256. Conditions Affecting Voids

The voids present in a given mass of sand will depend upon the shape of the grains, the degree of uniformity in size of grains, the amount of moisture present, and the amount of compacting to which the mass has been subjected. If all of the grains in a given mass of sand are of uniform size, the percentage of voids will be independent of what that size may be. In other words, the percentage of voids in a cubic foot of buckshot will be the same as in a cubic foot of bird shot; but if we take a cubic foot of a mixture of buck and bird shot we will find that the voids are much less.

257. Effect Of Shape Of Grain

M. Feret has published in France the results of a large number of experiments made by him as to the voids in sand and broken stone.1 Table 53 gives the results he obtained concerning the effect of the shape of the grains on the percentage of voids present. He first divided each sand into three parts by means of three sieves, which we will call A, B and C. Sieve A had four meshes per sq. cm. (about five meshes per linear inch), sieve B had 36 meshes per sq. cm. (about fifteen meshes per linear inch), and sieve C had 324 meshes per sq. cm. (about forty-five meshes per linear inch). The grains that passed A and were retained on B were designated G, the grains that passed B and were retained on C were designated M, and the grains that passed C were designated F. These different sizes were then recombined by taking five parts of G, three parts of M and two parts of F, and the resulting sand was designated G5 M3 F2. Thus, all of the sands tested had the same "granulometric" composition.

1 Abstracted in Engineering News, Vol. XXVII, p. 310.

Table 53. Voids In Sands Having Different Shaped Grains From M. Feret

It is seen that the rounded grains have the smallest percentage of voids, or about thirty-six per cent, unshaken, while the laminated grains gave the largest percentage. It may also be noticed that the angular grains were compacted more by shaking than any of the others.

258. Effect Of Granulometric Composition Of Sand On The Percentage Of Voids

To determine the effect of uniformity of size of grain upon the percentage of voids and the strength of mortars, the author has experimented with an artificial sand formed by crushing limestone. That portion of the product that passed the coarse screen of the crusher varied in fineness from particles three-eighths of an inch in one dimension to a very fine powder, the particles of which were less than .0065 inch in one dimension. Such material admits of division into parts that differ widely in fineness, but which are essentially of the same composition, and it is therefore excellent for an experiment of this kind.

The four sieves used in first separating the material into parts had, respectively, 10, 20, 40 and 80 meshes per linear inch, the sizes of the holes being, respectively, about as follows: 0.08 inch, 0.033 inch, 0.017 inch, and 0.007 inch square. The several sizes of grain are designated as follows: —

"C," Coarse, passing No. 10, retained on No. 20. "M," Medium, " " 20, " " 40.

"F," Fine, " " 40, " " 80.

"V," Very fine, " " 80.

M. Feret's method of designating the granulometric composition, namely, to represent by exponents the number of parts of each size of grain, has been adopted.

259. The voids were obtained by first weighing a given volume of the sand; dividing the weight by the specific gravity of the limestone, as previously determined, gives the amount of solid material in the measure, and this subtracted from the volume of the measure, gives the voids. This method is considered more nearly accurate than the usual one of measuring the amount of water required to fill the voids in a measure of sand, especially so for a sand of uniform character and one which absorbs water quite freely.

Table 54. Voids In Limestone Screenings, Showing Effect Of Variations In Granulometric Composition

The results obtained are given in Table 54. Comparing the voids in C, M, F and V, it is seen that the first three have nearly the same percentage, but V has less voids than the others. This is explained by the fact that this sample was made up of all sizes smaller than the holes in No. 80 sieve, down to the fine powder. Comparing the mixed sands, it is seen that the sample made up of equal parts of coarse and very fine had the least voids, the percentage being only a little more than half of that obtained with coarse particles alone. The next lowest percentage was given by the sample having equal parts of four sizes.

It is apparent that the granulometric composition has a very important effect on the percentage of voids. When one desires to make a compact mortar with as small a quantity of cement as possible, similar tests might well be made with the materials available for use.

260. Effect On Strength Of Mortars Of Varying The Granulometric Composition Of Sand

Table 55 gives the results of tensile tests of mortars made with limestone screenings of various granulometric compositions. The differences in strength are not very great, but it appears that with one-to-three mortars the highest strength is developed at six months, with the coarse grains alone, but when poorer mortars are in question the result is affected by the percentage of voids in the sand.

Table 55. Limestone Screenings With Portland Cement. Effect On Tensile Strength Of Variations In Granulometric Composition Of Sand

Cement

Portland, Brand R. For significance of composition of sand, see text.

261. Table 56 gives the results of similar tests of both Portland and natural cement with Point aux Pins sand dredged from St. Marys River and containing a very large percentage of quartz grains. The sand was divided into but three parts by sifting, and was then remixed, the proportion of each size being indicated in the table. The results verify the conclusions already drawn that the coarser sands give the higher strength. It appears that not more than one-half of the grains should be very fine if the best results are desired.