Porous Asphalt Maintenance Cleaning

In Europe, highway authorities routinely clean Porous Asphalt
pavements in order to retain hydraulic conductivity and low acoustic values, indeed some
sections of motorways around Rotterdam are cleaned three times annually.
The work is usually carried out under ‘rolling-block’ traffic management at night or at off peak
periods hours in the day- time.
Using the hydrology process, we apply water to the surface in the same way as other
activities but through wider angled jets (40 degrees) which in turn loosens and releases the
detritus deep down in the matrix and then the water and detritus are removed from the
surface by the vacuum recovery device.
At that time, the Porous Asphalt had been laid some 4 years and there were then problems
with surface spray during heavy rain because the draining ability of the material had reduced.
Water outflow tests were carried out and pre-treatment rates of outflow were recorded in
excess of 6 minutes! Newly laid material would normally give outflow rates of between 8 – 10
Using the captive hydrology process, we achieved post-treatment results of between 9-13
There is a strong economic and environmental case for regular cleaning of porous or other
materials with negative texture.
In environmental terms, regular cleaning maintains the low acoustic values inherent in the
asphalt meaning quieter surfaces for drivers and local residents alike.

Experience in Holland has shown that because of their regular cleaning regime, the life of the
pavement is enhanced by 3-5 years thus saving expensive replacement costs and further
protecting the environment because of the reduction in the need for resurfacing.
Other materials that would benefit from routine deep cleaning are Stone Mastic Asphalt’s and
the various thin surfaces that now abound the industry.

Concluding comments and acknowledgements

Captive hydrology is now widely accepted and used by most local authorities in Europe
mainly for Retexturing, however only a relatively small amount of work is done each year
amounting to around 1 million square metres.
A great deal more work could be done if financial constraints were eased and that highway
engineers would consider Retexturing instead of surface dressing. If a pavement is basically
structurally sound but is likely to be slippery when wet, then Retexturing is a viable, cost
effective solution and is environmentally responsible.
Using hydrology as a reusable intervention as suggested: - DMRB – Vol. 7 – Chap 11 derives
the following additional benefits for the Highway Engineer:
• No quarrying of new and irreplaceable materials
• No planing’s to dispose of
• Contaminants removed at the point of treatment and not abandoned to verges or
drainage systems
• A reduced need for fossil materials as bitumen use is reduced and fuel oil requirements
are minimized, thus saving on energy and keeping CO² emissions to a minimum.
• Minimal exposure of highway personnel to moving traffic.
It is still relatively early days in the development of hydrology in highway maintenance but
interest is growing as the positive benefits of the process are realized and there is a huge
potential market in the UK and Internationally.