Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 99

coefficient by one to two orders of magnitude when concrete had been
dried prior to testing. This highlights that permeability is strongly affected
by exposure and resultant microcracking.
5.3.3 Sorptivity
The transport of water in concrete due to surface tension within the capil-
lary network is known as capillary suction or sorptivity. Under conditions
of short-term contact with water, the penetration of water is proportional
to the square root of time (Ho and Lewis, 1984). The volume of water
absorbed by a dry surface will be influenced by the moisture content of the
concrete at the time of test. Dhir et al. (1987) compared drying at 20°C,
50°C, and 105°C to develop guidelines for repeatable pretreatment prior to
sorptivity testing. They showed that drying concrete with w/cm ratios of 0.4
and 0.55 at 50°C for 14 days removed 50% and 60%, respectively, of the
total evaporative water. Even drying at 50°C for 100 days did not achieve an
apparent equilibrium in any of the mixes tested as can be seen in Figure 5.3.
Hydrophobic admixtures, controlled permeability formliners, and silane
surface treatments all profoundly reduce sorptivity and the reduction
compared to an untreated concrete is greater in drier concrete.
5.3.4 Desorptivity
The rate of water loss from an initially saturated concrete surface or desorp-
tivity is also proportional to the square root of time (Dolch and Lovell,
1988). Parrott (1991) concluded that the initial weight loss after four days
of uniaxial drying can be regarded as an indicator of the moisture transport
properties in the cover concrete.
Baroghel-Bouny et al. (2001) established that isothermal drying of
cementitious materials gave a good indication of its permeability. Bentz
and Hansen (2000) used x-ray absorption to monitor the effect of drying in
cement paste. They tested layered specimens and found that the higher w/c
paste dried out first regardless of its location within the composite.
Aldred (2008) found desorptivity was correlated with an apparent steady-
state wick action for most concrete types. Therefore a simple desorptivity
test, which requires no special conditioning or equipment, appears to pro-
vide a good indicator of concrete water transport properties.
5.3.5 Water vapour diffusion
Water vapour diffusion is the movement of water vapour molecules (as a
gas not a liquid) due to a concentration gradient and is calculated using
Fick's law. Unlike permeability or sorptivity, transport by diffusion is due
to random motion of the molecules. Aldred (1999) showed that water
vapour diffusion coefficients for concretes incorporating chemical and