Hi, Eric here with thirty by forty design workshop, today we're going to be discussing
board formed concrete which we're using on two large site retaining walls on our Squid
Cove project here in Maine.
So, we've just cast a small mock-up and in the process I've learned quite a bit and I'm
eager to share that with you.
Now, we're all likely familiar with poured in place concrete.
It's one of the humblest of building materials and it's unique in that it begins as a highly
plastic slurry and after it cures it becomes its polar opposite - extremely hard and durable.
And, aside from its inherent utility in foundations and its weather-resistive properties, we can
exploit this transformation from slurry to sculpture in our architecture.
Typically, concrete is poured into large, panelized forms made of plywood or aluminum.
This results - to varying degrees - in a relatively smooth appearance.
Yet even smooth forms will telegraph knots and surface imperfections to the finished
face of concrete walls once the forms are stripped.
Now, board forming is actually an old technique that preceded the larger panelized systems
commonly used today and it's a method that accentuates concrete's plasticity.
In lieu of plywood or metal panels, boards comprise the container - or form - into which
the concrete mix is poured.
This results in a finished wall imparted with the reverse impression of the face of these
boards, including all the imperfections, the knots, and the gaps.
It's this texture that we're after when we specify board formed concrete.
It gives life to wall surfaces, subtly highlighting the process of their making and for our project
it also links it back to the other wood textures we're using and the surrounding wooded site.
So concrete is composed of four elements: water, cement, aggregate and sand.
By altering the mix between these ingredients we can control how the concrete will perform
over its lifetime and in the specific environment where we're using it.
For exterior walls we want a dense concrete and a high quality paste.
It's the paste that predominates the color of a concrete wall.
So, we're using a higher strength concrete which means more cement paste.
And to that we're adding a super-plasticizer technically known as a "high range water reducer."
This is an admixture that affects the cement particles, which are normally attracted to
each other and they tend to clump together in the presence of water.
A super-plasticizer binds to the cement particles and in so doing, leaves space between them
large enough for water molecules to enter.
Now what this does is it allows the water to hydrate the cement more efficiently.
Hydration is the chemical reaction that causes concrete to "cure" or harden.
Superplasticizer allows us to use less water - which makes for stronger concrete - and
still maintains its workability which is typically a problem when we use less water in the mix.
It also ensures that the concrete will readily flow into all the imperfections in the boards
that make up our forms.
The concrete mix is arguably the most important part of our recipe.
We want lots of texture so wood type and face figure is important.
For our project, we settled on a local spruce - which was readily available, it was inexpensive
and it was pretty easy to work with.
Most board forms use softwoods like spruce, pine, fir, or larch.
Now, we did experiment with finishes to accentuate the surface texture.
We tried pressure washing, wire brushing, sandblasting with walnut shells and finally
with coal slag.
For our test panel we also included regular mill-finished, un-planed boards.
And this is probably a good time to discuss the necessity for sample panels, they're just
an absolute must and you only get one chance to get the real thing cast properly and looking
the way you intended.
So Nate, our general contractor, built ours as a 2-foot by 2-foot by 8-inch thick panel
which was enough to test out the board textures and a few different edge finishing techniques.
Next, you'll have to choose board orientation and width.
Now, these are fundamentally aesthetic decisions: whether it's vertical, horizontal, fixed or
random width, broad or narrow but they can be used to underscore a particular design
vision.
We chose a horizontal board which helps with the site wall to gesture to the horizon which
is helping us to reach out into the site.
And we chose a fixed width board of 6-inches which is a comfortable, human-scaled proportion.
And 6-inches also allowed our coursing to easily correlate with other elements of the
architecture like the deck, our lighting and the interior and exterior steps which we've
designed which also relate back to the stepping site.
Vertical boards will tend to make elements feel taller, while narrower board widths - regardless
of orientation - will accentuate directionality even more, making vertical orientations seem
taller and horizontal ones seem longer.
The more boards though the more expensive the forming labor becomes so there's a delicate
balance to maintain there.
