Dynamic characteristics of coal specimens with varying static preloading levels under low-frequency disturbance load

The mechanical properties of residual coal pillars under the influence of upward mining disturbances significantly affect the safety of residual mining activities on working faces.This study conducted low-frequency disturbance dynamic uniaxial compression tests on coal specimens using a self-developed dynamic-static load coupling electro-hydraulic servo system,and studied the strength evolutions,surface deformations,acoustic emission(AE)characteristic parameters,and the failure modes of coal specimens with different static preloading levels were studied.The disturbance damage is positively correlated with the coal specimen static preload level.Specifically,the cumulative AE count rates of the initial accelerated damage stage for the coal specimens with static preloading level of 60%and 70%of the uniaxial compressive strength(UCS)were 2.66 and 3.19 times that of the 50%UCS specimens,respectively.Macroscopically,this behaviour manifested as a decrease in the compressive strength,and the mean strengths of the disturbance-damaged coal specimens with 60%and 70%of UCS static preloading decreased by 8.53%and 9.32%,respectively,compared to those of the specimens under pure static loading.The crack sources,such as the primary fissures,strongly control the dynamic response of the coal specimen.The difference between the dynamic responses of the coal specimens and that of dense rocks is significant.

Behavior of preloaded RC beams strengthened with CFRP laminates

Eighteen reinforced concrete beams, including 16 beams strengthened with CFRP laminate at different levels of preload and 2 control beams, were tested to investigate the influence of preload level on flexural behavior of CFRP-strengthened RC beam. The experimental parameters include rebar ratios, number of plies of CFRP laminates and preload level at the time of strengthening. Theoretical analysis was also carried out to explain the experimental phenomena and results. The experimental and theoretical results indicated that the preload level has more influence on the stiffness and deflection of the strengthened beam, both at post-cracking and post-yielding stage, than that on the yielding and ultimate flexural strength of the strengthened beam. The main failure mode of CFRP-strengthened beam is the intermediate crack-induced debonding of CFRP laminates, provided that the development length of CFRP laminates and shear capacity of the beam are sufficient.

Frost Resistance and Damage Velocity of Compressively Preloaded Concrete

The frost resistance and compressive strength degradation of concrete under the simultaneous action of compressive load and freeze-thaw cycles were experimentally investigated. Air-entrained and non-air-entrained specimens with different water/cement(w/c) ratios were subjected to different compressive stress by specially designed apparatus, while the specimens suffered freeze-thaw cycles. In order to track the strength degradation process, the nondestructive tests were carried out after each freeze-thaw cycle got the residual strength for each specimen. Based on the experimental data, a variable Kss was proposed to describe the damage velocity. Experimental results indicate that the deterioration processes are accelerated by the compressive loads, and the damage velocity increases with the increases of the preloading levels and w/c ratios. The air entrainment decreases the damage velocity and improves the frost resistance of non-air-entrained concrete, although it would reduce the compressive strength of concrete.