421. With the increasing use of concrete in arch bridges, in foundation piers and in columns of buildings, and especially in connection with steel in beams, etc., the compressive strength of the material becomes of the greatest importance. Moreover, the composition of concrete may vary so much, the range of available aggregates is so wide, and the methods of manipulation are so diverse, that many tests must be studied before one can judge of the probable strength of a given mixture.

For any very extended work, it may be found economical to make a series of tests using the materials available, and combining them as nearly as possible in the manner proposed in actual construction. This practice has been followed in several important works, and the data thus accumulated have added much to our hitherto somewhat vague notions of the probable strength of different mixtures under varying conditions of use. It is possible here to abstract but a few of the more reliable and complete tests of this kind, selecting those which indicate the value of certain special kinds of aggregate or the effect of certain variations in manipulation.

422. In connection with the design of the Boston Elevated R. R., Mr. George A. Kimball, Chief Engineer, prepared a series of concrete cubes of mixtures usually employed in practice, and with the materials available for the work in hand, and these cubes were tested at the Watertown Arsenal in 1899. A por tion of the results of these experiments are given in Table 130, where the details concerning character of the materials and the preparation of the specimens are shown. As each result in the table is the mean of at least twenty specimens, the irregularities frequently appearing in compressive tests have been largely eliminated, and the results are worthy of much confidence.

Table 130. Compressive Strength Of Concrete

Tests of 12 Inch Concrete Cubes for Boston Elevated Railroad.

Notes: — Materials: Cement, mean results with four brands Portland, two American, two German.

Sand, coarse, clean, sharp, voids 33 per cent, loose.

Broken stone, conglomerate passing 2 1/2 inch ring, voids 49 1/2 per cent, loose.

Mixing

Sand and cement turned twice, mortar and stone turned twice. Storage: — Cubes removed from molds three or four days after made and buried in wet ground until about a week before testing. Each result, mean of twenty or more tests.

Tests made at Watertown Arsenal, for George A. Kimball, Chief Engineer, Boston Elevated R.R. "Tests of Metals," 1899.

At the time of making these tests some cubes were crushed with a die having a smaller area than the face of the cube.

With a die 8 by 8 1/4 inches on one compression face, the area of the die being thus about .46 of the area of the cube face, the strength per square inch under the die was about twenty-five per cent, higher than when the entire face of the cube was pressed. This is in line with the behavior of all brittle substances under compression, as shown by Professor Bauschinger in testing sandstone specimens.

423. Tables 131 and 132 give a summary of a part of a very valuable series of tests of concrete cubes prepared by Mr. George W. Rafter and tested at the Watertown Arsenal for the State Engineer of New York.1

The results summarized in Table 131 are those obtained with four brands of Portland cement made in the State of New York, namely, Wayland, Genessee, Empire and Ironclad. Tests were also made with a sand-cement, and with one brand of natural, but these results are not included in the table. The aggregate was sandstone of the Portage group, broken by hand to pass a two inch ring.

The mortars used in making the cubes were of three degrees of consistency: (a) In the dryest blocks the mortar was only a little more moist than damp earth, and much ramming was required to flush water to the surface. (6) In another set the mortar was about the consistency of ordinary mason's mortar, (c) In the third set, the mortar was wet enough to quake like liver under moderate ramming.

424. The mortar was composed of one volume of loose cement to two, three or four volumes of loose sand. Other proportions were also employed, but in this table only those results are included in which the series of tests was complete as to variations in consistency and storage.

The voids in the stone were about forty-three per cent, when the measure was slightly shaken, and thirty-seven and a half per cent, when rammed without mortar. The amount of mortar used was made either thirty-three per cent, or forty per cent, of the volume of the loose stone.

Four methods of storage were used as follows: 1st, blocks immersed in water as soon as they were removed from the molds, and after three or four months they were buried in sand; 2d, blocks covered with burlap and wet frequently for several weeks, after which they were exposed to the weather; 3d, kept in a cool cellar from the time of fabrication until shipped for testing, and 4th, fully exposed to the weather throughout.

1 Report of State Engineer of New York, 1897.

Table 131. Compressive Strength Of Concrete

Mean Results with Four Brands Portland Cement, Illustrating Effects of Proportions, Consistency, and methods of Storage. Tests of Concrete Cubes, about Twenty Months Old, Made for State Engineer of New York.

425. In Table 131 each result is the mean of four cubes, one of each brand. The mean results are so arranged as to show the effects of variations in the amount, the richness, and the consistency of the mortar, and of the different methods of storage.

Taking up first the question of consistency, it appears from column "j" that the use of plastic mortar, marked "mason's," gave from 92 to 97 per cent, of the strength given by the dry mortar of about the consistency of "moist earth;" and that the "quaking" concrete gave from 89 to 95 per cent, of the strength of that marked " moist earth." From the three lines at the bottom of the table it is seen that in the poor concrete, one-to-four mortar, the wettest mortar gave nearly as good results as the dryest, while in the rich concrete, one-to-two mortar, the strength of the wet was but 89 per cent, of the dry. The explanation of this may be found in the fact that in the poor concrete the mortar was "brash," and the concrete did not ram well with a dry mortar, while the rich mortar was "fuller" and more plastic, so that the excess of water was not needed to make a compact mass.