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Cause of Shadowing Through an Overlay

By Chris Sullivan – ChemSystems, Inc.

Question:

We did a "rough trowel" overlay on a driveway in Pennsylvania, and three weeks after the job was complete dark marks appeared. At first, they would come and go, but now they are permanent. When the homeowner washes his car, the water that gets on the driveway surface also starts to darken the overlay. We think that moisture from underneath the overlay and penetrating from the top in areas where the overlay and sealer have chipped off is causing the darkening. We are now looking for a fix to this problem.

Answer:

This is what is known as "shadowing." Either contamination or moisture is causing a shadow or darkening to show through the overlay. The area where it is darker could be the shadowing of a crack, oil stain or repair material. Based on the photo, the cause appears to be moisture, especially in the areas around the control joint.

To fix the problem, you need to find where the moisture is getting under the overlay and stop its entry to prevent the shadowing from getting worse over time. The joints are easy – just caulk them with a flexible polyurethane caulk. Also, make sure to caulk all edges and places where water can migrate between the overlay and concrete substrate. The main areas are a bit trickier. You will need to be grind down the overlay, treat the concrete with a penetrating waterproofing sealer, and then reapply the overlay.

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