When properly completed, concrete overlays are a cost effective way to renovate a space for your clients and to add a lot of value to the property. Instead of having to hammer out the concrete to dispose of it and re-do the base, concrete that is in good condition can be overlaid with pavers or other natural stone material.
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When starting an overlay project, we need to make a judgement call on whether or not the concrete slab is structurally sound. This means ensuring that there is a proper slope away from the foundation to allow water that enters the system to move away from the house. Any imperfections, cracks, separations of +/-⅜” over 10 feet in the concrete slab may mean that the slab is likely to move and the base of the concrete slab was not properly prepared. Overlaying concrete that is questionable will come back to bite you. However, there are products on the market that allow you to address any inconsistencies in the concrete slab like crack repairs. We like to ask our potential clients questions like how long the concrete has been there and whether they have they had any issues with it before we start looking it over.
We like to use a 2” natural stone cap around the exterior of our overlays because we like the finish that it provides especially in high traffic areas like steps where the surface can become worn down. The good thing about natural stone is that it is the same all the way through. It also provides us with a ¼” difference in height between the pavers being laid in the field of this overlay allowing us to line the area with a non-woven geotextile and ¼” of concrete sand. Having the same height cap as the pavers would cause us to have to use just a very light dusting of fine masonry sand in order to create separation between the pavers and the concrete slab.
We use a polyurethane adhesive to glue the caps using perpendicular lines to allow any water to run right under the caps and out. If you ran parallel lines, water would be trapped under the caps causing issues during freeze-thaw cycles and the retention of moisture under the entire overlay system. When adhering these caps, we make sure each piece is in line with one another, level, but with a slight pitch towards the drainage area for water to run directly off the caps and out onto our walkway.
The pavers should never be laid directly on the concrete, as this will trap water that enters the system between the paver and the concrete which will cause issues during freeze-thaw cycles as well as the retained moisture causing effloresence issues which is a mineral deposit in the concrete that shows in the surface of the stone as a white residue.
Especially in concrete slabs that have control joints cut in or if a crack presents itself, the non-woven geotextile will act as a bridge containing the sand in the overlay system while still allowing for the drainage of water. It should be overlapped 12” at the seams. In the case of this concrete porch overlay, the fabric wraps up the sides of the caps to prevent the washout of concrete sand.
ICPI calls for holes to be drilled through the slab along these expansion joints to allow for the free flow of water through the system and into the base material and out to the drainage area. In the case of this front porch where it is covered and a polymeric sand suited for overlay applications allowing just 1% of water through it, not to mention that holes should not be drilled through a concrete front porch, we are not following this step.
Weeping holes in the masonry around the house should be addressed as well. These should never be blocked by the paving stones and an expansion cord or other material should be laid between the pavers and the stones to allow for the airflow behind the masonry. Failure to do so will cause moisture problems for the homeowner in the future.
When screeding the concrete sand on top of the geotextile, we want to match the slope of the concrete slab from our starting point to our ending point. At the caps, we want to have the pavers be approximately ⅛” higher than the caps to ensure the water runs off the caps properly. We also want to factor in the final compaction of the pavers after installing the polymeric sand. This will be minimal considering the ¼” screed layer, but still needs to be taken into account.
When installing the polymeric sand for your concrete porch overlay, you will want to ensure that you are using a polymeric sand that is specified for overlays.
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10 авг 2021
