Seismic behavior of composite columns with steel reinforced ECC permanent formwork and infilled concrete
Introduction
The performance of the RC frame structures that subjected to earthquake excitations strongly relies on the mechanical behavior of the critical structural elements, e.g., RC beams and columns. These components are required to exhibit sufficient inelastic deformation capacity and maintain a desirable load-carrying capacity under severe earthquake ground motions. However, the conventional RC beams or columns, particularly those having small shear-span ratios, are at the risk of irreparable catastrophic failures during the earthquakes. This is due to the fact that the inherently brittle defects of the concrete make it difficult for the RC members attain good shear resistance capacity [1]. Until now, one of the conventional approaches includes the use of a large number of stirrups in the RC cross-sections. This improved the shear capacity of the concrete members and prevented the buckling of longitudinal reinforcements [2], [3]. However, the critical portions of the structures such as plastic hinge regions were highly congested, which made it difficult to ensure the pouring quality of the concrete in these regions during construction.
In addition to the placement of additional stirrups, another critical approach for the improvement of the shear capacity and the seismic behavior of the RC elements is to enhance the ductility of the concrete at the material level. Many substitutes for the conventional concrete have been developed, such as fiber-reinforced high-performance cementitious materials [4], [5], [6]. As a kind of the fiber-reinforced cement-based composites, the engineered cementitious composites (ECCs) are distinct from the other kinds of cementitious materials due to their tailored-based design based on the basis of micromechanical models. The ECC is characterized by its pseudo strain-hardening behavior, multiple cracking (fine cracks with the width usually smaller than 100 μm), and the ultimate tensile strain exceeding 3% [7]. In recent years, the optimization of the mix proportions [8], [9], the durability of the ECC materials [10], [11], and the mechanical behavior of the ECC-based structural members [12], [13], [14], [15], [16] have been extensively investigated. Fischer and Li [17] studied the cyclic behavior of the ECC flexural beams and achieved a plump hysteretic loop (significant energy dissipation) in the ECC beams without using any stirrup in the members. Fukuyama et al. [18] investigated the seismic behavior of steel-reinforced ECC (RECC) shear elements, which exhibited favorable load-carrying and energy dissipation capacities. Gencturk et al. [19] experimentally evaluated the cyclic response of the ECC columns and showed that ECC columns were superior to the conventional RC counterparts in terms of initial stiffness, load-carrying capacity, energy dissipation capacity, and ductility.
However, the broad applications of the ECC materials are hindered by their high costs. Previous studies have shown several measures to reduce costs, e.g., partly replacing the oiled polyvinyl alcohol (PVA) fibers with the unoiled counterparts [9], using a high volume of fly ash [20], and appropriately reducing the volume fraction of PVA fibers [21]. In addition to these measures taken at the material level, many approaches have also been developed to reduce the costs of the ECC at the component and structural levels. A practical approach is the hybrid use of ECC and normal concrete for the production of the RECC/C composite elements. The ECC is commonly adopted in the tension side of the RECC/C member, owing to its favorable pseudo strain-hardening behavior. This leads to achieve a decrease in the crack width, an improvement in the energy dissipation capacity, and the mitigation of cover spalling. Maalej and Li [22] replaced the concrete, which surrounded the longitudinal reinforcing steel bars, with the ECC in the RC flexural members and found an improvement in the member durability. Liang et al. [23] developed a new kind of the composite columns with the ECC being used in the potential plastic regions, and experimentally studied the mechanical behavior of the columns. Leung et al. [24] conducted a series of experiments on the composite beams made with a U-shaped ECC permanent formwork (RECC/C composite beam). The results demonstrated the feasibility of the ECC formwork, which was used in the structures subjected to the aggressive environmental conditions for mitigating the issue of the severe steel corrosion. For an RECC/C composite member, the in-situ construction only requires the pouring of the plain concrete into the RECC permanent formwork that is prepared in the factory, and leads to the improvement of construction efficiency. Besides, the process also reduces the site wastes as compared to the established use of temporary wooden formworks. Due to this, the use of the ECC for a permanent formwork may be its promising application to the concrete structures. However, the studies on the seismic behavior of the RECC/C composite members are limited to date. This study was aimed to propose a new kind of the RECC/C composite column with a permanent formwork fabricated by the steel-reinforced ECC. The experimental and numerical methods were used to investigate the seismic behavior of the newly-developed RECC/C columns and the feasibility of using the RECC permanent formwork in the concrete columns for seismic regions was further verified.
