No abstract

No abstract

Summary This paper describes preliminary experiments to determine whether or not the rate of loading significantly affects the mechanical properties of concrete beams tested in flexure. Two rates of loading were used: 5xl0-7 in/in.s, the rate employed in normal mechanical tests; about 6 x 10-2 in./in.s when the beams were tested under impact conditions. This change of the rate of loading produced the following results: the load at fracture rose from static values of 749 and 792 lb to dynamic values of 990, 1,050 and 1,115 lb; the strain at fracture rose from static values of 185 and 243 μin./in. to 200, 287, and 293 μin./in. under impact conditions; the approximately linear portion of the load-strain line became curved at dynamic strains of 130 and 180μ in./in. compared with static values of 69 and 85 μin./in.—the corresponding loads were 720 and 900 lb compared with static values of 395 and 530 lb. If the onset of cracking is associated with the departure from linearity of the stress-strain curve, it appears that increase in the rate of loading markedly effects the strain at which cracking begins.

Summary A method is described for automatically recording load versus strain using a commercial X-Y plotter. Typical curves are given showing the results obtained with pavement-quality concrete and with lean concrete. The modulus of elasticity of the lean concrete was derived from the tangent to the load-strain curve at the origin and was found to be less than that obtained by resonance tests. A reasonable correlation was found between the elastic modulus and the strains at which micro-cracking was assumed to begin.

No abstract