How Environmental and Man-Made Factors Cause Damages On Asphalt Pavements

Asphalt undergoes chemical changes at the molecular level and the intermolecular level also known as microstructure. Because of the complex nature of molecular interactions in asphalt, the oxidation of this material is complicated by the different molecules interrelation. There is a mixture of polar and non-polar molecules in asphalt. Polar molecules have a positive charge on one side and a negative charge on the other side while non-polar molecules have their electrons distributed in a way that there is no abundance of charges on both sides. 

"Cracks developing on asphalt surface"

In essence, there are predominantly thousands of chemicals with open chain structure in asphalt, which makes the material to have a considerable level of un-saturation in its molecular structure. It is this open chain molecular structure of asphalt, which allows it to easily interact with weather elements, chemicals, and salts. Molecules from other substances can attack and disintegrate the asphaltic material molecules leading to changes in its structural composition.

In the microstructure of asphalt, there are three-dimensional relations of polar molecules, which are referred to as asphaltenes, and these are dispersed in a fluid made up of non-polar or low-polarity molecules, which are known as maltenes. The intermolecular bonds are weaker than the bonds of the fundamental hydrocarbons in asphalt material. This means that these intermolecular bonds break up first and determine the behavioral patterns of asphalt material.

The intramolecular and intermolecular relationships in asphalt material make it behave in two major ways. First, it is elastic due to the effects caused by polar molecules interrelation. Second, it is viscous since the different parts of the polar molecules are able to move relative to one another when dispersed in the fluid non-polar molecules.

When oxidation occurs in asphalt pavement, it makes the material become dry and brittle. Exposure of asphalt to oxygen triggers a molecular process, which leads to creation of new polar bonding sites or zones. What happens is that the increased polar sites or zones in asphalt materials make the molecules to rearrange and shuffle about as they search for bonds that can create equilibrium or bring a thermodynamic stable state.

This is a self-assemblage process, which continues for the rest of the life of the asphaltic pavement material. With time, as the polar sites discover molecules to attach on, the asphalt molecules bond firmly together with the aggregate material. The result is a pavement, which is stiffer, harder, and brittle. As asphalt oxidizes and dries up, it loses its elasticity and starts to fail.

One sign that signifies the failure of asphalt due to oxidation is the change in color. Initially, asphalt has a jet-black color but over time, due to oxidation process, it turns to light black and ultimately changes to gray. The drying asphaltic material begins to crack as it loses its bonding properties. Before the oxidation process reaches an equilibrium point, it may take about 5 to 15 years depending on how the material is affected by other factors like sunlight, chemicals, and water. This substantiates the reason why the estimated lifespan of asphalt pavements is about 20 to 25 years.

When oxidation process stiffens the paving materials, its ability to support heavy loads is reduced. This causes asphalt fatigue leading to formation of cracks and premature failure of asphalt. Heat from the sun causes the asphalt material to cure and dry fast than intended. Moisture from rainfall can accelerate the damage on oxidizing asphalt since it contains a large amount of polar-constituent molecules, which in turn attract the water molecules. Sealcoating can help delay or slow the rate at which asphalt naturally oxidizes.