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Measures to reduce cracking

The planning of a building to accommodate movement must be considered at the earliest possible stage of the design process. Key decisions must be made about the degree of cracking which will be acceptable and whether to provide movement joints. Having decided on the quality of finish and crack widths which will be acceptable, then, as discussed in Concrete Society Technical Report 67, Movement, restraint and cracking in concrete structures, a range of actions to reduce the risk of cracking become possible. Some decisions will be in the control of the designer, and may require a tighter control of the specification. Other areas will be in the control of the contractor.

One solution that is often proposed is the use of post tensioned concrete slabs. This can work reasonably well when the slabs are partially restrained or restrained at one end only. Where significant restraint can occur it might be necessary to decouple the slab to ensure that the prestress takes full effect.

The choice of aggregate can have a significant effect on the modulus of elasticity and hence the tensile strain capacity of the concrete. For thermal movements, the choice of aggregate can be highly significant. The type of cement affects both creep and shrinkage.

Early-age contractions are a significant source of movement in concrete. In extreme cases, it may be necessary to cool the concrete prior to placing or by to pass cooling water pipes through placed concrete for the first few days after casting. Early thermal effects are complete once the concrete temperature has fallen back to ambient, so it makes sense to plan the pouring sequence so that the cumulative effects are minimised. Another way of reducing the restraint forces is to introduce pour strips. This allows some of the movement to occur in the slabs before they are locked together or to cores or shear walls. The main benefits of pour strips are in reducing the forces due to shrinkage. However, if a pour strip is left open for 28 days the concrete will have completed less than 15% of its shrinkage; significantly longer periods (which may be impractical) are required to get any real benefit.

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TR67 Movement, restraint and cracking in concrete structures