Roads are a ubiquitous part of modern life. These essential structures are constantly exposed to environmental stressors, none more pervasive than temperature fluctuations. Hot weather, in particular, significantly impacts road pavement, influencing its physical properties and long-term durability. The behavior of the asphalt binder, often referred to as tar, within the pavement structure is central to understanding these effects.
How Heat Transforms Pavement Materials
Asphalt, the binding agent in road pavement, is a viscoelastic material, meaning its properties blend those of a viscous liquid and an elastic solid. This characteristic makes asphalt highly sensitive to temperature changes. At higher temperatures, the asphalt binder becomes less viscous and more fluid-like, akin to thick honey warming into syrup. This increased fluidity allows the asphalt particles to move more freely within the mixture.
Alongside this softening, both the asphalt binder and the aggregate (the stones and sand within the pavement) undergo thermal expansion. As temperatures rise, these materials expand, and if not properly accounted for in the pavement’s design, this expansion can create internal stresses. Prolonged exposure to heat, especially when combined with oxygen, also accelerates the aging process of the asphalt. This oxidation causes the binder to stiffen and become more brittle over time, even though its immediate effect at high temperatures is softening.
Common Road Problems from Extreme Heat
The transformations within the pavement materials lead to several observable issues on roads during hot weather. Rutting appears as linear depressions in the wheel paths. This occurs because the softened asphalt, under continuous pressure from vehicle tires, deforms and displaces, creating permanent indentations.
Another common issue is bleeding, also known as flushing. This happens when excess asphalt binder migrates to the surface of the pavement, forming a shiny, slick film. This oily layer can reduce the road’s skid resistance, posing a safety hazard, and often makes the surface appear wet or sticky. Shoving is also a concern, characterized by the formation of small, ripple-like bulges or waves across the pavement surface. This phenomenon is caused by the softened pavement accumulating and deforming under traffic loads.
Designing Roads for Hot Climates
To mitigate the effects of heat, engineers employ specific strategies in road design and material selection. A primary approach involves choosing specialized asphalt binders, often referred to as Performance Graded (PG) binders. These binders are engineered to resist softening at high temperatures, with their grade indicating the maximum pavement temperature they can withstand without significant deformation. For instance, a PG 64-22 binder is designed to perform in areas where the maximum pavement temperature reaches 64°C.
The mix design, which specifies the proportions of asphalt binder and aggregate, is carefully controlled to enhance heat resistance. The angularity and interlocking properties of the aggregate are particularly important for providing stability against deformation when the binder softens. Proper compaction during construction is also crucial; achieving optimal density and minimizing air voids within the pavement reduces its susceptibility to deformation under heat and traffic loads. Some surface treatments can also be applied to reflect sunlight and reduce heat absorption, thus keeping the pavement cooler.