Unloading and reloading stress-strain relationship of recycled aggregate concrete reinforced with steel/polypropylene fibers under uniaxial low-cycle loadings

To solve the key scientific problems such as low toughness and easy cracking of recycled aggregate concrete (RAC), A new sustainable material (fiber-reinforced recycled aggregate concrete - FRAC) was obtained by adding steel fiber (SF) and polypropylene fiber (PPF) into RAC matrix. Few experimental results were reported for the material under uniaxial low-cycle loading, and consequently, there is a gap in constitutive propositions to predict its behavior. Thus, one purpose of this research is to provide experimental results for FRAC characterizing damage growth and residual strain during cyclic compression in low-cycle tests with increasing strain amplitudes. Another purpose is to present a constitutive model to predict this behavior of FRAC accounting for fiber content. It is worth noting that the present results are relevant to displacement-controlled tests. The development law of residual strain (permanent strain) is explored, and the relationships between residual strain and unloading strain/reloading strain are proposed. The unloading stress-strain/reloading stress-strain equations are given. The damage evolution law of FRAC is revealed. Also, a new damage model represented by residual strain is suggested accounting for the fiber content. Furthermore, a stress-strain constitutive model coupling damage for FRAC is proposed. The model predicted with accuracy the unloading path, reloading path, residual strain development, and damage evolution for the composite accounting for fiber content.