40 to 50 mm, depending on the specification; they have a reduced ratio of 6% UV light stable polyurethane resin to 100 kg kiln dried aggregate. Resin bound gravel is particularly sensitive to moisture during the curing process and the presence of water in the mix can cause a reaction in which the polyurethane binder becomes white and bubbles start to form. This issue is quite common but is often due to poor installation practices. Moisture should be completely eliminated from any aggregate used in the mix and the use of kiln-dried sand and gravels is vital to the success and integrity of the surface. See also Sustainable urban drainage systems Bioretention Bioswale Impervious surface Permeability (earth sciences) Porousity Road surface Urban runoff References https://www.ferfa.org.uk/guidance/publications/ External links https://www.ferfa.org.uk/ The Environment Agency Scottish Environment Protection Agency https://www.limegatesolutions.co.uk/ https://www.the-driveway-company.com/resin/ Road infrastructure