Requisites of Good Gravel

REQUISITES OF GOOD GRAVEL. To be suitable for use in ma king concrete, gravel should have the following characteristics: (1) it should be composed of durable minerals; (2) it should be at least reasonably clean; and (3) it should have such a variety of sizes as to give a small per cent of voids.

The Minerals.

Practically all that was said about the durability of sand (see 4 186) applies with equal force to gravel. Most gravels are sufficiently durable for use in concrete In some localities, particularly in the foot-hills of the Appalachian and the Ozark Mountains, a material, locally called gravel, composed of angular fragments of chert, is found in the stream-beds; but such material is unsuitable for concrete, since it is checked and is easily broken, and because its flat glassy faces are too smooth for good adhe sion of the cement.

Cleanness.

All that was said under this head concerning sand (see § 188-90) applies also to gravel. A quantity of finely divided clay equal to 10 or 20 per cent of the gravel does no harm, and may add to the strength of the concrete,—particularly if the cement paste does not entirely fill the voids. The greater the proportion of clay, the more thorough should be the mixing.

Maximum Size.

The larger the maximum size of pebbles the denser and stronger will be the concrete; but experience has shown that for plain concrete it is impracticable to use fragments larger than about 3 inches in diameter, and that for reinforced concrete the maximum size should not be more than about 1 inch.

Voids.

For two methods of determining the per cent of voids, see 194-95. The specific gravity of gravel is practically constant and equal to 2.65. All that was said about voids of sand (I. 196-98) applies with equal force to gravel.

Since the fragments are larger in gravel than in sand, the former may have, and usually does have, a smaller per cent of voids. The voids of a well-proportioned gravel passing a 2-inch screen usually range from 20 to 30 per cent, and occasionally are as low as 15 per cent. Gravel is sometimes washed to remove an excess of loam and clay, and not infrequently the washing removes also some fine sand, which needlessly increases the per cent of voids. The washing should be done in such a manner as to remove no sand, and as a rule only part of the clay.

Coarse gravel is sometimes run through a crusher to reduce the size of the larger pebbles, after which the material has a larger per cent of voids because the sharp angles of the crushed gravel prevent it from packing so closely. On the other hand, the new faces of the broken pebbles usually offer a better surface for the adhesion of the cement than the original water-worn surfaces.

Before adopting a gravel for any important work, the per cent of voids should be determined; and if the result is not entirely satisfactory, the gravel should be separated into several sizes by screening, and the various sizes should be combined in different proportions to see if the per cent of voids can be reduced. For an

example of the large saving that may be made by screening the gravel, see the last paragraph in the following section. An advan tageous combination can sometimes be discovered by inspection, and may always be found by trial. An easy way of making this trial is as follows: Procure a piece of 10- or 12-inch vitrified pipe with a cement bottom, or a strong wooden box, preferably metal-lined, and fill it with the coarsest gravel to a depth of a foot or more, tamping the material as it is put in. Make a line around the pipe on the inside to indicate the depth of the stone. Weigh the vessel with the pebbles, empty out the latter and weigh the vessel alone, and determine the weight of the pebbles alone. Next take a new portion of the coarse material and add, say, one tenth of its weight of the next finer material, and repeat the above trial with this mixture. If the amount of this mixture compacted into the pipe or box weighs more than that of the corresponding coarse material, then this mixture is the better for making concrete; and vice versa, if the weight of this mixture is less, then this mixture is not as good for concrete as the corresponding coarse material. By successive trials find the most advantageous combination of sizes to produce a minimum per cent of voids, and this is the most economical combination.

For a direct but elaborate scientific method of determining the proportions of the various sizes'to be used to secure the minimum per cent of voids, see f 302-09.

The physical charac teristics of screened and unscreened gravel are given near the foot of Table 20, page 99. Judging from the little data that can be found in engineering literature and from all the information gathered by an extensive correspondence, gravels No. 16 and No. 17 of the table are representative of the gravels employed in actual work.

Concerning No. 18 notice that 65 per cent passed a No. 5 screen; and therefore this mixture could more properly be called gravelly sand. If one fifth of the material passing the No. 5 sieve he omitted, the voids in the remainder when rammed will be only 15 per cent instead of 20; and therefore if one tenth of this gravel were passed through a No. 5 sieve and that portion retained on the sieve were mixed with the remainder of the original, the voids would be reduced to 15 per cent, which would materially improve the quality of the gravel for making concrete. This is a valuable hint as to the pos sible advantage of sifting even a portion of the gravel. For an example of the saving secured by grading the materials, see the last paragraph 306.