Asphalt, also known as bitumen, is a thick, dark, petroleum-based material primarily used as the binder in pavement construction. This substance serves as the “glue” that holds together mineral aggregates like stone and gravel to form asphalt concrete. The material’s dual nature—appearing solid while retaining the ability to flow—has led to a persistent question: is asphalt a liquid or a solid? The scientific answer requires moving beyond simple definitions to understand its complex physical behavior.
Addressing the Supercooled Liquid Myth
The idea that asphalt is a liquid is often conflated with the widely circulated, though incorrect, belief that glass is a “supercooled liquid.” This misconception suggests that both materials are simply liquids that cooled without crystallizing. However, this terminology is misleading for both. A true supercooled liquid is thermodynamically unstable, but asphalt and glass are both classified as amorphous solids.
Amorphous solids appear solid and rigid but lack the ordered, repeating crystalline structure found in true solids. While they can flow, they are not in the unstable, supercooled state implied by the myth. The reference to old window panes being thicker at the bottom is caused by historical, uneven manufacturing processes, not centuries of flow. Asphalt’s behavior is explained more precisely by its unique mechanical properties.
Asphalt’s Viscoelastic Nature
The scientific classification that accurately describes asphalt is a viscoelastic material. This term defines a substance that exhibits a mixture of properties belonging to both a viscous fluid and an elastic solid. Asphalt’s behavior is entirely dependent on the applied stress and the timescale of observation.
When subjected to a rapid force, such as a hammer blow or impact from a vehicle tire, asphalt acts like a brittle elastic solid and can crack or shatter. This elastic response momentarily resists deformation. Conversely, when stress is applied over a very long duration, like the constant force of gravity, the material begins to flow like a highly viscous fluid. This time-dependent deformation is its viscous component, resulting in permanent change.
Temperature also plays a significant role in determining which property dominates. At high temperatures, the material becomes softer and more fluid, accelerating the viscous behavior. At low temperatures, the material becomes stiffer and more brittle, making the elastic-solid behavior more pronounced. This dual mechanical nature is dictated by strain rate and temperature.
Observing Extremely Slow Movement
The most famous demonstration of asphalt’s liquid-like property is the Pitch Drop Experiment at the University of Queensland in Australia. Initiated in 1927 by Professor Thomas Parnell, the experiment illustrates that substances appearing solid can be highly viscous fluids. Parnell used pitch, a tar derivative similar to asphalt, pouring it into a sealed funnel and allowing it to settle before cutting the stem in 1930.
Since the experiment began, only nine drops have fallen, with the average time between drops being around eight to ten years. The ninth drop fell in April 2014, demonstrating the extraordinary time scale required for flow. The pitch has a viscosity estimated to be about 100 billion times greater than that of water. This experiment offers tangible evidence that pitch flows continuously under gravity, confirming its classification as a highly viscous fluid.
How Engineering Accounts for Asphalt’s Properties
Civil engineers must manage asphalt’s viscoelastic nature to ensure durable roads that perform across a range of climates. The material’s tendency to flow slowly under sustained pressure creates road defects like rutting in hot summer months. Rutting occurs when the asphalt softens and the weight of vehicles causes permanent deformation in the wheel paths.
In contrast, asphalt’s brittle-solid behavior at low temperatures leads to thermal cracking in winter. As temperatures drop, the pavement shrinks and loses elasticity, making it prone to fracturing. To mitigate these opposing behaviors, engineers use specialized additives to modify the asphalt binder. Polymers, such as Styrene-Butadiene-Styrene (SBS) or various rubbers, are mixed into the binder to increase elasticity and stiffness.
These polymer modifiers allow the pavement to remain stiff enough to resist rutting in the heat while also staying flexible enough to prevent cracking in the cold. Performance-graded binder specifications ensure that the asphalt material is tailored to the expected range of service temperatures in a specific geographical location. This modification process manages the natural viscous flow and elastic brittleness, allowing asphalt to function as a reliable pavement material.