What is the moisture content of concrete?

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 )

Moisture content:
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.