How do road repairs work?
As well as improving the quality of local roads, Kempsey Shire Council is working to improve the community's understanding of how roads are managed and how repairs happen.
This simple introduction outlines how roads are built.
A road is made up of three layers:
- The surface
- The pavement
- The formation (or subgrade)
This is the part of the road that you drive on. Road seals usually consist of a mixture of gravel and bitumen which is spread over the area with a special machine to create a paved surface. By taking out bumps and cracks in the road, the road seal helps to restore grip and texture and creates a waterproof surface that’s safe to drive on. Underneath this, there are two layers of foundations.
The pavement is what supports the road surface and is what gives the road its strength. The pavement is made up of one or more layers of gravel known as the base and sub-base. The pavement is generally 300mm to 500mm thick. The pavement can be further strengthened by adding a 'binder' into the gravel during construction.
The lowest layer in a road is natural material known as the subgrade. This is basically the earth beneath the pavement. It generally consists of clay, ironstone and occasionally rock. The strength of the subgrade can also be improved by adding a binder to it.
Road maintenance and repair options
Road maintenance involves the repair of small failures (potholes) in the surface or underlying pavement so that the rest of the road can reach its nominated life in a safe and trafficable state.
As the road gets older, a number of factors will affect how much maintenance it needs. These factors include:
- changes in traffic volumes and heavy vehicle use
- weather patterns
- changes in the water table
- gravel quality and subgrade conditions
- installation of underground services.
Road repairs are usually tackled by ‘patching’.
There are generally two main types of patching.
- pothole patching
- heavy patching.
This involves filling holes that have developed in a sealed road with a mixture of bitumen and stone.
There are three options with the pothole patching used by Kempsey Shire Council.
This process involves the use of Council’s jet-patching truck.
The jet-patcher pumps out a mix of bitumen emulsion (a mixture of bitumen and water) and stone that is used to fill the hole. It is usually used on small to medium sized holes and doesn’t require rolling as it relies on the emulsion to 'break' and bind the stone together. This technique does not generally perform well when patching in wet conditions.
This type of pothole patching involves filling holes with a mixture of emulsion, stone and sand. Compacting the material into the hole using a roller is generally used to compress the different sized stone and sand together.
Depending on conditions, compaction is not always possible, and the holes are sometimes filled above the road surface for passing traffic to compress.
This approach is used to patch medium to large potholes, or where the underground pavement has given way in a heavily used wheel path, known as a ‘shove’. Cold-mix patching performs better than jet-patching in wet conditions, but is not ideal in wet weather.
Hot-mix (asphalt) patching
This is generally undertaken on larger areas where a portion of pavement and surface is removed and the asphalt is placed and compacted with a roller.
Asphalt is produced by using bitumen that’s been heated to approximately 180 degrees Celsius, added to a mixture of stone and sand, and delivered to the site hot.
As the mix cools down, it hardens and forms the patch. This type of patching can be used in large to very large potholes (and ‘shoves’) and performs best of all the patching techniques in wet conditions. It is also the most expensive and comes at a greater risk of injury to crews.
Heavy patching is used when large sections of the road are badly damaged or deformed.
The remaining surface is removed and the pavement gravel is pulverised by road reclaimer machinery.
Then a binder is added for strength, and the pavement is watered and re-compacted before a surface seal of either asphalt or sprayed seal is applied.
This is a more complicated and expensive type of patching and is usually only used when the road is in a very bad condition.