To keep the concrete forms held together under the immense weight and outward pressure of
wet concrete the two sides of the form walls must be tied together.
Now, typically these form ties are made of mild steel and the tips are broken off flush
with the wall face after the wall is cast.
In most cases this is fine, but mild steel will rust and because our board formed walls
are acting as aesthetic elements in our project - site walls near a large deck and a gathering
space we want them to look as good as possible.
Complicating this decision was the fact that we opted to use standard plywood forms for
the outer shell of our forms.
This was for ease of setup and bracing and allowed us to line the forms with our wood
boards.
What this also meant was that we have to use ties that worked with our concrete contractor's
standard forms.
So, we priced out stainless steel ties, but the added cost was significant and, as it
turned out, was more than our client wanted to spend.
So, we're planning to use the standard mild-steel ties and then we'll just come back and patch
the holes after they've been broken off.
Now, fiberglass rod ties are another option if you're not using a standard form.
They aren't expensive, they don't rust, and they're actually quite unobtrusive, but we
had a hard time sourcing them locally and again, they didn't work with our subcontractor's
forms so it really wasn't a viable option for us.
Now is also the time to think about the corner details.
Our retaining walls are doubling as a seating bench and a planter so the corners are becoming
really important.
Hard edges on concrete don't perform well over time as they tend to break and they sort
of look terrible.
So, our contractor Nate mocked-up a few different options for us to consider and we settled
on a small chamfer, which from a distance, creates a shadow line that actually looks
a lot like a hard-edged corner.
We tried a true corner and a quarter-round tooled edge as well but neither of those looked
quite as nice as the small chamfer.
After casting the sample panel and discussing it with our client, we all agreed that the
unaltered boards provided enough subtle texture for our needs.
And, this helped by reducing the extra labor cost of blasting every one of the boards.
There's reinforcing: all concrete should have reinforcing to prevent cracking and with that
you'll also need control joints per ACI requirements.
Concrete will crack and control joints essentially "control" where that cracking occurs keeping
it contained rather than letting it spider-web across a beautifully formed wall face.
Especially long walls will require them and you'll have to plan for these in the forms
as well.
Now, there's also things like planters, which we're using.
You'll have to form those and you'll have to reinforce them separately.
There are bond-outs for integrated equipment or devices, things like exterior lights, speakers,
recesses, utility connections, drainage if you're using a planter that's especially important.
And finally, you might choose to embellish the wall with any number of embedments - dates,
patterns, emblems, things like that.
That all needs to find its way into the formwork.
Now for the concrete placement.
Adding the superplasticizer will aid in the workability of the concrete as it will tend
to flow into voids readily.
However, this doesn't eliminate the need to vibrate the concrete as it's placed.
But, you have to be careful not to over-vibrate or over-consolidate or hit the rebar when
vibrating as that will tend to bring the aggregate to the surface and negate all the hard work
of the previous steps.
Vibrating keeps large voids or honeycombs from forming in the wall which are not only
unsightly, but they can impact the longevity of a wall allowing water to pool and freeze
and crack it over time.
Also think about how you want to finish the top surface.
For us, we opted for a hand-troweled, smooth finish.
Because of the variables involved it's hard for me to say precisely what this will add
to your project over a standard concrete wall.
It will depend on height, thickness, job scope, details, board size, ties; in short, all the
things we just discussed.
However, the more information you can provide your concrete sub and the more experience
they have in placing board-formed concrete the better your final price will be.
So you should come in prepared to discuss the requirements and expect to work through
the details together as you work to quantify the added costs.
So, to recap, the costs above and beyond a standard poured-in-place wall are: boards
and any prefinishing that you choose to do to them sandblasting or whatever, extra labor
to install the boarding, any special form ties, stainless, fiberglass, superplasticizer,
a higher strength concrete, any embedments or specialty bond-outs and potentially more
stripping time.
Now, for our project's budget all of this was pretty manageable and the trade-off - we
think - is priceless.
I'll be sure to update the video once the actual site walls are poured and complete
and you can leave any questions you have in the comments.