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Moisture transport in initially fully saturated concrete during drying

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Abstract

Moisture content changes during drying were investigated in the present work. Particular emphasis was placed on the initial stage of drying of saturated concrete, where moisture contents are high. For this stage of drying, experimental data are lacking, and no comprehensive theory exists to describe it.

The present investigation was performed experimentally and numerically for drying of cylinders with one exposed end, made of normal weight and lightweight concrete with varying water to cement ratio (w/c). The gravimetric technique was employed to obtain the spatial distribution of moisture content. The experimental results obtained indicate that drying of concrete becomes diffusion controlled when the average moisture content decreases below 70 to 80% of the initial saturation. Typical drying rates are in the order of magnitude of 0.18 kg/day/m2 and 0.02 kg/day/m2 for the first and the second stage of drying, respectively.

The lightweight concrete cylinders as compared to those made of normal weight concrete exhibited higher levels of moisture content throughout the process. At high w/c ratios, the moisture profiles for both types of cylinders, as expected, show steeper changes with time. Large, constant drying rates were observed both experimentally and numerically in the beginning of the drying.

The numerical model developed is based on a generalized mathematical formulation for mass and heat transfer in porous media, and its predictions are in agreement with the experimental data within the uncertainty range of the input data.

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Abbreviations

D :

diffusion coefficient, m2/s

DR:

drying rate, kg/(m2s)

(c p ):

specific heat, J/(kg K)

f att :

attenuation factor

g :

acceleration of gravity, m/s2

h s :

differential heat of sorption, J/kg

Δh vap :

heat of evaporation, J/kg

K :

specific permeability, m2

k i :

relative permeability of i phase

M i :

molecular weight, kg/mole

\(\overline {m_i }\) :

mass evaporation rate of i phase, kg/(m3s)

n boun :

unit vector normal to the boundary surface

n grav :

unit vector acting in the direction of gravity acceleration

p :

pressure, Pa

R :

universal gas constant

r :

pore diameter, m

S :

saturation

S d :

surface, m2

T :

temperature, K

t :

time, s

Δt :

time step, s

u :

x component of the velocity vector, m/s

V :

volume, m3

v :

velocity vector

v :

y component of the velocity vector, m/s

W :

weight of a disc, kg

w :

moisture content (mass of water / unit volume of concrete), kg/m3

x,y :

Cartesian coordinates, m

α :

heat transfer coefficient, W/(m2K)

β :

mass transfer coefficient, m/s

ɛ i :

volumetric fraction of phase i

φ :

arbitrary quantity

λ eff :

effective thermal conductivity, W/(mK)

μ :

viscosity, kg/(ms)

ξ :

underrelaxation factor

ϱ :

density, kg/m3

σ t :

surface tension, N/m

ψ e :

global energy residual, J

ψ m :

global mass residual, kg

O:

initial

1:

water vapour

2:

air

atm:

atmospheric

att:

attenuation

b :

bound water phase

boun:

boundary

c :

capillary

d :

drying

dry:

dry state

e :

energy

eff:

effective

g :

gaseous phase

grav:

gravitational

irr:

irreducible

l :

liquid water phase

m :

mass

ref:

reference

s :

solid phase

wet:

wet state

∞:

ambient

g :

intrinsic average over volume of gaseous phase

(i):

current iteration level

l :

intrinsic average over volume of liquid phase

(n):

time level at time (t)

(n + 1):

time level at time (t + Δt)

sat:

saturated

-:

averaged quantity over a REV

-i :

intrinsic average over phase i of a REV

/:

normalized

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Authors and Affiliations

  1. Department of Civil Engineering, University of New Brunswick, PO Box 4400, E3B 5A3, Fredericton, Canada

    Jana Šelih & Theodore W. Bremner

  2. Department of Mechanical Engineering, University of New Brunswick, PO Box 4400, E3B 5A3, Fredericton, anada

    Antonio C. M. Sousa

  3. Civil Engineering Institute ZRMK, Dimiceva 12, 61000, Ljubljana, Slovenia

    Jana Šelih

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  1. Jana Šelih
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  2. Antonio C. M. Sousa
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  3. Theodore W. Bremner
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Šelih, J., Sousa, A.C.M. & Bremner, T.W. Moisture transport in initially fully saturated concrete during drying. Transp Porous Med 24, 81–106 (1996). https://doi.org/10.1007/BF00175604

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  • DOI: https://doi.org/10.1007/BF00175604

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