# Blistering Mechanism Analysis of Hydraulic Asphalt Concrete Facing
## 5 Conclusions  
Through on-site observation and laboratory tests, the following conclusions can be drawn about the blister defects of Zhanghewan asphalt concrete slab:  
(1) Blisters with a diameter was 5 \(\sim\)100 cm, gradually appeared after two years of completion of the project, and new blisters are still appearing after ten years of operation. The blisters are distributed in the daily elevation range of the reservoir water level in the reservoir, indicating that blistering is related to the exposure of the asphalt facing.  
(2) Through the core inspection and the ground penetrating radar test, it was found that although the blistering was mainly located in the upper impervious layer, there was no phenomenon whereby the entire upper impervious layer was blistered, indicating that the blistering was not caused by blocking of the drainage layer.  
(3) The core inspection indicated that the air void content of the upper impervious layer was 2.47%-2.95%, i.e., less than 3%, which virtually eliminates the possibility that the water in the blister came from an external source (e.g., storage water, rainwater).  
(4) When moisture entered and was enclosed in the upper impermeable layer, the analysis showed that under the ambient temperature conditions of Zhanghewan, the enclosed moisture generated a vapor pressure of 13-22 kPa (related to the initial temperature selection). The model test showed that for an internal cavity of 50 cm diameter (which is common in the Zhanghewan site), the saturated steam pressure could make the cavity form a blister. Furthermore, for a cavity with a diameter of 50 cm and a depth of 5 cm, when the temperature was 55 \({}^{\circ}\)C and the steam pressure was 20 kPa, the blister would rupture. These test results are consistent with the situation of the on-site blisters.  
Based on the above analysis, it is estimated that the formation process of the blistering was as follows. The impervious layer was not fully pre-compacted after paving, and water entered when the vibrating roller was spraying water. The moisture was then sealed in the impervious layer after rolling, where many small water bodies were formed. After the reservoir commenced operation, these small water bodies continuously connected and grew under steam pressure, gradually forming blisters of different sizes according to the size and depth of the connected water bodies. In addition, there are virtually no blisters in the parts exposed above the water level. This is because these areas were artificially paved and no water spraying was used.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Thickness** **(cm)** & **Temperature** **(\({}^{\circ}\)C)** & **Pressure** **(kPa)** & **Deflection** **(mm)** & **Deflection** **Span Ratio** & **Remarks** \\ \hline  & 45 & 11 & 3 & 0.006 & No cracking, no leakage \\
10 & 50 & 14 & 11 & 0.022 & No cracking, no leakage \\  & 55 & 20 & 28 & 0.056 & No cracking, no leakage \\  & 45 & 11 & 11 & 0.022 & No cracking, no leakage \\
5 & 50 & 14 & 22 & 0.044 & No cracking, no leakage \\  & 55 & 20 & 58 & 0.116 & Cracked, leakage \\ \hline \end{tabular}
\end{table}
Table 6: Model test results.  
**Funding:** This research was funded by the State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, grant number SM0112B242018, and by the China Institute of Water Resources and Hydropower Research (IWHR), grant numbers SM0145B442016 and SM0145B632017.  
**Acknowledgments:** The authors would like to thank the anonymous reviewers for their constructive suggestions to improve the quality of the paper.  
**Conflicts of Interest:** The authors declare no conflicts of interest.