1. How long will your concrete bridge last?
Norbert Michel
Manager Infrastructure Disciplines

2. Dr Ahmad Shayan Acknowledgement Chief Scientist
Acknowledge my colleague at ARRB:
Dr Ahmad Shayan
Chief Scientist
Concrete Technology, Materials Sciences

3. Content of presentation
Introduction to durability of concrete structures
Durability problems affecting concrete structures
Examples of two deterioration mechanisms
Investigation of the two durability problems
Measures against these durability problems
Summary and recommendations

4. Concrete durability: Definition
Resistance against deterioration….

5. Concrete ingredients Coarse aggregate Water Fine aggregate (sand)
Cement
Chemical Admixtures
(Water reducer, Super-plasticiser)
Supplementary Cementitious Materials
(e.g., Fly ash, slag, silica fume)

6. Features of hardened concrete
Note different distribution of aggregate
- can influence properties of concrete, e.g., strength and drying shrinkage

7. Factors affecting durability of structures
Structural design
Quality of individual material components
Mix proportion parameters
Workmanship
Curing
Exposure environment

8. Exposure environment is an important factor in durability
Marine conditions:
Chloride-induced corrosion of reinforcing steel
Benign conditions
Wetting & drying cycles

9. 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
A combination of these problems can be present in some structures

10. Examples of corrosion
Quality and design….

11. Loss of cover concrete due to spalling Loss of steel cross section
Effects of corrosion
Loss of cover concrete due to spalling
Loss of steel cross section
Weakening of cement-steel bond
Result:
Reduction in load capacity

12. 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

13. Criteria for Half-Cell Potentials
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

14. Field investigation for reinforcement corrosion
No steel corrosion is likely
Steel corrosion probable
Measured electrical resistivity to understand corrosion potential…

15. Measurement of corrosion current density
Corrosion rate category
Icorr (µA/cm2)
< 0.1 No corrosion expected
0.1 to 0.5 Low to moderate rate
0.5 to 1.0 Moderate to high rate
> 1.0 High rate
No corrosion expected
Low to moderate rate expected

16. Stage 1 – Service life prediction based on chloride ingress profile
Threshold for corrosion

17. Service life prediction based on Chloride ingress profile

18. More realistic corrosion model
1st crack
Corrosion initiation
Major repair is needed at the end of service life

19. Mitigation of corrosion damage
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.

20. Alkali-Aggregate Reaction (AAR)
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 +

21. Visual features of concrete interior
View of reacted aggregate particles in concrete

22. Example of AAR in bridge pylon

23. Manifestation of combined AAR and corrosion
Appearance of a seriously deteriorated bridge pile affected by AAR and steel corrosion

24. Effects of AAR on concrete
Strength properties
Concrete cracking
Overall effect: Reduced service life

25. 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

26. Petrographic thin section

27. SEM/EDX of AAR Products

28. Repair of AAR-affected concrete
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

29. Prevention of AAR damage
Methodology for preventing AAR damage in new structures
Select aggregates that are not susceptible to AAR by:
testing aggregates by the Accelerated Mortar Bar Test
testing aggregates by Concrete Prism Test
If non-reactive aggregate is not available, modify concrete mix by using SCMs (slag, fly ash, silica fume)

30. 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...

31. The Reward: You won’t have to face premature deterioration!
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!

32. Thank you Norbert Michel Manager Infrastructure Disciplines
ARRB Group - Research and Consulting
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