Unless the character of the materials to be used is known, and ar'ess the qualities of the concrete made with certain proportions of the ingredients are known, this method should not be employed.
Proportioning by Voids. Since it has been proved by exper iment that the densest concrete is the strongest, and since it has been proved that cement paste is less dense than cement mortar and that cement mortar is less dense than a well-proportioned concrete, it follows that the densest and the strongest concrete that can be made with any proportion of cement and any combination of a particular sand and aggregate is that in which the cement paste fills the voids of the sand and the resulting mortar fills the voids in the coarse aggregate. Therefore, to determine the best proportions for any sand and aggregate, find the per cent of voids in the sand and in the stone, and use enough cement paste to fill the voids in the sand and enough mortar to fill the voids in the coarse aggregate.
The voids in the sand may be determined by either of the two methods discussed in § 194-95; and those in the stone by either method described in 4 214-15.
However, in using this method it should not be overlooked that the use of cement paste equal to the voids in the sand does not insure that the voids of the sand are filled, and that the use of mortar equal to the voids in the stone does not insure that the voids of the stone are filled. The cement paste surrounds the sand grains and virtually increases the size of all the grains and thereby increases the voids, since there are then no small grains to occupy the inter stices between the larger ones; and further, the water in the paste by its superficial tension keeps the sand grains apart and thus increases the per cent of voids. A similar effect occurs when mortar is mixed with the aggregate.
The increase in volume due to mixing cement paste with the sand is small in comparison with that due to mixing mortar with the aggregate, and hence will be neglected here. Besides, the deter mination of the best proportion of cement, i.e., of the strength or the mortar, to be used in any case is wholly a matter of judgment, and hence great refinement is inadmissible; and further, if a con crete of the best possible proportions is desired, either the third or fourth method of proportioning (§ 301 or 302-309) must be employed.
However, whatever grade of mortar is employed, it is always wise to use enough of it to fill the voids in the broken stone.
Table 27 gives the result of fifteen experiments to determine the amount of mortar required to fill the voids in broken stone. The mortar was moderately dry, and the concrete was quite dry, moisture flushing to the surface only after vigorous tamping. The broken stone was No. 10 of Table 20, page 99, and contained 28 per cent of voids when rammed.
Line 4 of Table 27 shows that if the mortar is equal to the voids, the volume of the rammed concrete is 7i per cent more than the volume of the rammed broken stone alone. Possibly part of the increase of volume was due to imperfect mixing, although it was believed that the mass was perfectly mixed. The table also shows that the voids in this concrete are equal to 7 per cent of its volume; in other words, even though the volume of the mortar is equal to the volume of the voids, the voids are not filled. Apparently the voids can be entirely filled with this grade of mortar only when the mortar is about 40 per cent in excess of the voids.
The increase in volume in Table 27 may be regarded as the maxi mum, since the mortar was quite dry and the stone unscreened. With moderately wet mortar and the same stone, the increase in volume was only about half that in the table; and with moist mortar and stone ranging between 2 inches and 1 inch, there was no appreciable increase of volume. With pebbles the increase is only about two thirds that with broken stone of the same size. With fine gravel (No. 18, page 99) the per cent of increase was considerably greater than in Table 27; with mortar equal to 150 per cent of the voids, it was possible to fill only about 5 to 7 per cent of the voids. The mortar used in Table 27 was 1 volume of cement to 2 volumes of sand, both measured loose; but with richer mortars the increase in volume was a little less, and with leaner mortars a little more.