Asphalt is a paving material composed of mineral aggregate held together by a bituminous binder. While the binder itself is hydrophobic and absorbs water very poorly, the pavement’s interaction with water is nuanced. Water movement is not about absorption into the material, but rather about its passage through the structure. Understanding this movement explains why asphalt roads eventually deteriorate.
Understanding Asphalt Permeability and Voids
Asphalt concrete is not perfectly dense; its structure contains small pockets of air, known as air voids, which form during mixing and compaction. These air voids are the primary pathway for water to enter the pavement structure. The key difference is between true absorption by the material components and permeability, which is the ability of water to flow through these interconnected void spaces.
The bituminous binder naturally resists water, ensuring minimal absorption into the binder itself. Water interaction is governed by the volume and connectivity of the air voids present in the final compacted layer. In conventional, dense-graded asphalt, engineers aim for a low void content, typically between 4% and 7%, to maintain strength and durability.
When the air void content exceeds approximately 8%, the individual air pockets become interconnected, creating channels that allow water to flow freely. This threshold causes the pavement to transition from functionally impermeable to highly permeable. Mix design heavily influences this void content, as the gradation of aggregate particles determines how tightly they pack together.
The Mechanics of Water Damage to Pavement
Once water permeates the asphalt structure through interconnected voids, it causes physical and chemical deterioration. One significant form of moisture-induced damage is stripping, which is the failure of the bond between the bituminous binder and the aggregate. Water infiltrates the interface between the asphalt and the rock, displacing the binder from the aggregate surface.
This chemical failure weakens the entire mixture, as the load-bearing capacity relies on the strong adhesion between the aggregate and the binder. Stripping is accelerated by traffic, which repeatedly applies pressure and forces water further into the weakened interface. The resulting loss of cohesion can lead to premature rutting and fatigue cracking.
Water trapped within the pavement also causes damage during cold weather through freeze-thaw cycles. When water freezes, its volume expands by about 9%, generating significant internal pressure. This expansion creates stresses that widen existing micro-cracks and air voids within the asphalt matrix.
Repeated freezing and thawing causes a progressive breakdown of the pavement, where small cracks grow into larger ones, eventually leading to the formation of potholes. The pressure exerted by the expanding ice can exceed 220 megapascals (MPa), which is enough to fracture the surrounding asphalt concrete. Water freezing in the underlying soil, known as frost heave, can also push the pavement upward, causing further cracking and structural failure.
Designing Asphalt for Optimal Water Drainage
Since water infiltration is the primary mechanism of asphalt failure, pavement design must include strategies to manage and remove water effectively. The simplest approach is surface drainage, which involves engineering the pavement with a slight cross-slope, or crown. This ensures water quickly runs off the surface into gutters or ditches, preventing pooling and minimizing contact time with the asphalt.
A counter-intuitive solution is the use of porous asphalt, a specialized mix designed to encourage water to pass through the pavement instead of running off. Porous asphalt uses an open-graded aggregate blend with very little fine material, resulting in a much higher void content, typically between 16% and 22%. These interconnected voids allow stormwater to drain rapidly through the surface layer into a stone recharge bed below.
This design effectively manages stormwater by preventing surface ponding and reducing the risk of internal moisture damage. The high void structure ensures water does not remain trapped, greatly reducing the potential for stripping and freeze-thaw damage. Porous asphalt is primarily used in low-volume areas like parking lots and residential streets, where it also helps filter pollutants before water infiltrates the ground.