Bond durability and degradation mechanism of GFRP bars in seawater sea-sand concrete under the coupling effect of seawater immersion and sustained load

Highlights

• Bond durability of GFRP bars in concrete under immersion and load was investigated.

• Degradation mechanism of GFRP bars in concrete was clarified.

• Sustained load could accelerate the degradation of the bond performance.

• Bond strength after 50 years was predicted according to fib Bulletin 40.

Abstract

In this paper, the bond durability of glass fiber-reinforced polymer (GFRP) bars in seawater sea-sand concrete (SWSSC) under the coupling effect of seawater immersion and sustained load was experimentally investigated. The test parameters included bar diameter (12 and 16 mm), immersion time (90, 180, and 270 days), and sustained load condition (loaded and unloaded). Test results showed that for the specimens with a small bar diameter, the bond strength had a slight growth at the initial stage of exposure mainly due to the water swelling of the bar, and then decreased due to the bond degradation. For the specimens with a large bar diameter, the bond strength increased with the increase of immersion time, which was determined by the increasing compressive strength of SWSSC. The sustained load reduced the bond strength of the specimens with a small bar diameter due to its acceleration of bond degradation, but had little effect on the bond strength of the specimens with a large bar diameter. The degradation mechanism was the rib deterioration of the bars caused by seawater immersion. Moreover, the sustained load could accelerate the degradation. The predicted bond strength retention of the specimen with a bar diameter of 12 mm under the coupling effect of the marine environment (moisture saturated and the mean annual temperature of 25–35 °C) and sustained load is 52% after 50 years of service.

Introduction

Fiber-reinforced polymer (FRP) bar is a suitable alternative of steel bar in corrosive environment due to its good corrosion resistance, high strength-to-weight ratio, et al. [1], [2], [3], [4], [5], [6], [7]. In recent years, the investigations on the short-term and long-term properties of FRP bars [8], [9] and their resin matrix [10], [11] have gained some achievements, which speed up the extension and utilization of the bars in civil engineering. The seawater sea-sand concrete (SWSSC) structure reinforced with FRP bars has a promising prospect for development [12], [13], [14]. The bond performance of FRP bars in SWSSC is the foundation of their joint work. It determines the stress transfer between FRP bars and SWSSC. Moreover, the bond durability significantly affects the long-term behavior of the structure. Therefore, it’s important to investigate the bond durability of FRP bars in SWSSC.

The influences of the seawater and sea sand on the concrete have been studied for years. Notably, opposite conclusions were reported according to the existing studies. For example, Dhondy et al. [15] found that the compressive strength of SWSSC at 28 days was 16% higher than that of the normal concrete, while Guo et al. [16] observed a lower compressive strength of SWSSC by comparing the two types of concrete. The various chemical and physical properties of seawater and sea sand from different regions may be one of the reasons for the difference in the research results [15]. However, it is generally accepted that the chemical ions in seawater and sea sand could increase the early compressive strength of concrete slightly [17], [18], [19]. The concrete incorporated with seawater and/or sea sand has a shorter setting time compared with the normal concrete [20], [21]. Shells in the sea sand harm the workability of concrete [22] and a small seashell content has a minor effect on the compressive strength [23]. Research on the durability of concrete with seawater and/or sea sand is insufficient. Some studies have concluded that the sea sand concrete suffers a lower freeze–thaw resistance than normal concrete, and the sea sand has little effect on carbonation [24].

