Approximately, the voids in gravel, if free from sand, may be estimated at from 25 to 30 per cent, of the bulk, and in broken brick or stone at from 10 to 5'J per cent.; but if it is desired to obtain an accurate estimate of the voids in any sample of aggregate, fill some known measure with the material, then add water until the measure is filled; the quantity of water necessary for the purpose will be the amount of the voids. When dealing with porous materials, the water should be measured beforehand, and added to the aggregate quickly; subtracting the remainder from the original measurement of water will then indicate the extent of the voids. But in calculating the amount of sand and cement necessary to fill the voids, it must be borne in mind that Portland cement and sand both lose bulk when water is added to them, the former by about 10 percent, and the latter by about double this percentage. It will thus be seen that the resultaint cubical measurement of the materials indicated in the question will be only about that of the rough aggregate, namely, 5 yd.; and if the concrete is consolidated by ramming, there will be a further decrease in bulk by about 20 per cent., thus reducing the bulk to about 4 cub. yd.