73. Sand is nearly always a constituent part of mortar and concrete. The strength of the masonry is dependent to a considerable extent on the qualities of the sand, and it is therefore important that the desirable and the defective qualities should be understood.

Fig. 6. Cement Testing Machine.

Fig. 6. Cement Testing Machine.

74. Object

The chief object of the sand is economy. If the joints between stones, especially in rubble masonry, were filled with a paste of neat cement, the cost would be excessive, and the increase in the strength of the masonry, if any, would be utterly disproportionate to the great increase in cost. Secondly, the use of sand is a practical necessity in lime mortar, since neat lime will contract and crack very badly when it hardens.

75. Essential Qualities

The word "sand" as used above is intended as a generic term to apply to any finely divided material which will not injuriously affect the cement or lime, and which is not subject to disintegration or decay. Sand is almost the only material that is sufficiently cheap, which will fulfil these requirements, although stone screenings (the finest material coming from a stone crusher), powdered slag, and even coal dust have occasionally been used as substitutes. Specifications usually demand that the sand shall be "sharp, clean, and coarse," and such terms have been repeated so often that they are accepted as standard notwithstanding the frequent demonstration that modifications of these terms are not only desirable but also economical. These words also ignore other qualities which should be considered, especially when deciding between two or more different sources of sand supply.

76. Geological Character

Quartz sand is the most durable and unchangeable Sands which consist largely of grains of feldspar, mica, hornblende, etc., which will decompose upon prolonged exposure to the atmosphere, are less desirable than quartz, although, after being made up into the mortar, they are virtually protected against further decomposition.

77. Coarseness

A mixture of coarse and fine grains, with the coarse grains predominating, is found very satisfactory, as it makes a denser and stronger concrete with a less amount of cement than when coarse-grained sand is used with the same proportion of cement. The small grains of sand fill the voids caused by the coarse grains so that there is not so great a volume of voids to be filled by the cement. The sharpness of sand can be determined approximately by rubbing a few grains in the hand or by crushing it near the ear and noting if a grating sound is produced; but an examination through a small lens is better.

78. Sharpness

Experiments have shown that round grains of sand have less voids than angular ones, and that water-worn sands have from 3 per cent to 5 per cent less voids than corresponding sharp grains. In many parts of the country where it is impossible, except at a great expense, to obtain the sharp sand, the round grain is used with very good results. Laboratory tests made under conditions as nearly as possible identical, show that the rounded-grain sand gives as good results as the sharp sand. In consequence of such tests, the requirement that sand shall be sharp is now considered useless by many engineers, especially when it leads to additional cost.

79. Cleanness

In all specifications for concrete work, is found the clause: "The sand shall be clean." This requirement is sometimes questioned, as experimenters have found that a small percentage of clay or loam often gives better results than when clean sand is used. "Lean" mortar may be improved by a small percentage of clay or loam, or by using dirty sand, for the fine material increases the density. In rich mortars, this fine material is not needed, as the cement furnishes all the fine material necessary, and if clay or loam or dirty sand were used, it might prove detrimental. Whether it is really a benefit or not, depends chiefly upon the richness of the concrete and the coarseness of the sand. Some idea of the cleanliness of sand may be obtained by placing it in the palm of one hand and rubbing it with the fingers of the other. If the sand is dirty, it will badly discolor the palm of the hand. When it is found necessary to use dirty sand, the strength of the concrete should be tested.

Sand containing loam or earthy material is cleansed by washing with water, either in a machine specially designed for the purpose, or by agitating the sand with water in boxes provided with holes to permit the dirty water to flow away.

Very fine sand may be used alone, but it makes a weaker concrete than either coarse sand or coarse and fine sand mixed. A mortar consisting of very fine sand and cement will not be so dense as one of coarse sand and the same cement, although, when measured or weighed dry, both contain the same proportion of voids and solid matter. In a unit measure of fine sand, there are more grains than in a unit measure of coarse sand, and therefore more points of contact. More water is required in gauging a mixture of fine sand and cement than in a mixture of coarse sand and the same cement.

The water forms a film and separates the grains, thus producing a larger volume having less density.

The screenings of broken stone are sometimes used instead of sand. Tests frequently show a stronger concrete when screenings are used than when sand is used. This is perhaps due to the variable sizes of the screenings, which would have a less percentage of voids.

80. Percentage Of Voids

As before stated, a mortar is strongest when composed of fine and coarse grains mixed in such proportion that the percentage of voids shall be the least. The simplest method of comparing two sands is to weigh a certain gross volume of each, the sand having been thoroughly shaken down. Assuming that the stone itself of each kind of sand has the same density, then the heavier volume of sand will have the least percentage of voids. The actual percentage of voids in packed sand may be approximately determined by measuring the volume of water which can be added to a given volume of packed sand. If the water is poured into the sand, it is quite certain that air will remain in the voids in the sand, which will not be dislodged by the water, and the apparent volume of voids will be less than the actual. The precise determination involves the measurement of the specific gravity of the stone of which the sand is composed, and the percentage of moisture in the sand, all of which is done with elaborate precautions. Ordinarily such precise determinations are of little practical value, since the product of any one sandbank is quite variable. While it would be theoretically possible to mix fine and coarse sand, varying the ratios according to the varying coarseness of the grains as obtained from the sand-pit, it is quite probable that an over-refinement in this particular would cost more than the possible saving is worth. Ordinarily sand has from 28 to 40 per cent of voids. An experimental test of sand of various degrees of fineness, 12 1/2 per cent of it passing a No. 100 sieve, showed only 22 per cent of voids; but such a value is of only theoretical interest.