Why Roads Crack, What is the Engineering Behind It, and How Engineers Attempt to Prevent It

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The United States spends roughly around $20,000,000 dollars per year in repairing and maintaining roads and pavement damaged by cracking, weather, and traffic stress (Federal Highway Administration). However, we have all felt and dealt with the frustration of having to put up with a crack road. It leads us to wonder what exactly causes these cracks and how much engineering and physics is really behind it. However, it must first be understood that roads are actually designed to crack in the eventual future. It isn’t a design failure that occurs, rather a predictable challenge that civil engineers actively attempt to create workarounds for and manage. 

Understanding this concept begins with knowing the fundamentals of the roads that keep our infrastructure and society successful. Roads consist of many layers, each containing their own structural purpose. The road design examined that will be explained in this blog is based on the Federal Highway Administration and other sources.  On the top, the surface layer, which is typically Hot Mix Asphalt or concrete, is the one that handles direct traffic loads. Below that is the base layer, which is designed to provide main structural support and provide load distribution. Under that is the subbase, which is the layer designed to help the base out with its functions while making sure the fine grained-soil from below doesn't intrude into the base. The fine grained soil is part of the layer under which is the subgrade, or the natural soil being built on.  

Now we will dive into the first major reason for road damage, which is traffic load and repeated stress. Every vehicle applies pressure to the road surface. Heavy vehicles like trucks add a significantly larger pressure to the surface. The roads are designed to spread this force out, however overtime, repeated loading weakens material. Heavy vehicles play a major role in this as the weight matters exponentially. The Fourth Power Law shows that doubling the axle weight can increase the damage to the road by almost 16 times. Hence, despite being low in numbers, trucks are a leading cause of road damage. The repeated loading of roadways leads to eventual cracks on the road, known as fatigue cracks. These occur from the development of small cracks at the bottom of the asphalt which eventually become visible.

A second major reason for road cracking is temperature expansion and contraction. When objects get hot, they expand and when objects get cold, they contract. Despite these movements being small, looking at them holistically in a large road can lead to it having a measurable effect. Despite asphalt being flexible, heating leads to softening and deformation of the material. Asphalt also becomes more brittle in the colder weather conditions. Thermal cracking occurs when the cold temperatures lead to improper contraction of the roads and transverse cracks perpendicular across the road form.

A third reason for the damaging of roads is damages due to water. Roads are, for the most part, designed in order for water to not be able to get to the structural parts. That being said, it is still able to enter through surface cracks, construction joints, potholes, etc. This leads to the weakening of the subbase and base layer, reducing load-bearing capacity and saturation of subgrade soil. This saturation leads to quicker formation of cracks. Improper or damaged drainage systems to get water off roadways can further enhance this issue

A fourth causation for the cracking of roads is the movement of ground under road and soil conditions. When the ground shifts underneath, even the slightest movements in the subgrade soil under can lead to road cracking. Furthermore, soil naturally compresses under the weight of the load from the roads. This process of settlement may occur unevenly, creating bending stress within the pavement. Clay-rich soils tend to expand and contract when wet and dry respectively. Special precautions are taken to deal with the up and down movement this causes. Finally, seismic events are able to crack roads, however minor events are accounted for in the design of roadways.

Road cracking may look like a surface-level issue at first glance, however understanding the forces behind why this occurs is crucial. Due to the unstoppable nature of these forces, engineers take the approach of using methods which will delay and manage the inevitable pavement deterioration. Strong pavement structures that follow the FHA’s guidelines, the selection of materials based on geographic considerations, the maintenance of proper water drainage systems and the stabilization of the base ground under the road are all ways that engineers are able to mitigate the impacts and minimize road cracking and damage. In summary, cracking isn’t a fault, rather it is a planned event which is properly maintained by engineers. As technologies begin to advance, newer ways to slow down road damage has led to the proliferation of longer-lasting roads. Next time you encounter a crack on a road, consider the different causes that could’ve been behind it.