Behaviour of steel-fibre-reinforced concrete beams under high-rate loading

The present study focuses on examining the structural behaviour of steel-fibre-reinforced concrete (SFRC) beams under high rates of loading largely associated with impact problems. Fibres are added to the concrete mix to enhance ductility and energy absorption, which is important for impact-resistant design. A simple, yet practical non-linear finite-element analysis (NLFEA) model was used in the present study. Experimental static and impact tests were also carried out on beams spanning 1.3 meter with weights dropped from heights of 1.5 m and 2.5 m, respectively. The numerical model realistically describes the fully-brittle tensile behaviour of plain concrete as well as the contribution of steel fibres to the post-cracking response (the latter was allowed for by conveniently adjusting the constitutive relations for plain concrete, mainly in uniaxial tension). Suitable material relations (describing compression, tension and shear) were selected for SFRC and incorporated into ABAQUS software Brittle Cracking concrete model. A more complex model (i.e. the Damaged Plasticity concrete model in ABAQUS) was also considered and it was found that the seemingly simple (but fundamental) Brittle Cracking model yielded reliable results. Published data obtained from drop-weight experimental tests on RC and SFRC beams indicates that there is an increase in the maximum load recorded (compared to the corresponding static one) and a reduction in the portion of the beam span reacting to the impact load. However, there is considerable scatter and the specimens were often tested to complete destruction and thus yielding post-failure characteristics of little design value and making it difficult to pinpoint the actual load-carrying capacity and identify the associated true ultimate limit state (ULS). To address this, dynamic NLFEA was employed and the impact load applied was reduced gradually and applied in pulses to pinpoint the actual failure point. Different case studies were considered covering impact loading responses at both the material and structural levels as well as comparisons between RC and SFRC specimens. Steel fibres were found to increase the load-carrying capacity and deformability by offering better control over the cracking process concrete undergoes and allowing the impact energy to be absorbed more effectively compared to conventional RC members. This is useful for impact-resistant design of SFRC beams

Assessing the accuracy of RC design code predictions through the use of artificial neural networks

Abstract In light of recently published work highlighting the incompatibility between the concepts underlying current code specifications and fundamental concrete properties, the work presented herein focuses on assessing the ability of the methods adopted by some of the most widely used codes of practice for the design of reinforced concrete structures to provide predictions concerning load-carrying capacity in agreement with their experimentally established counterparts. A comparative study is carried out between the available experimental data and the predictions obtained from (1) the design codes considered, (2) a published alternative method (the compressive force path method), the development of which is based on assumptions different (if not contradictory) to those adopted by the available design codes, as well as (3) artificial neural networks that have been calibrated based on the available test data (the later data are presented herein in the form of a database). The comparative study reveals that the predictions of the artificial neural networks provide a close fit to the available experimental data. In addition, the predictions of the alternative assessment method are often closer to the available test data compared to their counterparts provided by the design codes considered. This highlights the urgent need to re-assess the assumptions upon which the development of the design codes is based and identify the reasons that trigger the observed divergence between their predictions and the experimentally established values. Finally, it is demonstrated that reducing the incompatibility between the concepts underlying the development of the design methods and the fundamental material properties of concrete improves the effectiveness of these methods to a degree that calibration may eventually become unnecessary

Seismic response of steel fibre reinforced concrete beam-column joints

The present research work aims to investigate numerically the behaviour of steel fibre reinforced concrete beam-column joints under seismic action. Both exterior and interior joint types were examined and 3D nonlinear finite element analyses were carried out using ABAQUS software. The joints were subjected to reversed-cyclic loading, combined with a constant axial force on the column representing gravity loads. The joints were initially calibrated using existing experimental data – to ascertain the validity of the numerical model used – and then parametric studies were carried out using different steel fibre ratios coupled with increased spacing of shear links. The aim was to assess the effect of introducing steel fibres into the concrete mix in order to compensate for a reduced amount of conventional transverse steel reinforcement and hence lessen congestion of the latter. This is particularly useful for joints designed to withstand seismic loading as code requirements (e.g. Eurocode 8) lead to a high amount of shear links provided to protect critical regions. The spacing between shear links was increased by 0%, 50% and 100%, whilst the fibre volume fraction (Vf) was increased by 0%, 1%, 1.5%, 2% and 2.5%. Potential enhancement to ductility, a key requirement in seismic design, was investigated as well as potential improvements to energy absorption and confinement. The work also examined key structural issues such as strength, storey drift, plastic hinges formation and cracking patterns