1.1. Permanent deformation and factors affecting it
Strain is defined as a change in length expressed as a function of the length being changed i.e.
Strain, ε = (change in length) / (original length)
In a linear elastic material the strain increases linearly in relation to the increase in stress.
Deformation in an ideally elastic material is totally recoverable. Once the stress is no longer applied, the object returns to its original shape.
Strain is defined at a point, whereas deformation is defined as the change itself. In a pavement we are usually interested in the vertical change in position at the surface (e.g. due to a vehicle passing). In this, the deformation of the surface is the sum of all the vertical strains at every point beneath the surface. These can be a result of compressive forces, tensile forces, shear, bending or torsion (twisting).
Plastic deformation is not recoverable. However, an object in the plastic deformation range will first have undergone an elastic deformation, which is recoverable, so the object will return part way to its original shape.
Road construction materials are not ideally elastic, and they will accumulate some amount of permanent deformation as a result of every load application.
Laboratory test results obtained, both during the (ROADEX project) and on other occasions, indicate that the key factors influencing the accumulation rate of permanent deformation include:
- the grain size distribution of the material, especially the proportion of fine-grained particles and their quality
- the degree of compaction i.e. the dry density of the material,
- the amount of free water contained in the material and
- the stress conditions to which the material is exposed, especially the intensity of shear stresses. At low shear stress level the permanent deformation is likely to stabilize while at high shear stress level it may keep on accumulating continuously.
1.2. Problems caused by permanent deformation
The ROADEX project has shown that in the Northern Periphery permanent deformation is the main cause of undesirable rutting on low volume roads but according to recent results in Finland it has also a big role in rut development in highly trafficked roads. This rutting causes many kinds of problems to road users and road owners.
Traffic safety and truck driver health issues
Deep ruts can be a traffic safety risk. They collect water that can result in a risk of aquaplaning during rain and a skidding risk due to ice in winter. Permanent deformation, especially on road shoulders, can also cause warping in heavy vehicles. This can damage the health of drivers in the long term through unhealthy vibration
Reduced bearing capacity
Rutting of the aggregate and/or subgrade layers in a road can lead to failure of the upper asphaltic layers.
Where this happens, instead of draining off, the surface water in the rut infiltrates into the road structures and subgrade beneath the pavement causing them to soften.
Because of this, rutting is seldom uniform along the length of a road and unevenness in the road surface can arise leading to greater roughness levels and user discomfort.
Higher road user costs
High rutting can also cause extra costs to road users. More friction is developed against the side of the tyre leading to higher rates of fuel consumption and tyre wear.
Higher road owner costs
Ruts cause other types of problems to road owners. In Scandinavia, where studded tyres are used, the wearing rate of the asphalt surface increases significantly. This leads to deep ruts that collect water and decrease the pavement lifetime.
Ruts also create problems for the removal of ice and compacted snow during winter. This can be a very difficult operation with graders or underblades and can cause damage to the pavement.