# Experimental Study and Modelling on the Structural Response of Fiber Reinforced Concrete Beams
## 3 Experimental Results and Discussion
As soon as the concrete absorbs the initial load, fibres carry most of the subsequently applied load until macro-cracks are formed and stretched fibres become visually observable at the fractured walls of the macro-cracks. The concrete-steel combination ensures higher material integrity, fibre sliding starts at higher values of the bending load or deformation in the sample. Although steel fibres demonstrate good load bearing capacity, PP fibres play a major role in resisting  
Figure 4: Average graphs: Stress—middle of a span Deflection, for all groups. the loads due to their elastic nature. The number of micro-cracks increases, and a network of micro-cracks is formed. The formation of macro-cracks occurs in the perpendicular direction to the longitudinal axis, which indicates a good way for load application. Density of the network of micro-cracks depends on the distribution of the particles in the concrete matrix, as well as the sizes and concentration of the fibres. The complete pull-out of the fibres occurs during the last stages of application of the critical load. The intensity of load in each fibre depends on the material of the fibre, its shape, length and orientation in relation to the plane of the crack, direction of the applied force of pull-out and the depth of fibre inside the matrix of the concrete member. During the last stage of loading (deflection higher than 3-3.5 mm), fibres bridging the macro-crack with one tail are pulled out of the concrete. The process when the polymer fibres are pulled out needs more constant load, in the case with steel fibres, the necessary load is decreasing more rapidly. The pull-out process finishes with the partial or complete removal of the fibres from the concrete matrix in the areas, where friction caused by slipping occurs between the fibre and concrete matrix. Once the fibres are fully stretched (fully utilised), the crack width increases. The specimen breaks, indicating the maximum loading capacity and the maximum tensile force absorption by the FRC member. The differences between steel and PP fibres are worth further discussion. Figure 4 allows considering the major differences between these two types of fibre, which are observed during the breaking (failure) mechanism of the concrete members. At the initial stage of loading, each steel fibre interacts with the surrounding concrete as a common material. Mainly thanks to the hooks on its ends, the steel fibre does not slip out of the concrete when micro-crack split the concrete. The hooked ends of the steel fibre in combination with high value of plastic stress necessary to deform steel fibre and the value of friction force between the fibre and concrete are the factors that determine higher resistance of this material. Compared to PP fibres, a higher load is necessary to start the pull-out process (15 MPa for steel fibres as compared with 7.2 MPa for Strux PP fibres and 6.4 MPa for Durus PP fibres). After the first peak on the curves, all samples have macro-cracks. During this pull-out process, steel fibre ends form clouds of micro-cracks. The concrete matrix is rapidly losing elastic modulus and toughness, which results in rapidly decreasing load bearing capacity. Polymeric fibres are softer, the pull-out process of each fibre happens along with the deformation of fibre surface and fibre body. Fibres are not hooked, and the pull-out force is more stable along the entire pulling length. Several observations can be made considering the results of the research:  
1. It is necessary to take care about the concrete mix during addition of the fibres to make the mix homogeneous, for example, by adding the fibres in layers. Proper finishing of the top layer is necessary, making sure no marks of fibres are left on the sidewalks. 2. Fibre pull-out is a process dependant on how far each fibre is pulled out.