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

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

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

Concrete durability: Definition Resistance against deterioration….

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)

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

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

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

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

Examples of corrosion Quality and design….

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

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

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

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

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

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

Service life prediction based on Chloride ingress profile

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

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.

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 +

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

Example of AAR in bridge pylon

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

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

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

Petrographic thin section

SEM/EDX of AAR Products

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

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)

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...

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!

Thank you Norbert Michel Manager Infrastructure Disciplines ARRB Group - Research and Consulting P: +61 3 9881 1580  M: +61 (0) 412 357 249 norbert.michel@arrb.com.au