# Causes of Asphalt Pavement Blistering: A Review
## 1 Introduction  
The design and construction of road and airport pavements aim to meet structural and functional requirements [1] that ensure regular and safe service. Functional properties of a horizontal surface to be ridden require smoothness and adherence to allow for safe and comfortable vehicle mobility [2]. Several distresses (e.g., cracks, potholes, rutting, and raveling) have been recognized and investigated in the literature to counteract their appearance and manage their effects [3]. However, blistering negatively affects surface flatness and integrity [4], but it is still not fully understood [5]. It is frequent in hot climates and often appears upon exposure to rain, fog, or other water sources [6]. It is an irreversible (or partially reversible) process that consists of nonlinear blister growth dynamics that can start at the interface between two layers or within the upper layer and can have different consequences [7]. Indeed, blisters can reach a stable state, break, or lead to delamination, depending on the mechanical and adhesion properties of the materials [8; 9]. The uplifts have a circular shape with extremely variable dimensions: their diameter can reach 1 m, and their maximum elevation is usually a few centimeters over the ideal and initial flat surface [10]. It often affects asphalt mixtures on asphalt roof shingles [11], pipelines [12], pipes for drinking water supply [13], Portland cement concrete bridge decks [14; 15], steel deck bridge pavements [16], dams [7], pumped-storage power stations [7], runways [17], sidewalks [18], carriageways [5], waterproofing membranes [19], and asphalt-covered concrete structures [20] in south- and west-facing areas. According to the literature, the main physical mechanisms that drive blistering are temperature fluctuation [21], bottom-up pressure [22], loss of adhesion [23], peeling fracture [20], thermal expansion, and buckling [24]. Moreover, the blister failure mode has a direct relationship with the airvoid content, air and water permeability [25], stripping [23], the tensile/bond strength of the asphalt mixture [26, 27], and the surface roughness [28]. The properties of the lower layer play a pivotal role: asphalt over granular or permeable layers does not suffer from blisters [29]. Moreover, a rough lower surface (e.g., milled binder) ensures a higher bonding performance between the tack coat and the upper layer [30]. Additionally, subsurface void detection with ground penetrating radar allowed for the identification of cavities filled with water [31]. The complex mechanisms damage the surface of pavement, requiring maintenance and rehabilitation work. Indeed, neglecting the problem causes further distress with the onset of cracks, the infiltration of water, and removal of the bulge surface [32]. Cracks, potholes, raveling, surface wearing, and stripping affect pavement use when a low tolerance for out-of-flatness has to be applied (e.g., for airport runways where blistering can cause foreign object debris) [33]. Figure 1a,b shows night and day images of two runway pavements where blisters are in the red circles. Figure 1c,d shows the effects on sidewalks whose surface has numerous bulges; both images present a \(1\) euro coin (23.2 mm diameter). [PERSON] et al. [34] proposed a machine vision system to detect and grade the blistering defects of coatings according to [35]. Although there is a lack of standardized assessment methods to classify the severity and impact of pavement blisters, bulges higher than 3 mm prevent aircraft movements [36], and vertical differences in elevation higher than 1.25 cm are hazards for sidewalks and crosswalks [37]. Figure 2b shows the surface laser profile of the red alignment in Figure 2a and highlights the blister profile. The red circle in Figure 2a highlights the position of a blister. Figure 1: Blisters. (**a**) On runway pavement; (**b**) on runway pavement; (**c**) on sidewalks; (**d**) on sidewalks. This paper aims to present and discuss the causes of blistering according to the state of the art.