ADMIXTURES PREVENT CONCRETE CRACKING UP
October 28, 2004

Chemicals with the ability to reduce shrinkage in cementitious mortars have a long and successful history of use in proprietary, pre-packaged repair products, but recently they have become available in admixture form for use in conventional site-batched concrete, reports the UK Cement Admixtures Association (CAA).

Water reducing admixtures, especially superplasticisers have always had an important role in shrinkage reduction but their effect is relatively limited. New shrinkage reducing admixtures, typically based on glycol ethers, have been shown to give much larger reductions, often in excess of 50%.

In the absence of an admixture, a typical concrete will comprise 20% water when batched. Of this, only 9% is needed for cement hydration. The remaining 11% can evaporate over time, leaving voids and building-up shrinkage stresses that lead to cracking and debonding when the concrete is restrained. Water reducing admixtures assist by lowering the amount of evaporable water left after cement hydration but stresses still develop and cracking is not eliminated.

Shrinkage reducing admixtures work in a different way. They act on the evaporable water in the capillaries, preventing the shrinkage forces that build up between the liquid surfaces and the cement grains as the water is lost. Their effect is both short and long term and can typically reduce shrinkage by 30 to 70%. Shrinkage reducing admixtures are particularly effective if used in conjunction with the recently developed high range water reducing admixtures based upon Polycarboxylate Ethers (PCE).

This combination of shrinkage reducing and water reducing admixtures has opened up remarkable new possibilities for the use of concrete in situations that were previously the preserve of specialist products, says the Association. They can be particularly effective in repair and/or strengthening situations where the new concrete is restrained by the in situ concrete and where surface-to-volume ratio is often high. Such conditions cause a rapid loss of water through evaporation - often before the concrete has developed its full tensile strength and result in cracking of the new concrete and debonding from the substrate. As well as repair situations, shrinkage reducing admixtures have been shown to be very effective in floor slabs, significantly increasing joint spacing and also in water retaining structures where cracking can be the most common cause of leakage.

The development of shrinkage reducing admixtures has significantly increased the options available to engineers, not only in terms of design of new structures, but also in repairing and upgrading existing structures with concrete in order to meet current needs.

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