When summer temperatures cause pavement surfaces to soften, it demonstrates material science under stress. The familiar black surface absorbs solar radiation, causing internal components to heat significantly, often exceeding 140 degrees Fahrenheit. This intense heat fundamentally alters the properties of the binder holding the road together, leading to physical and structural problems. The softening pavement is a direct result of this material change, which sets the stage for surface stickiness and permanent road damage.
Clarifying the Pavement Material
The black, sticky substance in modern road pavement is almost never actual tar, despite the common use of the word “tarmac.” True tar is a product of distilling organic materials like coal or wood and is rarely used in road construction due to environmental and health concerns. The material used today is called asphalt binder or bitumen, which is a viscous, black, hydrocarbon material refined from crude oil. This petroleum-derived substance acts as the “glue” that binds together the crushed stone, sand, and gravel aggregate to form asphalt concrete.
Bitumen is a viscoelastic material, possessing both viscous (liquid-like) and elastic (solid-like) characteristics. At ambient temperatures, the asphalt binder behaves as a semi-solid, providing the necessary stiffness to support traffic loads. This dual nature allows the road to flex slightly under stress while maintaining structural integrity. The binder’s chemical composition, primarily complex hydrocarbons, makes it highly sensitive to temperature changes.
The Science of Viscosity Reduction
High summer temperatures dramatically affect the asphalt binder by increasing the kinetic energy of its constituent molecules. As the pavement absorbs solar radiation, the binder’s temperature can climb far higher than the ambient air temperature, sometimes by 50 degrees Fahrenheit or more. This influx of energy causes the long hydrocarbon chains within the bitumen to move more freely against one another. The material’s complex shear modulus, a measure of its stiffness, decreases as a result of this molecular movement.
This molecular change manifests as a reduction in viscosity, which is the material’s resistance to flow. The asphalt binder transitions from its semi-solid state toward a more fluid state. This softening can begin well below 140 degrees Fahrenheit, making the binder highly susceptible to deformation under load. The loss of internal friction and stiffness is the fundamental reason the pavement structure becomes pliable and weak.
Surface Phenomena: Bleeding and Tracking
The immediate, visible consequence of this reduced viscosity is a phenomenon known as “bleeding” or “flushing.” This occurs when the highly fluid asphalt binder rises to the pavement surface, filling the small voids between the aggregate particles. The road surface takes on a shiny, glass-like appearance and becomes sticky to the touch. This excess binder on the surface can reduce the pavement’s skid resistance, making the road slicker, especially when wet.
The sticky, softened binder also leads to a problem called “tracking.” As vehicle tires roll over the pliable surface, the excess asphalt material adheres to the rubber. The tire then pulls this sticky binder away from the road surface and deposits it elsewhere, such as on the sides of the vehicle or other paved areas. Tracking indicates that the asphalt binder has become too soft to resist the adhesion and shearing forces exerted by traffic.
Structural Failure: Rutting and Shoving
When traffic repeatedly drives over the softened pavement, the material’s reduced strength leads to long-term structural failures. The most common is “rutting,” which appears as longitudinal depressions in the wheel paths. Rutting is caused by the constant weight of vehicles displacing the pliable asphalt mix, forcing the material to consolidate or move sideways. This deformation is permanent and results from the binder’s inability to hold the aggregate structure rigidly under high temperatures and sustained traffic loading.
Another form of structural failure is “shoving,” characterized by localized ripples, waves, or bulges that form across the pavement surface. Shoving is often observed in areas where traffic frequently accelerates or brakes, such as intersections or bus stops. The horizontal force exerted by tires pushing against the soft material causes the asphalt mixture to deform and pile up in a wave perpendicular to the direction of traffic. Both rutting and shoving indicate that the viscoelastic binder has entered a state where its viscous nature dominates its elastic properties, resulting in plastic flow under load.
Engineering for Heat Resistance
Civil engineers combat the thermal softening of pavement through material and design strategies. One common method is the incorporation of Polymer Modified Asphalts (PMAs), where polymers like Styrene-Butadiene-Styrene (SBS) are blended into the bitumen. These additives increase the binder’s elasticity and stiffness across a wider temperature range, raising the softening point and improving resistance to flow. This modification allows the pavement to remain more solid even when surface temperatures are high.
Engineers also adjust the overall composition of the asphalt concrete mix. Reducing the proportion of asphalt binder relative to the aggregate minimizes the amount of excess binder available to bleed to the surface. Furthermore, lighter-colored aggregates or reflective surface coatings are employed to increase the pavement’s albedo, or reflectivity. By reflecting more solar energy, these lighter surfaces absorb less heat, which can reduce the internal pavement temperature and help preserve the binder’s stiffness.