City flooding? Goodbye to all that!

We are developing a new kind of asphalt that is permeable to water and that will prevent storm flooding in towns and cities. The solution is being tested at the moment in Tikaslaakso in Espoo, Finland.

Water-permeable, i.e. porous, asphalt is one way of dealing with storm water drainage and so preventing city floods.

“As a consequence of climate change, the annual amount of rainfall is predicted to increase by 12–24 percent over the course of this century. This means more flooding, and so steps must be taken to deal with the intensifying rains,” explains Lemminkäinen’s product development specialist Anniina Määttänen.

The rain problem is worsened by densification of city and town structures.  There are increasingly more surfaces in population centres that are impermeable to water, such as asphalt, close-spaced tiling, and paving. There are no longer enough greenery areas which would allow rainwater to soak into the ground. As a result, streets become flooded  when the drainage network is unable to cope with the volume of heavy rains.

Laboratory tests

In the 2012–2014 period Lemminkäinen was involved in a specialist capacity in the CLASS (Climate Adaptive Surfaces) project, which was led by the VTT Technical Research Centre of Finland. The project developed water-permeable surfacing that is designed to cope with the specific demands imposed by the Nordic climate.

Three different types of porous asphalt surfacing were developed in our central laboratory. Two of these are top surfacing for different traffic volumes, and the third is base surfacing to be laid beneath these. We gave our porous asphalts the brand name Lempore.

“We have used porous asphalt before,” Määttänen explains, “on sports fields and elsewhere, but now it is not only the top layer of asphalt surfacing that is porous. The basal layer is also open-structured and it stores and delays water. Both in our own central laboratory and in VTT’s research laboratories, studies were done of the resistance of open-asphalt surfacing to deformation and wear caused by studded tyres, resistance to sub-zero temperatures, water-permeability, clogging, and various cleaning techniques.”

The resistance of open asphalt to wear caused by metal-studded tyres decreases with the increase in the porousness of the asphalt, which makes it unsuitable for use on roads with heavy traffic. But open-asphalt surfacing is expected to be well suited to surfacing for lighter traffic, such as paths, park areas, town squares, and low-speed residential roads.

Testing the theory in practice

In the piloted areas in Tikaslaakso in Espoo, Finland, the behaviour of water permeability of open asphalt and other surfacing materials is being tested by VTT. As well as in Espoo, there are also trial areas in Helsinki, Vantaa, and Oulu. The pilot project in will be completed in October 2016.

According to Lemminkäinen site manager Antti Kuula, there were structural-technical challenges in sealing the structural layers of the pilot surfacing areas and in achieving the required load-bearing capacity.

“The structural layers consist of crushed stone, so the lack of finer material affects the consolidation of the surfacing. Because this is a pilot project, the various work phases had to be documented in much more detail than usual. This is to allow for precise examination of the outcome later on.”

Project Manager Pasi Marjamaa of the City of Espoo says that Espoo is in search of new solutions for managing storm water. Tikaslaakso was chosen for the pilot project for scheduling and other reasons.

“We tested different surfacing solutions that might be suitable for parking areas in Espoo. The aim was to determine how well the various solutions last over extended periods, and to find the best and most cost-effective solutions for conditions in Finland.”

Results over time

Researcher Hannele Kousa of VTT Technical Research Centre of Finland says that VTT had to begin data collection for the CLASS project from scratch. The idea was new in Finland. In some other places, including Japan and the United States, permeable surfacing is already common. In Europe, Belgium and Germany are among the countries where they’re used.

“In Tikaslaakso, we’re testing how the various surfacing types last over time. In addition to water-permeability, we’re also testing how the materials hold up in the freezing cold, how they withstand frost heaving, different upkeep methods, and the changes brought about by seasonal changes,” explains Kousa.

The measuring and monitoring of the preliminary stages of the pilot project continue until 2018, but overall monitoring will continue over the long term.