I built a stone retaining wall at the last playground i built. it is about 2.5 feet high at the peak. The top layer of stones is glued down with construction ashesive, but they keep coming loose. any suggestions?
I have had that problem in the past especially with tumbled block. We have had good success with hydraulic cement but you could also use any mortar with less expense. I like the hydraulic b/c it sets up so quickly. Too many times homeowners or their kids get on the block as soon as we pull off the site despite warning them against it and break the bond between the cap and the wall.
A few strategically appied dabs of a high quality silicone adhesive applied to a clean dust free surface works well. Especially well for a dry stack (Allan Block, Anchor, Keystone or Versalok) because it does not fight with the wall movement and relaxation when it returns. Any rigid mortar or adhesives will fail.
The flexible silicone sealer/adhesive dabs are used to let the units move slightly. This is because a SRW or unmortared stone retaining wall is designed to allow slight movements from temperature, moisture, soil pressures or seismic. That is why they are so popular. Using a unflexible adhesive is like fighting with Mother Nature.
Because it is a flexible wall, it CANNOT be used on a rigid concrete footing and does not have to be as deep as the frost line. Without a concrete footing an SRW retaing wall can be 40' high if engineered.
Any one of the international SRW licensing companies (Allan Block, Anchor Wall, Keystone oe Versalok) websites have a list of acceptable, proven products to use for caps on a SRW wall.
You are lucky unless they are light loads. The adhesive works well for SRW walls because the block are smooth on the top and bottom and the flexible adhesive allows the small movement. This is to attach a concrete block cap on the top of the wall.Obviously, no mortar or adhesive is allowed in the wall construction, just as a concrete foundation is not allowed even for a 40' high retaining wall.
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