# Blistering Mechanism Analysis of Hydraulic Asphalt Concrete Facing
## 2 Historic Review of Blister Defect in the Asphalt Impervious Facing Project  
The blisters on asphalt facing that have been reported in some cases [4] could be caused by different reasons:  
(1) Blisters from steam pressure in twice-paved impervious facings [5]. The two-layer impervious facings in the Innerfragant and Haselstein asphalt facing dams in Austria, completed in 1966 and 1967, respectively, are at least \(2\times 4\) cm thick on the slopes. The facings were placed on a binder course of 6 cm thickness, and the maximum particle size ranged around 10 mm. Blisters on these facings occurred during the first few years following completion, which locally led to the complete destruction of the upper layer and the lower layer being heavily attacked. This phenomenon was believed to result from steam pressure effects between the impervious layers, as well as from the freezing and thawing cycles [6; 7]. Blisters drilled open revealed progressive destruction in the lower layer through freezing and erosion. In some cases, the damage was seen to extend throughout almost the entire thickness, i.e., 4 or 5 cm, of the lower layer [5]. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Sheive Size**} & \multicolumn{8}{c}{**Passing Rate (\%)**} & \multicolumn{8}{c}{**Bitumen**} \\ \cline{2-11}  & **19** & **16** & **13.2** & **9.5** & **4.75** & **2.36** & **1.18** & **0.6** & **0.3** & **0.15** & **0.075** & **Content** \\ \hline Impervious layer & 100 & 100 & 100 & 92.4 & 68.7 & 58.1 & 46.2 & 26.2 & 17.2 & 13.3 & 11.7 & _7.7_ \\ Drainage layer & 100 & 98.2 & 48.2 & 32.8 & 16.4 & 12.0 & 10.6 & 8.1 & 5.8 & 4.0 & 3.0 & 3.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Asphalt concrete mix ratio of the Zhanghewan project. Figure 1: Cross section of Zhanghewan asphalt concrete facing. (2) Upheavals from uplift pressure beneath the facing [8]. The 85 m high Shibianyu dam in China started impounding in May of 1978. The dam body was rock-filled by directional blasting. The thickness of the binder layer of the asphalt facing was 12 cm. The impervious facings were placed in two or three layers according to their elevation (EL.) ranges: 7 cm +7 cm + 6 cm layers below EL. 690 m, 5 cm + 5 cm + 5 cm layers between EL. 690 m and EL. 705 m, and 5 cm + 5 cm layers above EL. 705 m. In early July of 1978, the water level reached EL. 703.5 m, and in early August dropped at 8 m per day to the maximum for irrigation. This revealed two upheavals 15 cm in height with an area of 15 m\({}^{2}\) at EL. 664-665; the tops of the upheavals were cracked radially. After another impounding and drawdown of the water level, eight upheavals, four caves, and three cracks were found, covering about 200 m\({}^{2}\) in total. The largest upheaval was 3 m in diameter and 17 cm in height, with a crack of 9 cm wide on the top of the upheaval, which penetrated through the whole facing. The cause of the upheavals was that the blasted rock-fill was blocking drainage. Seepage through the abutment, which was not grouted, and through the cracks created some uplift pressure of about 3.7 N/cm\({}^{2}\) during the fast drawdown of the reservoir level [8]. (3) Blisters from \"sick\" aggregate in asphalt concrete [9]. The Porabka-Zar pumped-storage power station in Poland was built between 1968 and 1978, with a multi-layer facing of asphalt concrete to seal its upper reservoir. Damage to the upper watertight layer was spotted for the first time in 1981, three years after the project began operating. Small heave blisters, 5 to 10 cm in diameter and 3 to 5 cm deep, were found to occur in this layer. As shown by observations, a number of new blisters appeared every year, always within a twenty-four hour zone of water level fluctuations. The petrography of the material from the blisters proved the substance originated from the alteration of \"sick\" basalt grit grains. The \"sick\" basalt grit used for the production of the asphalt concrete was, due to atmospheric influences, susceptible to weathering processes and likely to disintegrate.