Experimental and numerical investigation on the seismic performance of concrete-filled UHPC tubular columns

Highlights

• A novel concrete-filled UHPC tubular column was proposed.

• Seismic performance of the composite columns was experimentally investigated.

• Effects of design parameters were quantitatively analyzed.

• A detailed finite element model was developed.

Abstract

This paper presents an experimental and numerical investigation on a novel composite column, i.e., concrete-filled ultra-high performance concrete (UHPC) tubular (CFUT) column. One control RC column and three CFUT columns with different column parameters were tested under combined constant axial compression and lateral cyclic loading to investigate the influence on the seismic performance. The test results indicated that the good synergy work performance of the UHPC jackets and the core concrete allowed a continuous contribution of the UHPC jackets to the load-bearing capacities. Compared with the RC column, the CFUT column had a good seismic performance with significant enhancements in the initial lateral stiffness and lateral load-bearing capacity. With the wide-spread multi-cracking behavior due to the steel fiber bridging effect, the CFUT columns also exhibited improved deformation capacity together with decreased residual drift ratios. Besides, a finite element (FE) model was also developed to simulate the response of the composite columns. The experimental and simulated load-displacement curves agreed well, and the load versus deformation behavior of the composite columns could be properly captured.

Introduction

The concrete-filled tubular (CFT) columns have gained extensive research attention over the past several decades. Many types of CFT columns were developed, such as concrete-filled steel tubular (CFST) columns [[1], [2], [3], [4], [5]], concrete-filled fiber-reinforced polymer (FRP) tubular (CFFT) columns [[6], [7], [8], [9]]. Through a reasonable combination of different materials, the load-bearing capacity and ductility of CFT composite columns are greatly improved. To gain higher member stiffness and load-bearing capacities, the high-strength concrete and ultra-high-strength concrete were also applied in the CFST and CFFT columns [10,11]. However, although CFST and CFFT columns have been widely used in practical engineering, there are still some issues that need to be improved, such as the insufficient corrosion resistance of steel tubes and fire resistance of FRP tubes [[12], [13], [14]]. Many approaches have been proposed to tackle these disadvantages, such as the use of stainless steel tubes to improve the durability and intumescent coating to enhance the fire resistance of CFT columns [[15], [16], [17], [18]].

The development of material technology has brought many superior cement-based materials, such as textile-reinforced concrete (TRC), engineered cementitious composite (ECC), and ultra-high performance concrete (UHPC), which inspired new ideas to improve the durability and fire resistance of CFT composite columns. Therefore, recently, many experimental investigations have been carried out to study the mechanical performance of CFT columns with tubes made of emerging cement-based materials. For example, TRC was used to manufacture thin-walled jackets [[19], [20], [21], [22]]. Compression tests on concrete-filled TRC tubular columns indicated that TRC tubes were promising to be a reliable and cost-effective alternative to the conventional temporary formwork system, while no obvious enhancement was found in the load-bearing capacity and ductility of the composite columns. Afterward, with the development of ECC material with excellent tensile properties, the composite columns with ECC tubes were proposed and tested under varied load conditions [[23], [24], [25], [26], [27], [28]]. Compared with the thin-walled TRC tubes, the ECC tubes usually had a relatively larger wall thickness. In this way, the ECC jackets can not only act as supporting formworks during the construction period but also become an integral part of the structures after curing to contribute to greater load-bearing capacities during the service period, which generally resulted in the improved mechanical performance of the composite columns with ECC jackets compared with the RC counterparts.

As an emerging cement-based material, UHPC has received considerable research attention due to its outstanding mechanical properties, including ultra-high compressive strength, high tensile strength, and excellent toughness and durability [[29], [30], [31], [32]]. Nevertheless, the high cost of UHPC material partly limits its wide application in building structures. To overcome this disadvantage, this study adopted the idea of using the UHPC in some critical areas of structures, e.g., the UHPC jackets in composite CFT columns. Compared with the ECC jackets, the thickness of the UHPC jackets can be largely reduced because of the ultra-high compressive strength of UHPC, and thus the decreased cross-sectional dimensions of CFT columns can be achieved. Therefore, research has been conducted on the mechanical performance of concrete-filled UHPC tubular (CFUT) columns. Shan et al. [33] conducted an experimental investigation on the axial compressive performance of hollow UHPC tubes and circular CFUT columns. The test results indicated that the UHPC tubes could work well with the post-cast concrete and the composite columns exhibited a larger axial load-bearing capacity than that of the control RC columns. Shan et al. [34] also conducted another experimental investigation on the fire resistance of circular CFUT columns. The test results revealed that the composite column still had a high residual axial load-bearing capacity after evaluated temperature up to 800° centigrade, which indicated that the CFUT columns possess good fire resistance. Caluk et al. [35,36] manufactured two circular bridge pier columns by using the prefabricated UHPC jackets. The cyclic loading test results showed that the composite pier columns had an improved ductility and load-bearing capacity than that of the conventional RC column. Tian et al. [37] tested a batch of composite circular CFUT columns under axial compression and examined the influence of tube thickness, type, and amount of reinforcements in UHPC jackets. According to the above literature reviews, on the one hand, the UHPC jackets can function as a supporting system during concrete pouring to replace the traditional temporary wood or steel formworks, which leads to less on-site work and reduced construction time and cost. On the other hand, after pouring inner concrete, the UHPC jackets become an integral part of the composite structural members to sustain loads and act as a durable protective layer during its service period to make full use of the excellent mechanical properties and durability of UHPC [38]. However, the existing studies focused more on the compressive performance of this novel composite column, and no experimental study to data was reported on CFUT columns with a non-circular section.

To fulfill this research gap, this paper firstly conducted an experimental investigation on the seismic performance of the square CFUT columns. According to the literature review and the author's previous study [37], the UHPC jackets without transverse reinforcements or with insufficient reinforcements tended to detach from the core concrete after the peak load and thus not able to provide load-bearing capacity in the late stage of loading. Therefore, in this study, the square UHPC jacket is reinforced with steel reinforcements to avoid the detachment of the UHPC jacket from the concrete core. The post-cast concrete was poured into the UHPC jacket to form the composite column, as shown in Fig. 1. For columns with larger cross-section dimensions, additional cross-ties should be introduced to restrain longitudinal bars located away from the section corners from local buckling and to introduce a triaxial stress state to enhance concrete deformation capacity under seismic loading. To produce such UHPC tubes, centrifugal equipment in specialized precast concrete factories is required. In the present study, four column specimens with relatively small cross-section dimensions were manufactured and thus the reinforcements with cross-ties were not needed. The columns were tested under combined constant axial compression and lateral cyclic loading to investigate the influence of column parameters on the seismic performance. Besides, a finite element (FE) model was also developed to investigate the mechanical behavior and failure mechanism.