Short answer
NRMCA lists the usual causes of discoloration on new concrete: changes in the mix between loads, uneven admixtures, short mixing time, and finishing at the wrong time. Hard troweling a stiff surface, called trowel burning, leaves dark spots. Concrete with a higher water to cement ratio is generally lighter. Some slag-cement tints fade with sun and air.
Key facts
- Higher water to cement ratio generally means lighter concrete (NRMCA CIP 23)
- Trowel burning, hard troweling a stiffened surface, causes dark discoloration (NRMCA CIP 23)
- Flake calcium chloride that is not fully mixed discolors more than liquid (NRMCA CIP 23)
- Yellow-green tint from slag cement disappears with air and sunlight (NRMCA CIP 23)
Mix causes
NRMCA says color can change with the cementitious materials or sand from load to load, though that is generally rare and minor. Concrete with a higher water to cement ratio is generally lighter, some fly ashes make it darker, and slag cement makes it lighter. A yellowish to greenish tint from slag cement, usually on formed walls, disappears with exposure to air and sunlight and does not affect quality.
Construction causes
Inconsistent admixtures, short mixing time and badly timed finishing can all discolor a slab, according to NRMCA. Flake or pellet calcium chloride that does not fully dissolve discolors more than the liquid form. Trowel burning, hard troweling after the surface is too stiff, leaves dark areas by densifying the surface.
NRMCA does not count stains from things that land on the concrete later, such as irrigation water, plants, rust or oil, as discoloration.
From Tim’s jobs
