Long-term performance of FRP bars and FRP-reinforced seawater sea-sand concrete beams in real marine environments

Abstract

As coastal cities increasingly rely on resilient marine infrastructure to address the challenges posed by climate change and rising sea levels, the demand for sustainable construction materials has become more urgent. However, the construction industry faces significant challenges due to the scarcity of river sand and freshwater resources, as well as the issue of chloride-induced steel corrosion in concrete. In this context, a new type of reinforced concrete structure, composed of corrosion-resistant fibre-reinforced polymer (FRP) bars and seawater sea-sand concrete (SSC) (termed FRP-SSC structure) has been deemed as a promising solution due to its environmental and economic benefits. While existing accelerated aging test data on the durability of FRP bars, FRP bars embedded in concrete, and SSC suggests that FRP-SSC structures can be expected to be highly durable in marine environments, there remains a critical need to investigate their long-term performance and deterioration mechanisms under real-world conditions. Against this background, this thesis presents an in-depth study aimed at the investigation of long-term performance of FRP bars and FRP-reinforced SSC (FRP-SSC) beams after prolonged exposure to real marine environments. The PhD project consists of three parts, each addressing specific aspects of the durability performance of glass FRP (GFRP) bars, GFRP bar-to-SSC bonded joints, and GFRP-SSC beams. Following a comprehensive review and systematic analysis of existing research on the durability of FRP bars, FRP bar-to-concrete bonded joints, and FRP-reinforced concrete beams (based on three newly assembled databases containing over 15200 specimens), the first part of this thesis investigated the residual tensile properties of two batches of GFRP bars after exposure to different marine environmental conditions, including atmospheric, splash, tidal, and underwater zones, for up to four years. The results indicated that the GFRP bars conditioned in tidal and underwater environments exhibited a significant decrease in tensile strength compared to those exposed to atmospheric and splash environments. Besides, the bar size effect on the long-term tensile performance of GFRP bars was revealed. Based on these experimental findings, along with a critical review and assessment of existing models, a refined prediction model that incorporates a time-dependent correction factor was proposed, demonstrating enhanced accuracy in predicting their long-term tensile performance of GFRP bars under tidal exposure. The second part of this thesis focused on the bond durability between GFRP bars and SSC. Direct pullout tests were conducted on two series of GFRP bar-to-SSC bonded joints exposed to various marine environmental conditions over a period of four years. The test results revealed that the failure mode, shapes of bond stress-slip curves and bond strength of the specimens exhibited minimal changes after prolonged field exposure. However, specimens subjected to tidal conditions experienced a notable reduction in bond stiffness, with retention decreasing to approximately 64% after four years. Additionally, a time-dependent bond stress-slip model for GFRP bars in SSC was developed based on the modified BPE (mBPE) model, and its reliability was validated against the experimental data, showing satisfactory agreement and predictive capability. In the final part of this thesis, the flexural performance of two series of GFRP-SSC beams was experimentally investigated under four-point bending after field exposure to different marine environments. The results showed that long-term field exposure did not affect the failure modes and load-deflection behavior of the specimens, regardless of sustained loading conditions. While specimens exposed to atmospheric and splash environments exhibited no significant changes in flexural capacity over the exposure period, those subjected to tidal conditions exhibited a noticeable reduction in load carrying capacity, with retention stabilizing at approximately 90% after four years, accompanied by increased crack widths and decreased crack numbers. These changes were attributed primarily to the deterioration of bond stiffness between GFRP bars and SSC. To simulate the flexural behavior of these beams, a finite element (FE) model accounting for the deterioration of bond stiffness between FRP bars and SSC was developed, which demonstrated the capability to accurately predict the long-term flexural performance of GFRP-SSC beams under tidal conditions, particularly in terms of load-deflection responses and crack patterns.