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The mining stress formed in the surrounding rock after coal seam mining is the root cause of the damage and fracture of the stope surrounding rock. In order to study the evolution characteristics of mining stress and the fracture process of overlying strata on the mining face, a numerical model was established to simulate the evolution law of stress field and fracture field in the mining process of the working face by taking the 40101 mining face of Gaoliang Coal Mine as an example. The results show that the relationship between rock collapse and failure can be judged by using Mohr-Coulomb criterion to analyze the variation law of the difference between maximum and minimum principal stresses. The correctness of the time-varying evolution of fracture field and stress field in working face mining can be verified by comparing the numerical simulation results with the field measured results.

3D printed concrete technology has received a traffic attention in construction industry in recent years, due to the potential advantage: time and labor savings, cost reduction, low environmental impact. In this study, the influence of distribution of aggregate and steel fiber on the mechanical properties of 3 D printed concrete is experimentally investigated. The distribution characteristic of aggregate and steel fiber in 3 D printed concrete are evaluated by using X-ray computed tomography (X-CT) with 2 D image analysis method. The cast concrete is performed as reference. It is found that the aggregate and fiber in cast concrete are relatively evenly distributed and no alignment, while there is an orientation probability of more than 40% for aggregate, and 74% for fiber, respectively, along the printing direction in ± 20° in printed concrete. Meanwhile, using X-CT, the porosity, void sizes, shapes and distributions in printed and cast concrete are also investigated. Furthermore, it is observed that the printed and cast samples comprise of different voids, due to the different construction processes. Finally, the results prove that the performance difference for the printed concrete at different directions possibly depends on the orientation distribution of the aggregate/fiber, as well as the irregular voids, since the aggregates/fibers are orientated by the nozzle along the printing direction. [ABSTRACT FROM AUTHOR]

Supplementary cementitious materials (SCMs) are usually employed to replace partial cement and the pore-solution chemistry of blended cements is extremely essential for their cement hydration and durability. In this work, the pore-solutions of cement pastes with various water-to-cement (w/c) ratios and SCMs (fly ash, ground granulated blast-furnace slag and silica fume) are extracted for observing their ions and conductivities. The results illustrate that Na+, K+, OH−, Ca2+ and SO42− are the main and their concentrations are declined by the improving w/c ratios and SCMs dosages and thus decreasing their conductivities. The conductivity evolution shows three typical periods: induction (<5 h), acceleration (5 h-7d) and deceleration (>7d). The pore-solution conductivities of pastes with various w/c ratios are predicted well by the model established (R2 > 0.90) while the SCMs decline the model effectiveness. The OH−, K+ and SO42− provide the conductivities over 90% of the whole with the contributions in decreasing order. [ABSTRACT FROM AUTHOR]