## 4 Experimental Test Results
### Variable Amplitude Fatigue Loading
#### 4.2.3 Behavior of the Z-Jointed Precast FDDP under VAF Loading  
The compressive strengths of concrete cylinders for the NSC used to cast slab S6 were 43.26, 68.16, 64.99, and 65.74 MPa, with an average value of 60 MPa. The compressive strengths of the concrete cylinder for the UHPFRC used to fill the joints were 163.35, 183.31, and 153.28 MPa, with an average value of 167 MPa. The splitting tensile test for the UHPFRC resulted in tensile strength of 18.30, 20.47, and 21.69 MPa, with an average value of 20 MPa. The first 895,000 fatigue load cycles were performed at a frequency of 2 Hz, then 21,736 cycles at 1 Hz, and finally followed by 44,804 cycles at 0.5 Hz, leading to punching shear failure at a total number of cycles of 961,540. Figure 11e depicts the punching shear failure at the wheel footprint on top of the slab, while Figure 11f depicts the crack pattern at the bottom surface of the slab at failure. A fan-shaped crack pattern was observed at the bottom surface similar to those developed for the slab tested to collapse after passing the CAF loading. However, greater concrete spalling appeared along the perimeter on \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \begin{tabular}{c} **Slab** \\ **Number** \\ \end{tabular} & \begin{tabular}{c} **Joint Type** \\ \end{tabular} & \begin{tabular}{c} **Test \({}^{\star}\)** \\ \end{tabular} & \begin{tabular}{c} **Peak** \\ **Cyclic** \\ **Load (kN)** \\ \end{tabular} & \begin{tabular}{c} **Frequency** \\ **(Hz)** \\ \end{tabular} & \begin{tabular}{c} **No. of** \\ **Load** \\ **Cycles** \\ \end{tabular} & \begin{tabular}{c} **Ultimate** \\ **Load** \\ **(kN)** \\ \end{tabular} & \
\begin{tabular}{c} **Ultimate** \\ **Deflection** \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} **Failure** \\ **Mode** \\ \end{tabular} \\ \hline S1 & A & CAF + SUL & 137.5 & 4 & 4,000,000 & 930.92 & 23.47 & Punching \\ \hline  & A & VAF & 500.0 & 2–0.5 & 809,493 & 487.50 & 32.46 & Punching \\ \hline S2 & C & CAF + SUL & 137.5 & 4 & 4,000,000 & 973.00 & 26.09 & Punching \\ \hline  & C & VAF & 500.0 & 2–0.5 & 692,866 & 495.69 & 40.89 & Punching \\ \hline S3 & Z & CAF + SUL & 137.5 & 4 & 4,000,000 & 931.00 & 29.62 & Punching \\ \hline  & Z & VAF & 500.0 & 2–0.5 & 961,540 & 488.43 & 37.03 & Punching \\ \hline \hline \end{tabular}  
* CAF = constant amplitude fatigue loading; SUL = static ultimate load; VAF = variable amplitude fatigue load.  
\end{table} 
Table 4: Test results: loading data. Figure 12: Static load-deflection curves of slab S2 with A-joint after every 100,000 cycles of VAF loading. the punching shear plane at the bottom of the slab but only from one side of the closure strip when compared to the failure mode shown in Figure 8f for slab S5 subjected to CAF loading. This precast FDDP slab S6 failed at a jacking load of 488.43 kN, about 2.35 times the CHBDC factored design wheel load, and a maximum slab deflection of 37.03 mm. Figure 13 depicts the monotonic load-deflection relationship of the slab after each 100,000 fatigue load cycle. It can be observed that the slope of the curve decreased, leading to a reduction in slab flexural stiffness, with an increase in the number of VAF load cycles. To obtain a comprehensive overview of the punching shear failure at each side of the loaded area, slabs S1, S2, S3, S4, S5, and S6 were sliced at the load location in both the transverse direction (normal to the steel girders) and the longitudinal direction. Figure 14 shows schematic diagrams of the punching shear failure in the tested slab when sliced  
Figure 13: Monotonic load-deflection history for slab S6 at every 100,000 cycles of VAF loading.