet fuel). Tables (see Table 11) provide a
guide for supervisors to determine the correct amount of cutter to add, taking into account
traffic, weather conditions at the time of spraying and the next few days, condition of the
aggregate and pre-coating, size and type of aggregate etc.
Table 9: Cutting back C170 bitumen in the field
Pavement
temperature
Traffic
Aggregate
Aggregate
v/l/d
10 mm and larger
7 mm and smaller
<100
8
10
100 – 1500
6
8
> 1500
4
6
<100
6
8
100 – 1500
4
6
> 1500
2
4
<100
4
6
100 – 1500
2
4
> 1500
0
2
<100
2
4
100 – 1500
0
2
> 1500
0
0
<100
0
2
100 – 1500
0
0
> 1500
0
0
Range in °C
20 - 25
25 - 32
32 - 38
38 - 45
> 45
Note: A similar table is available for cutting back polymer modified binders.
Aggregate loading
Aggregates are usually stockpiled in advance of the work on prepared stack sites.
Clean, dry and uniformly precoated aggregate is required to minimise risk of failure of sprayed
seals.
Aggregates may be supplied uncoated to the stockpile, and aggregate is then loaded with a
purpose built aggregate loader that can load, screen and precoat in one operation.
Alternatively, aggregate may be precoated at the quarry prior to delivery, usually with a bitumen
based precoating material, and the trucks loaded with a front end loader.
If aggregate is to be left for considerable time on the stack site, it is covered with building plastic
to prevent contamination with dust and/or moisture.
All aggregate required to cover a sprayer run is loaded into trucks, and on-site, before any
binder is sprayed.
Aggregate spreading
To achieve a satisfactory seal it is important to spread the aggregate at the design spread rate,
and uniformly. Traditionally, the main aggregate spreader was the simple truck-mounted box
spreader, but many contractors are changing over to other types that provide more accurate
control over the spread rate and are able to spread at a more consistent and uniform rate. Some
of the box spreaders have been fitted with a roller to assist with the uniformity of spread, and the
truck speed is controlled more accurately using electronic controls. Figure 17 shows a medium
sized self propelled aggregate spreader in operation.
Figure 17: Medium self propelled aggregate spreader
Spreading widths vary from about 2.5 m up to 5.5 m for a large self propelled spreader.
It is recommended that spreaders are calibrated either at the start of a job or during the work.
The spread rate is checked by means of a square canvas mat (1 m x 1 m) placed on the
pavement, usually at the end of a run. The aggregate collected is weighed and the spread rate
calculated from the known conversion for the quarry, and if the spread rate is outside the
accepted tolerance the spreader is adjusted as required.
Some trucks and spreaders are fitted with load cells, or weigh in motion devices etc. to obtain a
more accurate measure of the quantity of aggregate loaded and spread.
Aggregate rolling
The traditional roller is the medium sized multi-wheel rubber tyred roller. Generally the minimum
requirement is a load of about 1 ton per wheel. A recent introduction is a combination roller fitted
with a rubber covered vibrating drum at the front, high frequency and low amplitude, and 4 large
rubber tyred wheels at the rear, as shown in Figure 18.
Specifications vary, but the recommended amount of rolling should take into account traffic and
assistance it can provide in the rolling process, and aggregate size. Generally two rollers are
sufficient on most jobs. Initial rolling speed is slow, but as aggregate is embedded into the
binder the rolling speed can be increased, up to 15 km/h, and this assists with moving loose
aggregate around similar to the effect by traffic.
As a rough estimate, one roller hour is required for about 1500 litres of binder sprayed.
Figure 18: Combination roller
Removing Loose Aggregate
With a reduced design spread rate, and improved control over actual spread rates applied, the
quantity of loose aggregate remaining on the surface after rolling and initial trafficking is at a
minimum. In the interest of traffic safety there is a requirement to remove any loose aggregate,
commonly on the same day or within 24 hours on high speed roads with medium to heavy
traffic, such as highways and main roads. Minor roads have a similar requirement to remove
loose aggregate but the specified period may be up to several days. Appropriate loose stones
warning signs are left in place until loose aggregate has been removed.
A simple test has been developed to measure loose particles remaining, and practical limits
developed as shown in Table 10, which appear to be satisfactory and accepted by the public.
Table 10: Loose aggregate particles remaining
Traffic
type/location
Average loose
particles
remaining/m²
Urban
20
Medium to high >
250 v/l/d
30
Low < 250 v/l/d
40
Comments
Similar to other statistically based
approaches a ‘lot’ is established, such
as 1 km length, work for the day,
similar type of treatment etc.
Loose aggregate particles are
collected from six 1 m² areas and the
result averaged for the lot. If the
number of loose stones exceeds the
specified minimum, than further
sweeping has to be carried out and
the lot re-assessed.
COMPETENCY TRAINING
Sprayed sealing is a specialist occupation and requires skilled operators and teamwork to be
successful. With the loss of experienced practitioners and some difficulty in attracting new
employees, a national training and assessment system has been established to train new and
existing employees. Training packages have been developed and their development funded by
the federal and state government. Training Packages for operators have been in place for some
time, but in 2007 national qualifications for supervisors, technical staff and managers have been
introduced.
Unfortunately, only a few training organisations are available to conduct specialist training and
assessment in asphalt and sprayed sealing operations mainly because the bituminous surfacing
operations are only a small part of the overall civil construction industry.
TYPES OF CONTRACTS
There are three main contractual approaches in sprayed sealing works. These are broadly:

Client specifies treatments and design rates of application. Most of the risk is with the
client and warranty is typically 3 months.

Client specifies treatment but contractor designs rates of application, and generally
warranty is extended to 12 months.

On long term maintenance contracts the maintenance contractor selects and designs the
treatment and has to ensure the treatment meets the specified surfacing requirements
(e.g. surface texture, skid resistance).
Contractors must submit a conforming tender, but may submit alternative treatments. With the
existing system it is difficult to evaluate this, and there is a desire by the clients to move to
‘performance based, specifications. This will allow the contractor more freedom but will shift
more of the risk to the contractor.
REFERENCES
Austroads 2006a, Update of the Austroads Sprayed Seal Design Method. Austroads Technical
Report, AP-T68/06, Austroads, Sydney.
Austroads 2006b, Specification framework for polymer modified binders and multigrade
bitumen. AP-T41/06, Austroads, Sydney.
Austroads/AAPA, Pavement Work Tips (sprayed sealing topics)
Work Tips may be downloaded free from the AAPA web site www.aapa.asn.au
Acknowledgement
The author wishes to thank the CEO and the AAPA Board for permission to prepare and
st
present this paper for the ARRB 1 International Sprayed Sealing Conference.
AUTHOR BIOGRAPHY
Walter Holtrop is a Professional Engineer and qualified Quarry Superintendent who has worked
in the construction and maintenance of flexible pavements since 1963, specialising in asphalt
and sprayed sealing works with VicRoads from 1974 until 1994.
In 1994 he joined AAPA as the Manager Training Centre to develop and present specialised
training courses nationally, represent AAPA on national competency training committees, and
provide technical advice to industry and its clients. His role changed in 2003 to become the
National Surfacing Engineer with the Training Centre now managed separately.
Walter is a member of the Austroads Bituminous Surfacing Research Reference Group and
various technical committees, including the committee working on the development of the
current Austroads seal design method.
He has been involved as a specialist consultant on large road projects within Australia and
overseas in Indonesia, Hong Kong, Saudi Arabia and India. He is a consultant lecturer for
Centre of Pavement Engineering Education (CPEE) in post graduate courses on Pavement
Wearing Surfaces, and is a member of the CPEE Academic Board.