# Causes of Asphalt Pavement Blistering: A Review
## 4 Physical Processes
### Thermodynamics Approach  
The laws of thermodynamics can support the hypothesis that blistering originates from thermal overpressures of the gases contained in the pavement. Assuming that only air is inside the asphalt and considering it is an ideal gas, Equation (1) for isochoric transformations allows for the calculation of the pressure variation in the gas contained in a given volume for an increase in temperature [46]:  
\[\mathrm{P_{g,2}}=\mathrm{P_{g,1}}\frac{\mathrm{T_{2}}}{\mathrm{T_{1}}}\frac{ \mathrm{V_{1}}}{\mathrm{V_{2}}} \tag{1}\]  
where \(\mathrm{P_{g}}\) is the air pressure, \(\mathrm{T}\) is the air temperature, \(\mathrm{V}\) is the air volume, and \(1\) and \(2\) refer to the initial and final conditions, respectively. According to Equation (1), [PERSON] [57] demonstrated it is possible to reach a volume increase of 30% with a 45 \({}^{\circ}\)C temperature increase. [PERSON] [68] produced a laboratory blister between a concrete surface and a waterproof asphalt layer, but there was not always enough air pressure to justify blistering. This result showed that the surface blister extension and the number of bubbles depended on the air volume initially trapped in the pavement. However, in some cases, a high trapped air volume and severe distress are difficult to justify when low-void-content asphalt is considered [69]. [PERSON] and [PERSON] [44] demonstrated the pivotal role of vapor pressure with a thermodynamic analysis. Assuming that water is in the pavement voids, it can be stated that the steam generated by water heating contributes to the increase in pressure. If water vapor and air are ideal gases according to [PERSON]'s law [70], the respective partial pressures can be added to determine the total pressure (P) in a blister (Equation (2))  
\[P=P_{g}+\varphi P_{sv} \tag{2}\]  
where \(\varphi\) is the relative humidity, and \(P_{sv}\) is the saturated vapor pressure. Therefore, Equation (3) describes the equilibrium between vertical stresses [71]:  
\[\sigma_{VT}=P_{g}+P_{w}+P_{r}-P \tag{3}\]  
where \(P_{w}\) is the weight per unit area of the asphalt, and \(P_{r}\) is the tensile strength or the bond strength of blisters within the layer or at the interface between two layers, respectively. Using Equations (1) and (2), [PERSON] and [PERSON] [44] demonstrated for temperatures from 25 \({}^{\circ}\)C to 60 \({}^{\circ}\)C that the increase in pressure overcomes the uplift resistance and can cause blistering even with an extremely modest amount of water. In the literature, blister tests have investigated the sealants and asphalt materials' bond strength [72]. [PERSON] [57] developed a pressurized blister test composed of a container partially filled with water and an asphalt slab laid on a perforated aluminum plate to verify whether the Norway climatic conditions caused the observed damage. Two slabs were investigated: they differed in width (3 cm and 4 cm, while their size in the plan was 30 \(\times\) 30 cm) and void content (2-5%). The results demonstrated that the expansion of air and steam under the slab allowed for the formation of a surface bubble. The pressure model cannot overlook the surface tension of the water because it prevents outflow through the interconnected voids if their diameters are less than 4 \(\times\) 10\({}^{-3}\) cm [40]. Under such geometrical conditions, and with modest hydraulic loads, the water in the accessible voids remains trapped in the narrow interconnecting channels, obstructs the passage of gases, and generates the conditions for blistering to start. The results from [57] demonstrate that water in asphalt voids contributes to the phenomenon.