In many cases it may he feasible and desirable to use materials of low grade in concrete work. Cinder concrete is preferred for some uses on account of its lightness, although it is low in strength. Local materials may be of poor quality, but usable by taking proper pre cautions and designing the work in accordance with the character of the concrete. Failures have sometimes resulted from the use of low-grade materials without investigation of their qualities. Many users of concrete have failed to recognize the importance of the quality of the aggregates and seem to have regarded any stone broken to proper size as good enough for concrete.
72. Tests for Coarse Aggregates.---There are at present no standard methods of making tests for concrete aggregates, or stand ard specifications for such materials. The methods usually employed in testing sand have been discussed in Art.. 7. A com mittee of the American Society for Testing Materials is making a study of concrete aggregates and of the methods of testing them, and it is hoped that this may result in a standard practice in snaking such tests, and in throwing light upon methods of proportioning and forming the concrete.
Mechanical Analysis.— To determine the relative quantities of various sizes of stone in aggregate, it is common to make a mechan ical analysis of the material. This consists in separating the various sizes by screening, and recording the amount retained upon each screen. The following has been adopted by the American Society for Testing Materials, upon recommendation of its Committee on Road Materials, as a standard method for making a mechanical analysis of broken stone or broken slag, except for aggregates used in cement concrete: The method shall consist of (I) drying at not over 110° C. (230° F.) to a con stant weight a sample weighing in pounds six times the diameter in inches of the largest holes required; (2) passing the sample through such of the following size screens having circular openings as are required or called for by the specifi cations, screens to he used in the order named: S.S9 (3 in.), 7.62 em. (3 in.), 6.35 em. (2 in.), 5.08 cm. (2 in.), 3.81 cm. (11 in.), 3.1S cm. (1; in.), 2.54 cm. (1 in.), 1.90 em. in.), 1.27 cm. (? in.), and 0.64 cm. (i in.); (3) determining the percentage by weight retained by each screen; and (4) recording the mechan ical analysis in the following manner: For materials in which sand is combined with the broken stone or broken slag, the scone method is employed together with the fine sieves used for sand (see Art. 7) and the results are recorded
in the same manner, beginning with the 200-mesh sieve.
Apparent Specific weight of a given volume of the solid material of which the aggregate is composed is often of impor tance in the determination of voids, or in proportioning concrete, a result obtained by determining the apparent specific gravity. The term apparent specific gravity as here used refers to the material as it exists, and includes the voids in the block of material tested; it may be somewhat less than the true specific gravity. For this purpose, the water to which it is referred need not be distilled, and determinations at ordinary air temperatures are sufficiently accurate.
The following method of determining apparent specific gravity of coarse aggregates has been adopted as standard by the American Society for Testing Materials.
The apparent specific gravity shall be determined in the follow ing manner: 1. The sample, weighing 1000 g. and composed of pieces approximately cubical or spherical in shape and retained on a screen having 1.27 cm. (1 in.) circular openings, shall be dried to constant weight at a temperature between 100 and 110° C. (212 and 230° F.), cooled, and weighed to the nearest 0.5 g. Record this weight as weight A. In the case of homogeneous material, the smallest particles in the sample may be retained on a screen having 11 in. cir cular openings.
2. Immerse the sample in water for twenty-four hours, surface-dry individual pieces with the aid of a towel or blotting paper, and weigh. Record this weight as weight B.
3. Place the sample in a wire basket of approximately . in. mesh, and about 12.7 cm. (5 in.) square and 10.3 cm. (4 in.) deep, suspend in water ' from center of scale pan, and weigh. Record the difference between this weight and the weight of the empty basket suspended in water as weight C. (Weight of saturated sample immersed in water.) 4. The apparent specific gravity shall he calculated by dividing the weight of the dry sample (A) by the difference between the weights of the saturated sample in air (B) and in water (C), as follows: 5. Attention is called to the distinction between apparent specific gravity and true specific gravity. Apparent specific gravity includes the voids in the specimen and is therefore always less than or equal to, but never greater than the true specific gravity of the material.