The answer is actually called: The mixing ratio.

Relative humidity indicates how near the air is to saturation, while mixing ratio shows the actual quantity of water vapor in the air.

The relative humidity will decrease

The water/concrete ratio, not mixing it well enough, quality of raw materials, the age of the concrete, and the temperature and humidity of the area surrounding the concrete.

One can calculate the absolute humidity (AH) from the relative humidity (r) using three equations: (1) the equation for mixing ratio, (2) an equation for relative humidity expressed in terms of mixing ratio, and (3) the Clausius-Clapeyron equation, which relates saturation vapor pressure to temperature.

The result of combining the three equations is:

AH = (1324 r/T) [exp {5417.75 (1/273 - 1/T)}]

where AH is expressed in grams per cubic meter, T is temperature in Kelvin, r is relative humidity (range is 0 to 1), and the relation holds true for T>273. For T<273, replace 5417.75 with 6139.81.