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Cracks in concrete structures: strategy and further actionHayman S. Ahmed

11Why concrete cracks?-PRINCIPALLY he tensile strength of the concrete has been exceeded due to either an induced action or an applied one.

-In other word, Strain = change in length original length. Hence there is movement of some form the movement MAY be damaging/harmful?the maximum crack width that can be permitted to develop is 0.3mm. BS EN 1992-3 concerns water retaining structures and their limits on crack widths are reduced to a range between 0.05 and 0.2mm.2Assess the significance-Is the structure showing the early signs of failure?- Is there a loss of moment capacity or shear strength?-Are they acceptable aesthetically? Will the cracks result in deterioration (e.g. corrosion)?-Will they result in poor performance (e.g. leakage from a water tank)?-Do they present a health hazard (e.g. radiation leakage from a nuclear reactor vessel)-Does urgent action need to be taken (propping or closure)?- Do the cracks affect structural integrity and/or robustness?3TYPES OF CRACKING STRUCTURAL CRACKING NON-STRUCTURAL CRACKING THERMAL CRACKING

4NON-STRUCTRAL CRACKING

Plastic settlement

Plastic shrinkage

1-re-vibrating2-stop bleeding by admixture1-appropriately troweled and/or floated2-prevent the bleed water from evaporating too quickly5

Crazing (map cracking)Rick mixes over travelling, poor curing NON-STRUCTRAL CRACKING

Thermal contraction cracksconcrete cooling down from the peak temperature ambient temperature1-limiting the peak temperature 2-providing steel reinforcement3-providing construction joints

edge restraint, end restraint, internal restraint

Improve curing and finishing

6NON-STRUCTRAL CRACKING Long term drying shrinkageCaused by insufficient jointsCorrosion of reinforcement inadequate cover, poor quality concrete1-reduce water content2-improve curingAlkali aggregate reactionReactive aggregate and high alkali cementEliminating the causes Eliminating the causes

7Evaluation

Early thermal contraction Plastic settlementPlastic shrinkageLong-term drying shrinkageCrazing Corrosion of reinforcementAlkali-silica reaction8QUESTIONS9