# Causes of Asphalt Pavement Blistering: A Review
## 3 What Are the Potential Causes of Blistering?
### Chemical Reactions within the Asphalt Surface  
In 1984, blistering appeared on Runway 14-32 of the Marine Corps Air Station in Beaufort, South Carolina. In summer, secondary blisters involving a thin asphalt overlay two years after its construction occurred during the cold period. The bubbles were observed in limited areas of the pavement, and chemical reactions between the pavement materials and/or the biological activity of organisms present have been investigated as the causes of blistering [44]. A complex bacterial composition was found in blisters on asphalt concrete linings in the Czech Republic [45]. Both chemical reactions and biological activities can lead to the production of gases that are responsible for the observed distress. [PERSON] and [PERSON] [44] came to rule out such a possibility. Firstly, they noticed the fast occurrence of high surface temperatures of the surface due to solar radiation. Daytime blister behavior occurs at the upper layer of asphalt pavement because the maximum daily temperature range occurs within the topmost 5 cm of the surface. This condition suggests a close and reasonable correlation with the climatic conditions to which the pavement was exposed. The bubbles raised when the temperature increased, receded when the temperature decreased, and deflated when punctured. Therefore, the blisters contained pressured gases and were not caused by buckling effects. Due to the deep (more than 3 m) groundwater table and the sandy subgrade, steam from the groundwater or soil could not cause gas pressure beneath the wearing layer. Four gas samples from the blisters were analyzed to investigate their composition. However, the chemical approach was incorrect:  
* According to [44], it is unlikely that chemical activity was responsible for the gas production since the materials used in the production process were chemically stable. The inversion of a chemical reaction requires a certain amount of energy and a catalyst, which were unavailable within the pavement considering the daily cyclicity. Finally, the amount of gas generated by reactions over time would have decreased due to the progressive consumption of available resources until exhaustion. Therefore, the daytime increase in gas pressure and the sudden appearance of blistering would not have been possible after many cycles;
* The chemical analyses of the gases contained under the surface uplifts revealed a high concentration of carbon dioxide and a low methane content, which is the main product of anaerobic activities. Such conditions suggest the presence of aerobic biological activities, which need a continuous oxygen supply. Blister gases do not differ from those in the normal air, and no other found gases could be responsible for the bubbles.  
However, the low void content of the asphalt and the lack of interaction of the latter with the outside make aerobic activities unlikely [45].  
A finite element analysis was implemented to estimate the pressure necessary to develop bubbles under a thin asphalt layer [44] and compare the numerical results with those from the thermodynamic equations of gases [46]. ADINA 1.0 software was used to develop a three-dimensional model of a blister with no bonding of the asphalt layer except at the blister perimeter. The calculated pressures were higher than those from the thermodynamic analysis due to the boundary conditions and the thermal-mechanical properties of the modeled materials. The field measurements, laboratory tests, and numerical results confirmed that the blistering was caused by the thermal expansion of air- and water vapor-trapped gases in the asphalt voids [44]. Voids and cracks in the asphalt allow gases to move into the pavement until the pavement matrix expands due to the diurnal temperature changes, trapping air and moisture. Blisters arise where the bonding between the asphalt layers or the tensile strength is deficient, and the gas pressure overcomes the adhesion forces.