In conclusion, then, we may say that good sand may consist of grains of almost any moderately hard rock that is not liable to future alteration in the work. The grains may be of any shape, but preferably should be sharp and angular or lenticular in form, not rounded and smooth. The sand should not contain such impurities as loam or humus, but for most purposes a small percentage of clay or fine rock dust is not objectionable. Clay should not, however, be permitted in sand for use in sea water.
Coarse grained sands are better than fine grained ones, but a mixture of fine and coarse is excellent, especially where but a small amount of cement is used, because such a mixture contains less voids and will make a less permeable mortar, while giving a good strength. As might be expected, the deleterious effect of poor sand is more apparent the larger the dose of sand used.
It is evident from what has preceded that the weight of sand per cubic foot will vary greatly, not only with the character of the rock from which it came, but also with its physical condition. Natural sand, as it ordinarily occurs, will weigh about as follows, according to its condition: —
Moist and loose......... 70 to 90 pounds per cu. ft.
Moist and shaken......... 75 to 100 " "
Dry and loose.......... 75 to 105 "
Dry and shaken......... 90 to 125 " "
When settled in water, weight of wet sand, voids full........100 to 140 " "
If the rock from which the sand is made weighs, say, one hundred sixty pounds per cubic foot solid (specific gravity, 2.56), then the sand will weigh per cubic foot 120, 100, and 80 pounds, for voids of 25, 37.5 and 50 per cent., respectively.
The cost of sand will, of course, vary with the locality. In exceptional cases where it is found directly at the works, it may not Cost more than twenty to thirty cents per cubic yard to deliver it on the mixing platform. If it has to be pumped from the bed of a river or lake and can be conveyed to the work in scows with a tow of not more than ten miles, it may be delivered at the work for from forty to sixty cents per cubic yard. If it must be hauled in wagons for some distance, it may Cost from fifty cents to one dollar per yard; and again, if sand is so difficult to obtain that it must be made by crushing rock, it may Cost from one dollar to three dollars per yard. Usually from sixty cents to a dollar is a fair price for sand. Several examples of cost of sand will be given in connection with the subject of cost of concrete.