In engineering literature but few definite specifications for the cleanness of sand can be found, a diligent search revealing only the following: For bridge work on the New York Central and Hudson River R. R., the specifications require that the sand shall be so clean as not to soil white paper when rubbed on it: For the retaining walls on the Chicago Sanitary Canal, the suspended matter when shaken with water was limited to 0.5 per cent. For the dam on the Monongahela River, built under the direction of the U. S. A. engi neers, the suspended matter was limited to 1 per cent. For the dam at Portage, N. Y., built by the State Engineer, the "aggregate of the impurities" was limited to 5 to 8 per cent. The contamination permissible in any particular case depends upon the cleanness of the sand available and upon the difficulty of obtaining perfectly clean sand. Sand employed in masonry construction frequently contains 5, and sometimes 10; per cent of suspended matter.
Under no consideration should the sand contain any leaves, straw, paper, shavings, chips, etc.
Effect of Clay. The effect of clay in the sand varies with the richness of the mortar, i.e., with the proportion of cement. The strength and density of neat cements and of mortars containing 1 or 2 parts of sand, are decreased by even slight additions of clay; but the strength, and also the density, of mortars containing three or four parts of sand are usually increased by the addition of 10 to 20 per cent of finely pulverized clay, and still leaner mortars are improved by even larger percentages of clay. The clay, if finely pulverized, helps to fill the voids of the sand and causes the cement ing material to coat the grains better and bind them together more strongly. The exact effect of the clay depends chiefly upon the fineness of the sand grains and upon the percentages of the voids (§ 193) in the clean sand; but depends also upon the thoroughness of mixing and the amount of water used, for if the clay forms a coating on the sand grains and is not removed in the mixing, a small amount of clay is very deleterious.
Lean mortars containing clay to a considerable per cent of the cement are more plastic and work better under the trowel than similar mortars made of clean sand; and clay is sometimes added to produce this effect. The presence of the clay retards the setting of the cement—natural usually more than portland—and makes the mortar more susceptible to the action of frost.
Washing Sand. Sand is sometimes washed. This may be done by placing it on a wire screen and playing upon it with a hose, or by placing it in an inclined revolving cylindrical screen and drenching it with water. When only comparatively small quan tities of clean sand are required, it can be washed by shoveling it into the upper end of an inclined V-shaped trough and playing upon it with a hose, the clay and lighter organic matter floating away and leaving the clean sand in the lower portion of the trough, from which it can be drawn off by removing plugs in the sides of the trough. Sand can be washed fairly clean by this method at an expense of about 10 cents per cubic yard exclusive of the cost of the water. For a sketch and description of an elaborate machine for washing sand by paddles revolving in a box, see Engineering News, Vol. xli, page 111 (Feb. 16, 1899). By this method the cost of thoroughly washing dirty sand is about 15 cents per cubic yard.
Washing may or may not improve the mortar-making qualities of a sand. The washing may carry away only the finer particles of the impurities, and thereby increase the strength of the mortar; or the washing may remove all the impurities and also the finer sand grains, and thereby increase the per cent of voids and hence weaken the mortar. Before deciding to wash any partic ular sand, a test should be made of the effect upon the strength of the mortar of any particular method of washing.