Theoretical study on mechanical behavior of steel conﬁned recycled aggregate concrete Yijie Huang a , Jianzhuang Xiao a, ⁎, Ch. Zhang b a Department of Building Engineering, Tongji University, Shanghai, 200092, PR China b Department of Civil Engineering, University of Siegen, D-57068 Siegen, Germany abstract article info Article history: Received 1 December 2011 Accepted 22 March 2012 Available online 23 May 2012 Keywords: Recycled aggregate Conﬁning pressure Concrete Constitutive relationship Elastoplasticity Axial compression A theoretical study of the mechanical response of recycled aggregate concrete (RAC) conﬁned by steel tubes under axial compression is conducted to examine the sensitivity of the conﬁning pressure on the strength and deformation of RAC. The main parameter in this analysis is the recycled coarse aggregate (RCA) replacement percentage. The axial stress–strain relationship of conﬁned RAC is obtained based on the elastoplasticity and veriﬁed by experimental results. It is found that the development of the conﬁning pressure can be divided into three sections: linear increase, nonlinear increase and relatively smooth increase. The mechanical proprieties of conﬁned RAC are dependent on the variation of the conﬁning pressure. The analysis results indicate that the assumption of constant conﬁnement or elastic–plastic conﬁnement is not appropriate for concrete conﬁned by steel tubes and the RCA replacement percentage has a moderate effect on the mechanical response of the conﬁned concrete. Finally, a simple model for the constitutive relationship of RAC under varying pressures and different RCA replacement percentages is proposed. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Concretes are characterized by high compressive strength but low deformability. Conﬁned concrete is an effective way to improve the properties of plain concrete. Some investigations have shown that due to the presence of lateral conﬁnement, the axial response of conﬁned concrete is enhanced with respect to the deformation and compressive strength. At present, conﬁned concrete members are widely applied because of their excellent properties. For example, the concrete ﬁlled steel tube (CFST) members have been already widely used in civil and military engineering. The design and detailed analyses of these elements can be performed in safe and economic ways by a thorough understanding of the conﬁning reinforcement- concrete interactions and the conﬁned concrete behavior. Since the performance of concrete is signiﬁcantly affected by the lateral pressure, there have been many researches to investigate the steel tube conﬁned concrete behavior and some constitutive relation- ships have been suggested for the conﬁned concrete in the CFST. For example, Tang et al. [1] proposed a model for the conﬁned concrete in circular CFST columns, which took into account of the effects of the material and geometrical properties of the column on the strength enhancement and the post-peak behavior. It is found that their model generally overestimates the effect of the lateral conﬁning pressure in circular CFST columns [2]. Based on the conﬁning pressure model proposed by Tang et al. [1], Susantha et al. [3] developed a uniaxial stress–strain relationship for concrete conﬁned by steel tubes with various shapes. Tomii et al. [4] and Hajjar's model [5] was applicable to square CFST, but the strength improvement due to the conﬁne- ment was neglected. Considering the effects of the conﬁning pressure, Liang and Fragomeni [6] developed a model for the conﬁned concrete in circular CFST. Hu et al. [7] proposed a stress–strain relationship for the core concrete in CFST, which was based on the model of Saenz [8] and Richart et al. [9]. Ellobody et al. [10] presented an axial stress– strain relationship for the conﬁned concrete in circular CFST. Both of these models were divided into three parts and took into account of the conﬁnement effects. Sheikh et al. [11] suggested a constitutive model for concrete under triaxial compression, which was divided into two stages and was applicable to the steel- and FRP-conﬁned concrete elements. Binici [12] presented a theoretical conﬁned concrete model for the mechanical behavior of CFST under different loading conditions. This model was capable of simulating both axial and lateral deformations of conﬁned concrete. However, among the above mentioned models, most of them were based on the following assumptions: (1) during the loading history the conﬁning pressure nearly remains constant or ideal elastic–plastic; (2) the outer steel tube carries negligible axial load or does not bear any axial load. The ﬁrst assumption may be appropriate when the outer tube is a perfectly plastic material and the outer tube does not carry any axial load. However, the steel is an elastic–plastic material with a strain hardening. So the conﬁning pressure will not remain as a constant. The second assumption may be right for the ﬁber reinforced plastic (FRP) members because their axial elastic modulus is negligible. In Journal of Constructional Steel Research 76 (2012) 100–111 ⁎ Corresponding author. Tel.: + 86 21 65982787; fax: + 86 21 65986345. E-mail address: jzx@tongji.edu.cn (J. Xiao). 0143-974X/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcsr.2012.03.020 Contents lists available at SciVerse ScienceDirect Journal of Constructional Steel Research