The effect of improper curing on properties that may affect concrete durability

Good curing, enabling prolonged hydration and the formation of a well-developed microstructure, is imperative if concrete is to perform at its full potential. This may become more important with the increasing use of composite cements containing more slowly reacting additions. Furthermore, the effects of improper curing – that is, compromised durability – may not become visible for many years. A series of concrete mixes have been prepared of 20 or 50 MPa target mean strength, using either CEM I or CEM I + 30% fly ash as the binder. Mixes were designed with two different workabilities (10–30 and 60–180 mm slump). Samples were cured in a fog room at 20 ± 3°C and 99 ± 1% relative humidity (RH) or under ambient conditions of 20 ± 3°C and 42 ± 5% RH. Performance was evaluated in terms of compressive strength, transport properties and resistance to carbonation. Equivalent paste samples were characterised by thermogravimetric analysis, X-ray diffraction analysis and scanning electron microscopy to follow hydration and microstructural development. Improper curing did not greatly affect compressive strength. However, the effects on transport properties, and therefore properties that may affect durability, were more profound. The effects of non-ideal curing were greater for lower strength mixes, those containing fly ash and, to a lesser extent, less workable mixes.