Figure 1: Results of multiple cause concrete damage.
This concrete in figure 1 above is suffering from alkali-aggregate reaction cracking that has also accelerated freeze-thaw deterioration of the surface. It is also being damaged by faulty design or construction techniques that located the electrical conduits too close to the exterior surface of the concrete. The proper use of air entraining admixtures in modern concrete has greatly increased the resistance of the concrete to freeze-thaw deterioration. Except in unusually severe exposures, freeze-thaw deterioration should not occur. This notwithstanding, freeze-thaw deterioration is often still blamed as the cause of damage to modern concrete. Before blaming freezing and thawing conditions, it is better to first determine why the air entraining admixture did not provide effective protection. Mix records and aggregate quality test results may indicate that the concrete was poorly proportioned or that the available aggregate was of low quality. Construction inspection records may indicate that placing and finishing techniques were inadequate. Petrographic examination of the affected concrete may reveal that alkali-aggregate reaction, sulfate exposure, or induced chlorides have weakened the concrete and allowed freeze-thaw damage to occur. All such findings might indicate that the problem is far more extensive than at first thought and requires more extensive preventative or corrective action than the simple replacement of the presently deteriorated concrete. The use of excessive mix water, the improper type of Portland cement, poor construction practices, improper mixture proportioning, dirty or low quality aggregate, and inadequate curing all contribute to low durability in concrete. Such concrete may have low resistance to normal weathering or to other hazards. Selection of the proper methods and materials for repair of concrete damaged by multiple causes depends on determining which is the weakening cause and which is the accelerated cause. Once the weakening cause is fully understood, it is commonly necessary to take preventative measures to protect the remaining original concrete from additional damage. The application of concrete sealing compounds or the thin polymer concrete overlay may prove useful in this respect. If no such preventative measures are judged useable, repair of the damaged concrete can be made by suitable methods, but short repair service life and the occurrence of future damage should be anticipated.