Toughness Properties of Steel-Polypropylene Fibre Reinforced Concrete under Elevated Temperature

Application of fibres in concrete which is also known as fibre reinforced concrete (FRC) has been found to improve the energy absorption or toughness of the material. However, only little information on FRC toughness is found when it is exposed to high temperature such as under prolonged heat or fire. The main objective of the study is to evaluate the toughness behaviour of FRC by experimentation when exposed to elevated temperature. The fibre used in the experimental work is steel (ST) or polypropylene (PP), and also the combination of both fibres. The fibre dosage varied between the ST and PP summarised as (100-0), (75-25), (50-50), (25-75) and (0-100). Meanwhile, the total volume fraction, Vf is fixed at 1.5% and they are exposed to elevated temperature at the following degree; room temperature (27 C), 200 C, and 400 C. The research shows that the addition of fibres in concrete enhanced the FRC toughness, however, it reduces as the exposure temperature increases. Introduction Toughness is the ability of the materials to absorb energy during deformation. It is determined by calculating the area under the load-deflection graph. In order to improve the concrete toughness, fibres are usually added in the mixture also known as fibre reinforced concrete (FRC). At the same time, it will also increase the flexural strength of the concrete, improved ductility and cracking resistance [1-3]. The crack growth can also be delayed when the total volume fraction of the fibre increases. Previous study found that adding steel (ST) fibre more than the polypropylene (PP) fibre in the same mixture can enhance the FRC toughness [3]. FRC toughness can be affected when it exposed to heat. Heat can come from many sources such as fire and prolonged high temperature on the exposed surface. Explosive spalling has been observed by many researchers often resulting in serious concrete deterioration [4-7]. High temperature causes dramatic physical and chemical changes, resulting in concrete deterioration [8,9]. Therefore, to reduce deterioration and spalling, additional material such as ST or PP fibre is added in the mixture which has sufficient fire protection [10, 11]. However, minimal or even negative effects of PP fibre on the residual performance of the heated concrete may also occur [12]. Most literature reviews agree that the addition of single type of fibre in concrete can significantly improve concrete resistance. However, only a small amount of research work is found which combined two or more types of fibre in concrete. Even more, the findings from the FRC exposure to heat and tested for its mechanical properties are still limited. Combining two or more types of fibre in a single mixture has many advantages. However, it has to be balanced between the macro and micro fibres. This is because the micro fibres will inhibit the growth of early cracks that occur in the concrete by blocking the channel for bleeding process and therefore reduced the bleeding effects. While macro fibres, usually act as a tier for the concrete materials under applied load. Adding macro fibres will give stages of failure compared with plain concrete which is sudden and can easily breaks into two parts at failure [13]. In this study, ST and PP fibre is added in concrete mixture singularly and also by combining both fibres. This is because there is still little information on flexural toughness when both macro and micro fibres are combined in a concrete mixture. Furthermore, the main focus of the study is exposing the specimens to an elevated temperature of 200 C and 400 C of which most previous study only tested at room temperature i.e. 27 C. CONCRETE & STEEL MATERIAL 629 Experimental Program In order to achieve the objectives of the study, experimental works is carried out in the laboratory. Ordinary Portland Cement (OPC) is used in this study where the water-to-cement ratio is fixed at 0.53. This is to achieve concrete grade of C40 and at the same time to ensure that the mixture is not too dry when it is mixed with fibres. Meanwhile, fine aggregates of below 10 mm are used and coarse aggregates are between 10 mm and 20mm. The total volume fraction of fibres, Vf are fixed at 1.5%from the total percentage of the concrete. The application of Vf = 1.5% is based on the findings by previous researcher which shows that the best proportion of the combined fibres for FRC is 75% ST fibres and 25% PP fibres [13]. The Vf = 1.5% is divided into five main batches to get the single and combined mixture: (i)100% ST fibres with 0% PP fibres (100-0), (ii) 75% ST fibres with 25% PP fibres (75-25), (iii) 50% ST fibres with 50% PP fibres (50-50), (iv) 25% ST fibres with 75% PP fibres (25-75), and (v) 0% ST fibres with 100% PP fibres (0-100). Each concrete batch is then left to cure and tested at room temperature (27 C), and also exposed to 200 C and 400 C for one hour or 60 minutes. This means that in every concrete batch there will be 3 sub-batches. For each concrete batch, 3 prisms of 150 mm 150 mm 550 mm are used. The volume of ST fibres is calculated using the following expression: