Influence of Multistage Target Temperature and Cyclic Loading/Unloading on the Permeability of Polypropylene Fiber Concrete

Abstract

Due to the combined effect of temperature and cyclic loading and unloading, the gas permeability of polypropylene fiber reinforced concrete structures changes during service. However, the current gas permeability test of polypropylene fiber reinforced concrete is based on a single influencing factor or a single test condition (monotonic loading), and the test conditions are quite different from the actual working conditions of the structure. To explore the permeability of polypropylene fiber reinforced concrete under cyclic loading and unloading under the influence of temperature, based on the stress principle that the specimen does not have structural damage and according to the steady-state equation of Darcy’s law, the Cembureau method is adopted. The gas permeability of polypropylene fiber reinforced concrete under single loading and unloading and multistage cyclic loading and unloading at eight target temperatures is tested by the triaxial permeability test system. The results showed that (1) when the target temperature was 120°C < T ≤ 200°C and 200°C < T ≤ 280°C, the fiber experienced two stages of “softening, melting-cooling recovery” and “melting and absorption,” which caused damage to the matrix pore structure. The gas permeability at 200°C and 280°C was 246 times and 350 times that at 22°C, respectively. (2) The damage degree of the matrix strength structure increases during cyclic loading and unloading, and the permeability loss rate during cyclic loading and unloading is 1.24∼1.57 times that of single loading and unloading. (3) The high target temperature leads to pore structure damage of the matrix, which not only affects the permeability of the matrix but also affects the strength structure of the matrix. When the stress ratio R ≥ 0.37, the pore structure damage and the strength structure damage of the specimen are superimposed, resulting in the antipermeability effect of the specimen developing in the unfavorable direction. The test simulated the actual working conditions of polypropylene fiber reinforced concrete, providing a reference for building fire protection, seismic design or postdisaster evaluation.