Relaxation behavior of BFRP tendon for prestressing application

Abstract

Relaxation is a key factor controlling the application of prestressing tendons. This paper focuses on the relaxation behavior of newly developed BFRP tendon for prestressing applications. A series of relaxation tests on BFRP tendons with three different initial stresses of 0.4 fu 0.5 fu and 0.6 fu (fu = tensile strength) were first conducted through a specially designed setup to eliminate the adverse impact of slippage at anchor zone.Theoretical value of relaxation rate based on an existing theory was calculated. An additional group of test was designed to clarify the effect of pretension on the relaxation behavior. The results show that the setup can effectively measure the relaxation loss of specimen, with monitoring the slippage at anchor zone simultaneously.Higher stress level can lead to larger relaxation loss. Theoretically calculated value of relaxation rate is consistent with the experimental results. The relaxation rates of BFRP tendon at 1000 h are 4.2 %, 5.3 % and 6.4 % under 0.4 fu, 0.5 fu and 0.6 fu, respectively. Pretension performs effective for the controlling of relaxation loss. Specimens with pretension treatment behave only 2.6 % relaxation rate under 0.5 fu at 1000 h, comparable to the relaxation rate of prestressing steel strand under 0.7 fu at 1000 h (2.5 %). The load retentions at one million hours are predicted to be 93.8 %, 91.8 % and 90.0 % for BFRP tendons under 0.4 fu 0.5 fu and 0.6 fu, respectively. Also, a load retention of 93.3 % is predicted for BFRP tendon with pretension treatment, under 0.5 fu, at one million hours.