Very few researchers have focused on the bond durability of FRP bars embedded in SWSSC or seawater concrete. Dong et al. [25] studied the bond properties of basalt FRP (BFRP) bars embedded in SWSSC under wet-dry cycles (a cycle included 6 h for seawater immersion at 40 °C and 6 h for drying) and seawater immersion at 50 °C. After 90 days of exposure, the bond strengths under the two conditions decreased by 8% and 22%, respectively. The studies have shown that SWSSC has similar mechanical properties to normal concrete [24]. Khatibmasjedi et al. [26] observed that the GFRP bars in seawater concrete had an 11% reduction in the bond strength after exposure to seawater for one year. Although researches on the bond durability of FRP bars in SWSSC are very limited, the bond durability of FRP bars in normal concrete had received much attention. Robert and Benmokrane [27] investigated the bond behavior of glass FRP (GFRP) bars embedded in concrete. The specimens were exposed to tap water for 180 days at 23, 40, and 50 °C, respectively. The test results showed that the bond strength decreased with the increase of duration and temperature. Chen et al. [28] reported that the bond strength of GFRP bars embedded in concrete achieved a 12% reduction after alkaline solution (pH = 12.7) immersion for 60 days at 60 °C. Zhou et al. [29] studied the influence of acidic solutions with different pH on the bond performance of GFRP bars in concrete. They found that the bond strength decreased with the increase of pH. Dong et al. [30] studied the long-term bond performance of different types of FRP bars immersed in seawater. The results indicated that the bond durability of fiber-reinforced epoxy bars was better than that of fiber-reinforced vinyl ester bars. Yan and Lin [31] studied the bond behavior of GFRP bars in concrete immersed in sodium chloride solution. The study concluded that the bond strength decreased with the increase of exposure time. The degradation rate at 70 °C was higher than that at 50 °C. Bazli et al. [32] reported that compared with alkaline solution and seawater, the acidic solution caused a more significant reduction in the compressive strength of concrete, and the bond performance between GFRP bars and concrete degraded more seriously. Altalmas et al. [33] and El Refai et al. [34] showed that at the initial stage of solution immersion, the adhesion between FRP bars and concrete increased due to the water swelling of the bar. Based on the acceleration test results, Hassan et al. [35] predicted the bond durability of BFRP bars in concrete for 50 years of service life at different temperatures and humidities. The bond strength retentions were not <71%. Through a microstructural analysis, Davalos et al. [36] proposed that the degradation of the bond between FRP bars and concrete was mainly caused by the bar degradation, while the effect of concrete was minor. A similar theory of bond degradation mechanism was also put forward by Dong et al [37]. Belarbi et al. [38] carried out an experimental investigation and proved that the bond durability of carbon FRP (CFRP) bars was better than that of GFRP bars in concrete under aggressive environments. Al-Tamimi et al [39] reported that the bond strength of GFRP bars showed little difference after exposure to sun and cyclic splash for 60 and 90 days.

The literature mentioned above worked on the effect of aggressive environments on the bond durability of FRP bars embedded in concrete. But during the service period, besides exposure to the surrounding environment, the concrete structures are inevitably subjected to sustained load at the same time. Only considering the influence of the environment on the bond durability may lead to a large deviation between research results and actual situations. Recently, several scholars have carried out investigations on the bond durability between FRP bars and normal concrete (not SWSSC) under the coupling effect of aggressive environments and sustained load. Alves et al. [40] and Bakis et al. [41] reported the bond behavior of FRP bars subjected to sustained load under different environments including indoor, outdoor, freeze–thaw cycles, and alkali solution. But they did not take the sustained load as an independent variable in their studies. Benmokrane et al. [42] studied the coupling effect of alkali solution and sustained load on the bond durability of an innovative GFRP bar (a head made of a thermoplastic matrix is cast at the end of the bar) in normal concrete. Compared with the individual effect of alkaline solution, the bond performance under the coupling effect was markedly degraded. Significantly, the test bar wasn’t the conventional deformational bar, which could induce different bond performance.

Few studies pay attention to the bond durability between FRP bars and SWSSC under the coupling effect of aggressive environment and sustained load. Moreover, the degradation mechanism of bond performance is not clear, which hinders the wide use of SWSSC structures reinforced with FRP bars. Therefore, it is necessary to make up for the research insufficiency. Seawater immersion is one of the most common corrosions for concrete structures in the marine environment. Among the commonly used FRP bars, GFRP bars have the highest performance-price ratio. Therefore, the research focuses on SWSSC structures reinforced with GFRP bars subjected to sustained load under seawater immersion.

Overall, the research objective in this paper was to explore the bond durability behavior of GFRP bars in SWSSC under the coupling effect of seawater immersion and sustained load and clarify the corresponding degradation mechanism. 42 pullout specimens were tested after conditioning. The effects of bar diameter, immersion time, and sustained load on the bond performance were investigated. Based on the microstructural analysis, the degradation mechanism of bond performance was also proposed. Furthermore, the long-term bond strength under the coupling effect of the marine environment and sustained load was predicted according to fib Bulletin 40 [43]. These results are expected to help people better understand the bond durability of GFRP bars in SWSSC, and provide a reference for the design and application of the SWSSC structures reinforced with GFRP bars.