452. Variations In Size Of Stone And Volume Of Voids

The bars given in Table 153 were all made with mortar composed of three parts sand to one of Portland cement by weight. The stone for these bars was sorted into different sizes, and these were recombined in the proportions indicated in the table. The sizes are denoted as follows: that passing one-half inch mesh and retained on one-quarter inch mesh, is called V; one-half inch to one inch is called F; one inch to two inches, M; two inches to three inches, C; and coarse sand, one-tenth inch to one-quarter inch, is called K.

The first five bars were made with broken sandstone, and it is seen that the coarsest stone, size one inch to two inches, gave the lowest result. The size V, one-quarter inch to one-half inch, although containing no smaller percentage of voids, gave a much higher strength. The highest result was given by the bar made with a mixture of four sizes, the voids in this mixture being only thirty-six per cent.

The bars containing gravel as aggregate indicate that the strength decreases as the size of stone and volume of voids increase, but a mixture of three sizes gives nearly as good a result as the fine gravel alone. Comparing the results with similar sizes of the two kinds of aggregate, it appears that the broken sandstone gives somewhat better results than gravel, notwithstanding that the proportion of voids in the former exceeds that in the latter.

453. Sandstone And Bowlder Stone Compared

The results given in Table 154 are from a series of tests made for the Michigan Lake Superior Power Company by Mr. H. von Schon, Chief Engineer,1 and show the strength of concretes made with two kinds of aggregate available at Sault Ste. Marie. Two samples of Portland cement, one made from marl and one from limestone, a slag cement, and a natural cement, are used in these tests.

The two samples of Portland cement give nearly the same result, the slag less than half the strength, and the natural quite weak. The ratio of the strength obtained with crushed bowlders to that made with sandstone is about 1.6 with Portland, and the superiority of the former is shown with all cements.

454. Various Kinds Of Aggregate

Table 155 gives the results obtained at St. Marys Falls Canal in using various kinds of stone. In bars 25 to 30, three kinds of stone are compared. The superiority of the Kelleys Island Limestone " shavings" from the stone planers is evident. The shape of the pieces may have had a considerable influence on this result, the planer shavings being flat, or lenticular in form. Bar 34 was made with a hard limestone from Drummond Island, 33 with gravel, and 31 and 32 with -gravel and stone mixed in equal proportions The gravel and hard limestone gave about the same result, but it is seen that the mixture gave a higher strength. Bars 154 to 159 were made to test the value of broken brick for use in concrete. It is seen that the strength obtained with brick is considerably lower than that obtained with the soft limestone. Had a poorer mortar been used, the brick would doubtless have given a better comparative result, since with the one-to-two mortar, the brick are not strong enough in themselves to utilize the full adhesive strength of the mortar.

1 Tests reported by H. von Schon in Trans. A. S. C. E., Vol. xlii, p. 135.

Table 154. Transverse Strength Of Concrete With Crushed Sandstone And Bowlders

Notes: — Cross breaking tests of 6 in. by 6 in. by 24 in. bars made for Michigan, Lake Superior Power Co.

Materials

Cement, representative brands of each of four classes.

Sand, river sand, " Point aux Pins," mostly quartz, 96 1/2 lbs. per cubic foot. Voids, 41.7 per cent. Fineness, 96 per cent, passing No. 20 sieve, 39 per cent, passing No. 40 sieve. Stone, Sandstone, broken Potsdam 1 to 1 1/2 inch size.

Bowlder stone, broken gneiss and granite bowlders, 1 to 1 1/2 inch size. Proportion in mortar, 1 part cement to 2.4 parts sand by volume. mixing: —Consistency, plastic; cement and sand mixed dry, then wet and mixed; mortar added to wet aggregate and concrete mixed by hand. Storage: — Bars stored in shed, protected from rain, fully exposed to air.

Age of specimens when broken, sixty days. Mixture: — 1. Mortar 15 per cent, in excess of quantity required to fill voids.

2. Mortar 10 per cent, in excess of quantity required to fill voids.

3. Mortar 5 per cent, in excess of quantity required to fill voids.

4. Mortar just sufficient to fill voids in stone.

5. Mortar 15 per cent, in excess of amount required to fill voids in stone, but this 15 per cent, excess made with lime instead of cement.

Table 155. Transverse Tests Of Concrete. Value Of Different Kinds And Sizes Of Aggregate

Aggregate

a = Potsdam sandstone; size, 3/8 inch to 1 inch.

b = Drummond Island limestone; size 3/8 inch to 3 inches.

c = Kelleys Island limestone; shavings from stone planers; size, 1/4 inch to 3 inches.

d = Gravel.

e = Broken brick.

Table 156. Transverse Tests Of Concrete. Use Of Screenings With Broken Stone

Notes: — Cement: Natural, Brand Gn, Sample 92 T, 80 lbs. Stone: —a = Drummond Island limestone, screened.

b = 10 parts screenings to 100 parts stone.

c = 17 parts screenings to 100 parts stone.

d = 17 parts screenings to 100 parts stone. Screenings replacing equal amount sand. e = 50 parts screenings to 100 parts stone. f = 100 parts screenings to 100 parts stone. g = Screenings only, no broken stone.

455. Use Of Screenings With Broken Stone

Table 156 gives the results of a number of tests made to show the effect of mixing screenings with the broken stone. A smaller amount of mortar is required to fill the voids in a given volume of stone and screenings mixed than is required for the same volume of stone. It is seen that, with natural cement, when the same volume of mortar is used in the two cases, the presence of screenings to the amount of one-third of the total aggregate does not make a material change in the strength of the resulting concrete, but when the screenings are allowed to take the place of a part of the sand in the mortar, as in bars 116 and 117, a much stronger concrete results. Natural cement mixed with sand and screenings alone, bar 122, does not make a strong concrete, but Portland cement with screenings without sand was found to give excellent results.