Practical Steps to Extend the Lives of
Bridges
Engineers Ireland 31 January 2014
Liam Duffy - National Roads Authority
Overview
• Introduction
• Inspection types
• Focus on Special Inspections
• Factors affecting bridge durability
• Load-carrying capacity assessment
• Repairs and Strengthening
Long Life Bridges: Adare Bridge Co.
Limerick
Irish Stone Bridges (O’Keeffe and Simington): “...there is no reason to dispute
the date range 1390-1410...”
Irish Stone Bridges (O’Keefffe and Simington): “...Richly mantled with ivy, this ancient
Bridge is very picturesque, blending as it were with the ivy-clad walls of the castle...”
Ensuring a Long Life for Bridges
Inspections
Principal, Special, Routine
Routine Maintenance
Effective Bridge
Management
Strengthening Works
Durability &
Repair Works
Structural Assessments
Inspection Types
• Routine Inspection
• Previously an annual inspection by LAs
• Engage consultants now
• Principal Inspection
• Structural inspection by experienced bridge engineers
• Interval 1-6 years
• Special Inspection
• Defect investigation, underwater inspection, assessment
Routine inspection and maintenance
Vegetation clearance prevents stonework deterioration,
debris clearance from watercourses prevents scour damage
PI – Inspect inaccessible components
Concrete Bridges – Factors affecting
durability
Leaking expansion joint or
failed waterproofing
Concrete Bridges – Factors affecting
durability
Heavy vehicles turning on
expansion joint, stop/start traffic lead
to joint durability issues and necessitate
component replacement
Waterproofing Application
& Testing
Concrete Bridges – Factors affecting
durability
Special Inspection of Concrete Bridges
- Testing
• Dust sample - depth of penetration of chlorides
• Chloride concentration at various depths (BA35/90 states upper
threshold 0.3% by mass of cement)
• Phenolphthalein test - depth of carbonation
• Half-cell Potential using Copper/Copper Sulphate electrode,
potentials <-350mV indicate probability of >90% that pitting
corrosion is occuring
• Concrete Resistivity <20Kohm-cm indicates moderate corrosion
rate, increasing to high corrosion rate for values <10Kohm-cm
Str
Ref
No
Spans
Total
Span
Form
Max
Cl @
Rebar
Max
Carb
Depth
Cover
to
Rebar
Half
Cell
Cu
<-350
Conc
Res.
<20
Conc
Str.
N/mm2
A
3
22m
RC
Slab
0.77%
15mm
24mm
min
46%
100%
43
B
5
61m
PC
Beam
0.12%
25mm
63mm
av
100%
100%
55
C
5
34m
PC
Beam
1.79%
Pier
12mm
77mm
av
12%
Pier
N/A
48
Pier
D
4
46m
PC
Beam
0.44%
15mm
46mm
av
27%
88%
62
E
3
28m
PC
Beam
0.11%
15mm
32mm
av
0%
0%
56
F
2
96m
RC
Arch
0.15%
16mm
32mm
av
2%
0%
57
G
5
268m
In Situ
P-T
0.21%
5mm
18mm
min
0%
44%
46
(0.65%)
Youghal Bridge
Post-Tensioning Special Inspection
• BA50/93 Post-Tensioned Concrete Bridges – Planning,
organisation and methods for carrying out Special Inspections
• Phase 1 Desk study and Phase 2 Preliminary site inspection to
indentify vulnerable details (eg segmental construction & half-joints)
and areas of deterioration
• Recommendations regarding priority for PTSI Phase 3 including
testing
• Aggressive marine environment, drop-in span with half joints, no
waterproofing, failed expansion joints and poor surfacing
Youghal Bridge PTSI Phase 3
Youghal Bridge Half-Joint
Component Replacement - bearings
Scour Assessment – DMRB BA74
Leenane Bridge collapse
Build-up of debris at bridge
following rainfall which
exceeded 1/100yr return
Assessments for load-carrying capacity
• Use UK DMRB – BD21, BA16, BD44 etc to ascertain whether a
structure can carry 40T GVW Assessment Live Loading
• NRA Stage 1 Screening Assessment uses MEXE and ArchieM
for masonry arches
• NRA Stage 1 Assessment uses strip analysis and grillage for
more modern structures
• Conservative analysis gives significant number of theoretical
failures
• Need to decide whether to strengthen bridge or consider more
refined analysis using other assessment packages
• Other influencing factors include durability, remaining service
life, extent of repairs required
Assessments (cont.)
• NRA Stage 2 Assessment methodology to be published in 2014
along with other NRA bridge inspection, assessment and repair
guidance in DMRB Vol.2 & 3 docs
• Preference is for ‘virtual strengthening’ using refined
assessment methods to attain structural assessment pass,
rather than strengthening
• If bridge passes assessment, durability issues can be
addressed:
• - repointing, masonry repair, improve drainage for masonry
arches
• - waterproofing, maintenance painting of steel, concrete
repairs for modern structures
Assessments (cont.)
• If bridge fails assessment then contract to address durability
issues (waterproofing, concrete repairs etc) should not proceed
until refined assessment confirms an assessment pass.
• Other factors in addition to durability, eg bridge geometry, road
width, accident statistics, pedestrian arrangements, special
inspection reports, scour inspection reports and other data
influence bridge management strategy relating to individual
structures
Assessments (cont.)
Coring to determine arch
barrel depth
Rare example of failure under load
Strengthening with External Plate Bonded
CFRP – Cast Iron Beam/Brick Jackarch
Failed a structural capacity assessment.
Strengthened with Advanced Fibre Composites
Repairs & Strengthening - Masonry
Repairs & Strengthening - Masonry
Wall strengthened with tie-bar & pattress
plates, and parapet replaced following
vegetation removal
Repair historic structures
sympathetically
New NRA Specification Series 2400. Lime mortar 1part NHL5 to 2.5 parts
well-graded sand, full joints, stonework to match existing
Refrain from unsympathetic repairs
Masonry arch construction