The addition of polypropylene fibers to concrete has no significant effect on the compressive strength and elastic modulus of concrete, but it does increase the tensile strength of concrete. The splitting tensile strength of fly ash concrete is about 9% to 13% of its compressive strength. Adding polypropylene fibers to concrete can increase the splitting tensile strength by about 20% to 50% [16].
1. Compressive strength: The compressive strength of concrete is a very important parameter as it determines other parameters such as tension, bending, etc. Many literature have discussed the influence of polypropylene fibers on the compressive strength of concrete and observed that polypropylene fibers reduce or increase the compressive strength of concrete, but in many cases, the overall effect can be ignored. In fact, the influence of small volume polypropylene fibers on the compressive strength of concrete may be masked by experimental errors.
2. Bending tensile strength: With the increase of fiber volume fraction, the bending tensile strength increases. As the aspect ratio of fibers increases, the strength of fibers also improves.
3. Fracture performance: The use of fibers effectively improves the failure performance of high-strength concrete. It can avoid typical shear bond fracture caused by strain localization. On the contrary, as shown in Figure 7, a large number of longitudinal cracks were formed on the entire concrete specimen, mainly oriented parallel or sub parallel to the direction of external compressive stress.
4. Creep and shrinkage characteristics of concrete: Fiber reduces creep strain, which refers to the time-dependent deformation of concrete under constant stress. However, the compressive creep value may only be 10% to 20% of that of ordinary concrete. Due to the extraction of water from the concrete during the drying process, the shrinkage of the concrete is also reduced by fibers. It displays the shrinkage, creep, and total time-varying deformation of various PFRC mixtures and non fiber concrete mixtures. From many perspectives, due to the presence of fibers, the shrinkage rate will decrease. Firstly, the fibers will not experience any shrinkage, thereby reducing the overall shrinkage of the mixture. In addition, the water retention effect of fibers in concrete mix proportions reaches a certain limit, which helps to delay shrinkage. Therefore, adding fibers to the concrete mix proportion is beneficial for reducing shrinkage deformation.
5. Bending impact characteristics: The number of hammer blows required to form the first visible crack on the lower surface of the beam is defined as the initial crack impact number (NCR). The failure impact number nf refers to the number of times a major macroscopic crack propagates from the bottom of the beam to the top of the beam. The impact ductility index is defined as the ratio of the number of failed impacts to the number of initial crack impacts, which can be used to represent the bending impact ductility.
6. Chlorine ion penetration: In addition to improving mechanical properties due to fiber wrapping, the presence of fibers also greatly reduces chloride ion penetration, depending on the orientation of the fibers. Antoni [17] studied the chloride permeation effect and found that due to the random orientation of short fibers, the permeation effect of short fibers is negligible compared to long fibers. In addition, in the direction perpendicular to chloride ion penetration, the presence of fibers as interface transition zones significantly reduces the movement of chloride ions into concrete, while fibers provide a easier pathway for chloride ions to migrate along the direction of fibers.