A Minute With...

Imad Al-Qadi, expert on pavement

3/6/2014  8:00 am

As the polar vortex wanes, having covered roadways with snow and ice, another scourge has arisen to beleaguer weary commuters: potholes. Imad Al-Qadi, the director of the Illinois Center for Transportation and a professor of civil and environmental engineering at the U. of I., talked about how and why potholes occur with News Bureau physical sciences editor Liz Ahlberg.

What causes potholes?

image of professor imad al-qadiPotholes are just one of many consequences of road deterioration. However, we notice them mostly after heavy rains or snowstorms because of the water action. Potholes mostly occur in concrete pavement near the joints where water enters the pavement. Centerlines and edges are also more vulnerable to the formation of potholes. However, the presence of moisture can only cause potholes if accompanied with vehicular loading.

Asphalt pavements are made of a skeleton of aggregate (rocks) held together by asphalt binder in a special design. The asphalt binder, which comprises about 5 percent by weight and 12 percent by volume of the mix, is viscoelastic – meaning it is soft at high temperatures and brittle at low temperatures. The brittleness of asphalt binder at low temperatures can lead to asphalt cracks. These cracks start as hairlines, but they still allow water to enter, thus accelerating the damage.

In the presence of excess moisture, a complex process known as “stripping” causes binder to lose its adhesion. Freezing and thawing cycles accelerate this process significantly, possibly as a result of aggregate degradation. Stripping leads to distress, which causes “raveling,” or the separation of aggregate and asphalt binder; hence, the formation of potholes. Potholes could result from poor quality paving materials, improper mix designs, or inadequate compaction, among other factors.

Has this winter been especially conducive to pothole formation? How do weather and snow removal efforts contribute to asphalt degradation?

The mechanisms of potholes are highly dependent on temperature swings, freeze-thaw cycles, and the amount of precipitation (snow and rain).

Sometimes, when pockets of trapped water repeatedly freeze and thaw with temperature swings, the expansion of frozen water pushes the pavement up, and when the temperature increases, water migrates away, leaving gaps or localized weak spots between the foundation bed and pavement-bound layer. Vehicles driving over these cavities cause the initiation of disintegration and accelerate the formation of potholes.

Snow removal efforts are considered non-harmful to original pavement materials. However, deicing salt causes corrosion of reinforcing steel in bridges and sometimes in roads, thus ultimately resulting in the formation of potholes. If not done properly, snowplowing may cause occasional damage to fixed potholes.

What are potholes repaired with? Does it really fix the problem?

Potholes are usually filled with patching materials called “cold-mix asphalt.” The constituents of cold-mix asphalt are very similar to the existing asphalt concrete layers, but they have lower quality ingredients – namely rocks and petroleum-based binder. The cold patches are manually compacted using compaction rods; however, they cannot reach the density levels or the quality of existing layers. With low quality ingredients and density problems, cold-mix asphalt has a short service life and is usually considered a short-term band-aid that does not last more than one winter season. For concrete potholes, there are proprietary cementations materials that can be more effective than cold mixes.

Are there pavement types that are less susceptible to potholes?

All pavement types and materials are bound to deteriorate with time, age, weather, and traffic loads. What matters most is the timely maintenance and preservation of roads. Timely intervention is critical to slow down deterioration, prevent failures, and maintain safe and comfortable riding quality for prolonged periods of time. For example, one of the most commonly used preservation techniques is the replacement of existing deteriorated layers with new thin asphalt concrete layers.

The selection of compatible aggregate, binder, proper mix design, and adequate binder thickness around the aggregate, as well as the implementation of strict quality control measures during construction, are all considered key factors for controlling the formation of potholes in asphalt concrete pavements. The use of proper material components, chemical admixtures, and joint sealing can also help control the development of potholes in concrete pavements.

Do you have any advice for motorists?

To stay safe and avoid damage to their vehicles, drivers need to slow down because potholes are misleading and can be masked with puddles. In the very near future, information on the physical condition of roads including potholes will be available in real-time. Web maps and apps will also become available in the near future to help locate potholes and will be connected to GPS devices for warning drivers of any potholes on the road.

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