Table 56. Varying The Granulometric Composition Of River Sand. Effect On Value Of, For Use In Cement Mortar

Note. — River sand, mostly quartz, obtained at Point aux Pins. Each result mean of five briquets, all made by one molder.

262. Effect Of Moisture

The effect of a small amount of moisture on the bulk of a given weight of sand is not usually appreciated, but it may easily be shown that it is very marked. The results in Table 57 were obtained by adding small amounts of water to a given bulk of dry sand. Each time, after the water was added, the sand was stirred up and the weight of a given volume of the moist sand was obtained. It appears that the finer sands are affected more than coarse ones.

In the case of the limestone screenings 40-80, if we add but 3.7 per cent, water to a given quantity of dry sand, the bulk of the sand is so increased that if we take 1,000 c.c. of the, moist sand it will contain but 720 c.c. of dry sand. The voids are, of course, correspondingly increased from 54.5 per cent, to 67.2 per cent.

The cause of this increase in bulk is that each grain of sand is surrounded by a film of water which prevents the grains from lying close together after they have been disturbed. A large amount of air is also imprisoned in the mass. It may be noticed that the difference in bulk between moist and dry sand is greater when the measurements are made " loose".

Table 57. Volume Of Sand And Voids As Affected By The Addition Of Water

* Not jarred down in measure as much as usual. water rose to surface, † Sand crumbled like damp earth.

Note. —10-20 = passing No. 10 sieve (holes about .08 in. sq.) and retained on No. 20 sieve.

263. This subject is of great importance in proportioning mortars, because, in construction, the amounts of cement and sand are usually measured. Suppose it is desired to use a mixture of one hundred pounds of cement to four hundred pounds of sand, and for convenience we will suppose the packed cement and dry sand each weigh one hundred pounds per cubic foot. If now we use damp sand, containing about 3.5 per cent, water, instead of dry sand, and measure the materials, we would have four cubic feet of damp sand to one cubic foot of cement; but damp sand would contain only about 4 X 75 = 300 pounds of dry sand, and we would really have a one-to-three mixture instead of a one-to-four.