Text 8. Properties And Manufacture of Concrete

A concrete structure, either plain or reinforced, maintains a unique position among the various systems of modern construction. With few exceptions it is the only type of structure that is completely manufactured from its compo­nent raw materials on the site of the work. In most instances, the quality of its essential raw materials is decidedly variable. Structures built of steel, stone masonry, or various other materials are composed of elementary units which are partial­ly or entirely prefabricated in factories or shops. These other materials are fitted or assembled on the work by skilled mechan­ics, but concrete is usually manufactured at the site of the structure by unskilled laborers. The designer of reinforced- concrete structures should remember this. He must know the useful properties and practical limitations of the materials with which his plan will be constructed. With this knowledge he should plan the work in such a manner that desirable re­sults are easily and correctly attained in the field.

Definition and Description of Concrete. Concrete is an artificial stone, cast in place in a plastic condition. Its essen­tial. ingredients are cement and water which react with each other chemically to form another material possessing struc­tural strength. A mixture of cement and water is termed cement paste. In order to increase the volume of artificial stone produced from a definite amount of cement it is custo­mary to add inert filler materials known as aggregates. A large amount of cement paste to which has been added a small amount of fine aggregate, to produce a mixture of fluid consistency, is called grout. When the amount of fine aggregate is increased to the extent that the mixture loses its fluidity and behaves as a cohesive plastic, the re­sulting mixture is termed mortar. With the further addition of coarse aggregate, the mixture is called concrete.

It is a custom of long standing to designate these mix­tures in terms of the relative volumes of cement, fine aggregate, and coarse aggregate of which they are composed; The ingre­dients are always indicated in the same order: cement first, fine aggregate next, and coarse aggregate last. For example, a 1:2:4 concrete is a mixture of 1 cu. ft. of cement, 2 cu. ft. of fine aggregate., and 4 cu. ft. of coarse aggregate plus a non-specified amount of water sufficient to produce a plas­tic consistency. A proportion given as 1:3 is intended to mean a mixture of cement and fine aggregate plus an indefinite amount of water but without the addition of coarse aggregate. Such a mixture would be classified as mortar.

This system of specifying concrete proportions by volume is rapidly becoming obsolete on major works but is still used for small projects. The current practice of progressive engi­neers is to indicate the proportions of materials in the same order but by weight and, frequently, to indicate the amount of water to be used.

Water, cement, and aggregates when mixed together in properly predetermined proportions produce concrete that is a plastic ma s capable of being poured or cast into molds. These molds, which are actually called forms, must be built of such size and shape as to restrain the plastic mass until it solidi­fies. With few exceptions the forms must be constructed in such a manner that the concrete, when poured, will be in its final position in the structure. Besides restraining the plastic mass until solidification occurs, the forms serve a less obvious pur­pose which should not be overlooked. They support the solid­ified mass until it has attained sufficient strength to support itself without undue deflection or complete collapse.

Concrete does not solidify or attain appreciable strength instantaneously. The chemical reaction of cement and water is slow and requires time for its completion. The reaction continues for many years. It is frequently divided, for purposes of description, into, three distinct phases. The first, called the initial setting time, requires approximately 45 min. to 8 hr. for completion. During this time, the freshly mixed concrete gradually decreases in plasticity and develops pronounced resistance to flow Disturbance of the mass, or remixing during this time, may cause serious damage. The second phase is an interval during which the concrete may be considered as a soft solid without surface hardness. It will support light loads without indentation, but it is easily abraded. Its surface can be scored, roughened, or otherwise marred without appreciable effort. This phase is frequently termed the interval of final set. Its duration is very indefinite but may be considered to exist for approx­imately 5 to 20 hr. after the original mixing operation. The third phase is one of progressive hardening and increase in strength. For concrete of good quality this progressive improvement continues indefinitely. It is rapid during early ages until about one month after mixing, at which time the mass has attained the major portion of its potential hard­ness and strength. After one month the improvement con­tinues at a greatly reduced rate.