Durable concrete repair - Importance of compatibility and low shrinkage

The durability of a concrete repair can depend on many factors. Those most often considered are compressive strength and low permeability. As reinforced concrete most often fails due to chloride ingress or carbonation this may seem logical. In fact, failure of a concrete repair material is more likely to occur due to incompatibility between the repair and concrete or high shrinkage levels. Either of these factors can lead to cracking and debonding. This in turn allows chloride and carbon dioxide to penetrate again. Concrete that requires repair is often of a low standard. The use of a high strength mortar is in this case inappropriate and can produce problems. A compatible mortar of lower strength and modulus of elasticity to match the concrete is indicated. The paper further describes the issue of compatibility of concrete and repair mortars and implications for durable repair. Test results for repair mortars illustrate this point. Excessive shrinkage of a repair mortar also causes failure. This problem is being increasingly recognised world-wide. Very low shrinkage cementitious mortars for concrete repair have been developed for this reason. The results of this work, and comparisons with traditional mortars including sand:cement mortars, are shown. Specifications that deal with shrinkage and compatibility factors have now appeared in several countries. Some broad suggestions for specification of cement based repair materials are discussed. (C) 1997 Published by Elsevier Science Ltd.