Maturity; shrinkage and creep

Maturity of concrete is the product of curing time and temperature above a datum temperature. It lets the strength of concrete cured at different temperatures be compared, since strength is a function of maturity (Saul's rule).

$$M = \sum (T - T_0)\,\Delta t$$

where $T$ = mean temperature of concrete during the interval $\Delta t$ (°C), $T_0$ = datum temperature (about $-10.5$ °C as per Saul; $-11$ °C is often used), and the unit of $M$ is °C-hours (or °C-days).

Concrete of the same mix that has the same maturity has about the same strength, whatever its temperature history.

Example. Concrete cured at 20 °C for 28 days has maturity $= (20+10)\times 28\times 24 = 20160$ °C-h. Concrete cured at 10 °C reaches the same maturity in $20160/((10+10)\times 24)= 42$ days. Concrete at 30 °C needs only $20160/((30+10)\times 24) = 21$ days.

Shrinkage

Shrinkage is the time-dependent reduction in volume of concrete, without any applied load, caused by loss of moisture and by chemical reaction (hydration and carbonation). Typical long-term values are 0.0003 to 0.0008 (300 to 800 microstrain). The main types are plastic, drying, autogenous and carbonation shrinkage.

Creep

Creep is the gradual increase in strain of concrete with time under a constant sustained stress, over and above the instantaneous elastic strain. It is caused mainly by seepage of gel water out of the C-S-H layers and by slip of gel particles. Creep continues for years, with about 50% occurring in the first 3 months and 75 to 80% in the first year.