Aggregate Shadowing

Summary

Aggregate Shadowing - is the development of dark spots on the surface of a recently laid concrete floor slab.

Technical Information

Aggregate shadowing is caused by the coarser aggregate remaining close to the visible surface of a concrete slab. It can be caused by the following mechanisms:

The paste directly above the near-to-surface aggregate will be subject to a higher pressure during power trowelling. The higher pressure will cause more water to be expelled from the paste, locally. This, in turn, causes a lower water/cement ratio and can cause localised darkening.
Aggregate can become saturated during mixing and release the moisture back into the local cement paste after curing. When the aggregate is closer to the surface, there is less cement for the water to diffuse into, making it more saturated than the surrounding cement giving it a potential darker appearance.
Aggregate which is dry can absorb water from the surrounding paste during curing. Above near-to-surface aggregate there is less paste present from which to absorb water meaning the paste will lose more water per unit volume. This will cause the paste in the area to have a lower water/cement ratio, potentially causing darker coloured regions.

Typically the aesthetic effect of aggregate shadowing will fade and blend with time.

If the surface is properly closed and cured, aggregate shadowing shouldn’t cause any problems to the durability or structural performance of the concrete. However, if the surface is not properly closed, spalling can more easily occur in areas with coarse aggregate close to the surface.