Effect of temperature on compressive strength

Temperature affects both the rate of hydration and the structure of the hydrated cement paste, and so affects strength.

Effect of casting (initial) temperature

• Higher temperature gives faster hydration and higher early strength (1 to 7 days).
• But the hydration products form quickly and are non-uniform, with a coarser pore structure, so the 28-day and later strength is lower (a concrete cast at 40 °C may have 10 to 20% less 28-day strength than the one cast at 20 °C).
• Low casting temperature (5 to 10 °C) slows the early strength but results in a higher long-term strength, since the gel is more dense and uniform.

Effect of curing temperature

• Curing at a higher temperature (for example steam curing) raises the early strength but lowers the long-term strength (the crossover effect).
• Low curing temperature slows the strength gain; hydration almost stops near 0 °C to -10 °C, and if concrete freezes before it has 3.5 MPa strength, it suffers permanent damage.
• Temperature fluctuations and rapid drying cause thermal and shrinkage cracks.

Maturity

Strength depends on both time and temperature, which is expressed by the maturity $M=\sum (T+10)\Delta t$ (T in °C, t in hours).

Hence a moderate temperature (15 to 25 °C) with continuous moist curing is best.