By a show of hands, who doesn’t like board-formed concrete? Anyone? If you want to be my friend, you better not have raised your hand.
So what exactly is “board formed concrete” and how do you get it to look its very best? You’re in luck because I’ve got the answers to those questions.
The picture above is from one of my projects and is a close up shot of the texture you get with board-formed concrete. It looks really, really awesome (if I do say so myself … even though I wasn’t the one that actually built this).
“Board-formed Concrete” is the name for a process of patterning concrete that leaves a wood grain image on the finished face of the concrete. It is a method of construction that is being used more frequently these days because concrete is extremely well-suited as a finished material in modern design, and by imprinting the wood grain on the surface, it visually softens and warms concrete’s somewhat cooler appearance while still allowing concrete to express the construction technique.
But how can you get this finish on your projects? Pretty simple really, just ask for it, most concrete contractors worth having on your job should know what you are asking for. On the chance that they don’t, or if you want your board formed concrete to look as good as the image above, I’m going to tell you how we do it.
When concrete walls gets placed (or poured, cast or set … whatever) there is formwork that gets built on site that holds the wet concrete in place until it has hardened. Typically, the formwork is built out of sheets of plywood that the wet concrete sits against, and the curing (drying) concrete takes on the patterns of the plywood surface … seems pretty straight-forward to me. When you want board formed concrete, you take that same plywood formwork and you add an additional layer of wood to the inside faces so that the wet concrete takes on this new pattern. In the picture above, you can see to concrete laborers adding this layer of wood to one side of the formwork.
If you want really nice board formed concrete, you have to kick it up a notch *BAM! (roundhouse kick to your formwork’s face!) To get a good understanding of the process I use, you need to learn something about how trees grow.
Most of us know that if you count the rings of a tree, you can determine how old a tree is … or at least how old it was at the time it was killed by getting chopped down. Maybe you never thought much beyond that BUT did you know that the dark rings are the winter growth and the lighter rings are the summer growth? Since trees generally grow slower in the winter months, the wood cells are denser and appear darker than the faster growing, more loosely spaced summer growth rings. The difference between these rings is the patterning that you see on board formed concrete.
But wait … it gets even better!
To really accentuate those patterns – this is what we do. We take 2x Spruce Pine Fir (1 1/2″ thick) and cut it down the middle so we now have two 3/4″ pieces. This process helps eliminate the tooling marks from the fabrication process as well as all the dents and dings that might be on the boards just from the process of being handled. Then …. we sandblast the surface.
You heard (read) me correctly. We sandblast the surface of the Spruce Pine Fir we just cut open. That is what the guy in the green HazMat suit is doing in the pictures above and below.
And why do we do this?
By sandblasting the surface, we accentuate the difference between the fast-growing (soft) summer wood and the slow-growing (hard) winter growth.
The sandblasting removes the soft portions of the wood faster than it does the hard portions so by sandblasting the surface, the pattern of the wood is more pronounced and not as subtle and irregular as it would have been if we had simply lined our formwork with 3/4″ cedar fence boards. Also, the horizontal line you see in the image above with where the Spruce Pine Fir boards came together and the concrete seeps into the gap a bit.
The horizontal pattern from the arrangement of the boards was left more pronounced in the picture above – which was at a project I worked on that was completed a few years ago.
So how much can you expect this sweet little detail to add to your project? (Are you sitting down?) If you live in the North Texas area of the world, you would need to budget an extra $20 per square foot, in addition to the cost of the concrete wall, to cover the expense of getting the wood, ripping it in half, sandblasting the surface, and then lining the interior faces of the plywood formwork.
There is no question you could accomplish this detail for less – you could just simply line the formwork with fence boards … but it won’t look anywhere near as good as the images you see above. It’s not for every project or budget and in the last 6 years, I’ve only used this treatment twice. But if you are judicious in your designs and consider carefully how you can use this treatment for maximum effect, I think there is room in almost any project for this detail.
Cheers, and happy sandblasting!