# Causes of Asphalt Pavement Blistering: A Review
## 6 Conclusions  
Blistering on asphalt pavements is a complex mechanism governed by several boundary conditions. Its behavior is diurnal, and the solar radiation and heat transmission within the upper pavement layers drive the phenomenon. Despite decades of research about this phenomenon, its origin and development mechanisms are unclear. Thermal conditions seem necessary but do not raise the pavement surface enough for safety and functional consequences.  
According to the scientific literature, two physical-mechanical processes drive the distress evolution: bottom-up pressure induced by gases trapped under or within the upper layer and buckling conditions. Intensive moistening of the structure in rainy periods and solar radiation in hot periods can contribute to blistering. Indeed, high asphalt temperatures cause decreases in the asphalt tensile stiffness and adhesion strength, the thermal expansion of entrapped gases in the asphalt voids, the evaporation of water under or within the upper layer, and out-of-plane deformation. According to the buckling theory of plate elements, critical thermal in-plane loads lead to instability and cause blisters. Both chemical and geometrical bonds between the upper and lower layers can reduce the risk of surface blistering. Further analyses should investigate the relationships among the void content, the water and air permeability, and the variation in the water/air permeability and void content/distribution due to cyclic temperature variations. In addition, the surface and gradient temperatures of asphalt varying the thermal performance of its components, the weather conditions, the shear friction between the wearing and binder layers, the adhesion bond between the wearing and binder layers, the influence of traffic, and the role of the smoothness and permeability of the lower layers are worthy of investigation. Due to the increasing extreme heat events, blistering occurrence may increase, and highly reflective materials and the physical properties of non-permeable-to-water but permeable-to-air mixtures could be ideal solutions. On the other hand, in roadworks, attention should be paid to cold and hot joints, the roughness of the lower layer, the tolerance for out-of-flatness, and the quantity and performance of the tack coat.  
Conceptualization, A.D.; methodology, L.M.; formal analysis, L.M. and L.P.; investigation, A.D.; resources, A.D.; data curation, L.M.; writing--original draft preparation, L.P.; writing--review and editing, L.M.; visualization, L.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by \"Experimental and analytical analyses to model blistering in asphalt pavements\" ID RM123188 DDB7 AA8D Sapienza University of Rome.  
Not applicable.  
\begin{table}
\begin{tabular}{l c c} \hline \hline \multirow{2}{*}{**Variables**} & \multicolumn{2}{c}{**Main Cause**} \\ \cline{2-3}  & **Buckling**[18] & **Thermal Expansion**[44; 57; 60] \\ \hline Necessary variables & Thin layers [24; 57; 64] & Water and voids within the asphalt [40] \\ Boundary conditions & Hot temperatures [18; 63] & Hot temperatures [74; 75] \\ Physical processes & Instability [64] & Overpressure, evaporation [70] \\  & & Imperfect bond between layers [42] \\  & Construction defects [66; 81] & Low tensile strength of the asphalt [43] \\ Concurrent causes & Imperfect foundation & Air void content and water \\  & layers [76] & conductivity [40; 79] \\  & Low friction coefficients [77] & Membrane behavior [68] \\  & & Stripping [27; 28] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main and concurrent causes and boundary conditions of blistering.