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Curling of ground-supported floor slabs

Curling of ground-supported floor slabs has been, and continues to be a problem. It is likely to occur in most floors but is usually insignificant. Its magnitude, however, is unpredictable.

Curling is caused by the differential shrinkage of the concrete. The exposed top surface dries and shrinks more than the bottom, causing the floor to curl upwards. It can occur at any time up to about 2 years after construction. In some instances surface regularity is affected and curling can also cause lack of sub-base support and slab rocking. Under-slab grouting can restore support.

As it is a function of shrinkage it is advisable to limit the shrinkage potential of the concrete, thus when specifying concrete for floors:
• Do not specify a higher strength than necessary
• Do not exceed a higher water:cement ratio than 0.55
• Consider water reducing admixtures
• Consider shrinkage reducing admixtures
• Specify the largest appropriate size of coarse aggregate (usually 20mm)
• Do not specify a high minimum cement content.

Effective curing of the concrete is also important. This will minimise the loss of moisture from the surface and reduce differencial shrinkage between top and base as the concrete hardens and develops strength.

Acknowledgement: Concrete Society

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Curling of ground floor slabs