Surface defects affect the top running surface of a pavement. These defects lead to discomfort for drivers and damage to vehicles and goods. They can eventually lead to greater pavement, or even subgrade, defects. Surface defects include corrugations, scour or ruts, and potholes.
9.5.1 Corrugations, potholes, and scouring
Corrugations are parallel ridges that lie at right angles to the direction of the traffic. They are often located on steep roads, particularly around corners, and are caused by vehicles bouncing as they travel over the pavement. The mass of a vehicle can determine the level of movement – a loaded vehicle will cause little damage, whereas an unloaded vehicle will bounce significantly more, especially when travelling up slopes. Once corrugations start to form, they induce further vehicle movement which increases the rate of damage to the pavement. Any irregular surface feature will initiate vehicle vibration, causing corrugations to occur. The type of vehicle suspension can also contribute to the problem. In dry conditions, using a grader to cut to the depth of the corrugation and respread the material usually rectifies the problem, as the underlying material beneath the corrugations is usually not affected. In wet conditions, the corrugations sometimes move lower and deform the base course and lower pavement layers. In these cases, the pavement should be left to dry, and reshaping and compaction of the material will be required to form a surface for water runoff. Longer-term corrective measures include the addition of a clay binder, or lime stabilisation of the pavement, and using higher quality crushed aggregate for isolated trouble spots. Where corrugations occur at the approaches of bridges, on steep grades, or on low-radius curves, sealing will provide the necessary control. There are several ways to prevent corrugations from forming, especially in critical problem areas like steep corners. Lightweight drags towed behind a vehicle can be an effective preventative treatment. Other equipment, such as a lightweight grader, or brushes attached to, or pulled by, a utility or tractor can be used to pass over the surface on a regular basis. Compaction after maintenance grading may also help.
Potholes are holes formed in the upper surfaces of the pavement. They generally occur in wheel paths in low lying areas, in shaded areas where the road is constantly wet, or where there is little or no crossfall. For example, at bridge approaches and intersections. Potholes are generally caused by poor pavement drainage due to low (or nonexistent) crossfall. Water will lie on the surface of the pavement, and seep into the top layers. Movement of vehicles over the area strips the surface material, allowing more water to enter the pavement. Fine material is then suspended in the water and carried away. The only remedy for potholes is to restore the crossfall and superelevation to its original level, to ensure adequate drainage. This can easily be achieved by cutting the surface and grading the pavement back to a correct crossfall. In severe situations, new material will be required to replace what has been lost and mixing and reshaping is required. Potholes can be prevented by increasing or constructing crossfall. This will encourage water runoff. Also, stabilisation of the pavement, or the use of a moisture retardant, can be used in areas where the pavement is constantly shaded and wet.
Scouring can occur along and across the pavement. Scouring not only causes traffic problems but also opens the pavement to the weather, which may cause further deterioration and defects. Pavements with high fine contents, and small aggregate sizes and insufficient binding, are more susceptible to scouring. Scouring is caused by the flow of water over the pavement, due to the lack of compaction or base course binding, excessive grades, lack of crossfall, or the build-up of debris on shoulders preventing drainage of surface water. The best option to overcome scouring is cutting, grading and compacting the affected material to ensure the crossfall and superelevation is reconstructed. Cleaning out ditches and berms will ensure that water is directed away from the roadway. The use of a high-quality mechanical interlocking aggregate, and stabilisation of the pavement can also be employed.
Surface pavement maintenance normally includes reshaping the pavement cross section, light grading to remove corrugations, ruts and pot holes, moving displaced running course back onto the roadway, and some regravelling (remetalling) the pavement, if required. It will help to improve vehicle safety and road drainage.
Between a good grader operator and metal truck driver, most surface pavement issues will be addressed rapidly. A poor grader operator could compound the problems. The grading frequency will depend on the traffic volume, axle weights, materials involved and the skill of the operator.
One of the most critical aspects of unsealed roads is correct crossfall and superelevation. Ensuring this is maintained, will provide effective water runoff from the pavement. The loss of road shape can be attributed to loss of material, settlement, poor construction or inadequate drainage, and improper grading practices. The only way to effectively reshape the road carriageway is to heavily grade the surface, reshape and then compact.
Road shoulders are part of the road cross section between the road running surface and the outside extent of the road. Shoulder maintenance will improve pavement drainage and reduce the risk of pavement edge defects. Shoulders can settle more than the pavement due to incorrect compaction at the time of construction. Vegetation and displaced road aggregate can build up on the shoulder, creating drainage issues like ponding. Also, road runoff can scour the roadside edge, because water is diverted along the roadway rather than across the shoulder into the ditch.
The best procedure to reshape and remove surface pavement defects, like corrugations and potholes, involves:
- Blading (scarifying) the surface
- Adding material. Either new metal, or metal pulled back from the edges of the road
- Mixing and shaping the surface to form a crown
- Finally, compacting at the optimum moisture content.
Maintaining the road crown or crossfall is critical. The grading should begin at the edge of the road, and work towards the centre. The cutting depth will depend on the reshaping procedure required. The windrowed material is positioned at the centre of the road, then spread evenly back across the cut surface on the final pass. This procedure is then repeated for the other side of the road carriageway. It is important that the grader does not make a final pass down the centre of the road with the blade horizontal, because this will remove the crown.
Maintaining superelevation on the curves provides effective drainage of water from the carriageway, as well as providing easier cornering for vehicles. The most important feature of superelevation is that it needs to gently transition between the corner and the straights. Any sudden change can adversely affect vehicle handling. Care must be taken not to create superelevation by dropping the inside edge of the road. Crossfall superelevation should not be too high for heavy vehicles – 6% is suggested as an absolute maximum.
9.5.3 Adding new running course
As a rough guide, running course should not be allowed to become less than 25 mm thick. It provides traffic with a smooth surface over which to travel, and stops it from wearing out the protective base course, which provides drainage and load distribution. A good metal truck operator will be able to effectively spread the metal along the road section to the required depth wanted.
If material removed from the running course is not replaced, the base course material will eventually be lost. This will reduce the depth of the pavement, ruts will appear due to the lack of load distribution, and the subgrade will fail under loading. Replacing gravel before subgrade failure occurs is an integral key to long lasting pavement performance.
All maintenance plans should include adding new running course material when needed. The road surface is worn away by traffic at varying rates due to:
- Weight, speed and frequency of traffic
- Road gradient
- Tightness of curves
- Depth of the running surface
- Type of pavement material used.
In wet conditions on steep roads, heavy traffic can accelerate the wearing process causing rapid deterioration. In dry conditions, aggregate is lost through raveling and dust.
In locations subject to freezing, shaded roadways may freeze during the winter months. Roadside trees can prevent the sun from melting the ice, resulting in the road being slippery all day. This is compounded if the cold weather continues. Using a broken stone and large aggregate will cause it to protrude above the ice formation, providing a safer running surface. Also, if necessary, trees can be removed from the sides of the road to allow sun onto the pavement.