Effect of internal moisture content and dynamic mechanical analysis testing conditions on the Tg values of CFRP tendons

Abstract

The high temperature and long-term performance of uni-directional pultruded carbon fibre reinforced polymer (CFRP) tendons in prestressed concrete relies on the integrity of the epoxy matrix and the degree of crosslinking. The dynamic glass transition temperature (Tg) obtained using Dynamic Mechanical Analysis (DMA) can be a useful indicator of the degree of cross-linking. However, variations in the epoxy plasticisation due to different initial hygrothermal equilibrium conditions in CFRP tendons can influence the measured Tg leading to inconsistent results. The dependency of Tg on the initial CFRP moisture content, and also DMA test parameters including the thermocouple position, the magnitude of static flexural strain, the static to dynamic strain ratio and the specimen geometry were investigated. Whereas the selected DMA test parameters had only a small influence on the Tg values, the initial moisture content of the CFRP tendon had a significant effect. A decrease in Tg of up to 38 °C was observed with an increase in average moisture content of up to 2.46%. Knowledge of the initial moisture content and the establishment of a comparative baseline are necessary to more accurately infer the crosslinking density and degree of cure of a resin from dynamic Tg results.