Water permeability of concrete can be reduced by up to 100 times through the use of Silica fume.
Test by some independence authorities
indicates that so called waterproofing admixtures had no effect on water
permeability of concrete. Rather that supervision and improved mix
designs associated with such systems can lead to a more impermeable
concrete. It should be noted that permeability of concrete is not a
simple function of its porosity but depends also on size, distribution
and continuity of the pores.
Pores in concrete are reduced
significantly with the inclusion of silica fume and illaries reduced
through reduced bleeding and production of denser paste. The
permeability is lower the higher the cement content of the paste i.e.
the lower the water/cement ratio. Values obtained for pastes in which
93% of cement has hydrated. It is the permeability of the paste that has
the greatest effect on the permeability of the concrete. Water vapor
transmission of concrete is reduced by a decrease in the water cement
ratio. A decrease in water cement ratio from 0.8 to 0.4 decreases the
vapour permeability by 50-65%.
Vapour permeability, however, decreases as the relative humidity decreases, an increase in relative humidity decreases the airfilled pore space available for diffusion. It follows the that if the moist side is for instance saturated an increase in the relative humidity of the dry side reduces the vapor permeability.
The permeability of concrete is reduced
by the use of silica fume in combination with superplasticizers which
reduce the water cement ratio up to 30%.
