Dients

Voids are usually referred to as a certain per centage of the whole volume. A gravel, for ex ample, with 35 per cent of voids, means that in a volume of the gravel occupying 100 cubic feet of space, 35 cubic feet consists of voids or empty spaces that have to be filled.

Having selected the available stone or gravel best adapted for the character of construction desired, any one of several different methods may be employed to determine the percentage of voids.

Voids in Coarse Aggregates.

A very simple method of determining voids in coarse aggre gates, described by Albert Moyer, is in substance as follows: Make a box that will contain 3 cubic feet (say, 1 by 14 by 2 feet, inside dimensions). Dry the aggregate by heating to 212 degrees F., the boiling point of water. If sand or screenings are present, they should be sifted out through a 1-inch mesh sieve, to be used as sand. Throw stone in box loose, a small quantity at a time, and level off top with straight-edge. Having first weighed the empty box, weigh the box when filled; deduct the weight of the empty box from the gross weight, and divide the net weight (which is the weight of the contents) by 3, which will give the actual weight of one cubic foot. Then simply use Table IV, which shows at a glance the per centage of voids in coarse aggregates of varying weight per cubic foot.

A gravel, for example, which on being well shaken clown weighs 110 lbs. per cubic foot, is shown by the table to contain 33 per cent of voids.

On account of the stone being measured loose, the voids shown in Table IV call for a little more of the fine material than is needed to just fill the actual voids in the concrete when tamped. They should, however, be used, as they make sufficient allowance to offset possible unevenness hi mixing.

Another method of determining the voids in coarse aggregates, is given by S. B. Newberry, substantially as follows: Use a metal box of exactly one cubic foot capacity. Fill with the material to be tested; shake down well; strike off level; and weigh. Now pour water in, at one corner, until it rises even with the surface; and weigh again. The difference in the weights will be the weight of the water that was added to fill the voids. Now, since one cubic foot of water weighs 62.5 lbs., to find the per centage of the volume occupied by the water that just filled the voids, we divide the weight of the latter by 62.5

and multiply by 100.

Suppose, for example, that the box of dry aggregate weighs 92 lbs., and, when the water is poured in, it weighs 120 lbs. Then 120-92=28 lbs., the weight of the water. The amount of the voids will therefore be (28-i-62.5) X100.44.8 per cent.

If the aggregate is dry and porous, it should be wetted before testing for voids, and then dried just enough to remove all moisture from the surface of the material. In this way the error that might occur through the absorption of water will be eliminated.

The object of pouring in the water at one point only, is to avoid entrapping bubbles of air.

Voids in gravel are seldom less than 30 per cent or more than 45 per cent. Broken stone, being angular, does not compact as readily as gravel, and hence has larger voids, though these will vary in amount, depending upon the charac ter of the stone, whether it is broken by hand or in a crusher, and whether run of crusher is taken or the material has been screened.

Table V., compiled by M. Feret, shows the percentage of voids to be found in round pebbles and broken stone of different sizes, and also in mixtures of different sizes of each of these ag gregates.

Voids in Sand or Screenings.

To determine the percentage of voids in sand or screenings, Mr. Newberry's method as described above for coarse aggregates, may be used with slight modi fication. Instead of pouring the water into the sand-filled vessel, the sand should be poured into a vessel containing water. If the water be poured on the sand, it will be impossible to get rid of all the air. The vessel should be filled not less than half-full of water, and the sand intro duced slowly. As most sands have less than 50 per cent of voids, the water will overflow the vessel before it is level full of sand. Keep on putting in sand, however, until the vessel is level full, being careful at the same time to have the overflowing water carry off as little as possible of the finer sand. When the vessel is level full of the water-soaked sand, strike off smoothly over the top, and weigh. The calculation of voids is then made in exactly the same manner as for coarse aggregates.