Investigation on fracture properties of concrete considering the viscoelastic characteristics

To study the effects of load levels and duration of loading on the fracture properties of concrete, basic creep and quasi -static loading tests were performed on the TPB specimens. The creep tests with duration of 90, 180 and 360 days were conducted on TPB specimens under 60% P max , 70% P max and 80% P max at the first, and then these specimens were tested under quasi -static loading until failure. The creep deformation versus time curves and load versus crack mouth opening displacement curves were obtained experimentally. Meanwhile, by introducing the Norton -Bailey model and the cohesive stress relaxation model to respectively characterize the viscoelastic properties in un-cracked concrete and in fracture process zone, the time dependent behavior of concrete was analyzed numerically. The results indicated that for concrete specimens under 60% P max less than initial cracking load P ini , the crack could not initiate in the creep process and the residual capacity of creep specimens increased obviously. For concrete under 70% P max and 80% P max larger than P ini , the crack propagated 1 mm and 10 mm under sustained loading and then maintained stability, and the residual capacity also increased obviously. Under sustained loading, the stress at the crack tip of concrete rapidly decreased at the first and then kept constant in the creep process due to the stress relaxation, while the strain at the crack tip gradually increased, which caused SIFs rapidly decreased at the first and then gradually increased with creep time. This caused led the residual capacity of creep specimens increased compared with that of the reference specimens and decreased with creep time.