Aggregates

AGGREGATES The first requisite in the selection of the stone for the concrete is cleanliness. This is absolutely essential to strength in the concrete. In selecting an aggregate, the character of the surfaces presented by the particles should always receive close attention; these must be hard and permanent. A covering of any fine material will interfere with the cement or mor tar getting into contact with the surface of the aggregate, and the strength will be reduced pro portionately. An excellent precaution in this respect is to avoid the use of dirty materials.

Some experimenters found that certain sands gave better strength with the addition of 10 or 15 per cent of finely divided clay than when tested without the clay. This, however, is no argument in favor of dirty materials. The addi tion of a small percentage of finely divided clay might be permissible when the clay is treated as a separate material, while even a much smaller quantity naturally occurring in the aggregate might make it wholly unfit for concrete purposes.

In order to obtain the best results, the aggre gates should be well graded; that is, they must not contain an excess of one-size particles, and must contain but a small percentage of fine par ticles. In the case of stone, the material will usually be quite satisfactory, provided the stone in itself is hard and durable and not affected by exposure to the elements, and provided it is pre pared and marketed under conditions which as sure its being clean and free from a covering of dust or other matter.

Some stone, though apparently quite hard, presents a chalky surface with which it is im possible for the cement to form a perfect bond. Stone of this character should be avoided, for it cannot possibly produce good concrete.

In

sand and gravel, one is dealing with en tirely different materials, but materials prob ably to be preferred to stone and screenings, when selected with sufficient care. The use of sand and gravel is very popular, owing to the ease with which they are obtained in many lo calities. Where these materials are readily se cured, they are frequently used as they come from the deposit, with little or no thought given to their fitness for the work in hand. The char acter of the materials which are sometimes used in concrete is surprising. Aggregates should

always be firm and hard, and should remain so when exposed indefinitely to the weather. It is quite common to find a considerable quantity of shaly pebbles in some of the glacial sands and gravels of the upper Mississippi Valley. These pebbles are not strong in the first place; and they disintegrate readily when exposed to the elements. They also absorb water readily when used in concrete, and expand under the com bined action of moisture and frost, injuring the concrete to a greater or less extent. Though the effect of the soft sand grains is not so apparent as is the effect of the larger pebbles, such sand cannot possibly produce first-class results if the shaly particles form any considerable portion of the sand content. In the territory referred to, it is not unusual to find sidewalks badly pitted and marred, as a result of the disintegration of this shaly material. These shaly particles are undesirable, because they are both weak and unstable. A concrete can never be stronger than the material making up the aggregate.

The size of the sand grains and the relative proportion of grains of different size, have a very marked effect on the value of the sand. At least 75 per cent of a sand should be retained on a 40-mesh sieve, with the particles well dis tributed between that size and the size passing a 4-mesh sieve, with an increasing proportion on the coarser sieves. Such a sand will have much less total surface than one composed of equal proportions of particles on the several sieves. A sand made up entirely of fine particles will present a very much larger surface which must be covered with cement, than either of the sands above mentioned. For instance, the total su perficial surface of a given volume of spheres one-sixty-fourth inch in diameter is sixteen times the surface of the same volume of spheres one-fourth inch in diameter. As the making of a first-class concrete necessitates the perfect cov ering of every particle of sand with cement, and every particle of the coarser aggregate with the cement-sand mortar, it is apparent that mate rials with an excess of fine particles should be avoided. The same line of reasoning is appli cable to the combined aggregate in the concrete.