Discolouration ~ manifests as a patchy surface finish. It is caused
where there are differences in hydration or moisture loss during the
concrete set, due to concentrations of cement or where
aggregates become segregated. Both of these will produce
moisture content differences at the surface. Areas with a darker
surface indicate the greater loss of moisture, possibly caused by
insufficient mixing and/or poorly sealed formwork producing
differences in surface absorption.Crazing ~ surface shrinkage cracks caused by a cement rich surface
skin or by too much water in the mix. Out-of-date cement can have
the same effect as well as impairing the strength of the concrete.Lime bloom ~ a chalky surface deposit produced when the calcium
present in cement reacts to contamination from moisture in the
atmosphere or rainwater during the hydration process. Generally
resolved by dry brushing or with a 20:1 water/hydrochloric acid
wash.Scabbing ~ small areas or surface patches of concrete falling away
as the formwork is struck. Caused by poor preparation of
formwork, ie. insufficient use of mould oil or by formwork having a
surface texture that is too rough.Blow holes ~ otherwise known as surface popping. Possible causes
are use of formwork finishes with nil or low absorbency or by
insufficient vibration of concrete during placement.Rust staining ~ if not caused by inadequate concrete cover to
reinforcement, this characteristic is quite common where iron rich
aggregates or pyrites are used. Rust-brown stains are a feature
and there may also be some cracking where the iron reacts with
the cement.Dusting ~ caused by unnaturally rapid hardening of concrete and
possibly where out-of-date cement is used. The surface of set
concrete is dusty and friable.Concrete -- Surface Defects