Influence of hybrid fibre reinforcement on the flexural performance of concrete beams under static, sustained and cyclic loadings

Cracking of concrete members adversely affects both their structural behaviour and the durability. One of the ways to resist as well as reduce crack growth at different stages of loading is to introduce a variety of fibres randomly into the concrete mix at the time of manufacturing these structural members. Fibre-reinforced concrete (FRC) has been increasingly used in construction for several decades to reduce crack growth by incorporating one or two types of fibre in reinforced concrete elements under different types of load. Hybrid fibre system can, through their different lengths and constitution, help control the propagation of cracks in both the short and long term loadings and at both the micro and macro scales. Previous studies to reduce crack growth in reinforced concrete elements have been carried out by incorporating one or two types of fibre in concrete and under the static, cyclic / fatigue and sustained loadings. This study investigates the synergistic effects of hybrid fibre systems, combining micro and macro polypropylene fibres with steel fibres, to reduce both micro- and macro-cracking and hence enhance the performance of reinforced concrete (RC) beams subjected to static, repeated and sustained loadings. The experimental programme involved casting and testing 41 short prismatic (100x100x500 mm) beams (0.5 m long) and twelve long (150x300x4200 mm) reinforced concrete beams (4.2 m long). The beams were split into two groups and each group was subjected to a different loading scenario (static and repeated for small prisms and static, repeated and sustained loadings for full-scale beams). Analysis of load-displacement curves, the width and spacing of cracks, crack distribution, and failure mode were used to evaluate the behaviour of the beams. Furthermore, this research utilised finite element analysis to verify the load deflection and cracking in experimental findings and obtain a deeper understanding of the flexural behaviour of the beams in terms of maximum load, deflection and cracking. FE models satisfactory match the experimental tests in terms of load-displacement curves under static and repeated loadings, hence they were acceptable to be utilised to conduct the parametric studies. From the analysis of the experimental data, it has been found that the hybrid fibre system lowered the number of cracks, decreased the deflection, reduced the strain in the steel reinforcement bars and reduced the crack width. The beams, when taken to failure, also exhibited a more ductile failure under all loading conditions. The test results indicated that the addition of different types of fibre and different volume fractions resulted in higher compressive and splitting tensile concrete strengths. According to the test data obtained from this research, it was found that the hybrid fibre system comprising micro and macro polypropylene fibres along with steel fibres considerably improved the behaviour of concrete beams by enhancing their stiffness in the post cracking stage and, consequently, restricting the crack openings and deformations. The inclusion of more than two different fibre types, especially the combination of steel, micro and macro polypropylene fibres, demonstrated a pronounced positive hybrid effect and suggest that the effect of hybrid fibres is very important to provide crack resistance at different scales/levels compared to the incorporation of a single fibre. In addition to what already established with the addition of steel or/and polypropylene fibre, this research has established the synergistic effect of the combination of steel, micro and macro polypropylene fibre and highlights that the inclusion of this combination in concrete results in superior performance and outweigh the using of single type of fibre even though the cost is higher. This combination of fibres positively influence post-cracking behaviour, demonstrate a higher maximum load, maintain higher stiffness, and exhibit smaller reductions in stiffness compared to other fibre types. Additionally, the combination of steel, micro and macro polypropylene fibres in the hybrid system helps mitigate crack propagation, indicating a positive synergy effect and improved crack resistance, particularly under higher stress levels. These results represent a notable advancement compared to previous knowledge in the field. The outcome of this research indicated the need for further research on the effect of this combination on long-term sustained loads and lower stress cyclic loads. The findings can be utilised to improve the design and evaluation of fibre-reinforced concrete structures subjected to static, repeated and sustained loadings.