SUSTAINED LOAD STRENGTH AND SHORT-TERM STRAIN BEHAVIOR OF HIGH-STRENGTH CONCRETE

A test program was carried out on 100 x 200 mm cylinders to study both the compressive strength under high sustained stresses and the short- term strain behavior of high-strength concrete. The sustained load spe cimens were subjected to sustained stresses for 3 months, if they did not fail sooner. The concretes had 56-day strengths of 65 to 120 MPa ( 9425 to 17,400 psi). Sustained load stresses ranged from 70 to 95 perc ent of the short-term strength. The effects of strain gradient and sil ica fume were also studied. In addition, stress-strain curves, volumet ric strains, and Poisson's ratio were measured in monotonic short-time tests. The ratio of the sustained load strength for specimens loaded for 90 days to the short-term strength of ultra-high-strength silica f ume concrete was found to be 10 to 15 percent higher than that of norm al-strength concrete while the sustained load strength ratio for high- strength concrete was found to be similar to normal-strength concrete. The sustained load strength ratio for high-strength concrete under sm all eccentric loads was found to be approximately 5 percent higher tha n that under concentric loads. There is a correlation between the sust ained load strength of high-strength concrete and the stress at which the short-term stress-strain curves deviate from a straight line. The volumetric strain, Poisson's ratio, and sustained load studies load st udies show that while the short-time stress-strain characteristic of h igh-strength concrete are similar to those of rock, its long-term beha vior is similar to normal-strength concrete and is dominated with its composite nonhomogeneous structure, creep of the paste, and time-depen dent deformation.