Itnpurities of an organic nature are always objectionable in sand for use in mortar. When it is necessary to use sand containing such impurities, it should be carefully washed and tested. A very small amount of vegetable matter in sand has sometimes caused the failure of mortar to harden properly.
Selection of differ so greatly in their qualities that it is difficult by mere inspection of the materials to judge of their relative values for use in mortal. In choosing sand for use in impor tant work, it is desirable not only to determine fully the physical characteristics of the available materials, but also to make actual tests of mortar by their use.
24. Tests for intended to determine the mortar making qualities of sand may be made in three ways: 1. Mechanical analysis of the sand, with determination of voids in the sand.
2. Density tests of mortars trade from the sand with the. cement to be used in the work.
3. Strength tests of mortars made from the sand in question with the cement to be used in the work.
The value of sand depends mainly upon its granulometric com position. The sand which, mixed with a given proportion of cement, gives the most dense mortar yields the strongest mortar. The sand which requires the least cement to make a mortar of maximum density is the most economical sand, when the mortar is properly proportioned.
The purpose in testing the sand should be to determine the pro portions of cement to sand necessary as well as to choose the best sand.
25. Mechanical Analysis.—To determine the relative sizes of grains cotnposing sand, the material is screened through a series of sieves of varying degrees of fineness. The sieves are made of standard size, 8 inches in diameter by 23 inches high, those with openings smaller than inch being made of woven brass wire, while the larger sizes are preferably drilled circular openings in sheet brass. These sieves are designated by numbers corresponding to the number of meshes to the linear inch, the size of opening depending upon the diameter of wire used. The size openings usually employed for sand analysis are approximately as follows: For ordinary examination of sand, when comparing or selecting sand for use, it is unnecessary to separate into so many sizes, and sieves Nos. 4, 10, 20, 50, and 100 are commonly employed. The sieves are made to fit together in nests with a cover and tight bottom to catch the residue from the finest sieves. The sifting may be done
by hand, by shaking and jarring the sieves, or mechanical shakers may be used. These may be obtained to work by hand or with small electric motors attached.
In making the tests, a sample weighing 50 g. is dried to constant weight at temperature not more than C. F.) and is then sifted through the sieves, so as to separate the grains into various sizes and determine the percentage of each by weight. The material properly classed as sand is that which passes through the No. 4 sieve and is retained on the No. 100 sieve. Sand retained by the No. 10 or No. 20 sieve may be classed a-s coarse sand; that caught between the No. 20 and No. 50 sieves is medium sand; that which passes the No. 50 sieve is fine sand. Material passing the No. 100 sieve is called dust.
Analysis Citrres.—Comparisons of the granulotnetric composi tions of sands are readily made by plotting the results of the sieve analysis as curves. It is usual to plot the sizes of openings as abscissa; and percentages passing each size as ordinates. The reciprocals of the numbers of the sieves may be used for size without impairing the value of the results, and probably represent more nearly the actual sizes of grains passing the sieves than does the computed width of opening. 'fable 1I gives the results of analyses of sands in common use for mortar, showing something of the variations which may frequently occur.
These results are plotted in Fig. 1. Sand No. 1 is a coarse bank sand containing a small amount of clay. No. 2 is a medium river sand of good quality. No. 3 is a fine sand. No. -t is screenings from broken limestone, containing rather high percentage of dust.
26. Determination of Voids.—The method most commonly used for void determination is known as the wet method, which consists in filling a measure with the sand to be tested and pouring in water until the voids are completely filled. The volume of water required to fill the voids divided by the volume of sand and multiplied by 100 is the percentage of voids; or the weight of water poured into the sand divided by the weight of water required to fill the measure and multiplied by 100 is the percentage of voids. It is very dif ficult to eliminate completely the air from the sand in making this test. The test is therefore liable to considerable error unless great. care be used in manipulating it.