# 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
#### 3.1.6 Crack Spacing and Widths  
The cracks' spacing and width were measured at the tension reinforcement level. The minimum, maximum and average crack spacing decreased with an increase in fibre content due to the rise in the beam load-bearing capacity. The crack spacing and final number of cracks are shown in Table 7.  
Figure 7: Beams failure modes. [PERSON] et al. [20] observed that average crack spacing in steel fibre RC beams with 0.5 to 0.75% fibre content was approximately 38% smaller compared with that of RC beams without fibres. [PERSON] et al. [29] reported that the average horizontal crack spacing for steel fibre RC beams without stirrups was approximately 0.4 d with 0.75 to 1.5% fibre content. [PERSON] et al. determined the average horizontal crack spacing at the mid-depth level was by considering all the distinct inclined cracks that propagated beyond the specimen mid-depth level at angles between 0 and 75\({}^{\circ}\) with respect to the longitudinal axis of the specimen. The average crack spacing at mid-depth level for beams with h = 308, 600, or 1000 mm ranged from 0.39 d to 0.6 d, 0.43 d to 0.6 d, and 0.35 d to 0.47 d, respectively [64]. The test results from the current study showed that the increase in fibre content for beam series B (from 0.53% to 1.05%) and BO (from 1.05% to 1.6%) in comparison to traditionally RC beams resulted in a drop in the average crack spacing by 7.9-86.3% and 4.9-6.2%, respectively. The average crack spacing for steel and polypropylene fibre RC beams without stirrups was 0.14 d to 0.24 d (for B beams with h = 400 mm). The crack spacing of the BO beams with openings and without stirrups was about 0.22 d with 1.05 to 1.6% fibre content. The reason for this behaviour is that crack spacing is dependent on the ratio of tensile strength to bond strength between the concrete and reinforcement (steel bars and fibres). The increase in tensile strength is lower than the increase in bond strength so their ratio decreases, thus a smaller distance between cracks is required to transfer the tensile stresses from the reinforcement and fibres the concrete. This means the smallest crack spacing will occur in the RC beams with the highest fibre content. It can be noted that the ratios of minimum/average crack spacing and maximum/average crack spacing for the beams with hybrid fibres was similar (except for the B3 beam with the highest number of cracks) and ranged between 0.160-0.195 and 1.805-1.827, respectively. The influence of fibre content on crack width is clear from Table 8. Increasing the fibre content from 1% steel fibres and 0.05% polypropylene fibres to 1.5% ST fibres and 0.1% PP fibres, resulted in a decrease in crack width at each of four loading stages. Interesting behaviour concerning the crack widths in the beams with fibres is given in Table 8, the crack widths in B2, B3 and BO3 are less than both the B1 and BO1 beams for all the loading levels. The flexural rigidities of the FRC beams are higher than traditionally RC beams. Smaller curvature and deflection resulted in narrower cracks in the FRC beams. This research and previous study [64] showed that the width of the diagonal crack in a FRC beams increases considerably at loads just before failure. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Beam**} & \multicolumn{6}{c}{**Crack Spacing**} \\ \cline{2-7}  & **Number** & **Minimum Crack** & **Maximum Crack** & **Average Crack** & **Minimum/Average** & **Maximum/Average** \\  & **of Cracks** & **Spacing (mm)** & **Spacing (mm)** & **Spacing (mm)** & **Crack Spacing** & **Crack Spacing** \\ \hline B1 & 42 & 26 & 164 & 95 & 0.274 & 1.726 \\ B2 & 30 & 17 & 159 & 88 & 0.193 & 1.807 \\ B3 & 57 & 23 & 79 & 51 & 0.451 & 1.549 \\ BO1 & 33 & 34 & 138 & 86 & 0.395 & 1.605 \\ BO2 & 36 & 16 & 148 & 82 & 0.195 & 1.805 \\ BO3 & 51 & 13 & 148 & 81 & 0.160 & 1.827 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Crack spacing at ultimate load.