# Causes of Asphalt Pavement Blistering: A Review
## 5 Concurrent Causes and Discussion  
The thermal conditions of asphalt and air drive the blistering phenomenon because they cause gas overpressure under or within the wearing layer and critical compressive stress in the plane of the initially flat plate. Nevertheless, the asphalt properties and boundary conditions can contribute to blistering:  
* The asphalt thickness over the bubble plays a pivotal role in whether or not distress occurs. The wearing layer acts as a deformable membrane that retains gases and prevents their expansion into the atmosphere [57] or suffers from buckling effects due to a ratchet process [18]. The opposing weight force and the bending stiffness of the membrane depend on its thickness: the higher the thickness of the material that covers the bubble, the greater the weight force that the upheaving gas pressure or the temperature compressive stress will have to exert on the surface. Assuming the wearing layer is a flat plate, the greater its thickness, the greater the stiffness against the flexural deformation outside of the plane. Thin layers [57] make the appearance of blistering more probable;
* Bitumen affects the complex constitutive law of asphalt and determines a variable viscous-elastic response depending on the surrounding conditions. The response of a rheological mixture depends on the load application method, delivery timing, and temperature [65]. Asphalt inherits variability in its deformation behavior from the binder. Thus, when a load is applied, it has a mixed viscous-elastic and plastic response by varying the boundary conditions [57]. Since the growth of bulges requires at least a few hours, the pressure generated by the expansion of the gases translates into a load applied slowly and a mixture whose surface temperature can reach 65 \({}^{\circ}\)C. At hot temperatures, creep behavior is dominant, the equivalent elastic modulus decreases, and the critical stress to create a buckling effect reduces;* The adhesive bond strength of asphalt material and aggregates prevents failures due to stripping [27; 28];
* Waterproof upper layers promote membrane behavior, and waterproof lower layers prevent overpressure from dissipating when thermodynamic processes cause or contribute to bubbles. The greater the quantities of gas trapped inside the pavement that are unable to interact with the outside due to the presence of waterproof membranes, the more likely blistering is to happen [68];
* The permeability of both water and air of asphalt plays a pivotal role in the blistering process: water is necessary for gas overpressures that contribute to buildup, while air voids make possible dynamic equilibrium conditions among the air, water, and bitumen. In particular, high surface temperatures can lead to leaking, which closes the voids and contributes to the impermeability of the wearing layer and its membrane effect [25];
* Concerning water permeability, [PERSON] et al. [79] demonstrated for void content values higher than 8% that the greater the void, the higher the water permeability. On the other hand, mixtures with a void content below 3% have the opposite condition because the voids are not interconnected and avoid filtration paths. Therefore, an air void content below 3% and a water permeability coefficient lower than \(1\times 10^{-8}\) m/s are assumed to be the impermeability indices [7]. The analysis of the role of water permeability therefore indicates that the prediction of the possibility or otherwise of blistering in a given superstructure can be trusted for the evaluation of hydraulic conductivity [40];
* Concerning air permeability, [PERSON] et al. [40] identified the threshold for the air permeability coefficient (i.e., \(10^{-9}\) m/s) below which blistering is observed;
* A proper mix design using bitumen with a high softening point can help prevent blistering, which can delay the loss of consistency of the asphalt mixture.