Durability of BFRP bars wrapped in seawater sea sand concrete

Abstract

To investigate the durability of structures containing basalt fibre-reinforced polymer (BFRP) bars wrapped in seawater sea sand concrete (SWSSC) in marine environments, three sizes (including bare BFRP bars) of SWSSC-wrapped BFRP bars were subjected to laboratory accelerated corrosion tests in simulated seawater environments. The SWSSC thicknesses were set at 10 mm and 20 mm; the immersion temperatures were room temperature (~28 °C), 40 °C, and 60 °C; and the immersion media included a seawater, tap-water, and alkaline solution. The tensile properties of the SWSSC-wrapped BFRP bars after immersion were determined, and the degradation mechanism of the BFRP bars was investigated. Based on the results, the alkaline solution is more damaging to the BFRP bars than tap-water or seawater. Alkalinity is a key factor causing the degradation of the BFRP bars. Thus, the tensile strength retention of BFRP bars wrapped with thicker SWSSC, which has a higher alkalinity environment, could be lower than that of bare BFRP bars. In addition, prediction of the tensile strength retention of the SWSSC-wrapped BFRP bars based on Arrhenius relationship was conducted, and the prediction resutls agree well with the experimental results.

Introduction

With the depletion of river sand resources and the shortage of fresh water, people have begun to focus their attention on the abundant seawater and sea sand resources in recent years [1]. Some experimental results have shown that the mechanical properties of seawater sea sand concrete (SWSSC) are similar to those of traditional concrete [2]. However, the high chloride content in SWSSC accelerates the corrosion process of the steel reinforcing bars in concrete structures [3], [4]. To prevent premature corrosion of the reinforcing bars in concrete, it is necessary to strictly control the chloride ion content in SWSSC or to apply protective measures to the steel reinforcing bars by traditional methods, such as protective coatings, electrochemical protection and mitigating agents [5]. In addition, much effort has been devoted to the development of new materials with stronger corrosion resistance to replace traditional steel reinforcing bars [6].

Among these new materials, fibre-reinforced polymer (FRP) composites are considered ideal for use in SWSSC due to their excellent resistance to chloride corrosion. Over the last few decades, the use of FRPs in the construction industry has increased [2], [7], [8], [9]. The most common types of fibre reinforcements are carbon fibre-reinforced polymers (CFRPs), aramid fibre-reinforced polymers (AFRPs) and glass fibre-reinforced polymers (GFRPs) [10], [11]. In recent years, basalt fibre-reinforced polymer (BFRP) composites have been rapidly developed and utilized; BFRPs cost less than CFRPs and have similar composition and mechanical properties to GFRPs [7], [12], [13]. Furthermore, basalt fibre is an environmentally friendly material because it is directly produced from volcanic rocks without additives [9], [14].

The use of FRP bars instead of steel bars in SWSSC can avoid corrosion of the reinforcing bars and enables the direct use of the original seawater and sea sand. Hence, FRP bars eliminate the restriction where original ecological seawater cannot be directly used as a raw material in concrete [4], [15], [16]. Although FRP bars have good corrosion resistance to chloride ions, their performance will deteriorate due to the highly alkaline environment in concrete [8], [13], [17]. Therefore, it is important to evaluate the long-term durability of FRP bars in marine environments.

In recent decades, the durability of FRP composites has been studied by many researchers [7], [17], [18], [19]. In these studies, few researchers have evaluated the durability characteristics of BFRP bars. Zhu et al. [7] found that chloride ions had little effect on the degradation of concrete-wrapped BFRP bars. Wei et al. [18] studied the mass growth rate and tensile strength retention rate of epoxy resin-based BFRP bars in a seawater environment and suggested that the reduction in the divalent iron ion content in basalt fibres would improve the durability of concrete structures reinforced with BFRP bars.

To study the long-term performance of BFRP bars applied to concrete structures, some researchers have used alkaline solutions to simulate the internal concrete environment (i.e., the pore water in concrete). Serbescu et al. [14] studied the durability of BFRP bars in alkaline solutions at 20, 40 and 60 °C and estimated that the bars retained approximately 72% or 80% of their original tensile strength after 100 years of exposure. Wu et al. [17], [19] concluded that the tensile strength retention of BFRP bars in an alkaline concrete pore solution decreased much more rapidly than that in distilled water, salt or acid environments; moreover, they found that the long-term performance degradation of BFRP bars in an alkaline solution was more significant than that in seawater because the OH^– destroyed the Si-O-Si bonds in the basalt fibres [15], [16]. Wang et al. [8] used two types of solutions to perform accelerated corrosion testing of BFRP bars at different pH values and immersion temperatures and for different durations; thereafter, they predicted the long-term behaviour of BFRP bars under service conditions using the Arrhenius degradation theory.

According to the abovementioned research on the durability of BFRP bars, existing studies have used a strong alkaline solution to study the durability of BFRP bars. Furthermore, methods to predict the long-term performance degradation of BFRP bars were proposed based on laboratory accelerated corrosion experiments. However, according to the laboratory simulation results, the long-term performance predictions for BFRP bars may not be sufficiently accurate because the alkaline solution of the simulated concrete is quite different from the real environment [20], [21]. Directly placing BFRP bars into an alkaline solution or seawater results can potentially produce conservative results. There is a lack of research on the durability of BFRP bars in a real concrete environment.

In this study, BFRP bars wrapped with two different thicknesses of SWSSC (10 mm and 20 mm) were designed. Furthermore, this study treated the bars with an alkaline solution, seawater, and tap-water at room temperature (RT), 40, and 60 °C. The tensile strength and Young’s modulus of each specimen were determined. To study the damage mechanism of the BFRP bars, scanning electron microscopy (SEM) was used to characterize the changes in the microstructure of the BFRP bars after immersion. Finally, a service life prediction of the tensile strength was performed based on the Arrhenius relationship.