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

A test program was carried out on 100-by-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 specimens were subjected to sustained stresses for 3 months, if they did not fail sooner. The concretes had 56-day strengths of 65-120 MPa (9,425-17,400 psi). Sustained load stresses ranged from 70% to 95% of the short-term strength. The effects of strain gradient and silica fume were also studied. In addition, stress-strain curves, volumetric 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 fume concrete was found to be 10-15% higher than that of normal-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 small eccentric loads was found to be approximately 5% higher than that under concentric loads. A correlation was found between the sustained load strength of high-strength concrete and the stress at which the short-time stress-strain curves deviate from a straight line. The volumetric strain, Poisson's ratio, and sustained load studies show that while the short-time stress-strain characteristics of high-strength concrete are similar to those of rock, its long-term behavior is similar to normal-strength concrete and is dominated with its composite nonhomogeneous structure, creep of the paste, and time-dependent deformation.