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The application of recycled concrete can alleviate environmental pollution and has great practical value. In this study, microwave irradiation technology is used to treat recycled concrete in order to obtain reinforced recycled coarse aggregate. Then, the quasi-static and dynamic mechanical properties of concrete (C), recycled concrete (RC), and recycled concrete irradiated by microwave (MRC) were studied, and the effect of the replacement rate of coarse aggregates on its mechanical properties was also studied. The results showed that the quasi-static compressive strength of MCR is higher than that of CR at all replacement rates. With the increase in the replacement rate, the quasi-static compressive strength of MCR decreases gradually, while the quasi-static compressive strength of CR decreases first and then increases, reaching the minimum value at 60%. The dynamic mechanical properties of CR and MCR all have an obvious strain rate effect. The dynamic increase factor (DIF) has an exponential relationship with the natural logarithm of the strain rate. The dynamic constitutive model of RC was established based on composite material mechanics and viscoelastic theory. The research content can provide guidance and basis for the study of mechanical properties and constitutive relationship of RC. [ABSTRACT FROM AUTHOR]

Abstract External confinement by wrapping with FRP sheets is a very effective method for the retrofit of a concrete structure subject to either static or seismic loads. For the reliable and underlying analysis of the confinement, an accurate and theoretical strength model is required for FRP-confined concrete. In this paper, the math-deductive method is introduced based on a review of the existing models. The theoretical method uses the failure criterion as the foundation to establish the axial-strength model. To consider the stress condition of FRP-confined concrete, an improved failure criterion is developed based on twin shear strength theory. Based on a proposed failure criterion, an axial-strength model is proposed for FRP-confined concrete columns. The coefficients in the proposed model are computed with respect to the range of the unconfined concrete strength, and the ranges of the coefficients show obvious trends. The predictions of the proposed model agree well with the comprehensive database. [ABSTRACT FROM AUTHOR]

For the environment protection and sustainable development in building construction, waste concrete can be processed into recycled aggregate to mix the recycled aggregate concrete (RAC). However, the existing mix design methods of RAC were complex, and the mechanical properties of RAC were more weakened than ordinary concrete. This paper presents a simple mix design method for RAC, including orthogonal test and single-factor test. Then, in order to study the behavior of confined RAC, this paper presents a comprehensive experimental study on the RAC filled in steel tube (RCFST) specimens and the RAC filled in GFRP tube (RCFST) specimens. The results show that the proposed mix design method can mix different stable strength grades of RAC promptly and efficiently. In addition, the steel tube and GFRP tube can provide a well confining effect on core RAC to improve the mechanical behavior of column. Moreover, the properties of core RAC in steel tube are the same as the common passive confined concrete, and the properties of core RAC in the GFRP tube are the same as the common active confined concrete. The study results can provide reference for other kinds of RAC mixtures as well as be a foundation for theoretical studies on confined RAC. [ABSTRACT FROM AUTHOR]

In this paper, new models were proposed for calculating the axial strain and the stress-strain curves of actively and passively confined (such as fiber-reinforced polymers confined) concrete columns. A new energy absorption method was introduced to establish a relationship between the energy absorbed by the concrete column and the lateral confinement. Based on energy-balance approach analysis, the characteristics of actively and passively confined concrete columns were thoroughly analyzed, and the transfer factor was presented. For actively confined concrete columns, the peak strain estimation model and the stress-strain relationship of the softening branch were proposed. For passively confined concrete columns, the models of passive strain, the demarcation point and the confined coefficient were proposed. Finally, the constitutive models of two types of confined concrete were established. The proposed models were established by strong theoretical foundation and clear mathematical derivation and could provide accurate prediction results. [ABSTRACT FROM AUTHOR]

• This paper presents the comprehensive study of the axial behaviors of three types of confined RAC. • This paper focuses on the double-confinement mechanism of double-confined RAC. • This paper proposes the constitutive models of the three types of confined RAC. Recycled aggregate concrete (RAC) is favorable for environmental protection and sustainable development in building construction, and confinement can improve the mechanical properties of RAC. This paper presents the detailed experimental and theoretical studies on the axial behaviors of three types of confined RAC cylinders (i.e., steel-tube-confined RAC, glass-fiber-reinforced-plastic-confined RAC, double-confined RAC). Results show that the confined RAC cylinders have excellent mechanical behaviors, the load–displacement curve trend changes with the confinement type, and the proposed constitutive models can provide accurate prediction results. [ABSTRACT FROM AUTHOR]