Elsevier

Structures

Volume 27, October 2020, Pages 2346-2364
Structures

Pre- and post-heating behavior of concrete-filled steel tube stub columns containing steel fiber and tire rubber

https://doi.org/10.1016/j.istruc.2020.07.034Get rights and content

Abstract

Concrete-filled steel tube (CFST) columns are increasingly used in different structures owing to their excellent load carrying capacity provided by the interaction between the concrete core and steel tube. However, there is lack of information on the design and prediction of the behavior of CFST columns with rubberized concrete core when they are exposed to elevated temperature. This paper presents a study on the axial compressive behavior of short CFST columns containing steel fiber and tire rubber particles exposed to elevated temperatures. To this effect, steel fibers were added at two volume fractions of 1% and 1.5% and tire rubber particles were added as fine aggregate at two replacement ratios of 5% and 10% to the concrete core. CFST columns were exposed to four temperatures of 20 (as control), 250, 500 and 750 °C, and then axial compression tests were conducted on them. The experimental test results show that an increase in the exposure temperature to 500 °C does not have a significant influence on the compressive strength of CFST columns, whereas an increased temperature to 750 °C results in a significant decrease (34%) in the compressive strength of the columns. It is also found that an increase in the exposure temperature, steel fiber volume fraction and rubber replacement ratio lead to an increase in the axial and lateral strain corresponding to the compressive strength of CFST columns. It is shown that the predictions of the compressive strength and axial stress–strain relationship by the proposed model are in good agreement with the tests results of both unheated and post-heated CFSTs.

Introduction

Concrete-filled steel tube (CFST) is a composite structural system that can be used in modern buildings [1]. Steel tubes not only act as permanent formwork, but also increase the strength and ductility of concrete [2]. Moreover, the uniform confining pressure provided by the steel tube is more efficient than that of stirrups in classic reinforced concrete structures [3]. As the application of CFST columns in high-rise buildings and bridge piers is increasing constantly, the need to adapt these columns to environmental demands is necessary [3].

The use of high-strength concrete in CFST columns has become popular in recent years due to the benefits it provides, including reduced material use and carbon dioxide emissions [4], [5]. Because these columns show undesirable post-peak behavior (sudden failure and load drop after reaching peak load), internal fibers are used in the concrete mixture to improve the ductility, compressive, tensile and flexural strength, and impact resistance of the concrete columns [6], [7], [8]. Lu et al. [9] performed an experimental study on CFST columns and reported a significant improvement in the ductility and energy dissipation capacity of the columns when steel fibers were added. It was shown that the use of tire rubber particles in concrete led to the improvement in the ductility and energy absorption capacity of the concrete [10]. Duarte et al. [11] studied the mechanical behavior of CFSTs with rubberized concrete core and found that adding tire particles to the concrete mixture resulted in an increase in the peak strain and a decrease in the load drop after the peak load of the CFSTs.

All structures are likely to experience fire incident during their service life. Therefore, understanding the capacity and mechanical properties of a structural element exposed to fire is necessary to optimize retrofitting efforts [12]. Yang and Hou [13] investigated the axial compressive behavior of CFST columns containing recycled concrete aggregate after exposure to 300, 600 and 800 °C heat, and reported that the exposure to 300 °C heat did not change the compressive strength and modulus of elasticity of the CFSTs. They also found that the compressive strength and modulus of elasticity significantly decreased when CFSTs exposed to fire of maximum temperature of 600 °C and 800 °C, and increasing the temperature increased the peak strain corresponding to the compressive strength of CFSTs. Ibanez et al. [12] presented a fiber beam model for post-heating response of CFST columns and then investigated the effect of load ratio and heating period on the post-heated behavior of the columns by a parametric study. They found that the post-heated ultimate load-carrying capacity of CFSTs was more sensitive to the increment of the heating time compared to the initial applied load.

