Dry Method.—A more accurate method of determining voids is to compare the weight of a measured volume of the sand with the weight of an equal volume of the solid material of which the sand is composed. In measuring the volume of sand, it is necessary to use care to secure the proper degree of compactness. For ordi nary comparisons the sand should be well compacted by shaking and jarring the measure. The weight of the solid rock is obtained by multiplying the weight of an equal volume of water by the specific gravity of the sand. The difference between the weight of the rock and that of the sand divided by the weight of the rock and multiplied by 100 is the percentage of voids.
If R is the weight of the solid rock and S, the weight of the sand, percentage of voids is This test supposes the sand to be dry. When it is desired to obtain the voids in moist sand, a weighed sample of the sand should be dried at 212° F. and the loss of weight determined. The weight of moisture in the measure of sand to be used in the test may then be computed. This weight is then to be subtracted from the total weight of the moist sand to find the weight of solid material in the sand.
If m is the weight of moisture in the volume of sand under test, percentage of voids is 27. Specific Gravity.—The specific gravity of siliceous sand is quite uniformly 2.65, or the weight per cubic foot of the solid rock is 165 pounds. To assume these values in determining the voids in such sand involves slight error in any case. Sands not strictly siliceous may vary in specific gravity from about 2.1i to 2.7.
The determination of specific gravity is made by immersing a sample of the material in water at lid° F. and dividing the weight of the sand by the weight of water displaced. This is most con veniently done by sifting the sand into the water in a graduated glass tube, and reading the increase of of the liquid in the tube. Care must be used to introduce the sand slowly so as to eliminate all air bubbles.
28. Density Test.—Comparative tests of sands may be made by determining the volume of mortar produced by definite weights of cement and dry sand. The sand that for a given weight of materi als. when mixed with the same proportion of cement to the required consistency, produces the smallest volume of mortar gives the most dense mortar. In making this test, molds in which the height is large in comparison with the section are convenient, the relative heights to which the mold is filled giving the proportionate volumes.
The volume of mortar after setting is what is required, but the measurement before setting, unless the mortar is quite wet, will give practically the same result.
Determination of Density.—The term density, as commonly applied to mortar, means the ratio of the volume of solid materials contained in the mortar to the whole volume of mortar. The density is obtained by weighing the ingredients before mixing and calculating their solid volumes from these weights and their specific gravities. The weight and volume of the resulting mortar are then measured. The weight of mortar should equal the sum of the weights of the several ingredients. The density equals the sum of the solid vol umes of sand and cement divided by the measured volume of the mortar. The density of mortars made from the sands shown in Fig. 1, one part cement to three parts sand by volume, are as follows: The method of computation is as follows: Taking specific gravity of cement as 3.1 and specific gravity of sand as 2.65, 29. Strength Tests.—Tests of the strength of mortars made from sands are the most conclusive evidence of the mortar-making properties of the sands. These tests to be of real value should extend over a period of at least twenty-eight days. They are made in the same manner as the mortar tests for judging cement, and comparisons are sometimes made with the results of tests with standard sand. Table III gives comparative results of tests of the sands shown in Fig. 1.
30. Washing Test.—'When it is necessary to examine sand for organic impurities, the silt, may be removed from the sand by washing. This is done by shaking a sample of the sand in a bottle with water, letting it settle for a few seconds, and then pouring off the turbid water. This is done repeatedly until the suspended matter is all removed. The wash water is then evaporated, and the amount of silt determined.
The silt is ignited in a platinum crucible and the loss on ignition is the percentage of organic matter present.
A very small amount, not more than 1 per cent, of organic matter may be a serious detriment, sand containing such impurities should be carefully tested and may need to be washed in order to give satis factory results ill use.