Keeping Wellington’s roads in good shape is a crucial part of the maintenance work we do every year to keep the city moving.
Road surfaces are not permanent and deteriorate over time. They wear and tear from traffic, trenching work for underground services, and oxidation of the surface from sunlight all take a toll.
These issues cause cracks in the surface. They start small and become progressively larger with time. These cracks cause water to ingress into the structural granular layers beneath the surface. This then weakens, and will eventually fail and materialise as potholes.
As a result, we need to resurface roads. On average, we resurface about 68km of road every year – about 10 percent of the city’s 690km of roads. This mostly happens between November and March because warm air and ground temperatures are essential to get the best results.
When we resurface a road, we typically use one of the two following options – or a combination of both will be selected:
- Chipseal – consists of a layer of sprayed bitumen followed by one or two layers of stone spread evenly over the surface of the road. It waterproofs the road and provides enhanced skid resistance.
- Asphaltic concrete (asphalt) – is a hot-applied pre mixture of aggregates and bitumen that is paved as a pavement layer of varying thicknesses starting from as thin as 25 millimetres to whatever thickness is required. It improves the levels and structure of the road.
Asphalt and chipseal have different purposes. Asphalt is used for structural and ride quality improvements. Chipseal is used for waterproofing and enhanced skid resistance.
Every road's lifecycle includes waterproofing intervals and structural improvement intervals. The current health state of a road will dictate which treatment is needed at any given time.
This is why you may see chipseal being applied on an asphalt road, or an existing chipseal road being re-laid with asphalt. For example, if the asphalt on a road is aged and dry but structurally sound, then it only requires waterproofing, which we can do with chipseal.
Deciding which surface to use
When deciding which type of resurfacing to use, we carry out numerous surveys of the entire roading network. These assess the condition of the existing surface and records things like cracking, potholes, broken edges and patching as well as the ride quality of the road.
What our surveys find, and the way the road is used, will impact which resurfacing treatment we choose.
Asphalt is chosen for areas that have a lot of turning traffic or are often used by heavy vehicles. This is why asphalt is used for main arterial roads, in the inner city and in front of shopping centres. This is also why you might see turning circles at the end of cul-de-sacs surfaced in asphalt while the rest of the road is chipseal.
Chipseal is a key tool for preventative maintenance on our roads. Roughly every 8 years, depending on conditions, the small cracks that naturally appear in a road will mean that waterproofing needs to be applied. Often, if a road that seemed to be fine is being resurfaced with chipseal, this is the reason why.
Find out more about the differences between asphalt and bitumen.