Experimental study on dynamic mechanical properties and damage law of steel fiber concrete

Investigating the effect of steel fiber blending schemes on the dynamic mechanical properties and damage evolution of concrete is crucial for deep-shaft construction projects. Dynamic impact tests were conducted on steel fiber concrete specimens with varying mixing schemes and grades, according to the settings of the ultra-deep shaft-lining support project of a mine in Yunnan Province. The evolution of strain fields on the specimen surfaces under impact loads was analyzed using digital image correlation techniques. The steel fiber mixing schemes were as follows: single mixing, in which 55 kg of end-hook long fibers were added per cubic meter of concrete; double mixing, which involved the addition of 15 kg of end-hook long fibers and 40 kg of short copper-plated straight fibers per cubic meter of concrete; and triple mixing, which involved the addition of 40 kg of end-hook long fibers, 5 kg of copper-plated straight medium-length fibers, and 10 kg of short copper-plated straight fibers per cubic meter of concrete. The test results indicated that the dynamic compressive strength and dissipated energy ratio of the triple-mixed steel fiber concrete specimens were greater than those of the single- and double-mixed steel fiber concrete specimens. The smaller dynamic strength of plain concrete, the more pronounced the enhancement in the dynamic strength observed in concrete specimens with a steel fiber admixture. A high-speed camera was used to record the complete failure process of the steel fiber concrete specimens. The camera revealed that the failure mode was influenced by the concrete grade and steel fiber mixing scheme. The observed failure modes included shear, splitting, and shear-splitting failures. Compared with plain concrete specimens, steel fiber concrete specimens exhibited fewer cracks, a lower percentage of reflected energy, and higher percentages of transmitted and dissipated energy under impact loading. This indicates that the steel fibers effectively inhibited crack initiation and propagation, thereby enhancing the stability of the shaft wall concrete. Non-destructive impact tests on steel fiber concrete specimens showed that the triple-mixed steel fiber scheme inhibited concrete damage under impact loading to the maximum extent. It is recommended that the lining support for the deep shaft of the mine be constructed using grade C50 concrete with steel fibers mixed with 40 kg of end-hooked long fibers, 5 kg of copper-plated straight medium-length fibers, and 10 kg of short copper-plated straight fibers per cubic meter of concrete.