This study presented the experimental and numerical results for the mechanical behavior of the RECC/C composite columns. Firstly, the experimental program included six RECC/C composite columns prepared with the ECC permanent formwork. For comparison, a counterpart RC column was also prepared. Based on the results of the cyclic tests on the seven specimens, the evaluations were conducted to study the effects of various parameters, which included the matrix type, transverse reinforcement ratio, shear-span ratio, and the column axial load, on the failure modes, energy dissipation capacity, ductility, and the evolution of stiffness. Secondly, the numerical studies were performed based on the platform OpenSees, which complemented to the experimental investigations. The effects of the design parameters (i.e., axial compression, shear-span ratio, compressive strength, the ultimate compressive strain of the ECC, and thickness of the ECC cover) on the flexural response of the composite column were examined through numerical simulations.
Section snippets
Specimen design
Seven specimens, including the six RECC/C composite columns and a conventional RC counterpart, were prepared and tested in this experimental study. All the specimens had identical cross-sectional dimensions (300 mm × 300 mm). Three different column heights, i.e., 540 mm, 600 mm, 800 mm, were considered in the experimental program, which resulted in three different shear-span ratios λ = 1.55, 1.75, 2.42 (λ = H0/h, where H0 = vertical distance from the lateral loading point to the bottom of the
Failure modes and crack patterns
The two types of failure modes were observed during the tests, i.e., the flexure failure (F) and the flexure-shear failure (FS). In the flexural failure mode, the specimens underwent a large plastic deformation after the yield of longitudinal reinforcement. The ultimate failure was led by the propagation of a major flexural crack formed at the bottom of the column and was indicated by the crushing of the ECC/concrete in the compression zone at the bottom of the column. The flexure-shear failure
Numerical simulations
This section aimed to provide the further insights into the mechanical behavior of the RECC/C composite columns by numerical studies, which were complementary to the previous experimental investigations. Numerical simulations were conducted based on the open-source platform OpenSees [33]. A nonlinear model of the RECC/C composite column was developed and validated against the experimental results. Subsequently, parametric studies were performed using the verified model in order to investigate
Summary and conclusions
This study developed a new type of the steel-reinforced ECC/concrete composite (RECC/C) column, which consisted of the RECC layer used as the permanent formwork and the core concrete infilled into the RECC layers. Experimental and numerical studies of the RECC/C composite columns were carried out in order to investigate the seismic behavior of the proposed composite columns. In the experimental program, six RECC/C composite columns and a conventional RC column were tested for the examination
CRediT authorship contribution statement
Zuanfeng Pan: Conceptualization, Methodology, Writing - review & editing. Yazhi Zhu: Writing - review & editing, Writing - original draft. Zhi Qiao: Investigation, Data curation, Formal analysis. Shaoping Meng: Supervision, Funding acquisition.
Acknowledgements
The authors acknowledge the financial support of the National Natural Science Foundation of China (Grant Nos. 51778462, 51908416) and the National Key Research and Development Plan of China (Grant Nos. 2017YFC1500700 and 2016YFC0701400). This work is also sponsored by Shanghai Pujiang Program, China (Grant No. 19PJ1409500). Any opinions, findings, and conclusions or recommendations provided in this paper are those of the authors and do not necessarily reflect the views of the sponsors.
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