Flexural behavior of seawater sea-sand concrete beams reinforced with BFRP bars/grids and BFRP-wrapped steel tubes

Highlights

• The flexural performance of a new type of BFRP bars reinforced SWSSC beam was tested and numerically simulated.

• Transversally placed BFRP grid units were used as shear stirrups.

• The bottom tension area of some beams was equipped with a BFRP-wrapped steel tube.

• The proposed hybrid beam has the potential to be applied in ocean engineering.

Abstract

This paper presents an investigation of a new type of seawater sea-sand concrete (SWSSC) beam which consists of BFRP bars/grids and BFRP-wrapped steel tubes (BWST). The transversally placed basalt fiber-reinforced polymer (BFRP) grid units act as shear stirrups, while the longitudinal BWST near the bottom area is used to improve the tension resistance. The flexural behavior of the hybrid SWSSC beams under different BFRP reinforcement ratios with or without the BWST were experimentally tested and compared. In addition, a finite element model (FEM) was established and a parametric study was conducted to show the influence of key parameters. The results showed that the placement of the BWST could significantly improve the mechanical properties of the hybrid beams, especially in terms of improving the flexural stiffness and reducing crack width. In addition, the full-length embedded BWST could shorten the height of the vertical cracks developed along the FRP grids, and denser diagonal cracks developed in the shear span.

Introduction

The traditional method of concrete production consumes a significant amount of freshwater and river sand resources and is currently facing the bottleneck problem of lack of resources (especially for high-quality river sand). In the construction of offshore engineering projects, there are large amounts of seawater and sea sand resources present from which concrete can be prepared for use. However, due to the excessive amounts of chloride ions, the direct use of seawater and sea sand will cause accelerated corrosion of internal metal reinforcements and thus bring great hidden danger to the long-term performance of structures. It is well accepted that fiber-reinforced polymers (FRPs) have excellent corrosion resistance to chloride ions, and their combination with abundant seawater and sea sand has good application prospects in the field of newly built structures, especially for marine infrastructures. Since Prof. Teng from the Hong Kong Polytechnic University proposed the concept of combining FRP and seawater sea-sand concrete (SWSSC) in the construction of new structures in 2011 [1], [2], researchers have designed a variety of ways to use FRP and SWSSC in combination, including SWSSC-filled FRP tubular columns [3], [4], [5], [6], [7], [8], [9], [10], FRP tube-FRP profile-sea sand concrete composite columns [11], and FRP bar-reinforced SWSSC members (beams, walls, and slabs) [12], [13], [14], [15], [16], [17], [18], [19], [20]. In recent years, research on this topic has focused on the evaluation of long-term durability [21], [22], [23], [24], [25], [26], [27], [28] and the design of new types of composite members using a combination of FRP, SWSSC, and ultrahigh performance concrete (UHPC) [29], [30].

However, FRP products have the characteristics of linear elasticity and low elastic modulus (except for high-elastic-modulus carbon-FRP). Pure FRP bar-reinforced concrete (RC) members, for example, have shortcomings such as low flexural stiffness and poor ductility [31]. As a result, the design of non-prestressed FRP bars RC beams is generally controlled by the serviceability limit state, and the strength of FRP bars cannot be fully utilized, which is uneconomical. For this reason, scholars have proposed using the hybrid method of FRP bars and steel bars to improve the above defects to some extent [32], [33], [34], [35]. Studies have shown that hybrid-reinforced beams have good comprehensive performance and that their flexural stiffness and ductility can be significantly improved. In addition, referring to steel-reinforced concrete (SRC) beams, FRP bar-reinforced concrete-encased steel composite beams were also proposed and investigated by scholars [36], [37]. Studies have shown that the flexural stiffness, ductility, and load-bearing capacity of pure FRP bar-reinforced concrete beams can be significantly improved when they contain a steel I-beam. To the best of our knowledge, it is common to use a steel I-beam as the additional inner reinforcement, while relatively few studies have used a longitudinally embedded steel tube as the inner steel reinforcement. Zhao et al. [38] proposed a double-skin tubular beam that consists of an outer FRP tube and an inner steel tube, with the space between them filled with concrete. The inner steel tube was shifted to the tension side of the cross-section for improved flexural performance, and headed shear studs were welded onto the surface for improved bond performance. The test results showed that the proposed tubular beams possessed a very ductile response and that the headed shear studs effectively reduced or eliminated slips between the steel tube and the concrete. However, when the concrete type was SWSSC, ordinary steel tubes were not allowed to be used directly, and anti-corrosion treatments were needed on the surface. Wrapping FRP on the surface of the steel tube was an effective and convenient method to isolate the steel from chloride ions. Zhang et al. [39] tested the axial compression performance of SWSSC-filled steel tube columns. To prevent corrosion of the steel tube, FRP layers were attached to both the inner and outer walls of the steel tube. However, there are few studies on using a FRP-wrapped steel tube as the inner reinforcement in the tension area of concrete beams [30].

It is also worth noting that when the concrete type is SWSSC, the anti-corrosion problem of the shear stirrups also needs to be considered. Traditional steel stirrups should not be used, and FRP stirrups are recommended [12]. Although FRP stirrups are commercially available [40], the manufacturing process is complicated, especially at corners where the strength reduction is large. The recommended strain value of FRP stirrups in the specification is only 0.004 [31], and the corresponding tensile strength is only approximately 200 MPa to 300 MPa, which results in a very low material utilization ratio. FRP grids have a grid structure that crosses vertically and horizontally. These grids can replace FRP stirrups to form a reinforcement cage with longitudinal FRP reinforcements. At present, FRP grids are mainly used to strengthen concrete or masonry structures by the external bonding method with polymer cement mortar (PCM) [41], [42], [43], [44], [45], [46], [47]. Only a few studies have tried to use FRP grids as stirrups for newly built structures [48], [49], [50], [51]. For example, Jeong [48] tested the flexural behavior of eight glass-FRP (GFRP)-reinforced concrete beams with carbon-FRP (CFRP) grid shear reinforcement and it was confirmed that CFRP grids can be used effectively in flexural concrete members with tension reinforcement using GFRP bars. Sha [50] investigated the axial compressive behavior of reinforced concrete (RC) columns transversely reinforced with FRP grids. The tests indicated that using FRP grids as transverse reinforcements in RC columns can significantly improve the strength by confining the concrete core. Kazem [49] investigated the seismic performance of three large-scale concrete structural joints using CFRP bars as longitudinal reinforcement and CFRP grids as transverse reinforcement.

In light of the abovementioned practical needs and research status, this paper designed a new type of hybrid FRP bar-reinforced SWSSC beam, which is especially suitable for infrastructure in the ocean environment. The innovation lies in the following three aspects. First, the concrete adopted was the SWSSC. Second, basalt-FRP (BFRP) grid units were transversely placed as stirrups. Third, a BFRP-wrapped steel tube (BWST) was embedded in the tensile zone of the hybrid beam to further improve the bending performance. It is worth noting that the full-length longitudinally embedded BWST can also bear part of the shear force in the shear span while playing the role of tensile reinforcement. For the developed BWST, the fiber orientation of the outer FRP layer was along the axial direction of the steel tube, which allowed it not only to isolate chloride ions but also to bear tensile stress. In this paper, the flexural behavior of a hybrid SWSSC beam was tested, and based on finite element (FE) modeling, the influence of key parameters such as the BWST and BFRP reinforcement ratio, the yield stress of steel tube was compared and analyzed. Research shows that the new hybrid beam has excellent comprehensive performance and has the potential to be well accepted and applied in ocean engineering.