01 Apr Designing a Concrete Fireplace – Part 3: Strength and Tectonics
This is part three in a series of posts about designing fireplace mantels, surrounds, tiles and other fire features with precast concrete. I hope to share with you some of the appreciation for this fantastic material that I have gained over the last fifteen years, as well as provide some tips and info for designers and homeowners who are considering designing a concrete fireplace.
To many, the word ‘concrete’ is synonymous with strength. We think of huge spanning bridges, tall buildings and pavement – things that are massive, heavy, rough and industrial. By necessity, economies of scale are an important aspect of concrete for infrastructure and construction. When you are pouring millions and millions of cubic yards of the stuff, a balance must always be struck between the concrete’s performance and it’s cost. Hence, when concrete is specified for these types of applications, strength per dollar is extremely important. So although most concrete in the world is indeed strong it is, like all things, relative.
Concrete for interior design applications such as fireplace mantels and tiles is much less hampered by the constraint of cost per cubic yard. These types of products use much less raw material and command a relatively higher market price. Accordingly, their concrete mix designs (the combinations and ratios of raw materials that go into the final concrete – the recipe) can achieve much higher strengths than their industrial counterparts. This means that the designs of products used for fireplace applications: tiles, mantels and surrounds, can express some of the unique qualities of concrete that aren’t economically possible in massive scales.
Concrete for interior design can have three or more times the strength of your typical sidewalk. Loosely translated, this means that these castings can be thin. High strength has a side-benefit of increased density which means that concrete fireplace mantels can have a very smooth, touchable surface. Now, one might say that neither of these qualities express traditional ‘concreteness’ as I described in the opening paragraph, and there is some truth to this but really, these are just part of concrete’s underlying identity that is not revealed in say, a sidewalk. It is kind of like comparing a Formula One car to a station wagon. Both have wheels…
The beauty of concrete for fireplace applications, is that despite its thickness (or thinness, as the case may be), it expresses mass and substance extremely well. So cladding a fireplace wall with tiles that are 3/4″ thick has the effect of creating something that feels monolithic. You can achieve the substantial aesthetic of a structural concrete wall that is smooth and inviting to the touch and applied like a tile. High strength concrete also allows for cleaner designs, crisper lines and less overall weight.
When designing fireplaces using cast concrete elements, like tiles and mantel pieces it is important to respect what is tectonically appropriate. That is “just because you can, doesn’t mean you should.” As with all materials it usually looks best if you use concrete in ways that express its intrinsic qualities. Avoid designs that convey ‘thinness’ or ‘lightness,’ or arrangements where pieces seem to inelegantly defy gravity. When laying out patterns, keep structure and lines of force in mind so that pieces bear on other pieces appropriately.
With modern concrete formulations, one can achieve strength and finish properties unheard of in typical, industrial applications. This not only allows for an incredible amount of flexibility when designing concrete fireplaces, but reveals a side of concrete that is sleek, soft and wholly modern.