The future is due to bring more and more rainstorms which our sewers and roads will be unable to handle. Permeable road surfaces which drain away the rainwater will therefore come into increasing use as climate protection against the more frequent severe rain events.
The increased frequency of rainstorms and the ensuing cascades of concentrated stormwater create a requirement for bigger and more expensive sewers for tomorrow's Denmark. Since the major rainstorm in Copenhagen in July 2011, NCC has been working intensively to develop new ways of handling the enormous volumes of stormwater, for instance by means of water-permeable asphalt in parking lots and on suburban and inner-city roads – places where there is normally no room for retarding basins and where large pipe dimensions can only be installed at enormous expense.
The product is called NCC Permavej® and consists of a stable base course which is able to absorb large volumes of water (DrænStabil®) and a screed layer able to transport water without losing its load-bearing capacity (DrænAf®).
A permeable road surface starts with DrænStabil® from NCC, a base course with the usual properties of conventional products. However, unlike conventional base courses, DrænStabil® has a pore volume which is able to absorb large quantities of water very quickly, not unlike a soakaway.
DrænStabil® is so composed that its drainage capacity does not significantly impair its load-bearing ability.
On top of the stable base course is a screed layer of DrænAf, which is able to transport water without losing its load-bearing capacity.
The surface may be topped off with, e.g., Perma-Drain, a specially developed paving stone from Danish IBF Betonvarer, which leads water through large joints filled with the water-transporting DrænAf. Perma-Drain is designed to allow large joints in the finished surface but without compromising its stability.
DrænStabil® has a pore volume of 30% and can thus absorb even very large volumes of water. An example:
An extreme downpour of 190 liters/hectare over 10 minutes, a so-called 5-year event, produces 11.4 mm of water over 10 minutes. This quantity of water will only fill the pores in DrænStabil® to a height of 3.8 cm (11.4/10x100/30 = 3.8). And this doesn't take into account the fact that the water will continue to drain into the soil during the 10 minutes.
In the Copenhagen rainstorm of 2011, volumes of water equivalent to those in the calculation only occurred for very short periods of time.