Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 101

Wet face in
contact with
water

Dry face
exposed to
RH < 100%

Concrete

Water
absorption

Water vapour
diffusion

Figure 5.4 Schematic diagram of the interaction of various transport mechanisms during
wick action. (From Buenfeld, N. R. et al., in Chloride Penetration into Concrete ,
eds. L. O. Nilsson and J. P. Ollivier, RILEM, Paris, France, 1997.)

describe water transport into air-filled concrete structures appears to have
been first used by Aldred (1988). Water transport through concrete due to
wick action is many times that due to pressure permeability under typical
environmental conditions. Therefore wick action plays an important role
in the watertightness and durability of concrete structures (Aldred, 2008).
Wick action was considered a combination of sorptivity and water vapour
diffusion with evaporation being the linking process as shown in Figure 5.4.
However, Aldred (2008) showed that wick action was poorly correlated to
sorptivity but well correlated to desorptivity as shown in Figure 5.5.
James developed a simple equation for estimating steady-state wick
action from a simple 14-day desorptivity test:

Q w ′ = 0.1/ L ′ × (0.19 D 14 - 22.4) × 10 −9

where
Q w ′ = estimated steady-state mass flux (kg/m²/s)
D 14 = average desorptivity rate over 14 days (kg/m²/s)
L ′ = section thickness in metres (dimensionless)

5.3.7 Chloride diffusion
Chloride diffusion is the movement of chloride ions as a result of a con-
centration gradient. Under steady-state conditions, the diffusion coefficient
is usually calculated using Fick's first law of diffusion. Under conditions
of uniaxial penetration of chloride ions, diffusion is usually calculated by
Fick's second law. However, Fick's law is based on the material through