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Crack damage evolution in concrete coarse aggregates under microwave‑induced thermal stress

Yuan Yuan, Shao Zhushan, Qiao Rujia, Guo Xuan, Wang Weitao

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 3

art. no. e108

Coarse aggregates of waste concrete can be efficiently separated from mortar under microwave irradiation. However, the microwave-induced damage in aggregates are restricting mechanical properties of the aggregates for replacing natural aggregates. Since damage evolution in rocks treated by microwave are influenced by mineralogy and microwave operating parameters, such as power and irradiation time, understanding the microwave weakening mechanism of rocks is necessary to assess and control the damage of aggregates for recovery of high-quality concrete coarse aggregates. This article develops an approach for evaluating crack damage evolution in aggregates exposed to microwave by combining theoretical analysis with experimental investigation. A theoretical heat source-matrix model based on electromagnetic and thermal properties of mineral components is established for microwave heated aggregates. Substituting microwave irradiating parameters and mineralogy of the aggregates into the model, corresponding temperature fields and thermal stress fields are solved. Cracks in aggregates after microwave exposure are observed using scanning electron microscopy (SEM) and quantified in terms of crack length, density and intensity. Crack damage varied with microwave energy is assessed by crack length and density. Crack propagation is further discussed by contrast of stress intensity factor (SIF) at the crack tip and fracture toughness of the aggregate. Cracking behavior analyzed by SIF of cracks is consistent with that obtained from quantitative analysis on SEM images. The results suggest that granite shows a stronger resistance to thermal stress damage compared with basalt under microwave exposure, and a multistage microwave treatment should be adopted for recovery of various aggregates.

An analysis model for the temperature and residual stress of tunnel liner exposed to fire

Qiao Rujia, Shao Zhushan, Yuan Yuan, Zhou Hang, Guo Yinbo

Archives of Civil and Mechanical Engineering

2021

Vol. 21, no. 4

388--398

The temperature and stress analysis of tunnel liner is the basis of the damage assessment of the tunnel, and it is also have a great significance to tunnel fire protection design. In this study, a thermo-mechanical coupling model is derived to study the temperature and stresses of tunnel liner under the RABT fire curve. In contrast to consideration the effects of flame impingement on the heated surface only, the heat transfer coefficient (HTC) of the heated surface of tunnel liner is considered in the proposed model. The applicability of theoretical method is verified by comparing with the fire tests. According to maximum temperature experienced and material degradation, the residual stress of tunnel liner after fire is discussed, which could provide the basic for the damage assessment after fire. Contributions of the HTC of tunnel liner on the temperature and stresses were quantitatively described. This theoretical model explains the temperature and residual stress evolution of tunnel liner under fire when considering the effect of HTC, which provides a theoretical basis for the tunnel fire proofing.