Influence of elevated temperature on the flexural behavior of spliced CFRP bars in UHPFRC beams

Carbon fiber-reinforced polymer bars in ultra-high-performance fiber-reinforced concrete (UHPFRC) form a synergistic combination, and their practical application can be further expanded, particularly in cases involving spliced connections. However, the splice behavior of CFRP bars in UHPFRC after fire exposure requires further investigation, given the thermal sensitivity of CFRP bars. In this investigation, thirteen UHPFRC members were fabricated and subjected four-point bending to examine the splice behavior of CFRP bars after thermal exposure. The primary test parameters include splice lengths (20d, 40d, 60d) and exposure temperatures (200, 400, 600 °C), with comparisons between both un-spliced and un-exposed specimens. The load-carrying capacity slightly increased after exposure to 200 °C, but significantly declined at higher temperatures—by up to 50 % at 400 °C and over 80 % at 600 °C. Except for the specimens exposed to 600 °C, similar failure patterns were observed once the splice length exceeded 40d. In addition, the onset of bar slip rose in proportion to temperature, resulting in brittle failure due to weakened steel fibers as well as delamination of the CFRP bars in UHPFRC following 600 °C exposure. Prior to investigating splice lengths, the developed tensile strength was assessed, showing that a specimen with a splice length of 60d provided higher developed tensile strength than un-spliced specimens, irrespective of exposure temperatures. After exposure to 400 °C, the required splice length for CFRP bars was approximately 59d - about 41 % greater than that of un-exposed specimens. The proposed splice length can be applied to the practical design and implementation of CFPR bars, taking their thermal characteristics into account.