Inherent colour variation, as opposed to surface staining or moisture and grout loss though leaking joints, is almost inevitable. It can be caused by differences between different batches of concrete, due to variations in the mix proportions and variability in the cement and aggregates. Variations may be minimised by ensuring continuity of supply of materials for the duration of the contract and by careful attention to batch control and workmanship.
Colour variations may also be due to the type and quality of the surface of the formwork. For example, dark discolouration and glazing of the surface can be caused by formwork with an impermeable surface with insufficient release agent. Leaving forms on longer in one area compared to another alters the curing conditions and may leave a darker surface.
Colour variations may also become apparent at some time after the formwork is removed, due to uneven curing or exposure to different drying conditions. This can be avoided by protecting all surfaces from drying out and by correct curing.