Durability of concrete and cement composites

Conclusions Dimensional stability and cracking processes in concrete

The knowledge of movements and cracking processes presented in this chapter has been developed from theories, research and technical field data published over many years for conventional types of concrete, namely low-strength mixes using natural aggregates. Although the principles are not expected to change, in the future design methods given in Codes of Practice will be continuously amended to account for the use of more sophisticated concreting materials, such as high performance concrete, which refers to both high durability (low permeability) and/or high strength concrete. In some cases, the high strength is required to design smaller concrete elements or low deflections through a corresponding high modulus of elasticity, low creep and low shrinkage. Such concrete tends to be more brittle and prone to cracking so that ductility needs to added, say, in the form of fibre reinforcement. High performance is achieved by the combined use of chemical and mineral admixtures: high range water- reducers (superplasticisers) and fine cement replacement materials to replace part of cement, e.g. silica fume.

On the other hand, admixtures designed to reduce shrinkage, creep and the risk of cracking are likely to be used more frequently in conjunction with, say, superplasticisers to offset any reduction in strength. Little is known about the long-term behaviour of concretes made with cocktails of different admixtures and the possible interactions between supplementary cementing materials (or mineral admixtures) and multi-functional chemical admixtures need to be assessed. Advanced methodologies may have to be developed for evaluating the early age cracking characteristics of some of the newer formulations being considered for use in high strength applications and repair situations (Bentur and Kovler, 2003). The increased usage of recycled concrete and other waste materials to replace or augment natural aggregates in concrete has also to be considered in the context of prediction models and design documents. It should be emphasised that it has been only possible to present brief details of movements and cracking processes experienced by concrete in this chapter, there being a huge amount of research and technical literature available.

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