Originally published by The Green Scene, July 23, 2024
My Favorite Cast Member: Cast in Place Concrete
The Green Scene is a high-end design-build Landscaping and Swimming Pool firm in sunny California, specializing in developing custom designs for our clients. For decades, we have worked with cast concrete, guided by the expertise of Garden Artisan, Scott Cohen. With his extensive background in ceramics and custom mold creation, Cohen supervises our design teams to achieve outstanding results.
“Concrete is a versatile material that we can cast into virtually any shape, making it my preferred material for freeform swimming pools,” says Cohen. “Working with pre-manufactured radiuses from factories can be a nightmare, but casting coping in place offers us unparalleled flexibility in the field. One significant benefit is the ability to color-coordinate our coping with steps, caps, countertops, bars, and fire features.”
Frame of Mind:
Here’s how Cohen describes coping: “Coping is the frame that wraps around the art piece that is the pool and spa. Sometimes we select a darker frame, sometimes a lighter frame, and sometimes one that matches the decking so it appears as if there is no frame at all.” Contrasting the color between decking and coping can add a safety feature, making the water’s edge more visible when the coping is poured in a contrasting color.
Pop Culture:
Cast concrete products adjacent to wet locations like pools and spas are highly susceptible to issues with reactive aggregates, such as pop-outs and ASR. I recommend using white cement instead of gray to mitigate these problems or adding fly ash or slag to hand-mixed materials to reduce the potential for reactive aggregate issues.
Natural Beauty:
The downside of any cast-in-place material is that the color will be mottled and inconsistent. In my opinion, this is the secret behind the beauty of cast concrete products—the inherent imperfection. However, some clients prefer a more consistent color and texture, which can only be achieved with pre-cast concrete products manufactured in a factory environment. It’s essential to discuss this preference with clients in advance.
Good Vibrations:
Always vibrate your cast concrete coping into the molds. This helps consolidate the concrete, reduce voids, and improve the quality of your castings. Vibration allows you to use less water in your mix, resulting in less porous concrete and fewer shrinkage cracks. We score our coping every 1 1/2 times the width of the coping, or about every 18” to 24”. We do not use reinforcing bars in coping that is 3” or less in thickness.
Additional casting tips:
Use a release agent on your forms. Pull the forms while the concrete is firm but still wet enough to finish the edge. Concrete that cures in the molds never looks professionally finished. Forms that are pulled early enough can often be reused on another project.
Get the Edge:
Pool coping can be made of precast concrete, stone, concrete pavers, and cast in place. It is easier to do a custom edge detail when you cast it in place. The three most popular edge details on cast concrete pool coping are square, bullnosed, and “mantle.”
Coping with Isolation:
Because pool structures are dug down into the “inactive” zone of soil, below the freeze/thaw line in most areas, it is a good idea to pour the concrete coping with an isolation joint between the decking and the coping. This foam gap is usually backfilled with sand and topped with a weatherproof sealant (like Dec-O-Seal). I don’t recommend monolithically poured, cantilevered concrete coping in California clay soils or anywhere that can freeze.
Between a Rock and a Hard Place:
A recent practice in pool construction is to utilize a soft joint (typically silicone) between the bottom of the coping and the top of the waterline tile. Instead of a cementitious grout that will easily crack with any movement, a soft joint allows the concrete coping to shrink and expand without influencing the tile. This results in less tile cracking and delamination. The remainder of tile is grouted with standard sanded grout, but returned to silicone in the corners or any “change of plane”.
Slippery When Wet:
One of the benefits of selecting cast concrete over polished stone coping is that stone can be slippery when wet. The coping surface should be finished with a sand, broom, exposed, or somewhat textured top to provide slip resistance.
Cast concrete coping works great with pool cover track systems.
