Foam Core Concrete: Drop the Weight. Keep the Mass

light weight foam in concrete countertop

Let’s face it, if you’re a fan of concrete in the home, you probably like it big and chunky. I’m talking about concrete that makes it’s presence known by being bold and unmistakably thick. This can be a contradiction of sorts; on one hand we want pieces that are thick and blocky, on the other hand those very pieces can be impossibly heavy. Weight is a major limitation for concrete products. In trying to please our customers and push the limit of our medium, we have experimented with some radical solutions. Let me tell you about one such case:

We were commissioned for a 128 x 40” concrete island. This is a beast of a single piece. This customer was also interested in 2” thick concrete. There is a cut-out for an under-mount sink, but the total weight of this concrete behemoth would be about 731 lb. Although Chris and I fancy ourselves as strong fellows, this would be a bit much! We’ve been able to reduce weight in concrete countertop construction before, so, we took this as another opportunity to lighten things up.

The basic idea is to remove weight, which is obvious, but this was to be an island piece with an overhang, and, not only does it need to sit on cabinets in an asymmetrical fashion, the overhang needs to be strong. Some concrete artisans utilize a “drop edge” that is the idea of making a thin countertop say for example 0.75” thick or so and then the edges are extended to appear thick. This is a valid approach although it’s not quite as strong as other techniques and also looks funny if you see it from below. We’ve also had some trouble with parts warping or cracking when using this technique. So although it could be fine for others, we do it a little differently.

Principles first: The best place to remove weight from such a piece is exactly in the middle. The overhang is a classic cantilever beam bending problem. When somebody leans on the side sticking out, tension forces occur in the top of the beam and compression forces occur on the bottom.

When a structure like this is loaded, most stress occurs on the top and bottom surfaces. The middle is loaded but with sheer force that is much lower than the tension and compression on the top and bottom respectively of the beam. To remove weight we try to remove mass from the beams middle so as to keep the top and bottom strong. Our plan used 1” foam set directly in the middle of the concrete. This yielded 0.5” of concrete on either side, which, really is not much! But not to worry, not only is the concrete we use heavily fortified with 3/4” alcali resistant glass fiber, but we applied two layers of fiberglass lath (cross hatch of filaments) to each of the sides.

construction of a large concrete countertop flat forms
Concrete facecoat applied

We laid out the mold on a nice big flat table (more on that later perhaps). The mold then represented the dimensions we took at time of template. We also cut a foam blank for the sink cut-out and located it appropriately. After carefully applying release agent we poured a “face coat” or specifically colored layer of concrete to the tune of about 0.15” thick. After troweling this layer on the flat and mold sides, we waited to let it set up slightly. When a finger depression leaves only a slight mark in the face coat we apply the first layer of structural concrete. This layer is just under 0.5” and is fortified with fiberglass. Six percent of the total weight of the structural mix is composed of short strand fiberglass. After carefully troweling this layer flat we applied two layers of fiberglass lath. The next step was to apply the foam.

perforated foam in concrete countertop
Foam laid out on top of the first structural layer
expanded polystyrene foam sitting on top of fiberglass mesh reinforced concrete
Close shot of the foam sitting on top of the fiberglass lath

The foam left a 1.5” wall around the perimeter and also was split into three pieces so as to provide some structural ribs. The other important and key part of the foam are the perforation holes. This is a little trick we learned from working in the composite industry. We cut 1” diameter holes at some forgotten spacing not only to provide a mechanical tie-in between the top and bottom face, but also to carry some of that lesser, but still existing, shear stress.

After massaging the foam in place we poured the next layer of structural mix on top of the foam ensuring that the perforation holes were filled. And, to keep the structure symmetric, we applied two layers of fiberglass lath pushing it in with trowels. We made sure the mold was filled up to the top of our 2” form boards by screeding the back side. Finally letting the exposed mix stiffen up a bit, we covered the work with plastic to keep it from losing moisture and placed some blankets on top to keep the piece appropriately warm during curing.

This technique reduces the weight of the casting to 430lb ( A 41% reduction). Not light, but not too shabby for this massive single piece island…At least now the two of us can move it around!

Here are some shots of the piece as completed.

Rough concrete countertop
Massive Concrete countertop with foam core
Concrete countertop sitting on saw horses in wet grinding area
Our large concrete island sitting on some horses

More shots to come!