433. For such purposes as floors for buildings, cinders are used in concrete to a considerable extent on account of their light weight. Cinder concrete weighs only from two-thirds to three-fourths as much as broken stone or gravel concrete. The strength, however, is correspondingly less, and whether for a given strength a floor may be made lighter by the use of cinders will depend upon the conditions of use and the character of the reinforcement.

Table 137 gives the results of the tests of eight-inch cylinders, fifteen inches high, made by Mr. George Hill.1 In these cylinders, cinders, broken stone, and gravel were used as aggregates. The character of the materials is shown in the foot-note of the -table. As the specimens were but one month old when tested, the results are low, but since in the construction of floor arches the centers are usually removed in less than one month, the strength developed in a short time has a special interest.

1 Trans. Am. Soc. C. E., Vol. xxxix, p. 632.

Table 137. Compressive Strength Of Concrete About One Month Old

Tests of Cylinders, Eight Inches Diameter, Fifteen Inches High.

Notes: —

Cement, American Portland, tensile strength 624 lbs. per sq. in., neat, seven days.

Slag cement, a little less than 400 lbs. per sq. in., neat, seven days. Sand, clean, sharp, bank sand of mixed sizes, from moderately fine up to some pebbles size of bean. Cinders, ordinary steam, dust to 3/4 inch size. Stone, broken trap, nearly uniform size passing 1 1/2 inch ring. Gravel, clean, washed, 1/2 in. to 1 1/2 in.

Abstract of tests by Mr. George Hill, M. Am. Soc. C. E., Vol. xxxix, p. 632.

It is evident that cinder concrete should not be loaded very heavily within a month after made. The gravel gives a better result than broken stone.

434. In Table 138 are given the results of some tests of twelve-inch cubes of cinder concrete made at the Watertown Arsenal for the Eastern Expanded Metal Companies. Steam cinders were used, practically as they came from the furnace, only the larger clinkers being broken. Two proportions were used and the specimens were broken at one month and three months. It is seen that the one-one-three mixture is about twice as strong as the one-two-five with all brands. The variations between the several brands are also very great.

Table 138. Crushing Strength Of Cinder Concrete. Portland Cement

Tests of Twelve-inch Cubes at Watertown Arsenal.

Notes: — Concretes mixed rather dry, 10 to 12 1/2 pounds of water per cubic foot of concrete.

Mixture "A," one part cement, one part sand, three parts cinders. Mixture "B," one part cement, two parts sand, five parts cinders. Weight of concrete, 104 to 116 pounds per cubic foot.

Tests For Eastern Expanded Metal Companies

Data from "Tests of Metals," 1898.

435. Table 139 gives the results of other tests in the same series, using a single brand of cement and five mixtures, the richest containing three parts cinders and one part sand to one volume cement, and the poorest six parts cinders and three parts sand to one cement. The weight per cubic foot of the several concretes is also given.

Tests of cinder concrete prisms made by the late Prof. J. B. Johnson at Washington University 1 indicated that the mixture containing one part sand and three parts cinders to one volume cement gave the highest strength, or about twelve hundred pounds per square inch, at one month. The same mixture gave the highest values for the ratios of strength to Cost, and of strength to weight per cubic foot.

1 "Materials of Construction," p. 628.

Table 139. Crushing Strength Of Cinder Concrete. Various Proportions With Germania Portland Cement

Tests of Twelve-inch Cubes at Watertown Arsenal.

Note: —Tests for Eastern Expanded Metal Companies, "Tests of Metals," 1898.

436. Clay In Concrete

The effect of clay on the tensile strength of mortars has already been shown (Art. 49). Aggregates available for concrete frequently contain a certain amount of clay, and the question arises whether such aggregate must be washed, or whether certain small percentages may be permitted in the concrete, using, perhaps, a trifle richer mortar. The results in Table 140 were made to determine the effect of clay on the crushing strength of concrete.1

The test specimens were six-inch cubes, and were broken when one week to twelve weeks old in an Olsen machine. The proportions were two parts sand and six parts gravel by weight to one of Portland cement, or two parts sand and four parts gravel by weight to one of natural cement. The clay is apparently expressed as the per cent, of total aggregates. It is seen that while six or twelve per cent, clay retards the hardening of both Portland and natural cement concrete, the strength of the Portland concrete after four weeks is increased by six per cent, clay, while at the same age the strength of the natural cement concrete is not greatly affected. The ramming of concrete is facilitated by the presence of a small amount of clay, but larger amounts may render the mass sticky and difficult to ram.

1 Tests by Messrs. J. J Richey and B. H. Prater, Technograph, 1902-03.

Table 140. Effect Of Clay On Crushing Strength Of Concrete. Six-Inch Cubes