Behavior of FRP grid-reinforced ultra-high performance concrete (UHPC) pipes under lateral compression

In this paper, ultra-high performance concrete (UHPC) pipes internally reinforced with fiber-reinforced polymer (FRP) grid (abbreviated as “FRP grid-UHPC pipe”) are proposed. The lateral load carrying performance of FRP grid-UHPC pipes was explored by testing 15 pipe specimens. The test variables included the presence and amount of FRP grid reinforcements, pipe thickness, as well as the presence of carbon FRP (CFRP) jacketing. Results showed that separation failure and flexural fracture were the primary failure modes for the pipes with and without FRP grid reinforcements, respectively. The typical load-vertical displacement curve of a FRP grid-UHPC pipe exhibited a two-stage ascending behavior. Without CFRP jacketing, the two ascending segments were linked by a load reduction, while the transition was smooth with CFRP jacketing. For the variables, it is found that the presence of FRP grid reinforcements could enhance the pipe initial cracking load by approximately 30%, while increasing the FRP grid layers could lead to a much steeper rising in the second ascending segment. Increasing pipe thickness and applying CFRP jacketing could increase the slopes of both initial and second ascending segments. Therefore, increasing pipe thickness (from 30 mm to 45 mm) and applying 1-ply CFRP jacketing could enhance the pipe ring stiffness by 422% and 115% respectively, which was more efficient that increasing the FRP grid layers.