Concrete goes through a lot during its lifespan, and its surface often suffers as a result. Poor finishing, scaling, popouts, gouges, abrasion, and general weathering all detract from a slab’s appearance.
On most occasions, the owner’s inclination is to have the slab completely removed and replaced with fresh concrete. But there’s usually a more cost-effective and less disruptive solution. Many slabs, both interior and exterior, can be saved by the simple application of a thin overlay.
Tough and tenacious
Designed to rejuvenate worn, damaged, or ugly concrete, there are overlays to fit virtually any need. They can be used to correct elevation problems, restore the look of plain gray brushed concrete, or transform a drab floor or wall into something colorful and dramatic. Overlays can be integrally colored or tinted with liquid stain after the fact, so the owner’s design choices are nearly unlimited.
Overlays are exceptionally durable. Compressive strengths of polymer-modified overlays range from around 6,000 psi to well over 8,000 psi, whereas the concrete beneath is typically about 4,000 psi. This harder surface is very abrasion-resistant.
Due to the polymers contained within them, overlays adhere tenaciously. They stick far better than un-modified mortar. Anyone who’s ever allowed polymer overlay material to dry on his trowel or mixing paddle can attest to the fact that it’s extremely difficult to remove.
It sticks so well, in fact, that when a delamination of an overlaid surface does occur, one often finds about 1/8 inch of the concrete substrate stuck to the back of the peeled-off coating. This indicates that the bond between the overlay material and the concrete surface was stronger than the bond between the concrete’s surface and the matrix beneath it.
Not for crack repair
This brings up an important point: An overlay is only as good as the surface to which it’s applied. If the concrete surface is weak, scaling, crumbling, or dusting, it must be stabilized before applying an overlay. All loose material should be completely removed before attempting to coat it.
Depending on the condition of the surface, preparation may be as simple as pressure washing. However, more extreme measures such as shotblasting, grinding, or scarifying are often needed. Inadequate surface preparation is by far the most common cause of overlay delamination.
Likewise, overlays are not crack repair products. An existing crack in the substrate will transfer up through the overlay within a very short time. This is especially true on exterior concrete which undergoes temperature fluctuations.
There are crack repair materials intended for mending cracks before overlayment is applied. These patching products work with varying results. If the customer’s aim is to eliminate cracks altogether by coating over them, he will likely be disappointed.