SAND 73. Sand is nearly always a constituent part of mortar and concrete. The strength of the masonry is dependent to a consider able extent on the qualities of the sand, and it is therefore important that the desirable and the defective qualities should be understood.
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 dispropor tionate 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 ma terial 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 re peated 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 feld spar, 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 approxi mately 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 examina tion 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 re quirement 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 some times questioned, as experimenters have found that a small per centage 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 con crete 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 nec essary to use dirty sand, the strength of the concrete should be tested.