State highways carry large traffic volumes over long periods and through varying climatic conditions. Their strength comes from a layer of carefully graded and compacted stone immediately below the surfacing, called the 'base course layer'. This layer is supported by varying thicknesses of compacted gravel on top of the natural ground.
On medium to high-traffic highways, the best base course layer is good-quality crushed local stone (aggregate). Where this isn't available, we use lime or cement to improve the strength of the crushed stone. For even heavier traffic, such as on motorways, we use an asphaltic concrete (bitumen) mix instead of stone.
Although concrete roads were built in New Zealand between World War One and World War Two, they are currently not being used because:
they are difficult to maintain where services such as water, gas and sewers are located underneath
they do not handle the long-term earth movements to which New Zealand is susceptible very well
they are more expensive than asphaltic concrete
they tend to be more noisy than asphaltic concrete without a noise-absorbing surfacing.
The volume of traffic on our highways means the pavements do deteriorate with time and sometimes need re-strengthening. How often depends on the road, but they typically last for between 20 and 50 years. We usually recycle the existing base course layer with additional new material.
Research undertaken by the CAPTIF Road Research Centre has significantly improved our pavement maintenance over recent years.
Find out how we monitor pavement condition.