Dark discolouration on smooth formed concrete surfaces (mottling)
Patches of localised dark discolouration, or mottling, very often occur on concrete surfaces cast against smooth impermeable form faces, e.g. film faced plywood, plastics, GRP and steel. The instances of mottling may have increased in recent years due to a fashion for designers to specify smooth flat concrete surfaces and the common use of high quality film faced plywood panels (both for high quality finishes and for multiple re-use purposes).
Although the phenomenon of ‘mottling’ has been known about for many years (it was researched nearly 30 years ago, there is still considerable ignorance amongst designers, specifiers and contractors on the causes.
Concrete Advice 21 aims to explain the causes and ways of reducing the risk of its occurrence.