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Impact behaviour of concrete under multiaxial loading Sukontasukkul, Piti

Abstract

Impact testing of concrete under different types of loading conditions (compression, tension and bending) has been carried out worldwide. However, it has been found that the results are mostly not comparable due to the differences in testing techniques and specimen configurations. In this study, a series of impact tests: compression, 1-dimensional bending (beam tests) and 2-dimensional bending (plate tests) were carried out using the same testing technique (drop weight machine) and similar configuration (hammer weight, height and impact energy). Concrete exhibited more rate sensitivity under 1-D flexure than under compression. However, in 2D flexure, the rate sensitivity of concrete decreased significantly, to a level about the same as that under compression. In addition, the impact behavior of concrete under confining stress was also investigated. The confinement technique adopted here was of the active type using the instrumented confinement apparatus designed and constructed at the University o f British Columbia. These experiments were also carried out in compression and flexure ( ID and 2D). In all cases, the measured properties of concrete (failure mode, stress-strain response, strength, inertial load, and rate sensitivity) changed significantly with increasing confining stress. Under confined compression, the failure mode of concrete changed from a shear cone to splitting failure and resulted in higher stress rate sensitivity. However, under confined flexure, the stress rate sensitivity decreased with increasing confinement due to the change of the failure mode from flexure to shear. To complete the confinement study, a further investigation on the effect of the loading platen stiffness (soft or hard) and tup diameter (plate specimens only) was also carried out. Analytically, scalar damage mechanics (SDM) was used to predict the behavior of confined and unconfined concretes under both static and impact loading. The damage measurement technique was selected differently, depending upon the kind of loading (static or impact). Under static loading, the damage was defined in terms of the change or degradation of elastic modulus (E). However, the strain rate variation was used to define the damage of concrete under impact loading. It was found that the use of S DM to predict the response of concrete works quite well; the predicted responses of concrete under both loading conditions agreed reasonably with the measured responses.

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