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Plain Concrete

stone, materials, sand, mortar, gravel, broken and aggregate

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PLAIN CONCRETE Materials Used for Aggregates.—Concrete as used in con struction is essentially a mixture of cement mortar with broken stone, gravel, or other coarse material. The mortar serves to fill the voids in the stone and the whole is bound into a solid monolith by the setting and hardening of the cemont.

The materials mixed with the cement in forming concretes are known as aggregates. The sand or stone chips in the mortar is called the fine aggregate and the coarser gravel or broken stone is the coarse aggregate. In the manufacture of good concrete it is essential that each of the materials be of proper quality, and that they be properly proportioned and incorporated into the mixture.

Fine Aggregate.—Material which will pass a ;-inch screen is usually included under the term fine aggregate, or sand. The requirements for sand and its use in mortar have been discussed in Chapter II. Ordinarily, the sand which makes the strongest and most dense mortar will also give the best results in concrete, though this may not always be the case. The grading of the sand should be such as to reach maximum density when combined in proper proportions with the coarse aggregate to be used in the concrete.

Coarse Aggregate.—This may consist of any hard mineral sub stance broken to proper size—usually broken stone or gravel, although sometimes broken slag, cinders, or broken brick is used.

The value of stone as an aggregate depends upon much the same qualities as are needed for building stone. For high-class concrete work, it is important that the stone should possess strength, and absorb but little water. Stones breaking to cubical shapes give better results than those of shaly or slaty character, while rounded pieces pack closer and show less voids than those with sharp corners. Trap and granite are usually the best of concrete materials. When the concrete is to he subjected to abrasive wear, trap is a superior material. For resistance to direct compression, good granite is to be preferred. Limestones and sandstones vary greatly in their values as concrete materials, hard limestones and some of the more compact sandstones being desirable materials, while the softer vari eties are not generally suitable for first-class concrete work. Gravel,

when of flint or other hard material, may make excellent concrete.

Sizes for Broken sizes to which concrete stone should be broken depends upon the use to which the concrete is to be put. In heavy walls or massive work, the upper limit of size may be 2 or 3 inches in diameter. It is desirable to have the stones as large as can be easily incorporated into the mixture. In reinforced work, where the concrete must pass between and under the reinforcing rods, it may not be feasible to use stone of more than 1 inch diameter.

In stone or gravel for coarse aggregate, as in sand for mortar, the grading of sizes should be such as to give maximum density. For a given stone, the strongest concrete will ordinarily be inacie by that arrangement of sizes which requires the least mortar to completely fill the voids in the stone, as a surplus of mortar beyond that required for completely filling the voids is an element of weak ness in the concrete, as well as a waste of the more expensive materials. Stone as ordinarily used in concrete contains all sizes, from the largest allowed to the size of the largest sand. All material retained on a 1- or i-inch screen is commonly regarded as coarse aggregate, and stone is used as it comes from the crusher with all the sizes included, only the chips being screened out.

Gravel containing sand is sometimes used without screening by mixing with cement. This is not desirable practice, as the sand is seldom in proper quantity or uniformly distributed through the gravel, it should be screened out and proportioned properly to the cement and gravel.

In concrete work it is usually necessary to use the materials available in the locality of the work, but where important work is to be clone, careful attention should be given to the character of these materials and of the concrete made from them. The design of concrete structures should be based upon full information concern ing the properties of the concrete to be used, and this is largely a question of aggregates. Poor concrete work has much more fre quently resulted from the use of poor aggregates than from the use of inferior cement.

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