447. Variations In Richness Of Mortar

In Table 148 several concretes made with mortars having different proportions of sand are compared, and the results of briquet tests on similar mortars are also given. Although the briquets were not broken at the same age as the bars, the tests on the latter at the different ages show that they were not gaining strength rapidly, and the results may therefore be compared without serious error.

Table 148. Transverse Tests Of Concrete. Variations In Richness Of Mortar

Notes: —

Portland, Brand R, Sample 82 M. Natural, Brand Gn, Sample 83 T. Sand, from " Point aux Pins" (river sand).

Stone, Potsdam sandstone, retained on 3/8 inch square mesh, and no pieces larger than 3 inches in one dimension. Amount mortar used in each case equal to voids in stone measured loose, except in case 1-2 natural, when mortar exceeded voids by seven per cent.

The fracture showed concrete very compact in nearly all cases.

The results obtained with natural cement show that the tensile strength of the mortar in pounds per square inch was greater than the modulus of rupture obtained for the concrete.

This is also the case with rich mortars of Portland cement, but for Portland mortars containing more than three parts sand to one of cement the concrete gives the higher result. The strength of the concrete with one-to-four mortar is fifty-five per cent, of the strength with one-to-one mortar for Portland, and forty-five per cent, for natural. The decrease in strength due to larger proportions of sand in the mortar is usually greater than the decrease in Cost.

Table 149. Transverse Tests Of Concrete. Variations In Quantity Of Mortar

Notes: — Cement, Portland, Brand R, Sample 64 T. Sand, " Point aux Pins," three parts by weight dry to one cement. Stone, Drummond Island limestone, passing 1 inch slits and retained on | inch slits.

448. Variations In Amount Of Mortar Used

Bars 37 to 43, Table 149, were all made with the same kind and quality of stone and the same quality of mortar, three parts sand to one cement by weight, but the amount of mortar varied; thus, in bars 41 and 38 sufficient mortar was used to fill the voids in the stone, while the bars above were deficient in mortar, and those below contained an excess. It is seen that the highest result is given by the bars in which the mortar was just sufficient to fill the voids in the stone, though the bars containing an excess of mortar gave practically the same result, while a deficiency of mortar resulted in decreased strength.

449. Variations In The Amount Of Sand For Fixed Quantities Of Cement And Stone

In Table 150, bars 68 to 75 were all made with the same kind and quantity of cement and stone, but the amount of sand, and consequently the quantity and quality of the mortar, varied. The highest strength is given by the concrete in which the weight of the sand was three times the weight of the cement; this quantity of sand gave sufficient mortar to fill the voids in the stone. The richer mortars, though stronger, were deficient in quantity, while four parts sand made an excess of mortar having a lower strength.

Table 150. Transverse Tests Of Concrete. Variations In Quantity Of Sand For Fixed Quantities Of Cement And Stone

Notes: — Cement, Portland, Brand R, Sample 768. Sand, " Point aux Pins".

Stone, Potsdam sandstone, screened with 3/8 inch mesh, and all pieces larger than 3 inches in one dimension rejected. Appearance of fracture: a, very porous; 6, many voids; c, some voids; d, few voids.

450. Consistency Of Concrete

The bars, the results of which are given in Table 151, were made to show the effect of the consistency of the concrete on the strength obtained. It is seen that the highest strength is given when the consistency is such that a little moisture is shown when ramming is completed; the decrease in strength from an excess of water is much less than that caused by a corresponding deficiency. The results of briquet tests on similar mortar are also given in the table, and it appears that the highest result is given by the mortar containing the least water, which shows the familiar fact that the mortar for concrete should be more moist than that which gives the best results in briquet tests.

451. Value Of Thorough Mixing

Bars 182 to 189, Table 152, were made to show the effect of thorough mixing of the concrete. Comparing the concrete turned once or twice, and back, with that turned three or four times, and back, it is seen that the mean strength of twelve tests with the former is 328 pounds per square inch, while the mean strength of the same number of tests with the more thoroughly mixed concrete is 388 pounds per square inch, an increase of eighteen per cent.

Table 151. Transverse Tests Of Concrete. Variations In Consistency

Notes: —Portland cement, Brand R, Sample M.

Natural cement, Brand Gn, Sample 88 T. Sand, " Point aux Pins" (river sand). Stone, Potsdam sandstone, 7 cubic feet to each batch. Results in last column give tensile strength at one year of briquets made from similar mortar. Consistency: — a, very dry; no moisture shown on ramming.

6, slight moisture appeared at surface after continued ramming.

c, quaked somewhat.

d, quaked and water rose to surface in ramming.

e, too wet to ram.

Table 152. Transverse Tests Of Concrete Bars. Value Of Thorough Mixing

Notes: —Cement, Portland, Brand X, 200 lbs. Sand, " Point aux Pins," 600 lbs. Stone, Potsdam sandstone, 15 cubic feet.

Table 153. Transverse Tests Of Concrete. Variation In Size Of Aggregate

Notes: — All mortar, three parts sand to one part Portland cement by weight. Quantity of mortar about one-third volume of compact stone. Stone: — a = Potsdam sandstone; d = gravel. size: K = 1/10 inch to 1/4 inch.

V = 1/4 " 1/2 "

F = 1/2 " 1 "

M = 1 " 2 "