Hot weather concreting can be defined as any combination of high air temperature, low relative humidity, solar radiation, and wind velocity tending to impair the quality of fresh or hardened concrete.  These conditions adversely influence concrete quality primarily by accelerating the rates of moisture loss and cement hydration.

The potential effects of hot weather on fresh concrete include:

Increased rate of slump loss

Increased rate of setting resulting in greater difficulty with handling and finishing

Increased risk of cold joints

Increased difficulty controlling entrained air content

Increased tendency for plastic shrinkage cracking.  The most common effect of hot weather on fresh concrete is shrinkage cracks.  They appear on the surface of freshly mixed concrete soon after it has been placed, while it is being finished or shortly thereafter.

The potential effects of hot weather on hardened concrete include:

Decreased 28 day strength resulting from high water content

Increased tendency for drying shrinkage and differential thermal cracking

Decreased durability from cracking

Increased permeability

Preparations before concreting

Several precautions can be taken to limit the damage caused by hot weather.  These are:

Dampen the sub-grade or slab bed while keeping it free of standing water.

Erect temporary windbreaks to reduce wind velocity over concrete surface.

Erect temporary sunshades to reduce concrete surface temperatures.

Protect concrete with temporary coverings such as curing paper, plastic sheets or burlap.

Fog the slab immediately after placing and before finishing taking care to prevent accumulation of water that may reduce the quality of the cement paste on the slab surface.

Add plastic  fibers to the concrete mixture to help reduce plastic shrinkage crack formation.

Spray a retarding evaporation-limiting compound, Con Film, (available at Universal Concrete Supplies) on the slab immediately after placing the concrete but before finishing it.  This will retard the set of the slab surface and reduce surface evaporation.