Concrete is often used on industrial pavement sites within New Zealand that have high volumes of heavy traffic.  However, the initial cost to the public roading network of using concrete has meant that bitumen-bound materials have been preferred.

Even so, short sections of concrete pavements in high-stress areas can provide a long-term solution with minimal maintenance.  Therefore, Transit New Zealand is now trialling short concrete road sections to take advantage of the superior strength and robustness of concrete.

case study 6_photo2.jpgThe Peanut Roundabout in Napier is an example of a trial site that is part of Transit New Zealand’s programme.  This section of SH50 carries most of the fully laden trucks bound for the Port of Napier and has a history of surfacing distress due to the tight curvature and high volumes of heavy vehicle traffic.

The brief for the project was to design and construct a sound and rigid concrete pavement that provided adequate skid resistance for the traffic environment.  In addition, construction of the road had to be finished with minimal delays to the travelling public.

Construction of the roundabout was completed in just five days.  On the first day, the existing pavement was removed and stockpiled, the subgrade prepared and the proof rolled.  On day two, the stockpiled pavement was stabilised and the surface prepared for concrete.

On day three, the road was concreted and cured.  Concrete with a compressive strength of 30MPa was used along with mixed grades of sealing chip and crushed aggregates.  Polypropylene and structural synthetic fibres were used to reduce plastic shrinkage and increase flexural strength.  The concrete was screeded by hand, delivered at 60mm to 80mm slump, and superplasticised to around 110mm in order to achieve the desired workability.

Poorly designed and constructed joints can lead to differential settlement.  To overcome this, dowelled joints at 200mm centres were employed at regular 5m intervals transversely along the road to suit the road geometry.  The concrete was poured directly up to the cut edges, with no local thickenings or slab anchors used.

On day four the joints were saw-cut, and on day five the line marking was completed, and the road opened once adequate concrete strength was confirmed.

Since the roundabout was completed, the pavement has performed well – no cracking is visible and no differential settlement is evident.  Expected wearing of the surface has occurred in the running path, while in the low-speed environment skid resistance is not compromised.

The use of concrete in road pavements is a practical way of providing a long-term durable solution for highly stressed local sections of roads.  As demonstrated by this trial site, a sound pavement structure, adequate surface texture, and constructability during a short time period can all be achieved with the use of concrete.

*Hart, Gordon & Johnson, Richard. (2006) Concrete roads for high stress applications. Proceedings of the Transportation and the Pursuit of Excellence, NZIHT & Transit NZ 8th Annual Conference. Auckland, New Zealand.