Concrete Curing

A new use that is providing a profitable market for the retail lumber dealer ii in connection with the new method of curing concrete floors. This is exactly what has happened to a number of lumber dealers recently; architects are specifying the use of a reenforced waterproof paper for this purpose and the contractors naturally buy this paper from their lumber dealers.

"It is through curing that the internal structure of the concrete is built up to provide strength and watertightness. As concrete hardens, water continues to enter into combination with the cement; therefore, it is necessary to keep the surface of the hardening concrete moist. The longer this curing period can be extended, the harder and stronger will be the concrete. It is usually recommended that the curing be continued over a period of a week to ten days, but even longer curing will be found profitable in improving the desirable qualities of the concrete."

While most of the concrete used in building construction is placed without any special consideration for curing, it has become common practice for architects and.engineers on big concrete building jobs to specify.the curing of the concrete floors by covering with sand which is kept wet. This layer of moist sand over the surface of the concrete is designed to retard the evaporation of water from the concrete itself. The disadvantages are the cost of securing and placing this sand layer and the difiiculty of keeping it uniformly wet during the curing period. It also results in placing a great deal of dust in contact with the surface of the fresh concrete.

Approrimateliy 300,00 sq. ft. of Sisalkralt is being used f or curing and 'protecting concrete floors in Giannini Hall, Universit! of Colifornia, Berkele5t, Calif. The architect uas Williom C. Hoyes, San Francisco. Contractor P. l. Walker Compang, Son Francisco.

Experts in concrete work have known for many years that the strength and density of concrete is affected by the rate at which the material dries out after it has been poured. A simple statement of this scientific fact is the following abstracted from a recent booklet issued bv the Portland Cement Associati-on:

The development of Sisalkraft, a reenforced waterproof building paper, with strength sufficient to withstand the abuse to which it is subjected on boncrete floors, makes possible an improvement in this method of curing. By simply unrolling the paper over the concrete floor, a waterproof and airproof cover is provided which prevents the rapid escape of moisture from the concrete and protects the surface from construction dust of all kinds until the building is completed. The accompanying photograph shows a typical application of this paper for curing floors in big buildings.

While skyscrapers offer a wonderful market for this paper wherever they are being built, most dealers must look to the smaller construction jobs for their sales. The desirability of clean, hard, dustless concrete is not limited to skyscrapers. There is every reason for securing these

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