Behaviour of CFRP-prestressed concrete beams under sustained loading and high-cycle fatigue at low temperature.

摘要

Fibre-reinforced polymers (FRPs) are becoming increasingly accepted in structural engineering applications. In particular, Carbon-FRP (CFRP) tendons are proving to be promising as prestressing reinforcement for concrete structures. While several studies have been conducted on CFRP-prestressed concrete beams, very little attention has been given to their long-term behaviour at low temperatures. This thesis investigates the behaviour of CFRP-prestressed concrete beams in two studies: (a) under sustained loading at low temperature, and (b) under high-cycle fatigue at low temperature. Seven 13-year-old, 4.4 m long precast concrete T-beams were tested, of which five were prestressed to various levels with CFRP tendons and two with conventional steel strands.After being subjected to sustained loading, all seven beams were tested in the second study. Only three of the five CFRP-prestressed beams were subjected to cyclic loading, one at -28°C and two at room temperature, while only one of the two steel-prestressed beams was subjected to cyclic loading, at -28°C. Cyclic loading consisted of 3 million cycles at a frequency of 0.85 Hz. The load range represented 21 to 42% of the flexural capacity of the CFRP-prestressed beams and 30 to 60% of that of the steel-prestressed beam. Monotonic tests were run every 1 million cycles. Finally, all seven beams were monotonically loaded to failure. All CFRP-prestressed beams survived the 3 million cycles but the steel-prestressed beam failed after 185,000 cycles. However, the CFRP-concrete bond was weakened by high prestress levels, cyclic loading, and low temperature during sustained loading and loading to failure. This resulted in bond failure at loads ranging from 69 to 91% of the full flexural capacity. Stiffness and camber gradually decreased during cyclic loading.In the first study, three beams were exposed to -27°C while being subjected to a sustained load of 25% of their flexural capacity for 163 days. The sustained load produced cracking in two beams with lower prestress levels. Results were compared to those obtained from three similar beams subjected to the same sustained load at room temperature. Deflection increase under sustained load at low temperature was generally small and similar to that at room temperature. Prestressing strain had a direct relationship with temperature in the CFRP-prestressed beams.