These are defined in EN 934-2 as admixtures that reduce the capillary absorption of water into the hardened concrete. There are, in addition, ‘permeability-reducing’ admixtures that reduce the passage of water through the concrete under a pressure head. Most products function in one or more of the following ways:
All these ‘waterproofing’ admixtures reduce surface absorption and water permeability of the concrete by acting on the capillary structure of the cement paste. They will not significantly reduce water penetrating through cracks or through poorly compacted concrete which are two of the more common reasons for water leakage through concrete.
Dosage varies widely, depending on the type and the required level of performance. Two per cent is common for the hydrophobic types, but may be 5% or more for the pore blockers. Dosage is often given as a weight or volume per cubic metre rather than on cement weight as their action is largely independent of cement content.
Lowering the free water content of the mix reduces the size and continuity of capillary pores and this can be achieved with water-reducing or high range water-reducing materials. Reducing capillary size alone does not necessarily reduce surface absorption and may even increase it as capillary suction increases with smaller diameter pores. However, the overall pressure permeability of the concrete will be improved due to greater capillary discontinuity.
Pore blocking can be achieved by the addition of very fine unreactive or reactive additions such as silica fume or by the use of insoluble organic polymers such as bitumen introduced as an emulsion. The hydrophobic admixtures are usually derivatives of longchain fatty acid of which stearate and oleate are most commonly used. These may be supplied in liquid or powder forms. Some admixtures are combinations of two or more of these systems.