If concrete must be placed in freezing weather, it is better to keep the structure above the freezing point with fires than to lower the freezing point with salt. If the sand and rock are frozen, hot water may be poured upon them before mixing. Concrete may even be made with hot water, a safe limit being 150F. but hot cement sets quickly and is not so strong as a slow setting concrete made with cold water.

To make a concrete of maximum strength it is necessary to know the amount of air space or "voids" in the sand and rock for the spaces in the sand must be filled with cement and those in the rock with the cement and sand mortar. To find the amount of voids in the rock or sand weigh a bucketful and then the amount of water to cover when levelled off. Knowing the specific gravity of the material we may find the amount of voids by the following formula: W / (R/SG + W) where W is the weight of the water, R the weight of the rock or sand and SG its specific gravity. Another method, avoiding the necessity of knowing the specific gravity, may be devised as follows : Suppose we fill a bucket with water and weigh the water in the bucket, which will be the weight of the bucket and water less the weight of the bucket. Now fill the bucket with rock or sand; catch and weigh the water overflowing which will be the amount of water displaced by the rock. The amount of water remaining in the bucket will be that occupying the air spaces. To find the proportion of voids use this formula : (W - w) / W where W is the weight of the bucketful of water and w the weight of the water displaced.

To illustrate we will consider that the weight of the rock is 80 lb., the amount of water needed to cover this is 30 Ib. and the specific gravity of the rock 2.50. Using the first formula we have 30 / (80/2.5 + 30) = 15/31 or 48.3 per cent, voids.

The sand by the same process has shown 40 per cent, voids; therefore the concrete will require for each 100 parts of rock by volume, 48.3 parts of sand and 19.3 parts of cement, this being 40 per cent, of 48.3. These ingredients will represent a mixture of 1 cement : 2.5 sand and 5.1 rock or familiarly 1:2.5:5.1. This mixture will stand a compressive strain of about 2,000 lb. to the square inch.

To obtain maximum strength in concrete the voids in the rock and sand must be filled and likewise enough of the cementing material to completely envelop both rock and sand. Engineers usually allow from 5 to 15 per cent, for voids over that found by experiment to cover this condition. If we allow an increase of 5 per cent, to cover this cementing material, we then have a mixture containing 100 parts rock, 53.3 sand and 24.5 parts cement or a 1 : 2.2: 4.1 mixture. This will stand a compressive strain of 2,400 Ib. per square inch, or a tensile strain of 240 Ib. per square inch, taking the extreme ratio. If we consider the strains due to vibrations to be six times those due to tension, the compressive strains on our foundations should not be over 40 Ib. to the square inch. As we have but a fourth of this we may consider the mixture sufficiently strong for our purpose. As the base of the block covers more area than the top, the lower portion may be made of a leaner mixture either by increasing the proportion of rock and sand or by ramming in large pieces of rock as the foundation is built up.