Properties of Mortars Mixed with Polystyrene and Hemp Fiber Wastes

When polystyrene (PS) and hemp fiber waste were mixed into the sand aggregate, some physical-mechanical properties of mortar changed. The PS and hemp fiber were tested as partial replacements for sand in mortar with three designated percentages of 2.5, 5.0 and 10.0% by mass. The properties of mortar with PS were found to be better than that of the mortar with hemp fiber. The water absorption of mortar with PS was comparable with the reference mortar but lower than that of mortar with hemp fiber. The compressive strength of the mortar with PS was higher than that with hemp fiber whereas the tensile strength of the mortar with 2.5% PS and hemp fiber was comparable and was higher than that of the reference mortar. The thermal conductivity of a wall plastered by mortar containing PS decreased as the PS content was increased, whereas the thermal conductivity of a wall plastered by mortar containing hemp fiber increased as the hemp fiber content was increased. Thick crack was detected in the reference wall while hair line crack occurred from the wall plastered with PS and hemp fiber mortars. The results indicated that 10.0% PS could be used as a partial replacement for sand in mortar with an improvement in some of the properties of the mortar.

Effects of Bedding Geometry and Cementation Strength on Shale Tensile Strength Based on Discrete Element Method

To study the effects of anisotropy and heterogeneity on the shale failure mode and tensile strength, Brazilian splitting tests were performed from both directions of the bedding and layer thickness. Layers containing different bedding and loading angles and layer thicknesses were obtained separately. The results show that, at 0° and 90° angles, the shale cracks grow “linearly”; at 15°, the shale cracks have “arc type” growth; and at 30°–75°, the shale-splitting displays “broken line” crack propagation. The tensile strength from 0° to 90° exhibits an increasing trend. Water has a significant softening effect on the tensile strength of shale—the higher the water content, the lower the tensile strength. In addition, a 3DEC numerical simulation was used to simulate the tests, establishing shale specimen particles with random blocks. In the shale disc, uneven parallel bedding and uniform parallel bedding were set up with different loading angles and layer thicknesses to generate simulated stress-displacement curves, and the effect of layering on shale cleavage was analyzed from a mesoscopic perspective. The tensile strength of shale with uniform parallel bedding was found to be higher under the same conditions, which is consistent with the experimental results. By comparing the experimental and simulation results, from both the macro- and mesoperspectives, the Brazilian splitting crack growth of shale is affected by bedding, displaying a process from disorder to order. This study is of great significance for further exploration of the mechanical properties of shale under loading failure.

New perspective of fracture mechanics inspired by gap test with crack-parallel compression

The line crack models, including linear elastic fracture mechanics (LEFM), cohesive crack model (CCM), and extended finite element method (XFEM), rest on the century-old hypothesis of constancy of materials’ fracture energy. However, the type of fracture test presented here, named the gap test, reveals that, in concrete and probably all quasibrittle materials, including coarse-grained ceramics, rocks, stiff foams, fiber composites, wood, and sea ice, the effective mode I fracture energy depends strongly on the crack-parallel normal stress, in-plane or out-of-plane. This stress can double the fracture energy or reduce it to zero. Why hasn’t this been detected earlier? Because the crack-parallel stress in all standard fracture specimens is negligible, and is, anyway, unaccountable by line crack models. To simulate this phenomenon by finite elements (FE), the fracture process zone must have a finite width, and must be characterized by a realistic tensorial softening damage model whose vectorial constitutive law captures oriented mesoscale frictional slip, microcrack opening, and splitting with microbuckling. This is best accomplished by the FE crack band model which, when coupled with microplane model M7, fits the test results satisfactorily. The lattice discrete particle model also works. However, the scalar stress–displacement softening law of CCM and tensorial models with a single-parameter damage law are inadequate. The experiment is proposed as a standard. It represents a simple modification of the three-point-bend test in which both the bending and crack-parallel compression are statically determinate. Finally, a perspective of various far-reaching consequences and limitations of CCM, LEFM, and XFEM is discussed.