Studies of Crack Widths and Deformation under Sustained and Fatigue Loading

Abstract

In connection with partial prestressing, cracks may develop and excessive deformation may take place. It is, therefore, essential to investigate safe limits of the maximum width of cracks to ensure freedom from corrosion of the steel and from excessive deflection. In Fig. 1 the relationship between the deflection and concrete stress at the tensile face of a beam is illustrated. The load-deflection diagram is shown in a simplified manner comprising two straight lines: one based on the calculated deflection of a homogeneous section and the other parallel to the calculated ,deflection of a cracked section computed using the appropriate value •of n corresponding to the modular ratio of the actual E-value of the -steel and concrete. In fact, this deflection curve will be slightly rounded off near the change of incline in deflection beginning from the load at which micro-cracks develop. This corresponds to the tensile strength, i.e., a tensile stress of, say, 400 to 600 psi for high strength concrete; but the load at which the cracks become visible to the unaided eye is much higher and approximates the point at which the change in deflection occurs in the