Causes of Pavement Deterioration

Pavement deterioration, if unchecked gives rise to problems in two principal areas:

Safety: This can be divided into two further categories:

• Skid resistance – low skid resistance is a key contributory factor to a large proportion of accidents
• Unevenness – this gives rise to a poor ride quality and the danger that the motorist underestimates the difficulty in controlling a vehicle; the problem becomes particularly acute in wet weather if unevenness allows standing water in the wheel paths.

Economics: While there is a direct economic cost if safety levels falls, there is an additional and sometimes very significant cost to the economy of a region or a country if a key highway starts to deteriorate. In broad terms, increased roughness leads to slower travelling speeds and less efficient transportation of goods. It also leads to increased fuel consumption and wear and tear on the vehicles. This economic equation is the basis of many pavement management systems in use in various parts of the world.

The following are the main causes of pavement deterioration:

Deterioration Cause No: 1 – Environment

Whilst the principal distress mechanism acting on a pavement are usually traffic related, there are several which stems directly from the environment.

Binder Ageing: The phenomenon of hardening of bitumen is well known. It is leads to stiffening of the mixture and so a reduction in the strains developed in a pavement under traffic load. However, it also makes bitumen become more brittle. This can lead to more rapid deterioration under the action of traffic. Ageing is caused principally by:

• Oxidation: The rate of oxidation will be greater in a material with high air voids since this will allow the oxygen easy access to the bitumen; it also occurs more quickly at high temperature and under the action of ultra-violet sun rays
• Loss of Volatiles: In some cases, the aggregate used in a bituminous mixture can, over a period of time, absorbs some of the volatile elements of a bitumen, reducing the quality of the bond between the binder and the stone.
• Flow in Hot Weather: Bitumen behaves as a liquid under most conditions. In hot weather, the temperature of the surface can reach 50C even in the UK and the material is sufficiently soft to flow. This would mainly be seen as increased rutting under the action of traffic but can also take the form of flown down a slope if the road is on a gradient.
• Frost: A properly designed and constructed bituminous surface should not normally be susceptible to damage by frost. However, the danger exists that freezing of water trapped in the mixture may cause stress which damage the structure of the material.
• Settlement and Heave: Ground movement is one of the design considerations for roads. Bituminous mixtures can crack under the action of a slow build up of strain over a period of days, weeks or months, although the slow the movement the more strain can be accommodated before the material fails.

Deterioration Cause No: 2 – Surface Wear Due to Traffic

 The effects of this type of deterioration can be quite serious in terms of safety and ride quality although the damage is restricted to the surface. It is possible to identify several basic types of problems:

• Wear to the stones at the surface damages the microstructure at the surface of the stone. This linked directly to the level of achievable skid resistance and therefore to safety. When the level of microstructure drops too far, it is necessary to apply some form of surface treatment to restore it to acceptable levels.
• Abrasion of the stones leads to a loss of textural depth. This is a problem in wet conditions since it increases the likelihood of water coming directly between the tyre and road surface. The result is aquaplaning.
• Stones can be plucked from a brittle surface giving a very rough texture and a very poor ride quality. This is most likely to happen in hot climates, where the binders harden most severely. The resulting phenomenon is termed as ‘ravelling’ and it extreme conditions, it can result in pot holes development.
• ‘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.

Deterioration Cause No: 3 – Structure Damage Due to Traffic

Asphalt Rutting: Asphalt rutting is very familiar sight on the road all over the world, particularly as the weight and volume of goods transported on the roads has seen sustained and dramatic increase.

Rutting is the longitudinal depression in the wheel path in bituminous pavements, which can be attributed to excessive consolidation, formed by an accumulation of permanent deformations caused by repeated heavy loads. It is due to two modes of 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 the 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 by the problem.

Cracking of Bituminous Material: 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 upward from the bottom of layer. This is still the assumption made in most analytical pavement design. However, experience clearly shows that it 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 effects:

• Increased brittleness at the surface due to ageing
• Horizontal stresses caused by the way the tyre deforms when in contact with road surface
• Shear forces at the edge of tyre contact area.

If the cracking is allowed to proceed to proceed unchecked, then it will penetrate the full depth of the bituminous layer. Cracks will then multiply and grows 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.

Bond Problems: One of the inevitable problems with a bituminous road is that the materials have to be applied in layers and in order to maximise the effectiveness of the pavement as a whole, these layers have to be well bonded. Unfortunately, this is a little understood area and pavement life is sometimes reduced significantly because of inadequate bond. In general, traffic action helps to build up the strength of the bond, but this can only happen where, there is sufficient bituminous bond coat in place.

The results of poor bond are that the pavement is no longer so good at spreading load and strains are therefore higher both in the bituminous material itself and in the underlying materials.

Conclusion

It is clear from the above discussion that pavement deterioration is not a straightforward thing to analyse and monitor. It is therefore necessary to make sure that measurements and investigations are aimed at finding out the underlying causes. In this way maintenance can be targeted. For instance, it is grossly inefficient to have a philosophy which sees a certain thickness of overlay as a standard remedial measure without a proper assessment to ascertain whether there may not be a more cost-effective solution. It may be as simple as repairing the drainage.