CRUSHING STRENGTH OF CINDER CONCRETE. Table 35, page 202, is the summary of fifty-two tests of portland-cement cinder-concrete cubes.* The cinders were used as they came from the furnace without sifting, the. larger clinkers only having been broken. t Thrisiu STRENGTH OF CONCRETE. A knowledge of the ten sile strength of concrete is much less important than that of its corn pressive strength, for in massive construction the tensile strength is of no importance, and in the construction of slabs, beams, and girders no dependence is placed upon the tensile strength of concrete on account of its brittleness and its liability to crack from shrinkage. However, a knowledge of the tensile strength of concrete is important in problems relating to the use of reinforced concrete (Chap. VIII), and hence will be briefly considered.
Table 36, gives values of the tensile strength as determined by three experimenters. All of the tests were made upon gigantic briquettes.
The tensile strength of concrete is usually obtained by determining the transverse strength of concrete beams and com puting the stress upon the extreme fiber (the modulus of rupture) under the assumption that the neutral axis is at the center of the beam. Table 37 gives the modulus of rupture of concretes of various proportions of portland cement, stone screenings, and crusher-run stone from 0.1 to 2.25 inches in diameter. The age of the concrete when tested was 112 days.
Table 38 shows the modulus of rupture of concrete made with four different aggregates and three degrees of wetness, tested at four ages. Each result is the mean of three experiments.
The determination of the transverse strength of beams affords a convenient method of comparing the strength of concretes of different proportions and ages. Tables 37 and 38 are given partly to permit of such comparisons. Compare the relative transverse strengths for different proportions and ages as given in Tables 37 and 38 with the relative compressive strengths as given in Table 32, page 198, and Table 33, page 199.
Ratio of Tensile to Compressive Strength. The ratio of the tensile to the compressive strength of concrete usually varies from 6 to 12 when the compressive strength is determined by crushing cubes. Candlot gives the results of forty sets of experiments on
cement mortars of various proportions, tested at ages varying from 1 week to 3 years, in which the ratio varies from 5 to 12i, which ratio seems to be independent of the proportion or of the age.
The shearing strength of concrete is of no importance in massive structures, but is important in the design of reinforced concrete beams. It is difficult to arrange an experi ment to determine the shearing strength of concrete without in volving cutting action or bending stress. In Bulletin No. 8 of the University of Illinois Engineering Experiment Station the various methods employed to determine the shearing strength of concrete are discussed, and the conclusion is drawn that most of the methods are deceptive and give results which are too small.
Table 39 gives the results obtained by Professor Talbot; and Table 40, page 206, shows results obtained by Professor Spofford at the Massachusetts Institute of Technology. These two series of experiments are much the most satisfactory of any that have been made, and give results much larger than any former experiments. Professor Talbot used four methods of testing,—in three of which a hole was punched through a plate (for details see Table 39), and in the other the ends of a square beam were securely clamped between stiff metal blocks and the load was applied through a flat bearing block. Professor Spofford used a cylindrical beam the ends of which were clamped in cylindrical bearings, the load being applied through a semi-cylindrical bearing.
The modulus of elasticity is the ratio of the unit stress to the corresponding unit elastic deformation, and is an important factor in the design of reinforced concrete structures. The value of the modulus increases with the age and the richness of the concrete, and decreases with the increase of the load. Apparently the modulus is the same for tension as for compression.