# Blistering Mechanism Analysis of Hydraulic Asphalt Concrete Facing
## 3 Blister Defect Characteristics of Zhanghewan Asphalt Concrete Facing  
In April 2016, a comprehensive survey of the Zhanghewan asphalt concrete facing was conducted. A total of 282 defects were found, including 193 blisters, 87 local small-scale flows, a 40 cm long horizontal crack, and a blister on the sealing layer. The blisters were distributed between EL. 795 m and EL. 811 m, which was more or less consistent with the zone of the water level fluctuations; for example, the number of blisters from EL. 808 m to EL. 809 m was 41 (shown in Figure 2), indicating that blistering was related to exposure. In terms of size, blisters with diameters between 10 cm and 30 cm accounted for 78%, and those with a diameter of 10 cm were the most common with about 42 (shown in Figure 3). Blister size was substantially evenly distributed along the elevations. In terms of appearance, half of the  
A \(\Phi\)30 cm core was drilled from a \(\Phi\)40 cm blister at station number 0 + 325, EL. 801 m; the blister was cracked at surface. When drilling to about 5 cm deep, pressure water poured out, and the upper impervious layer of the core was found to separate into two layers at a depth of about 4-5 cm, as shown in Figure 5; the lower part of the upper impervious layer had no cracks or defects. This means there was enclosed water in the middle of the upper impervious layer, and the blister was likely to have been caused by the steam pressure of the water. Since the diameter of the blister was larger than the diameter of the core, the separation marks could still be seen on the inner wall of the drill hole.  
In order to further inspect the blister, 9 \(\Phi\)10 cm core samples were drilled in three typical areas, and the void content of the upper impervious layer of the core samples were tested. The void content and the permeability coefficient of the drainage layer were also tested and are listed in Table 2. It was found that all the core samples showed separation in the upper impervious layer, as shown in Figure 6. Geological radar test results also indicated that the blister defect was located in the upper impervious layer [14]. It can be seen from the test results that the void content and permeability coefficient of the drainage layer were moderate, the drainage layer was smooth, and no leakage water was seen in the drainage gallery, indicating that the moisture in the blister was less likely to have come from the drainage layer. At the same time, the void content of the impervious layer was less than 3%. As we know, when the void content of asphalt concrete is below 3%, the permeability coefficient will be less than 10\({}^{-8}\) cm/s [15] and it is almost impermeable, which indicates that reservoir water is not likely to diffuse into the asphalt concrete. Therefore, the moisture in the blister was not likely to have come from reservoir water.  
Figure 5: Core of the blister (\(\Phi\) 30 cm).  
Figure 6: Core samples (the coarse part is the drainage layer).  
In addition, there were no problems of \"sick\" aggregate or impure aggregates in the asphalt concrete in the core samples. Therefore, according to these actual conditions, the moisture in the blister was more likely to have entered the impervious layer during the construction period. How the moisture in the impervious layer triggered the blisters currently seen requires further research.