429. It has already been stated that the character of the aggregates is second only to the quality of the mortar in its effect on the strength of concrete. The materials available for aggregate in different localities are so varied that only a general idea of their relative values may be obtained from a limited number of tests.

The results given in Table 134 are from tests made at the Watertown Arsenal,1 and show the compressive strengths of concretes made with broken trap and gravel of different sizes. The concretes are all very rich, and the strengths correspondingly high, although the oldest specimens have hardened less than three months. The results are somewhat irregular, and the conclusion to be drawn concerning the best size for the aggregate is not very clearly brought out. The one-inch trap gives uniformly good results, as do the mixtures of two or more sizes. The trap rock gives a higher result than the gravel, the mortar being sufficient to fill the voids in the trap, and in excess for the gravel.

1 "Tests of Metals," 1898.

Table 134. Compressive Strengths Of Rich Concretes At Different Ages. Tests Of Twelve-Inch Cubes

Notes: —Tests made at Watertown Arsenal, "Tests of Metals," 1898.

All concretes composed of one cubic foot of Alpha Portland cement, weight 96 1/2 to 106 lbs. per cu. ft., one cu. ft. of bank sand, weight 93 1/2 to 104 lbs. per cu. ft., and 3 cu. ft. of aggregate, weighing from 93 to 105 lbs. per cu. ft.

The size of aggregate indicated gives the larger of the two screens used in separating it into different sizes; thus, "3/4 inch" means passing 3/4 inch mesh and retained on 1/2 inch mesh.

The compressive strength of twelve inch cubes of one-to-one mortar alone was 3,833 lbs. per sq. in. at seven days, and 4,800 lbs. per sq. in. at seventy-five days.

430. In 1896-97 Mr. A. W. Dow2 prepared a number of twelve-inch cubes of concrete for the Engineer Commissioner of the District of Columbia. These cubes are of interest as showing the strength of natural cement concrete as well as Portland, and the results are abstracted in Table 135.

1 Not fractured.

2 Report Operations, Engineer Department, District of Columbia, 1897. Also Baker's "Masonry Construction," p. 112 r.

Table 135. Compressive Strength Of Concrete

Tests of Twelve-inch Cubes for the Engineer Commissioner of the District of Columbia.

Notes: — Materials:

Cement, Portland, "Atlas" (American), 104 lbs. per cu. ft.; Natural, "Round Top," 70 lbs. per cu. ft. Sand, 15 per cent, retained on No. 8 mesh, 75 per cent, between 8 and 40 mesh, 10 per cent, passing 40 mesh. Sand was used damp, and weighed in that condition 90 lbs. per cu. ft. Stone, Bluestone, "Average," 93 percent, between 1/3 inch and 2 inches.

"Coarse," 89 per cent, between 1 1/2 inches and 2 1/2 inches. Gravel, "Average," 90 per cent, between 1/6 inch and 1 1/2 inches.

"Small," 90 per cent, between 1/8 inch and 3/4 inch. Granolithic, 92 per cent, between 1/10 inch and 1/3 inch. mixing, thorough by experienced man.

Tamping, light, in 4 inch layers, just sufficient to bring mortar to surface. Storage, cubes thoroughly wet twice a day. Age of specimens when broken, one year.

The concretes all contained two parts sand and six parts aggregate to one cement, but the character of the aggregate varied as shown. The natural cement concrete gave from one-quarter to one-third the strength of the Portland concrete. The best result seems to be given by the average size broken stone, which was in reality a mixture of various sizes, ninetythree per cent, of it being retained on a one-third inch mesh and passing a two-inch mesh. The mortar was probably insufficient to fill the voids in the stone for the first three cubes in the table, and under these conditions the gravel, with its smaller percentage of voids, makes a good showing. This illustrates what we have already said, that the relative value of broken stone and gravel for aggregate depends upon the proportion of mortar used.

1 Mixture of one part granolithic size to one of concrete stone.

Table 136. Compressive Strength Of Concrete. Portland Cement

Tests of Six-inch Cubes of Various Mixtures.

Note : — Results of Messrs. Ketchum and Honens.

431. The results in Table 136 were obtained by Messrs. R. B. Ketchum and F. W. Honens at the laboratory of the University of Illinois,3 and illustrate the rate of gain in strength -of several mixtures. The cement used was Savior's Portland, fine and of good quality. The sand and gravel were composed principally of silica, with 10 to 30 per cent, of limestone. About 60 per cent, of the sand passed a "number thirty" sieve. The unscreened gravel had about 42 per cent, caught on a " number five" sieve and eighteen per cent, of it passed a "number thirty." Except in one mixture, however, the gravel and broken stone were screened, and only that portion passing a two-inch ring and retained on a "number five" sieve was used. The stone was a magnesian limestone.

1 Unscreened.

2 Result irregular.

3 Technograph, 1897-98.

The concrete was mixed dry, so that considerable tamping was required to bring water to the surface. The cubes were first kept under a damp cloth for one day, immersed six days, and then stored in air in a room until broken. In crushing, "the direction of the force applied was parallel to the tamped surface".

432. Each result in the table is the mean of six specimens. Comparing number 2 with number 9 indicates that the strength obtained with one part cement to three parts unscreened gravel is much higher than with mortar of one part cement to three parts sand. Comparing 9 and 10 indicates that seven parts gravel and stone may be mixed with one-to-three mortar and give higher strength than the mortar alone. A comparison of 6, 7, and 8 shows that in case there is sufficient mortar to fill the voids in the aggregate, angular fragments give a somewhat higher strength than rounded ones, but that a mixture of broken stone and gravel is better than either alone. One of the most important points brought out by the tests is that the strength at seven days is 46 per cent., and at thirty days is 88 per cent., of the strength attained at three months.