Testing - Concrete Mix Design, Quality Control and Specification - page 178

7.6.4 Water permeability
Water permeability is often used in a generic sense for all forms of water
movement. However, the American Concrete Institute (ACI) defines it as “the
rate of discharge of water under laminar flow conditions through a unit panel
cross-sectional area of a porous medium under a unit hydraulic gradient and
standard temperature conditions, usually 20°C”. Therefore, water perme-
ability necessarily involves the hydraulic pressure through saturated concrete.
Deterioration of concrete is most prevalent where concrete is subjected
to wetting and drying or partially submerged. Under these conditions, the
concrete is neither subjected to a hydraulic gradient nor saturated and thus
permeability cannot be the driving force for water movement or salt accu-
mulation. Even in tunnels and other structures exposed to hydrostatic pres-
sure the effect of evaporation from the surface appears the dominant factor
(Aldred, 2008). Aldred et al. (2001) calculated that water flux through OPC
concrete with w/c 0.4 due to water permeability alone would be an order
of magnitude less than that due to wick action (i.e., flow due to water on
one face and air on the other without pressure) for a typical retaining wall
of 300 mm thickness exposed to 20 metre hydrostatic pressure. Therefore,
water transport through uncracked concrete under most practical situa-
tions is not dominated by the external hydrostatic pressure. Accordingly
a water permeability coefficient as measured by the pressure differential
tests would not appear the dominant driving mechanism of water or salt
penetration in most severe environments.
Vuorinen (1985) found that oven drying and resaturation increased the
water permeability by about l00 times that of a specimen that had not been
oven dried due to the resultant microcracking. A similar detrimental effect
of gradual drying to 79% RH was reported by Powers et al. (1954). This
highlights one of the important variables in water permeability measure-
ments. Pressure permeability is relatively difficult to measure accurately
and with repeatability. It also requires specialised equipment.
The most commonly used pressure penetration procedure is BS EN
12390-8 (based on the previous DIN 1048 procedure). Compliance tests
on one grade of high performance concrete measured a range of water pen-
etration depths from 0 to 12 mm, approximately 70% of the approximately
150 values reported by an independent laboratory were 0 mm. Pocock and
Corrans (2007) for a different grade of concrete reported a mean water
penetration of 8 mm with a range of 30 mm and a coefficient of variation of
125%. The target mean penetration depth was -6.5 mm. Concrete Society
Technical Report 31 refers to a study in the United Arab Emirates where
the coefficient of variation for 399 results was 65%.
These limitations suggest that the pressure permeability tests are not par-
ticularly suitable for durability performance specification and certainly not
ongoing compliance testing.