Experimental and numerical study of cementitious composites subjected to high strain rate tensile loadings

Over the last decades, many attacks involving explosive materials have taken part in sensitive building and structures in different parts of the world. These events have stimulated scientists to study and provide solutions for this problem from different standpoints. From the point of view of the structural engineers, the studies have focused on the behaviour of structural materials subjected to blast loading and on the development of new construction materials with enhanced mechanical properties when subjected to high strain rates. According to the experience gained from this kind of events, many victims of these attacks were not caused by the direct impact of the explosion but by the so called progressive collapse. Previous researches have shown that the use of ductile materials may improve the structural behaviour against such progressive collapse. In this sense, self-compacting concrete reinforced with different steel fiber shapes can be considered a good candidate to improve structural behaviour against collapse due to its enhanced ductility. For these reasons this thesis focus on self-compacting concrete reinforced with two types of fibers, to investigate its properties under a wide range of loading rates. Three different self-compacting fiber-reinforced concretes, named A, B, and C, having the same cementitious matrix but different quantity and types of fibers were produced. The types of fibers considered were straight and hooked-end types. 40 kg/m3 of straight fibers were used in the three mixes, while the amount of hooked-end fibers was 0, 20 and 60 kg/m3 for the concretes A, B, and C, respectively. These three concretes exhibited different mechanical behavior in all tests performed on them, even under quasi-static loading rates. For the three concretes, three-point bending tests were conducted on prismatic notched beams with two different instruments, a servo-hydraulic testing machine and a drop-weight impact machine. The recommendations of the RILEM TC 162- TDF committee and the standard of EN 14651 were followed. With the servo-hydraulic machine two different loading rates were used: 2.20× mm/s (quasi-static) and 2.20× mm/s. The result showed an increase in peak load and fracture energy due to the increase in fibers content and in the loading rate. About the tests conducted with the drop-weight impact machine, three different heights were used (40mm, 160mm and 360mm) and a fixed weight for the impactor (120.6 kg), producing therefore three different impact energies and velocities. The results of these tests demonstrated again that the higher the strain-rate, the higher the specific fracture energy for the three concretes. However, such increase of the specific fracture energy is lower as the quasi-static strength of the concrete is higher. Besides the abovementioned laboratory tests, blast test were conducted on four slabs of each concrete type. To this end a steel structure with the capability of testing up to four slabs subjected to the same explosive load was used. For each concrete, one slab was subjected to an explosive load of 2.46kg of TNT equivalent, while three slabs were subjected to 3.18kg of TNT equivalent. In all cases the standoff distance of the explosive was kept fixed at 1.5m. The results indicated that increasing the amount of fiber prevents fragmentation of the specimens, leading to a more distributed crack pattern. Besides this, these tests were used to validate the numerical model for fracture of concrete under high strain rates presented in this thesis. In order to study the residual strength of the concretes, some of the slabs previously tested under explosive loading were retested under static loading to investigate the flexural load and to compare them with intact slabs. The results provides valuable experimental information concerning the behaviour of this kind of material against progressive collapse. In the numerical field, a new approach for the simulation of fracture of fiber reinforced concretes under high strain rates has been presented. Such approach is based on the cohesive zone model (CZM) in conjunction with the strong discontinuity approach. The original contribution of this new approach consists on the definition of the dynamic increase factor (DIF) for the tensile strength as a function of the cohesive crack opening. This new approach has been implemented in the finite element commercial code LS-DYNA through a user subroutine. The numerical model has been successfully applied to the experimental tests conducted in this thesis, with the only exception of the residual strength tests. The good agreement achieved between the numerical simulations and the experimental results supports the hypotheses made in this new model