426. Turning to the question of the amount of mortar, it is plainly shown that the concrete containing forty per cent, is but little better than that containing thirty-three per cent. This is in line with what has been said elsewhere, that an excess of mortar, as well as a deficiency, may be an actual detriment to the strength of the concrete. In this case the thirty-three per cent, mortar was not quite sufficient to fill the voids in the stone, and forty per cent, was a very slight excess.

Some interesting conclusions are indicated by the results in the line marked "ratios," near the bottom of the table. The ratios of the strength of the concrete containing thirty-three per cent, mortar to the strength of that containing forty per cent, are 91.6 per cent., 98.5 per cent, and 102.6 per cent., respectively, for one-to-two, one-to-three, and one-to-four mortars. That is, with a rich mortar forty per cent, may be used to advantage, but if the mortar is of poor quality, the strength of the concrete is not increased by an excess of mortar.

Finally, as to the strength developed under different conditions of storage, column " k " shows that for these cements the highest strengths are attained by immersing the concrete in water. In comparison, the strength developed by the concrete covered with wet burlap is 84 per cent.; in cool cellar, 82 per cent.; and in the open air fully exposed to the weather, 81 per cent.

427. The results given in Table 132 are the mean crushing strengths obtained in the same series of tests as described above, so arranged as to bring out the effect of the richness of the mortar. Although several brands were tested, only the results obtained with a single brand of Portland, namely, Millen's "Wayland," are included here, since the series was not completed with other brands. From similar tests with concretes containing one-to-two and one-to-three mortars only, it was found that three other brands of Portland gave from 91 to 102 per cent, of the strength obtained with the Wayland, and a brand of sand-cement gave 66 per cent.

Table 132. Compressive Strength Of Concrete. Effect Of Richness Of Mortar

Mean Results, Four methods of Storage.

Notes: —One brand Portland cement.

Aggregate, Portage sandstone, broken to pass two-inch ring. Age of cubes about twenty months. Each result, mean of four cubes.

Each result in the table is the mean of four cubes, each stored in a different manner. Tests with four brands (Table 131) where the concretes were made with one-to-two, one-to-three and one-to-four mortars, indicated that the percentages of the mean strength developed in the several methods of storage were as follows: If stored in water, the cubes developed 115 per cent, of the mean result; covered with burlap kept wet, the cubes developed 97 per cent.; stored in a cool cellar, 95 per cent.; and fully exposed to weather, 93 per cent, of the mean strength.

The mean results given at the bottom of the table represent each a mean of twenty-four cubes made with two different amounts of mortar, three degrees of consistency, and four methods of storage. By applying the percentages given above the probable corresponding result for any set of conditions may be obtained. The last line of the table shows the proportions that the strength of the concretes made with poorer mortars, bear to the strength obtained with one-to-one mortar.

428. Table 133 gives the results of a series of tests made by J. W. Sussex at the University of Illinois.1 The materials used were "Chicago AA Portland cement, sand containing a small percentage of fine gravel, and crushed limestone which would pass through a sieve with one-inch mesh." The proportions were three parts sand and six of broken stone to one volume of loose cement. The cubes were six inches on a side. The treatment during storage is not stated. The consistency of the concrete was as follows: "Dry," water 6.0 per cent., as moist as damp earth, no free water flushed to surface in ramming. "Medium," 7.8 per cent, water; water flushed to surface and concrete quaked only after being well rammed. "Wet," water 9.4 per cent., concrete quaked in handling and could be tamped but lightly.

Table 133. Compressive Strength Of Concrete. Relative Strength Of Dry, Medium, And Wet Mixtures

Notes: — Concrete composition:

Cement, Portland, one volume.

Sand, containing some fine gravel, three volumes.

Six volumes broken limestone passing one-inch mesh. Specimens, six-inch cubes. Results by J. W. Sussex, Univ. of III.

1 Technograph, 1902-03.

Each result in the table is the mean of three cubes. The concrete was tamped in layers about one inch thick with a rammer weighing 11 1/3 pounds and dropped six inches. Ten blows of the rammer constituted "light" tamping and twenty blows "hard" tamping. The results show that the "medium" concrete gains its strength more rapidly than the "wet," but that at one month the "wet" concrete has a higher strength than the dry, and that at three months the wet surpasses in strength both the dry and the medium.