## 3 Results and Discussion
### Ib-Value Analysis
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Ib-Value Curves for RC Beams** & **Maximum Value** & **Minimum Value** \\ \hline Uncorroded RC beam & 0.129 & 0.032 \\
10\% corroded RC beam & 0.116 & 0.031 \\
20\% corroded RC beam & 0.104 & 0.031 \\
30\% corroded RC beam & 0.087 & 0.046 \\
40\% corroded RC beam & 0.083 & 0.028 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Maximum and minimum values of the improved b (Ib)-value curves. proportions of macro-cracks and micro-cracks are nearly the same, which indicates that the RC beam is in the crack stable extension stage. In the load stage after 350 s, the curve fluctuates violently. Such a situation demonstrates that a large amount of both micro- and macro-cracks were formed in the RC beam continually, and the RC beam had entered into the unstable fracture extension stage. Finally, unstable failure of the beam occurs. In summary, the uncorroded RC beam represents a poor bending strength because it formed a large number of macro-cracks and was crushed at the load point in the early stage of loading. Therefore, the Ib-value curve shows that the uncorroded RC beam conforms to the failure mode of over-reinforced, which is consistent with the previous analysis results in Section 3.1. From the Ib-value curve of the 10% corroded beam, it can be seen that in the early load stage from 0s to 100 s, the curve, at first, decreases slowly, which indicates that the RC beam is in the crack initiation stage. While for the period of 100-180 s, curve gradually rises and then falls, showing that lots of macro-cracks are formed, and then the micro-cracks continue to extend; the RC beam is in the crack stable extension stage. Later on, in the load stage after 180 s, the curve fluctuates violently, a large number of AE signals generated by the initiation of cracks or relative movement between steel and concrete are detected by sensors, meaning that the RC beam is in the unstable fracture extension stage. In summary, the Ib-value curve shows that the 10% corroded RC beam conforms to the failure mode of balanced-reinforced. From the Ib-value curve of the 20% corroded beam, it can be observed from the curve that proportions of the formed micro- and macro-cracks are nearly the same from 0 to 350 s, suggesting that the RC beam is in the crack stable extension stage. After 350 s, the curve fluctuates violently, indicating that the RC beam is in the unstable fracture extension stage. Comparing with the 10% corroded beam, the 20% corroded beam enters into the crack stable extension stage earlier, and its' duration over which its curve violently fluctuates is shorter. It can be seen from the Ib-value curve of the 30% corroded beam, there are macro-cracks formed in the RC beam at the early load stage of 0 to 20 s, which is caused by cracking at the bottom of the beam. Then, the RC beam enters in the crack stable extension stage from the 20 to 180 s. Finally, after 180 s, the RC beam is in the unstable fracture extension stage. Comparing with the 30% corroded RC beam, the amplitude of AE signals for the 40% corroded RC beam is stable during the early stage from 0 to 50 s. This phenomenon indicates that the bond behavior between the steel bar and concrete is weak and almost all the AE events are produced by concrete. Later on, cracks extend steadily from 20 s to 150 s. After 150 s, a large number of macro-cracks are formed within a very short time, and the specimen is destroyed immediately. In summary, it can be concluded that the steel bar has little contribution to the flexural strength of the 40% corroded RC beam, and this beam conforms to the failure mode of rare-reinforced. In summary, it can be concluded that steel corrosion reduces the maximum and minimum values (i.e., the variation range) of the Ib-value curves due to the generation of macro-cracks in concrete due to the expansion stress of steel bars.