Frequently Asked Questions
Q: What is the moisture content of concrete?
A: The moisture content of concrete
must be viewed from the context of total water content of the fresh
concrete mixture and the available moisture content of the hardened
concrete. The total water content of a fresh concrete mixture is
a function of the total cementitious materials and water cement
ratio (w/cm). Typical fresh concrete mixtures vary in cementitious
material content in a range of 279 kg/m3 to 415 kg/m3
(470 lb/yd3 to 700 lb/yd3). Water cement ratios
typically vary from 0.4 to 0.55. To estimate the available moisture
content of hardened concrete one must start with the total water
content of the fresh mixture and define the service condition of
the hardened concrete with regard to relative humidity (%). In addition
the water that is chemically bound with the cement in the hydration
process must be accounted for. The water bound with the cement is
in the range of 0.22 to 0.24 of the cement content.
As an example, the moisture content of a concrete mixture with
334 kg/m3 (564 lb/yd3) of cement and a w/c
of 0.45 and in a service environment with a 50% relative humidity
could be estimated as follows:
Total water content:
334 kg cement/m3 times 0.45 w/c ~ 150 kg water/m3
(564 lb cement/yd3 times 0.45 w/c ~ 254 lb water/yd3
Chemically bound water at 0.24 w/c:
334 kg cement/m3 times 0.24 w/c ~ 80 kg water/m3
(564 lb cement/yd3 times 0.24 ~ 135 lb water/yd3
150 kg water/m3 - 80 kg water/m3times .50
relative humidity ~ 35 kg water/m3
(254 lb water/yd3 – 135 lb water/yd3 times .50
relative humidity ~ 60 lb water/yd3)
In reality the relative humidity of the concrete will only reach
50% at the near surface of the concrete and the moisture gradient
with depth will increase toward 100% relative humidity so this method
of estimation would typically overstate the quantity of moisture
available to leave the concrete due to the initial mixing water
in the mixture.
This is only an estimate of the moisture available to leave the
concrete, but may help to give some perspective to the limited amount
of water that the concrete can contribute when considering the drying
time of hardened concrete.
Relaltive Humidity Profile
Kosmatka, Steven H., Kerkhoff, Beatrix, Panarese, William C., Design
and Control of Concrete Mixtures, 14th Edition, 2002 (revised
2008), 372 pg.
Kanare, Howard M., Concrete
Floors and Moisture, 2nd Edition, 2008, 172 pg.