PSI - Issue 45

Yipu Guo et al. / Procedia Structural Integrity 45 (2023) 66–73

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Yipu Guo et al. / Structural Integrity Procedia 00 (2019) 000 – 000

activities generate massive quantities of c onstruction and demolition waste (CDW). Traditionally, CDW is often disposed by means of landfilling or open dumps considering the economic feasibility, which is classified as the top of the hierarchy in terms of threatening extent to raise the environmental issues (Lei et al., 2023; Peng et al., 1997). Therefore, it is necessary to find a feasible and efficient approach for recycling or reusing of CDW with economic, environmental, and social benefits. Waste concrete is the most common and indispensable CDW. Over the past few decades, scholars and practitioners recycled waste concrete to prepare recycled aggregate (RA) instead of un-renewable natural sand and natural stone to produce recycled aggregate concrete (RAC) (Thomas et al., 2020). Using RA is an efficient way that benefits the most two prime pillars of sustainability in construction, natural resource consumption and mitigation of environmental threats. Recently, on the basis of extensive investigations on RAC from material to the structural application level and many positive results received, the sustainable idea of further introducing recycling cycles on waste RAC after the end of service life into the next generation of RA to manufacture the multi-recycled aggregate concrete (Multi-RAC) has been generalized by academia, to promote the cleaner production of concrete industry (Wang et al., 2022). In comparison to natural aggregate (NA), RA has the characteristics of high water absorption and larger crushing index, making RAC behaves generally inferior under mechanical loadings (Feng et al., 2021). Besides, the source diversity of parent concrete and complexity of exposed environments during its service life, as well as the nonuniform crushing and sorting treatments, lead to the material properties of RAC showing the great variability (Wang et al., 2021). Additional recycling cycles on Multi-RAC are deduced to further complicate the case and amplify the variability (Silva et al., 2021). For example, both de Brito et al (2006) and Huda & Alam (2014) observed increasing water absorption with the increased number of recycling cycles whereas Zhu et al. (2016) found that water absorption with the higher number of recycling cycles was not increased expectedly, which is probably owing to the inclusion of supplemental cementitious materials. In contrast to many studies showing incrementally declined mechanical properties, Salesa et al. (2017) prepared Multi-RAC with Multi-RA sourced from precast concrete rejects and obtained the favorably higher compressive strength of Multi-RAC compared to natural one. Overall, the current research on the properties of multi-recycled aggregate (Multi-RA) and the concrete contain it is yet at budding stage, which is generally limited in the physicochemical properties of Multi-RA, basic mechanical properties, shrinkage and durability performance of Multi-RAC. Due to the instability of Multi-RA, it is still unclear how the sequential recycling cycles will affect the mechanical behavior of Multi-RAC. On the other hand, to promote the utilization of Multi-RAC as structural concrete, it is necessary to investigate the mechanical behavior of Multi-RAC under the multiaxial stress state. In practical engineering, confined triaxial compressive performance is one of the crucial concerns, which is of vital significance to understanding confined concrete in its structural level (Xu et al., 2021). Although there are few attempts focusing on the experimental and theoretical analysis of RAC’s triaxial mechanical properties and some failure criteri a have been proposed (Chen et al., 2022; Meng et al., 2022), the relevant study on Multi-RAC is absent. Against such background, 3 recycling cycles and 4 magnitudes of lateral confining pressure (5-20 MPa) were designed as experimental parameters to investigate the triaxial mechanical properties of Multi-RAC in this study. In addition, based on the experimental results, a failure criterion for Multi-RAC was determined. 2.1 Mix proportions and specimen preparation Ordinary cement (PO. 42.5) and natural river sand (fineness modulus of 2.8) were used to prepare natural aggregate concrete (NAC) and 3 generations of Multi-RAC. The previous generation of Multi-RAC was crushed and sorted to prepare the next generation of Multi-RAC. To eliminate the effect brought by the effective water-to-cement ratio, Multi-RAC was made with Multi-RA presaturation method. The gradation of 3 generations of Multi-RAC is kept identical to NAC. The specimen size is 50 mm×100 mm cylinders. The detailed mixture proportion, mixing procedure, physical properties, and macro and micro morphologies of NCA can be found in a previous study (Lei et al., 2023). 2.2 Test program The RTX-4000 testing machine is used for the uniaxial and triaxial compressive tests. Triaxial compressive tests were performed with four lateral confining pressures of 5 MPa, 10 MPa, 15 MPa, and 20 MP. The loading rate for