SELECTING A SAND. Natural sands differ greatly in fineness, in the per cent of voids, in cleanness, and consequently in their effect upon the strength and quality of the mortar in which they are used. Therefore, before commencing any considerable work all available natural sands and all possible substitutes should be exam ined to determind their value for use in mortar.
Before beginning the comparison of the different sands a sieve analysis of each should be made to determine whether or not the sand has a proper proportion of different sized grains. In the present condition of our knowledge we do not know the exact gradation of sizes which will give the best results; but sometimes a sieve analysis will show that a sand is unfit for mortar, and sometimes that the sand can be materially improved by screening out some portion of it or by adding either fine or coarse sand. As a check upon the conclusions drawn from the results of the fineness test, a determina tion should be made of the per cent of voids in each sand (§ 194-95). Further, an investigation should be made to see whether or not any particular sand would be improved by washing (§ 190).
After a preliminary examination of each sand as above, briquettes having the proportions of sand and cement to be used in the work should be made of each, and should be tested at different agea,—if time permits, at least at 1 week and 1 month. The test should be made with the particular cement to be used in the work, since the fineness of the cement affects the result differently with different sands. The mortar for the briquettes should preferably have the plasticity to be used in the work rather than the normal consistency prescribed for laboratory tests of cement (* 161), since the fineness and cleanness of different sands affect the plasticity of the mortar. Owing to lack of time it is sometimes impossible to wait for a complete test as above, in which case sands may be compared by determining which produces the smallest volume of mortar for the same quantities of dry materials. The cement and the dry sand should be weighed out in the proportions to be used in practice, and should be mixed to the consistency to be used in practice; then the mortar should be introduced into a cylinder, and the volume of the compacted mortar noted. Dry mortar will have the same volume after setting as when it was green, but wet mortar will contract in setting, owing to the expulsion of the surplus water; and therefore after the mortar has nearly set (but not too hard to be removed from the cylinder), the surplus water should be poured off and the volume of the mortar be again noted. If equal dry weights of two sands are
mixed with the same proportion of a cement, that sand is best which gives the least volume of mortar determined as above. The pro portion of cement and also the degree of plasticity used in making this test should be that to be employed in actual practice, since differences in the amount of cement or water will change the relative volume of mortar produced.
This method of comparing sands is more accurate than by measur ing the voids, for three reasons: 1. The cement paste coats the grains of the sand and increases the volume of the mortar, and this increase varies considerably with the fineness of the sand; and hence it is more accurate to compare sands by making mortars of them and comparing their densities than to compare sands by their voids. 2. The per cent of voids in a mass of sand varies with its compactness, and hence there may be considerable error in comparing different sands by measuring their voids. 3. A small amount of moisture in a mass of sand affects its weight much less than its volume; and hence it is more accurate to compare the sands by mixing •the mortars by weight and comparing their densities than to compare the sands by measuring the voids directly. Of course the sands could be dried before determining the voids; but that requires more labor, and does not remove the preceding objections to the method of comparing sands by measuring the voids directly. The method of comparing sands by the direct determination of the voids may be useful in reducing the number of sands to be tested by determining the relative density of the mortars.
By one of the preceding methods compare all available sands and screenings, and after determining the one giving the mortar of the greatest strength or of the greatest density, inquire into the relative cost of each. It may be economy to pay consider able for transportation of a better sand than to use an inferior local sand; or it may be more economical to use a local sand with an in creased proportion of cement than to bring in a better sand from a distance.* In this connection the possibility of using stone screen ings (§ 199) should not be overlooked.