how long will your concrete bridge last? norbert michel manager infrastructure disciplines
TRANSCRIPT
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How long will your concrete bridge last?
Norbert MichelManager Infrastructure Disciplines
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Acknowledge my colleague at ARRB:
Dr Ahmad ShayanChief Scientist Concrete Technology, Materials Sciences
Acknowledgement
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Content of presentation
1. Introduction to durability of concrete structures
2. Durability problems affecting concrete structures
3. Examples of two deterioration mechanisms
4. Investigation of the two durability problems
5. Measures against these durability problems
6. Summary and recommendations
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Concrete durability: Definition
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• Resistance against deterioration….
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Concrete ingredients
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Fine aggregate (sand)
Coarse aggregate Water
Cement
Chemical Admixtures (Water reducer, Super-plasticiser)
Supplementary Cementitious Materials(e.g., Fly ash, slag, silica fume)
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Features of hardened concrete
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Note different distribution of aggregate- can influence properties of concrete, e.g., strength and drying shrinkage
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Factors affecting durability of structures
Structural design
Quality of individual material components
Mix proportion parameters
Workmanship
Curing
Exposure environment
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Exposure environment is an important factor in durability
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Marine conditions:
Chloride-induced corrosion
of reinforcing steel
Wetting & drying cycles
Benign conditions
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Major durability problems
Shrinkage and thermal cracking Carbonation-induced corrosion of reinforcement Chloride-induced corrosion of reinforcement Alkali-aggregate reaction (AAR) Sulfate attack Salt attack Frost attack (not serious in Australia) Fire
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A combination of these problems can be present in some structures
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Examples of corrosion
Quality and design….
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Effects of corrosion
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Loss of cover concrete due to spalling Loss of steel cross section Weakening of cement-steel bond
Result: Reduction in load capacity
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Field investigation of steel corrosion
Electrochemical properties of steel Half-cell potential mapping Corrosion rate measurement
Resistivity of concrete (ease of current flow)
High resistivity is desirable
Determination of chloride ingress
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Criteria for Half-Cell Potentials
Potential Indication of corrosion activity
More positive than –0.20 V
A greater than 90% probability that no reinforcing steel corrosion
is occurring
Between –0.20 V and –0.35 V
Corrosion activity of the reinforcing steel is uncertain
More negative than –0.35 V
A greater than 90% probability that reinforcing steel corrosion is
occurring
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Field investigation for reinforcement corrosion
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X1 X2 X3
Y1
Y2
Y3
Y4
Y5
Y6
Y7
Half Cell Potential MappingColumn 4, Pier 2, Lynchs Bridge
-150-50
-350--150
-550--350
-750--550
mV
0.044 µA/cm²-230 mV
0.185 µA/cm²-422 mV
0.315 µA/cm²-605 mV
Icorr / CSE
Measured electrical resistivity to understand corrosion potential…
No steel corrosion is likely
Steel corrosion probable
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Measurement of corrosion current density
Corrosion rate categoryIcorr (µA/cm2)
< 0.1 No corrosion expected0.1 to 0.5 Low to moderate rate0.5 to 1.0 Moderate to high rate> 1.0 High rate
X1 X2 X3
Y1
Y2
Y3
Y4
Y5
Y6
Y7
Half Cell Potential MappingColumn 4, Pier 2, Lynchs Bridge
-150-50
-350--150
-550--350
-750--550
mV
0.044 µA/cm²-230 mV
0.185 µA/cm²-422 mV
0.315 µA/cm²-605 mV
Icorr / CSE
No corrosion expected
Low to moderate rate expected
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Stage 1 – Service life prediction based on chloride ingress profile
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ID Location Cl,surf (%) D (m²/s)
Lyn2 Pilecap of Pier 2 1.07 2.85E-12Lyn3 Column 5 of Pier 2, waterline 0.70 1.84E-12Lyn4 Column 5 of Pier 2, 1.5 m above base 0.07 2.13E-12
0
0.2
0.4
0.6
0.8
1
1.2
0 20 40 60 80 100
Depth (mm)
Cl- (
% o
f c
on
cre
te)
0.8 kg Cl-/m3
Lyn2
Lyn3
Lyn4
Cover depth of pier column
Threshold for corrosion
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0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60
Time (years)
Dep
th o
f T
hre
sho
ld C
hlo
rid
e (m
m)
Cap 2006 C5 tidal 2006 Cap 2009 C4 tidal 2009
Present exposure age
Reinforcing bar
Figure 3: Advance of the threshold chloride in concrete. Threshold Cl- =0.4% cement mass. Solid lines: data of 2006; dotted lines: data of 2009.
