443. Tensile, Transverse And Compressive Strengths Of Mortar Compared

The tests given in Tables 146 and 147 were designed to compare the strengths of cement mortars in tension, bending and compression, and to show the relative effect on the three kinds of strength of certain variations in manipulation.

The tensile specimens were briquets of the ordinary form, made in brass molds. The transverse and compressive specimens were made in wooden molds, the bars for transverse tests being two by two by eight inches and molded horizontally, while the specimens for compressive tests were two-inch cubes. Specimens of the three forms were made from the same batch of mortar to obviate, as far as possible, variations due to difference in gaging. Two cubes, two briquets and one bar were usually made from one gaging of mortar.

The briquets were broken in the usual manner on a Riehle cement testing machine. The bars were broken on a homemade lever machine. Two fixed knife edges were placed five and one-third inches apart, and the breaking stress was applied through a third knife edge at mid-span. The lengths of the lever arms of the testing machine were in the ratio of one to twenty-five, and water was allowed to run gently into a vessel at the end of the longer arm. The span of five and one-third inches was chosen because at this length the modulus of rupture, for a two inch square specimen, has the same numerical value as the center load applied.

The cubes were crushed in a crude machine, improvised for the purpose, consisting of two iron plates, two hydraulic jacks, with hydraulic weighing gage and proper framework. The upper plate was fastened to the base of the framework by means of two bolts which worked freely in the lower plate, and the latter was connected to the weighing gage at the top of the framework by two bolts which worked freely in the upper plate. An hydraulic jack was placed under either end of a yoke, at the middle of which was supported the weighing gage, While the tensile and transverse tests are doubtless good, the compressive tests are lacking in accuracy because of the crude method of crushing.

444. Table 146 shows the comparative tensile, transverse and compressive strengths of two samples of cement, one of Portland and one of natural, with different proportions of sand. It is seen that the modulus of rupture, or stress on the extreme fiber in transverse tests, computed by the ordinary formula, is considerably greater than the strength obtained in direct tensile tests. The ratio of the transverse to the tensile strength varies from 1.25 to 1.90 for Portland and from 0.95 to 2.19 for natural.

These tests indicate that the ratio of the compressive strength to the tensile strength diminishes with the addition of sand, but the reverse has been found to be true in other series of tests where the facilities for making compressive tests were better. The result obtained here may be attributed to the fact that richer mixtures gave cubes with smoother and more regular faces, and thus less subject to eccentric loading. The compressive strength increases between three months and one year much more than the tensile and transverse strengths. Tests on ten brands of Portland and ten brands of natural showed that in general the brands giving the highest strength in tension gave also the highest strength in transverse and compressive tests.

445. A few results to show the effect of consistency of the mortar on the three kinds of strength are given in Table 147. With Portland cement the highest strength in transverse and compressive tests is given by a wetter mortar than that giving the highest strength in tension, but with natural cement the compressive strength is lowered more than the tensile strength by an excess of water. All of the specimens were one year old when broken.

446. Transverse Tests Of Concrete Bars

The effect on the strength of concrete of variations in manipulation and treatment is most satisfactorily investigated by tests of large sized specimens either in compression or bending. In the preparation of such large specimens the conditions of actual construction may be closely reproduced, and the results, although likely to be quite irregular, as the strength of concrete in structures is not uniform throughout, are nevertheless very valuable On account of the expense connected with such tests, the number of specimens is usually so limited that the natural irregularities in strength mask the true conclusions.

Table 146. Comparative Tensile, Transverse, And Compressive Tests. Varying Proportions Of Sand

Note: —Cement P = Portland, Brand R ; N = Natural, Brand An; Sand, crushed quartz, passing No. 20. retained on

No. 30 sieve.

Table 147. Comparative Tensile, Transverse And Compressive Tests. Effect Of Varying Consistency Of Mortar

Notes : — Cement, P = Portland, Brand R; N = Natural, Brand In; Sand, "Point aux Pins," pass No. 10 sieve. Age of specimens, one year. Two parts sand to one cement by weight.

In Tables 148 to 156 are given some of the results obtained in testing over two hundred concrete bars at St. Marys Falls Canal. The molds for making the concrete bars were ten inches square by four and one-half feet long inside. The concrete was rammed into the mold with a light wooden rammer. The bars were, in general, covered with moist earth soon after completed, to await the time of breaking. To break them they were supported on knife edges placed four feet apart, and the load was applied at mid-span through an iron bolt laid across the bar. In the earlier tests a direct load was imposed by means of a platform which was gradually loaded with one-man stone, but in the later tests the load was applied by means of hydraulic jacks, an hydraulic gage being used to measure the force. In many cases the half bars were again broken at a later date with a twenty-inch span, as shown in the tables.