The loading capacity is one of the mechanical properties for design of composite columns in structural applications. As was reported previously [14], [15], existing design codes (such as AISC, ACI 318 and AS) provide conservative predictions of the load capacity of CFSTs with conventional concrete without internal fibers. A few studies have been conducted to predict the compressive strength and axial load–strain behavior of CFST columns considering the effect of steel fiber and heat (e.g. [9], [16], [17], [18], [19], [20], [21]). Lu et al. [9] proposed expressions for the load capacity and ductility of CFSTs columns containing steel fiber, and their predictions agreed well with the experimental results of their study and the literature. Li et al. [16] developed a formula to predict the compressive strength of CFST specimens of their study exposed to 200, 500 and 700 °C heat containing recycled concrete aggregate. Lai and Ho [18] developed a formula for predicting the axial stress–strain relationship of circular CFST columns based on a database containing 442 specimens, considering different geometrical and material properties. Yang et al. [19] developed a stress–strain model using finite element method to study the behavior of CFST columns in particular in the cooling phase after the heat exposure and validated their model with some experimental data existed in the literature. Tang et al. [20] proposed a model for predicting the axial stress–strain relationship of the conventional concrete core in CFSTs considering the effect of geometrical properties of the specimens, and reported that their model overestimated the lateral confining pressure of circular CFST columns. Huang et al. [21] developed a model to predict the axial stress–strain behavior of CFST columns containing recycled concrete aggregate considering the elastoplastic behavior for the steel tube. They found that the development trend of confining pressure involved three parts, namely linear, nonlinear and relatively smooth increase. They suggested that assumption of constant or elastoplastic confinement is not appropriate for concrete confined with steel tubes.

Based on the literature review, one study has been reported on the effect of temperature on the compressive behavior of CFSTs [16]. However, the concrete core was recycled aggregate concrete and no study has been reported to date on the compressive behavior of unheated and post-heated CFSTs containing steel fiber and tire rubber particles. Owing to the increased interest in using waste tire rubber particles and steel fibers in concrete production, the study on the behavior of CFSTs with the concrete core produced with tire rubber particle and steel fiber materials exposed to elevated temperatures and development of an accurate model for predicting the behavior is of great importance. Therefore, a study is presented to investigate the behavior of short CFSTs with high-strength concrete core containing steel fiber and tire rubber particles before and after exposure to elevated temperatures. The paper initially provides the results of the experimental tests on CFST specimens considering different parameters, including the tire rubber replacement ratio, steel fiber volume fraction, diameter-to-thickness ratio of the steel tube and exposure temperature. Then, a theoretical analysis and finite element modeling are conducted to develop a model for predicting the axial load–strain relationship of the CFSTs.

Section snippets

Specimen preparation

19 groups of CFSTs with variables of tire rubber particle content, steel fiber content, steel tube thickness, and temperature were manufactured. In addition, 19 groups of concrete specimens companion to the CFSTs were prepared to measure the compressive strength of the concrete cores. Three nominally similar specimens were used in the compression test for each specimen configuration. Therefore, 57 CFSTs and 57 unconfined concrete specimens were provided in this study. High-strength concrete (in

Test results and discussions

Table 2 presents the compression test results of the CFST specimens. The values reported in the table are the mean values obtained from three similar specimens. In this table, D, t, fcu, fcc, fyc and ξ are external diameter of steel tube, thickness of steel tube, compressive strength of concrete core, compressive strength of CFST specimen, strength of CFST specimen when the steel tube yields and confinement index, respectively. CFST specimens shown in Table 1, Table 2 are labelled as follows:

Comparison of experimental peak loads with predictions by codes and other studies

This section studies the comparison between the obtained peak loads from the experiments and the peak load predictions by codes (ACI, AS, Eurocode 4, CSA and AISC) and two existing studies in the literature. Fig. 5 and Table 3 present this comparison.

Theoretical analysis of axial stress–strain relationship

This section presents an accurate expression for predicting the compressive strength and axial stress–strain relationship of unheated and post-heated CFSTs containing steel fiber and tire rubber. The proposed axial stress–strain curve consists of three branches, including pre-peak linear, pre-peak nonlinear and post-peak branches. This method was also used by the previous study on predicting the axial stress–strain relationship of CFSTs with recycled aggregate concrete [53]. Based on the

Finite element modeling

In this section, the experimental results of axial load–strain relationships of post-heated CFSTs are compared with those of modeling predictions obtained from simulations conducted by the commercially-available software ABAQUS. To model the CFST specimens, the material properties of steel tube and concrete core together with their interaction are required [54]. The nonlinear analysis of the compressive behavior of the CFSTs was conducted with load–displacement output. The axial loading was

Conclusions

A study on the axial compressive behavior of unheated and post-heated CFSTs containing steel fiber and tire rubber particles has been presented. The effect of different parameters, including tire rubber replacement ratio, steel fiber volume fraction, D/t ratio and exposure temperature, on the compressive behavior of these columns has been investigated. The following conclusions are drawn from the results and discussions:

  • 1.

    An increase in the exposure temperature from ambient temperature to 500 °C

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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