Service life prediction based on Chloride ingress profile
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More realistic corrosion model
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Corrosion Damage Curve
0
20
40
60
80
100
Time (Year)
Dam
ag
e
1st crackCorrosion initiation
Major repair is needed at the end of service life
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Mitigation of corrosion damage
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Existing structures: How to address these in-situ? What is practical? What is efficient and suitable?
New structures in aggressive environment: What are the design considerations that are needed? What would be effective? How much does it cost?
The above needs testing and verification.
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Alkali-Aggregate Reaction (AAR)
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AAR gel is highly hydrated in the presence of water and reacted aggregate develops expansion
Result: Expansion and cracking of concrete
Alkali hydroxide
Silica in aggregate
Water AAR gel+ +
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Visual features of concrete interior
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View of reacted aggregate particles in concrete
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Example of AAR in bridge pylon
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Manifestation of combined AAR and corrosion
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Appearance of a seriously deteriorated bridge pile affected by AAR and steel corrosion
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Effects of AAR on concrete
Strength properties
Concrete cracking
Overall effect: Reduced service life
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Diagnosis of AAR
Visual observation
Microstructural examinations, including petrographic examination and Scanning Electron Microscopy (SEM) / Energy Dispersive X-ray (EDX)
Residual alkali content
Residual expansion
Residual strength
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Petrographic thin section
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SEM/EDX of AAR Products
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0 2 4 6 8 1 0E n e rg y (k e V )
0
2 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0C o u n ts
C
O
N a
M gA l
S i
SC l K
C a
C a
0 2 4 6 8 1 0E n e rg y (k e V )
0
2 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0C o u n ts
C
O
N aM g
A l
S i
SC l
KC a
C a
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Repair of AAR-affected concrete
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38°C, 100%RH
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0 50 100 150 200 250 300 350 400
Exposure time (day)
Exp
ansi
on
(%
)
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0 50 100 150 200 250 300 350 400
Exposure (day)
Exp
ansi
on
(%
)
Pier5 Pile18 Pier3 Pile28 Pier6 Pile18
Residual expansion of concrete must be determined
Expansion ongoing = more cracking expected
Minimal expansion
Repair technique would depend on condition of affected concrete, residual expansion and exposure
conditions
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Prevention of AAR damage
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Methodology for preventing AAR damage in new structuresSelect aggregates that are not susceptible to AAR by: testing aggregates by the Accelerated Mortar Bar Test testing aggregates by Concrete Prism Test
0
0.05
0.1
0.15
0.2
0.25
0.3
0 5 10 15 20 25
Time (day)
Exp
ansi
on (%
)
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0 100 200 300 400 500
Exposure time (day)
Exp
ansi
on (%
)
If non-reactive aggregate is not available, modify concrete mix by using SCMs (slag, fly ash, silica fume)
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So how long will your bridge last?
Corrosion of reinforcement ?
Alkali-aggregate reaction ?
End of life ?
Investigations such as those described would determine the end of service life...
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Recommendations
To avoid these deterioration problems…. Select non-reactive aggregate Check cement composition (C3A, SO3, alkali) Use supplementary cementitious materials in correct quantity Use appropriate admixtures to reduce water content Verify low permeability of cover concrete
The Reward: You won’t have to face premature deterioration!
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Norbert MichelManager Infrastructure DisciplinesARRB Group - Research and Consulting
P: +61 3 9881 1580 M: +61 (0) 412 357 [email protected]
Thank you