# Hybrid Fibres as Shear Reinforcement in High-Performance Concrete Beams with and without Openings
## 3 Experimental Results and Discussion
### Load-Deflection Curves and Cracking Behaviour
#### 3.1.2 Cracking and Yielding Load  
The development of cracks at each stage of loading (every 25 kN) was monitored, marked and measured on the beams until the load level at which new cracks did not occur. At the last stage the beams were tested up to failure under monotonic loading. The tests were controlled by displacement and the deformation rate was 0.4 mm/min.  
External loads result in bending and direct stresses, initiating flexural, bond, and shear cracks. When the tensile stress in the concrete exceeds its tensile strength, internal microcracks occur. These cracks generate into macroracks propagating to the external edges of the member. After the first crack developed in RC beam, the stress in the concrete at the cracking zone is reduced to zero and is assumed by the steel bar. The first cracking load values were obtained from the load-deflection curves, the visual identification of crack, as well as the major strains images measured and calculated in the ARAMIS system. The secant stiffness at the point of two-thirds of the maximum load is applied to idealize the elastoplastic curve that passes through the maximum point of the load-deflection relationship. Then the deflection at an intersecting point between the two lines is used to determine the yielding load on the curve [62]. Table 4 summarizes the test results including the loading stages and corresponding beam deflections at cracking and yielding load. Cracking load is the point on the curve where the first flexural crack appears. It is used to calculate the initial stiffness of beams. The yielding load values were obtained from monitoring the load-deflection results.  
It can be seen from this table that increasing the fibre volume content of the beams resulted in increases in their cracking load, even for the beams with the openings. The increment in cracking loads in the FRC beams with openings arises from the corresponding increase in flexural strength of the fibre concrete and a decrease in the W/B ratio. These increments in cracking load were 4-23% and 45-47% for series B and BO, respectively, by doubling the fibre content. A rise in the yielding loads was observed with an increase in fibre content. For the 1.05% fibre content the increase was 23% for the B3 beam, and 25% for the BO2 beam with the openings as compared to the beams without fibres. The lower increase in the yielding load of the beam B3 than that in BO2 is effect of difference in the  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Beam** & **Fibre Volume** & \multicolumn{2}{c}{**Cracking**} & \multicolumn{2}{c}{**Initial Stiffness**} & \multicolumn{2}{c}{**Yielding**} \\ \cline{3-6}
**Notation** & **Content (\%)** & **Load (kN)** & **Deflection (mm)** & **(kN/mm)** & **Load (kN)** & **Deflection (mm)** \\ \hline B1 & – & 89.4 & 0.64 & 139.69 & 350.9 & 10.19 \\ B2 & 0.5 ST + 0.025 PP & 93.3 & 0.87 & 107.24 & – & \({}^{1}\) \\ B3 & 1 ST + 0.05 PP & 109.8 & 0.52 & 211.16 & 432.5 & 7.57 \\ BO1 & – & 85.2 & 1.45 & 58.76 & 331.3 & 10.03 \\ BO2 & 1 ST + 0.05 PP & 123.6 & 2.17 & 56.96 & 414.1 & 10.48 \\ BO3 & 1.5 ST +0.1 PP & 125.3 & 2.08 & 60.24 & 414.3 & 11.88 \\ \hline \hline \end{tabular} \({}^{1}\) Sudden beam failure due to fibre concrete shear failure.  
\end{table}
Table 4: Load-deflection response parameters of beams at cracking and yielding loads.  
Figure 4: Load-deflection curves for beams: (**a**) idealized response; (**b**) experimental response.  
W/B ratio associated with a different arrangement of reinforcement. On the other hand when the fibre content was 1.6%, the increase was 25% for beam BO3. The yielding loads of the BO beams with openings were slightly lower than those of the B beams. The difference was about 4.3%. This proves that using hybrid fibres, especially in beams with openings is beneficial.