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Water resisting admixtures

Water-resisting admixtures are more commonly, though strictly incorrectly, called ‘waterproofing’ admixtures and may also be called ‘permeability-reducing’ admixtures. Their main function is to reduce surface absorption and/or passage of water through the hardened concrete. To do this, most function in one or more of the following ways:

- reducing the size, number and continuity of the capillary pore structure
- blocking the capillary pore structure
- lining the capillaries with a hydrophobic material to prevent water being drawn in by absorption/capillary suction.

They are designed for use in conventional ‘wet’ concrete. They will not significantly reduce water penetrating through cracks or through poorly compacted concrete.

Water-resisting admixtures lower the risk of corrosion of reinforcement in concrete subject to aggressive environments as long as appropriate admixture types or combinations are used. They have other uses including the reduction of efflorescence.

Water-resisting admixtures benefit durability by reducing the ingress of water and hence other water-soluble aggressive chemicals such as chloride or sulfate that could otherwise cause more rapid structural deterioration. A particularly important feature is the ability to reduce surface absorption. This prevents surface saturation and reduces surface concentration of salts where there is cyclic wetting and drying. Such absorption has been shown to be a major factor in deterioration.

Concrete Society Technical Report 61, Enhancing reinforced concrete durability, gives information on the effects of water-resisting admixtures in concrete on the rates of carbonation and chloride ingress.

Acknowledgement: The Concrete Society

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TR18 A guide to the selection of admixtures for concrete

The influence of integral water-resisting admixtures on the durability of concrete