There are many discussions throughout our country about the serious problems with our pavements. There are some basic material properties and environmental conditions which are fundamental in how pavements perform that need to be recognized. The list below is based on over 50 years of experience. We are able to readily determine the cause of pavement failure from visual inspections with very good reliability.
- While the tensile strength of a rock might be as much as 10 Mpa, the tensile strength of compacted rocks (aggregate) in a road is zero.
- The strength of a rock in compression is 20-30 times greater than its tensile strength.
- The compressive strength of aggregate greatly depends on the gradation. If the gradation is over sanded so that the large rocks cannot interlock, traffic will cause the mix to flow even in compression.
- Failure can occur when materials containing aggregate are no longer in compression.
- Asphalt is a liquid and an adhesive and works the best when it can act as a liquid.
- Although the tensile strength of asphalt is low, it is the only source of strength when a pavement is under tension. That strength increases with increased shear rate (traffic speed) and decreases with increased temperature.
- The asphalt in a pavement must be able to relax tensile stresses faster than they build up or the pavement will come apart (crack) once the tensile strength of the asphalt is reached.
- Water greatly reduces the strength of asphalt pavement materials.
- Most aggregates like water better than asphalt and will reject asphalt if given the chance. That is called stripping.
- While additives can be added to asphalt so that a mix can meet specification requirements, there are data suggesting that the protection with certain popular additives will not be permanent.
- The presence of salt or magnesium chloride in wet pavement makes stripping problems worse.
A pavement will not form cracks unless it is in tension. An asphalt pavement is compressible. If it undergoes a rapid cycle in tension, and that tension does not exceed the strength of the asphalt, the pavement can rebound. However if the tension is prolonged, the asphalt will flow allowing the rocks to separate thus forming cracks. Although generally the cracks will remain open, on roads with heavy traffic, the traffic will knead the cracks back together in hot weather. Potholes form in areas where there is serious cracking. The presence on water can make any damage worse. Raveling occurs at the surface and is greatly increased if water is present. Again traffic causes tensile stresses to pull the aggregate from the surface.
In brief, cracking, potholes and raveling are all caused by tension on the asphalt bond between aggregate particles.
Rutting has four causes, studded tires, under designed pavement, under-compacted subgrade and poor gradation of the aggregate. The problem with studded tires has not been solved. With under-designed pavements, the structural strength is not strong enough to prevent failure of the subgrade causing a rut in the surface. With under-compacted subgrade, traffic on the pavement will complete the compaction resulting in a rut or a low spot. If the aggregate gradation is over sanded, the mix will flow away from the tire track and leave a rut.
A more detailed discussion can be found on www.petroleumsciences.com at a later date.
Robert L. Dunning, email@example.com