Evaluation of Prestress Losses, Transfer Length and Harping Characteristics of CFRP Systems
Carbon fiber reinforced polymers (CFRP) are becoming a recognized alternative to traditional construction materials with a wide range of applications in current civil engineering practices. An example of such applications is the use of CFRP systems as prestressing reinforcement for concrete bridge girders, especially in aggressive environments, where steel strands are susceptible to corrosion. The main objective of this dissertation is to address some of the challenges associated with the use of prestressing CFRP systems in bridge beams. Prestress losses, transfer length, and harping characteristics of CFRP systems were evaluated in this study as critical parameters, which affect the design and long-term behavior of CFRP prestressed concrete bridge beams. This dissertation aims at evaluating prestress relaxation loss of two types of prestressing CFRP materials (cable and bars). Effects of initial prestressing level, CFRP length, and anchorage loss on total prestress relaxation of CFRP systems were investigated experimentally, and prestress relaxation equations were developed for both pre- and post-tensioned applications. One of the challenges of using CFRP reinforcement in concrete structures is thermal incompatibility of the CFRP and concrete, which can result in thermally induced loss/gain and deterioration of the bond between CFRP and concrete. Several CFRP pre-tensioned prisms with different jacking stress levels were fabricated to evaluate transfer length and associated long-term prestress losses. After one year from prestress transfer, specimens were exposed to thermal fluctuation cycles to represent the severe seasonal temperature change during a bridge service life. Experimental results suggest degradation of the bond between prestressing CFRP and increase of the transfer length for prestressing CFRP cable and bar. In deviated or harped configurations, prestressing CFRP systems demonstrated substantial reduction in their jacking capacity due to combination of axial and flexural stresses at harping points. New deviators with large diameters and contact surfaces were developed and fabricated to maximize the harping capacity retention of CFRP systems. In addition, a finite element (FE) model was developed and calibrated based on test results to evaluate harping characteristics of CFRP bar in different harping configurations. Then, a parametric study was conducted to provide jacking stress limits for harped prestressing CFRP bars in different harping angles.