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TECHNICAL PAPER

the absorption of water from the surface exposed to rain and transport due to vapor diffusion, surface creep, and liquid
increase in the degree of saturation of the concrete medium. assisted vapor transport in addition to the bulk movement
The surface becomes saturated, followed by the ingress of of liquid in partially and fully saturated pore spaces. The
water by capillary suction as the rain pursues. Drying, on the differences in the underlying mechanisms of moisture transport,
other hand, involves gradual reduction in degree of saturation result in a separate functional form for drying diffusivity. For
of the medium on account of evaporative losses. Moisture loss concrete, the moisture dependency of drying diffusivity has
during drying occurs in two distinct stages. During the initial been represented using a ‘S’ shaped curve [75-76] .
stage, the rate of dying remains practically constant, while in
the subsequent stage the rate of drying gradually reduces with (54)
[66]
time . Thus, a typical wetting-drying cycle can be divided into
the following four distinct stages, namely, 1: moisture ingress where, D Wdry (m /s) is the drying diffusivity of totally wet medium,
2
under unsaturated surface condition; 2: moisture ingress under α o represents the ratio of minimum to maximum drying diffusivity
saturated surface condition; 3: constant rate drying and 4: falling and the parameters θ c and n* characterize the location of drop
rate drying. and the spread of the curve respectively. Recently, some work
on extension of Richards’ equation and diffusivity function
The pioneering work on water ingress in concrete was initiated
by Christofer Hall [67-69] by considering extended Darcy’s Law for are reported by Sarkar and co-workers and are worthy of
unsaturated condition. Fundamental ideas of wetting and drying consideration for future research workers continuing to work in
[77-78]
have been comprehensively reviewed [66,70-71] . More details on this this field .
aspect can be found in reference literatures [66-74] . Models for estimating hydraulic diffusivity of concrete from
porosity and pore size distribution have been proposed [19,79] .
The phenomenon of moisture flow in an unsaturated porous Hence, it can be estimated from w/c, paste content etc.
medium is conventionally represented using the extended However, in general further research is needed for accurate
Darcy’s Law, stated as, prediction of hydraulic diffusivity. The strong dependence of

(51) hydraulic diffusivity on moisture content renders the unsaturated
flow problem highly non-linear. A plausible analysis of the
For a one-dimensional case such as vertical wall or exposed problem is, therefore, dependent on the application of a robust
flat horizontal concrete surface (neglecting hydraulic head) the numerical scheme.
governing equation reduces to,
The boundary condition mentioned in Table 20 and 21 i.e.,
(52) wind driven rain (WDR) refers to joint occurrence of wind and
rain which causes an oblique rain intensity vector [71,72] . From the
with the term, representing a flux acting in the direction viewpoint of the interaction between rain and vertical facades
of outward normal to the surface of the domain. Here, θ (m /m ) of concrete structures, the term WDR intensity takes on the
3
3
is the volumetric moisture content, t (s) is the time variable, x (m) narrower meaning of the component of rain intensity vector
2
is the space variable and D m (θ) (m /s) is the moisture dependent causing rain flux through a vertical plane. A semi empirical
hydraulic diffusivity function. The distribution of moisture in method is mostly used to calculate the WDR intensity using
a porous medium is characterized by the nature of hydraulic commonly available meteorological data and is given by the
diffusivity function, which is known to assume typical trends expression after Lacy , as,
[80]
for wetting and drying conditions respectively. Absorption of
rainwater in building materials is primarily caused due to the (55)
action of capillary forces, and the associated hydraulic diffusivity where, V 0 (mm/h) is the WDR intensity, W (m/s) is the wind speed
has been successfully represented using an exponential function and R (mm/h) is rainfall intensity.
of the form [66,70-71] given in Equations 53 and 54.
During constant rate drying, due to evaporative cooling, the
(53) surface of the medium maintains itself at wet bulb temperature.
The heat transferred to the wet concrete surface from the
2
where, D dWet (m /s) is the wetting diffusivity corresponding to ambient air results in an evaporation flux given as,
totally dry state of the medium. The value of n in the stated
equation ranges between 6 - 8 for building materials [66,67] and (56)
for concrete in an initially dry state, a value of n = 6 has been 2
suggested [72-74] . where, J c (kg/m .s) is the constant rate evaporation flux, h f (W/
2
m .K) is the film heat transfer coefficient, also referred as surface
The phase of drying, on the other hand, involves moisture conductance, γ w (J/kg) is the latent heat of vaporization of water

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