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Dynamic fracture toughness of fiber reinforced concrete Bindiganavile, Vivek Srinivasan

Abstract

The technique of instrumented drop-weight impact testing is often adopted to perform impact tests on cement-based materials. However, to date, no uniformity in the testing methods exists, which makes it difficult to enable reasonable comparisons between data emerging from different laboratories. Test machines come with widely varying hammermass systems, different ranges of drop-heights and various release mechanisms. As the University of British Columbia houses three such machines yielding a wide range of impact possibilities, one of the objectives for this research was to conduct a parametric study of drop-weight impact testing to study the effect of drop-height and hammer mass on the impact response of plain concrete. It was found that the test machine significantly influences the apparent stress-rate sensitivity. For the same incident energy, a heavier mass simulates a flatter pulse (i.e. a slower impact rate) but a higher drop-height (or approach velocity) simulates a sharper pulse (and consequently, a higher rate of impact). It is established that drop-height is the most important and critical parameter for a comparison of data across machines. Any future standard for impact testing of cementbased materials should emphasize drop-height of impact and not the hammer mass. Fiber reinforced concrete (FRC) is a heterogeneous material comprising of distinct components so that the mechanical properties of this composite are in effect, a sum of individual responses, which are affected by their mutual interactions. Many applications demand from FRC, an enhanced resistance to impact loading. Designing for impact involves understanding the impact response of each of the various phases within the material, viz. the concrete matrix, the fibers, and the fiber-matrix interface, the last one being the most critical component. Although plain concrete and, to a lesser extent, steel fiber reinforced concrete have been the subject of high stress-rate testing, limited data exists on the impact response of polymeric fiber reinforced concrete. Given that both the matrix (plain concrete) and the fibers (metallic or polymeric) depict widely varying stress-rate sensitivities, it is a moot point as to how the resultant multi-phase material behaves under impact loading. The properties of FRC were investigated at the level of a) the fiber-matrix interface, b) crack growth & bridging and c) as a structural material. To this end, over 300 single fiber pull-out tests, 60 crack growth tests and over 300 flexural tests were carried out. In this program, three drop-weight impact machines and an air-gun driven dynamic pull-out machine were utilized. For the first time, a drop-weight impact machine was configured to conduct fracture studies of Contoured Double Cantilevered FRC beams under impact loading. The thesis reports a complete dynamic analysis, which was performed to identify and account for the inertial effects during crack growth testing. The results reveal that inertial correction was significant in the case of plain and polypropylene fiber reinforced concrete but was negligible when steel fiber was used. Pull-out of single-fibers reveal that bond stiffening occurred under impact. This was evident through higher peak loads and lower corresponding slip values. Polymeric fibers had higher slip values under static conditions, but under impact, their slip values approached that of steel fibers at all angles of orientation. This capacity of polymeric fibers to approach the behaviour of a higher modulus material such as steel was repeatedly evident in fracture tests as well as flexural tests. The flexural toughness of steel and polypropylene FRC converged at higher drop-heights. Specimen size-effect on the impact response of FRC has not received adequate attention and hence forms a significant part of this study. The results indicate that, provided the self-weight is ignored, both plain and fiber reinforced concrete exhibit size-effects on their flexural strength under impact. However, the nature of this size-effect was not clear from the present work as the data appeared to fit conflicting empirical models as given by Bazant's Size Effect Law and by the Multifractal Scale Law. Under impact, the flexural toughness with both types of fibers demonstrated size effects, a phenomenon that was not seen during quasi-static tests. Size effect under impact also appeared to intensify with an increase in the drop-height for both plain and fiber reinforced concrete.

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