What makes roads last?

SANRAL is a state-owned agency responsible for the planning, design, construction and maintenance of the national road network. 

SANRAL is also developing a rating system that is dedicated for road projects and which promotes the use of sustainable best-practices in the planning, design and construction of roads.

Five factors influence the performance of a pavement (road surface):  

TRAFFIC: Traffic is the most important factor influencing pavement performance. The performance of pavements is mostly influenced by the loading magnitude, configuration and the number of load repetitions by heavy vehicles. The damage caused per pass to a pavement by an axle is defined relative to the damage per pass of a standard axle load, which is defined as an 80kN single axle load (E80). Thus, a pavement is designed to withstand a certain number of standard axle load repetitions (E80s) that will result in a certain terminal condition of deterioration at the end of its design life, which typically has been 20 years in South Africa. Roads are not designed to last 20 calendar years, but for axle load repetitions estimated to occur over a 20-year period. If the estimates are correct, a road could reach end of its life after only five calendar years.

MOISTURE (WATER): Moisture can significantly weaken the support strength of natural gravel materials, especially the subgrade. Moisture can enter the pavement structure through cracks and holes in the surface, laterally through the subgrade, and from the underlying water table through capillary action. The result of moisture ingress is the lubrication of soil particles, loss of particle interlock and subsequent particle displacement resulting in pavement failure.

SUBGRADE: The subgrade is the underlying soil that supports the applied wheel loads. If the subgrade is too weak to support the wheel loads, the pavement will flex excessively, which ultimately causes the pavement to fail. If natural variations in the composition of the subgrade are not adequately addressed by the pavement design, significant differences in pavement performance will be experienced.

CONSTRUCTION QUALITY: Failure to obtain proper compaction, improper moisture conditions during construction, quality of materials and accurate layer thickness (after compaction) all directly affect the performance of a pavement. These conditions stress the need for skilled staff, and the importance of good inspection and quality control procedures during construction. 

MAINTENANCE: Pavement performance depends on what, when and how maintenance is performed. No matter how well the pavement is built, it will deteriorate over time based upon the factors mentioned above. The timing of maintenance is very important.