## 4 Discussion of Test Results
### ITBF Test Results
#### 4.2.2 Analysis of the ITBF Test Results  
In order to speed up the test process and avoid UTM equipment damaged by long-time working conditions, a stress ratio 0.5 of ECC was chosen during this test based on SCB fatigue test results. Under this status, the highest internal stress level of ECC in composite beam specimens was 5.48 MPa. According to the mechanical calculation results, the loading force should be 6.2 kN and the stress amplification factor was 2.96. Under this loading condition, the test load on ECC was equivalent to about 40 tons of axle load on actual OSBD pavement. The displacement curve obtained in the UTM controlling software interface during this test is shown as Figure 12.  
Figure 11: ECC-steel composite specimen fatigue test according to orthotropic steel bridge deck (OSBD) pavement actual working conditions.  
Figure 12: Displacement curve of loading point of composite beam under stress ratio 0.5.  
By observing the test results shown in Figure 12, it can be seen that many mutation points occurred on the displacement curve due to the crack propagation on the surface or inside of the ECC specimen. However, the displacement of the loading point quickly returned to the original level after mutation point. This phenomenon reflects that the only inorganic cement mastic may crack in ECC during this stage and the existence of PVA fibers enabled the ECC to bear loading after the cement mastic was damaged. Due to this characteristic, cracks propagated in the ECC, but the load-carrying capability was not significantly weakened. With the increase of loading cycles, cracking damage became more and more serious, but no sudden brittle fracture occurred. However, the bending and tensile capacities of ECC were further decreased and the loading point displacement increased slowly while cracks were propagating. The fatigue crack morphologies on the ECC specimen surfaces in a composite beam are shown in Figure 13.  
Finally, two ECC-steel composite beams were tested, the fatigue lives were obtained and converted into the reference fatigue life under normal loading, as shown in Table 3.  
As shown in Table 3, the average fatigue life of these two composite beam specimens was about 240,000 loading cycles. According to the fatigue equation, the tested fatigue life was converted into reference fatigue life, which represents the fatigue life under normal vehicle loading without the load amplification factor. In this research, the reference fatigue life of ECC materials was 5.530 million cycles. In order to better reflect the fatigue cracking resistance of ECC, the reference fatigue lives of epoxy asphalt concrete (EAC) and high strength asphalt concrete (HSAC) were tested using the same test conditions, as shown in Figure 14.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Specimen No.**} & **Test Loading** & **Load** & **Test Fatigue** & **Average Test** & **Reference** \\  & \(F_{\text{max (kN)}}\) & **Amplification** & **Life** & **Fatigue Life** & **Fatigue Life** \\  & & **Factor \(\alpha\)** & **(Million cycles)** & **(Million Cycles)** & **(Million Cycles)** \\ \hline
1 & & & 0.2134 & & \\ \hline
2 & 6.2 kN & 2.96 & 0.2667 & 0.24 & 54.53 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Loading parameters and fatigue life results in ITBF test.  
Figure 13: ECC fatigue cracks in composite beams under the improved three-point bending fatigue test (ITBF) test.  
As Figure 14 shows, the reference fatigue life of ECC is 4.23 times and 16.63 times better than EAC and HSAC respectively, reflecting excellent fatigue cracking resistance. Besides, the fatigue cracking process of ECC is slow, tough, and has a strong working capability with existing cracks, which is beneficial in maintenance and repair work after cracking occurs.