Like other urban local authorities around the country, there are three main ways we resurface roads:
- chipseal – which consists of a layer of sprayed bitumen followed by one or two layers of stone chips and provides a flexible, waterproof, highly skid-resistant surface
- asphaltic concrete (AC) – which is a mixture of chip and bitumen and applied hot to the road surface
- slurry seal - which is a thin mixture of bitumen and chip.
On average, about 60% of the resurfacing work we do is chipseal, 35% is asphaltic concrete, and 5% slurry seal.
The difference between the seals
Chipseals are mainly used to stop water entering and damaging the underlying foundation layers of the road and generally have a life of 3-10 years. Their use is like painting your house on a regular basis to prevent the weatherboards rotting.
Why we chipseal your road Brochure (501KB PDF)
Asphaltic concrete is used to improve the smoothness of ride for people in vehicles and on bikes and helps reduce vehicle maintenance costs. We use it to even out roads, or sections of road, that have an uneven or undulating surface.
We also use it a lot in the inner city and in shopping centres because of high amount of both traffic and pedestrians.
However, there are disadvantages to using asphaltic concrete:
- It's not as waterproof as other seals unless it is used in thick layers (usually over 60mm). Because most of our roads have an underlying layer or foundation that 'gives' or flexes under heavy loads, it usually isn’t wise to cover them in thick layers of asphalt because the asphalt will crack and fall apart.
- It's much more expensive than chipseal.
We've been using slurry seal more in residential streets over the last few years as it gives a smoother surface with fewer problems caused by loose chips.
However, slurry seals have limitations:
- They cost about twice as much as chipseal and only last half as long.
- They are also very brittle, which makes them unsuitable for cracked or highly flexible roads.
How we select the resurfacing treatment for your street
Surveying the roads
When deciding which type of resurfacing to use, we carry out two surveys of all our roads. One survey assesses the condition of the existing surface and records things like cracking, potholes, broken edges and patching. The other survey measures road ‘roughness’ – using a number of lasers attached to a vehicle.
Analysing benefits against costs
The surveys are entered into our computerised road assessment and maintenance management system which helps to identify those sections that need waterproofing and those that need reshaping. A benefit/cost analysis - which takes into account traffic volumes, pavement loadings, and the results of the ‘roughness’ survey - is an important part of the system.
Most of our suburban streets have a flexible structure that need waterproofing every now and again. Chipseal is the best engineering solution for this. At times, we'll cover new chipseal with a layer of asphaltic concrete - but only if our assessment shows the road needs some reshaping as well as waterproofing.
Keeping costs down
The NZ Transport Agency provides about half the funding for the resealing work and Wellington City Council pays the rest.
We try to keep costs to you - as a ratepayer and/or taxpayer - down, while making sure that the best engineering option is chosen in each case.
Sometimes compromises have to be made. For example, chipseal in suburban streets can cause some inconvenience with loose chips and a little more noise. However, if we used the smoother asphaltic concrete, the cost would be 3-5 times higher. As the NZ Transport Agency would most likely not cover this cost, we'd have to pass it on to ratepayers.