Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 97

has been established). The coefficient of water permeability is calculated
according to D'Arcy's equation. Considering the tortuous nature of the
capillary network within concrete, particularly of high quality, the time to
achieve a steady-state condition may be very long. Accordingly, a coefficient
of water permeability is sometimes calculated from uniaxial penetration
based on Valenta's equation. There are three avenues by which water can
penetrate through concrete under pressure:
1. Gross voids arising from incomplete compaction or segregation
2. Micro (or macro) cracks resulting from plastic, autogenous, or drying
shrinkage; thermal stresses; or plastic settlement
3. Pores or capillaries resulting from mixing water in excess of approxi-
mately that which can combine with the cement, that is, water in
excess of 0.38 by mass of cement
Gross voids may be regarded as too obvious a cause to be included.
However, they are worth mentioning because they may be made more likely
by action that may otherwise reduce porosity, such as a harsh, low slump
mix will have a lower water content or a richer mortar (higher cement/sand
ratio) than a sandier mix of equal strength. Obviously a low permeability
concrete must be fully compacted by the means available. It must not depend
on unrealistic expectations of workmanship. Of course the development
of self-compacting concrete is an excellent answer to permeability since it
is inherently of low permeability and, at least theoretically, cannot suffer
from segregation or a lack of compaction.
Water occupies 15% to 20% of the total volume of fresh concrete and,
when the w/c ratio exceeds 0.38 by mass, not all of this water can be
consumed in the hydration of the cement. The resultant voids left by the
excess water will provide pathways for water transport. If they become
discontinuous, they will not provide easy passage for water.
The latest packing theories of mix design have demonstrated that close
attention to the packing of fine material of cement size and smaller can
reduce total void space in the paste fraction, especially when accompanied
by superplasticisers.
The total amount of pore space is not the only factor determining per-
meability. Another important factor is the distribution of the pores and
their discontinuity. Bleeding is a source of continuous or semicontinu-
ous pores. Bleeding is initiated by the settlement of cement particles in
the surrounding mixing water, after compaction in place. This tends to
leave minute pockets of water under fine aggregate grains. There may be
enough water to allow the fine aggregate grains to settle slightly and the
water to escape around them and rise up through the concrete. The process
occurs on a larger scale under the coarse aggregate particles and eventu-
ally the whole mass of the concrete settles slightly, leaving a film of water