In road construction, there is an infinite number of pinwheels to make the process as perfect as possible. Everything has to be considered and taken into account, starting with the planning, the tendering process, the specification of machines and staff, the choice of material and of course the paving process. Looking at the process, I would like to go a little deeper into the different asphalt layers, because they have a significant impact on the resilience and many other characteristics that are often marginally mentioned.

The different asphalt layers and their tasks

First, I would like to give a short introduction into the different layers and their functions. Every layer is important for the reliability of the road.  

I. Base course

The base course, as the name implies - along with the binder course - is the load carrying element of the road and gives the binder and top layer a consistent hold. Furthermore, the base course protects quickly and effectively from rainfall and other environmental conditions. Exposed traffic forces are removed and evenly distributed on the base. The layer thickness should be selected in such a way that the required compaction is achieved at each point of the street. Often, with base course thicknesses of more than 15cm, the layer is paved in two production steps.

II. Binder course

The binder course ensures that shear stresses are absorbed and thus deformations in the road are prevented. Uneven surfaces remaining from the base course are further optimized in the binder installation, so that the perfect smoothness is achieved as soon as possible after the binder installation. The layer thicknesses stagger depending on the material and type of the road between 5cm - 12cm.

III. Top layer

The top layer is the most heavily used layer of the road, because it is subject to the direct influence of traffic, weather and thawing agents. Thus, the top layer must be particularly resistant to wear and deformation. Durable evenness, grip, environmental friendliness and low noise are other properties required by the top layer. In addition, the layer must be designed with structures, textures, colors and shapes. The specified layer thicknesses can be very different, depending on the requirement and the type of material.

The classification of the road in its different asphalt layers and their components has an economic as well as a technical background. Every single layer has its own special task as described above. Its position, strength and material composition make it a part of the load-bearing capacity and resilience of the finished road. The individual layers are combined into a compact structure in order to fulfill the desired specification, which is essential for the durability of the entire construction. Not to be forgotten is the infrastructure aspect of every street, because the users of the road are also very interested in a stable road that meets the requirements.

How do unstable or even too low layer thicknesses affect the overall road?

1. Lifetime of the road

Unstable layer thicknesses prevent the individual layers from fulfilling their function. For example, if a road has changing thicknesses in the binder layer, it will absorb the resulting shear stresses better and worse. Early road damage is the result. However, if the binder course is made uniformly and consistently, the building structure will be evenly stressed and will be able to maintain its originally specified lifespan.

If there are irregular layer thicknesses in the top layer, premature road damage will be the result. Especially in the top layer you need to have a consistent layer thicknesses in order to avoid cracks or waves.

If you are able to have perfectly leveled layers, stable thicknesses can much easier be achieved. Good logistics, stable material temperatures, and constant paving speeds are some more prerequisites for a constant layer thicknesses in the paving process.

It gets more critical if the layer thicknesses are irregular across all layers. Critical points over the entire course of the road are the result. There will be spots where the road will take early damage and not reach the desired lifespan. In the following sketch you can see, even as a layman, that something is wrong. What is not critical in the base course, changes dramatically with the binder and even more in the top layer. If the binder course is irregular and even too thin in some places, it can no longer fulfill its task as a tension absorber. This problem gets even bigger in the base course, because in this case the problem becomes visible first. Cracks and holes are the result of too thin and irregular layers. The durability of the road decreases significantly.

2. Economic and infrastructure aspects

The economic aspect is especially important for the contractor because he has a great interest that the completed project achieves the quality to reach the specified and planned lifespan. The contractor will not have any warranty claims. Especially contractors, but also road owners and we as the tax-payers are interested in roads with high quality and a long lifespan. One particularly essential basis for this are constant layer thicknesses, which are paved exactly as required.

Furthermore, by working on the tolerance limits, the contractor gets the opportunity to save material and thus costs. If one considers that with a material price of 80.00 EUR per ton, a paving scope of 100km with an average screed width of 6m can result in a saving of 80,000.00 EUR for every saving of 0.1cm layer thickness, the cost savings for a year can easily be calculated.

Even if the guidelines and specifications all over the world vary: At some point the road construction project and thus the paving material has to be paid. At the end of the day, after each project, there are people using the street. No matter if it´s on two-wheeled vehicles, cars, trucks or others. Whether business or private purposes. We all have a great interest in traveling safe and relaxed but still quick to reach our destination.