Hot Weather Concreting

by Curtis C. Went!. Midwest Testing Laboratory

Concrete in hot weather poses some special problems. Among these are strength reduction and cracking of flat surfaces due to rapid drying not to mention difficulty in the finishing operations.

Most of these problems can be minimized by the following precautions:

1. Plan Ahead

Be prepared with necessary equipment and material well in advance of hot weather.

Be sure of an ample water supply for sprinkling subgrades, wood forms and aggregates, and for curing.

Have tarpaulins or polyethylene sheets and lumber ready for sunshades and windbreaks.

Schedule work so that concrete can be placed with the least delay. Jobs could be started late in the afternoon during extremely hot periods.

2. Use Cool Materials

Stockpiles of coarse aggregate should be sprinkled with water to cool the aggregate by evaporation. Hot aggregate stockpiles mean dry aggregate which then absorbs water from the mix causing rapid slump loss.

3. Prevent Absorption

Sprinkle subgrade and wood forms just before placing concrete so they will not absorb water from the mix.

4. Protect Against Evaporation Erect windbreaks to prevent strong, hot winds from drying exposed concrete flat-work surfaces while they are being finished.

5. Placement and Finishing

Don't delay in placing concrete. Strike it off and darby it at once.

Place temporary covers, such as burlap kept continuously wet, over the fresh concrete immediately after striking and darby ing.

When ready for final finishing, uncover only a small section immediately ahead of the finishers. Cover again at once after final finish and keep the cover wet.

6. Cure Immediately

Start curing as soon as surfaces are hard enough to resist marring.

If curing compound is to be used, apply it immediately after final finishing. See that adequate and uniform coverage is obtained. In extremely hot weather, it is advisable to cover the slab with water for 12 hours before using curing compound.

Keep the concrete surface constantly wet to avoid alternate wetting and drying during the curing period..