Strength of Concrete

STRENGTH OF CONCRETE Ice only an inch thick covering a stream or lake will carry safely the weight of a loaded team, whereas a covering of snow a foot thick will not support the weight of a child. What makes the difference in strength between the ice and the snow? Both are of the same chem ical composition. The only difference between them—and the only cause of weakness in the snow as compared with the ice—is the presence, in the snow, of a large proportion of air-spaces or voids, greatly lowering its density.

It is much the same with cement mortar and concrete. The quality of the materials entering into the mixture will, of course, affect its strength. Trap rock or granite, for example, will give a stronger concrete than sandstone; and a good quality of cement will prove superior to a poorly ground and poorly calcined product. But, other things being equal, the most important factor affecting the strength of concrete is its density, and this, in turn, depends upon the quantity of cement used—that is, the amount measured proportionately to the volume of the mixed concrete; upon the sizes of the aggregates and their proportional grading of coarse and fine; and upon the thoroughness with which the mixing is done and the ingredients compacted together.

With given proportions of cement, sand, and gravel or crushed stone, the strength will depend chiefly on the quality of the materials and the thoroughness of the mixing. The loss of strength due to the use of improper sand may amount to as much as 50 per cent; and a 25 per cent loss of strength may result from carelessness or laxity of methods in mixing.

The strength of concrete as shown under di rect compression affords a guide to the safe vertical loads that may be placed upon it. The results of tests by Taylor and Thompson are given in Table XVII, showing the safe vertical loads which may be placed upon Portland cement concrete of various mixtures after one month's setting, where the height of the column or mass is not over, say, twelve times its least horizontal dimension.

In case of large mass foundations, values one-eighth greater than those given in the above table may be taken.

Where the concrete mass is subjected to vibrating or pound ing loads, the values given in the table should be reduced one-half.

The tensile strength of concrete is very much less than the compressive strength, rang ing from one-tenth to one-fifth of the latter. It is also a more uncertain quantity. Experiments by Taylor and Thompson, with mixtures of average proportions gave the ultimate fiber stress in beams as about one-eighth the break ing strength in compression. As a general thing, the tensile strength is not considered in com puting the structural strength of reinforced con crete. It has a certain bearing, however, on the problem, as it affects the location of the neutral axis in beams, and further has an important bearing on the calculated deflection of a beam. The tensile strength of good concrete suitable for reinforced construction, well aged, ranges from 200 to 500 pounds per square inch.

In the matter of the shearing strength of concrete, experimenters have reported widely different results, varying all the way from equality with the tensile strength to equality with the compressive strength. The variation depends upon the extent to which the tensile stress in the section in shear is overcome by com pression at the same section. Unless a section in shear is in compression at the same time to over come the tension resulting from the shear, the shearing strength of the section cannot exceed the tensile strength of the concrete.

Weight of Concrete. The weight of a cubic foot of concrete is ordinarily considered as 150 pounds, its variations above or below this de pending on the specific gravity of the materials used and the compactness of the mixture. Cinder concrete weighs only about 110 pounds per cubic foot, and its strength varies from one-half to two-thirds that of stone or gravel concrete.