Drying of Concrete
How long does it take concrete to dry?
A concrete surface may look dry, but the slab can still contain
sufficient moisture to cause problems when covered. The term “concrete
moisture” is understood to mean the total water used in the
concrete batch, plus curing water, minus the water bound in hardened
cement due to hydration. The amount of concrete moisture can be
considerable. In practical terms, several pounds of water must evaporate
from every square foot of concrete for the slab to be considered
adequately dry for floor finishes. An industry rule of thumb for
estimating the drying time necessary for concrete floors to reach
acceptable moisture content is 1 month of drying for each inch of
concrete thickness (1 mm per day).
|Moisture in concrete.
Drying begins when water is no longer available at the exposed
surface. If concrete is moist cured by sealing in the original mix
water with wet burlap or plastic sheets, drying will begin when
these covers are removed. Spray-applied curing membranes are somewhat
breathable, and therefore, drying begins shortly after the membrane
is applied. However, curing compounds can drastically reduce the
drying rate and significantly extend the drying period.
It is possible to calculate the drying time for a given concrete.
For this calculation, information is needed about the absorption
characteristics, diffusion coefficients for water and water vapor,
porosity and pore size distribution, and degree of hydration. Since
such information usually is not available, practice relies on experimental
data combined with measurements of the actual moisture condition
of the concrete slab in the field.
A method of characterizing the moisture condition of a concrete
slab is to measure the relative humidity of the air in the concrete
pore system. This is done by placing a relative humidity probe into
a hole drilled in the concrete. The relative humidity achieved within
a concrete slab depends on a combination of factors including the
initial water-to-cement ratio, drying history, pore structure, and
concentration of soluble ions in the pore water solution.
The Swedish Concrete Association describes a method to estimate
drying times for concrete slabs using relative humidity. Correction
factors for thickness, number of drying sides, ambient conditions,
and curing conditions make it possible to adjust for deviations
from a set w/cm and target relative humidity. The purpose of this
calculation is to enable a contractor or construction manager to
estimate minimum drying times for concrete slabs during the planning
stage of a project. See Concrete
Floors and Moisture, 2nd edition,
EB119, for the details of this method and to understand moisture