Common Defects in Bituminous Pavements and their Causes
Rutting or Deformation
Rutting or deformation is a very familiar sight on roads in the UK and all over the world, particularly as the weight and volume of goods transported on the roads has seen sustained and dramatic increase. It is possible to discern two modes of rutting or deformation.
Densification occurs if the material has been under-compacted. This can give rise to the relatively early development of ruts to a depth of perhaps 5mm, depending on the thickness of bituminous material affected.
Shear flow represents more damaging form of rutting. It is commonly seen at traffic lights or bus stops, where heavy vehicles are travelling slowly. The shape of the resulting rut gives an indication of the layer or layers which are affected.
Traditionally analytical pavement design has been based largely on protecting the subgrade and other pavement foundation layers from overstressing, which could result in deformation and rut development. The consequence of deformation in the pavement foundation is a broad rut which, while slightly less dangerous than the sharply defined ruts due to asphalt deformation, nonetheless represent a hazard if they are deep enough to trap surface water.
Fatigue Cracking
Fatigue cracking occurs in a bituminous material due to high and repeated tensile strains. Conventional analysis shows the highest tensile strain occurring at the base of the bituminous material, causing a crack to propagate upwards from the bottom of the layer. This is still the assumption made in most analytical pavement design. However, experience clearly shows that is much more usual to find cracks originating at the surface of the road if the bituminous materials are greater than a certain thickness, perhaps 200mm. This can be attributed to a number of reasons:
- Increased brittleness at the surface due to ageing;
- Horizontal stresses by the way the tyre deforms when in contact with the road surface;
- Shear force at the edge of the tyre contact area.
The effect is a crack which can look much worse than it actually is. It may only penetrate to a depth of 50mm for instance, but the deformation which then occurs around the crack under repeated load may give it a wide appearance.
It is also an observed fact that the stiffness of a bituminous mixture tends to decrease under the influence of repeated load even before any crack becomes visible. In reality, it is assumed that the stiffness loss is due to micro cracks which develop throughout the structure of the material before coalescing to form a visible crack.
If cracking is allowed to proceed unchecked, then it will penetrate the full depth of the bituminous layers. Cracks will then multiply and grow in width to form crazing. In parallel, water will enter the pavement through these cracks, softening the underlying unbound materials and accelerating the failure process.
Fretting or Raveling
Abrasion of the stones leads to a loss of textural depth. This is a problem in wet conditions since it is most likely to happen in hot climates, where the binder hardens most severely. The resulting phenomenon is termed ‘ravelling’ or 'fretting' and in extreme condition, it can result in pot-hole development.
Bleeding
Bleeding is the appearance of a film of bitumen at the surface, giving a smooth and dangerous condition. This is caused by excess binder in the surface layer which is squeezed out under the action of traffic in hot weather.