ROB GAIMSTER, CEMENT & CONCRETE ASSOCIATION AND NZ READY MIXED CONCRETE ASSOCIATION
EVER PULLED INTO the car park of your local shopping precinct, come to a stop at the traffic lights in the centre of town, or turned at a roundabout close to your kid’s school, only to feel as if you have driven over a cattle grid?
Intersections of various types are routinely damaged by traffic more than almost any other point on the road.
Heavy vehicles stopping and turning stress the road surface (pavement) at intersections, which by their very nature experience greater traffic volumes compared to the approaching lanes.
The asphalt surface at these busy junctions is prone to premature deterioration, rutting and ravelling under the strain of buses and trucks travelling at low speed around a tight curvature.
The deformed driving surface becomes an expensive maintenance problem for the roading authority, as well as a comfort and ultimately a safety concern for road users.
While intersections form only a tiny percentage of our total roading network, their importance to the overall level of functionality warrants the careful consideration of alternative design and construction models.
While not traditionally a widely used pavement material in New Zealand, concrete pavements for intersections offer a range of advantages that are worth summarising here:
- As a relatively low maintenance asset a concrete pavement is competitive in terms of repair or replacement;
- Reduced maintenance translates to less disruption to traffic and local businesses;
- Generally thinner than alternatives, a concrete pavement requires less excavation;
- A concrete pavement is less susceptible to softening or deterioration caused by oil and fuel spillage;
- Good light reflectivity of a concrete pavement enhances pedestrian and vehicle safety at night and during inclement weather;
- A concrete pavement’s surface texture and skid resistance can be tailored to meet specific requirements;
- Constructability of concrete pavements over a short time period allows for rapid re-use of the intersection.
In short, the rigidity of a suitably designed and constructed concrete pavement is well suited to withstand the loading and turning movements of heavy vehicles at busy intersections, which in turn means the expense associated with frequent maintenance is substantially reduced.
When planning to construct an intersection, the concrete pavement should cover at least the ‘functional’ area. This includes the longitudinal limits of the approach and exit lanes, as the distress caused by braking of turning heavy vehicles extends beyond the ‘physical’ area.
Concrete slab thickness will be influenced by traffic density and vehicle characteristics, but is most likely to be thicker in the physical (central ) area of the intersection, compared to the approach and exit lanes.
The most important design aspect of concrete pavement intersection is jointing. However, by applying simple jointing fundamentals that consider spacing, type and layout concrete expansion and contraction can be managed effectively for long-term performance.
Concrete mix and construction
An appropriate concrete mix is essential to ensure a successful outcome for any concrete pavement. The capability to meet strength requirements within the specified timeframe is particularly crucial.
Adequate strength goes a long way to safeguarding a hard, durable, skid-resistant surface, as well as accommodating the tensile stresses resulting from shrinkage and loading.
The construction of concrete intersection pavements can incorporate a variety of methods, including fixed-form and slip-form equipment. The latter will most likely be used if the intersection is considered large enough by the contractor.
Although various types of equipment can be used, a relatively standard set of construction steps apply to the majority of concrete intersection projects. These include, the removal of the existing intersection, subgrade / subbase preparation, setting forms, placing in-pavement objects, concrete placement, surface texturing and jointing.
Uptake and exerience
Overseas experience with concrete pavement intersections has been encouraging. The concrete roundabouts constructed across New South Wales for instance have performed with very low maintenance demands.
In South Africa, Canada and the United States concrete pavement intersections are becoming more commonplace, supported in-part by the work of the relevant concrete associations.
Here in New Zealand, where investment in roading is at levels not seen for decades, the adoption of concrete pavements to provide long term durable solutions for road sections that are highly stressed merits consideration.