Concrete Furniture
Architectoid · Material · Concrete
James Perry · Conner & Perry Architects
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| Concrete furniture designed and photographed by Conner & Perry Architects |
Of all the materials available to a designer working outdoors, concrete is the one that most honestly rewards understanding. It is not simply durable. It is plastic, in the truest sense of the word: formable, shapeable, capable of becoming almost anything you can imagine, provided you can build a form for it. That is a fundamentally different proposition from wood, which wants to be a plank, or steel, which wants to be a tube. Concrete will be whatever you ask it to be.
Years of work at the Sheats-Goldstein Residence, John Lautner's 1963 masterwork above Benedict Canyon, have given me a working feel for this material that no textbook delivers. You learn where the reinforcing has to be before you can settle the shape. You develop an intuition for the geometry of stress. And you learn the hard discipline of the material, which is that the design work happens in the formwork. Once the concrete is placed, the decision is permanent.
The Structure
Where the Steel Goes
Here is the structural truth that separates good concrete design from bad: reinforcing steel must be located in the zones of tension. Concrete is extraordinarily strong in compression, resisting being squeezed, and comparatively weak in tension, in being pulled apart. Its tensile capacity runs on the order of a tenth of its compressive strength, which is why the steel exists at all. The steel is not there to make the concrete stronger in general. It is there to do the one job the concrete cannot do, in the one place the concrete is being asked to do it.
The cantilever is the case that gets drawn backwards most often. A bench cantilevered from a wall is bent by gravity, but the bending reverses relative to a simple span: the top of the bench goes into tension and the bottom goes into compression, and the maximum tension occurs at the face of the support, where the bench meets the wall. The primary steel belongs near the top surface, and it has to run back into the wall or the footing far enough to develop, not stop politely at the joint. Cantilevers that fail, fail at the support. They fail because the bars were short or the bars were low, rarely because the concrete was weak.
Reverse the support condition and the logic reverses with it. A slab of bench spanning between two piers is a simple span: tension in the bottom, steel low, maximum moment at midspan. Same material, same load, opposite answer. Trace the moment before you place a single bar.
Cover is the other half of the detail, and it is the half that gets value-engineered away on site. Steel needs concrete around it to stay passivated and out of the weather. The conventional minimums for exterior work are three inches where concrete is cast against and permanently exposed to earth, two inches for #6 bars and larger exposed to weather, and an inch and a half for #5 and smaller. Skimp on this and you get rust staining in five years and spalling in fifteen, as the corroding bar expands and pushes the face off the element. Cover is also what limits how aggressively you can treat the surface, which is where the next decision starts.
At the Sheats-Goldstein Residence, every surface participates in structure. That is the lesson concrete teaches you. The furniture is not placed in the space. It is part of the same continuous form.
The Surface
A Finish for Every Intention
One of concrete's most underappreciated qualities is the range of finishes it will accept. The same structural pour yields radically different results depending on how the face is treated, and the choice is worth making at the drawing stage rather than discovering it on the day the forms come off.
Board-formed concrete preserves the grain and texture of the formwork lumber directly in the face of the element. Each plank seam and knot becomes part of the finished surface, which means the form carpenter is the finish carpenter, and the board layout deserves a drawing of its own. Sandblasting opens the aggregate and gives a matte, slightly rough texture that reads as stone-like and grounded. Bush hammering goes further, fracturing the surface matrix to expose aggregate in a rougher, more uniform pattern with a strong directional grain under raking light.
Both of the last two remove material, and that is the connection back to the reinforcing. A light sandblast takes perhaps a sixteenth of an inch. An aggressive bush hammer can take a quarter inch or more. That depth comes out of your cover, so either detail the mock-up depth into the cover dimension or specify the finish before the shop drawings are approved.
At the other end of the spectrum, a steel-troweled finish produces a smooth, dense, nearly reflective surface, the kind you see on interior floors and high-finish countertops. Outdoors it is worth thinking twice about, because dense and smooth is also slick when wet. Ground and sealed concrete takes the same direction further, with progressive diamond pads reaching a clarity and depth that approaches terrazzo, and revealing every bit of the aggregate you chose or failed to choose.
And then there is the pebble-seeded aggregate finish, which is the one most often specified incompletely. Broadcasting and pressing the stones into the surface toward the end of the pour is only the first half. The finish does not exist until the cement paste covering those stones is removed, either with a surface retarder applied to the fresh concrete and washed off the following day, or with a carefully timed wash before the paste takes final set. Get the timing wrong in one direction and you wash out the matrix and lose the stones. Get it wrong in the other and the stones stay buried under a gray film. The reward for getting it right is a naturally textured surface with real slip resistance and a quality genuinely tied to the landscape beneath it.
The Body
Seventeen Inches, and a Slope You Cannot See
A concrete bench that is beautifully formed and uncomfortable to sit on is a failure, and the dimensions that prevent this are unforgiving because you only get one attempt at them. Finish height wants to land between seventeen and eighteen inches, measured after the finish treatment and after any paving that abuts it. Seat depth of sixteen to eighteen inches works for a bench people perch on; go deeper only if there is a back to lean against, or you have simply built a wide ledge.
Slope the seat about an eighth of an inch per foot toward the open edge. It is invisible to the eye and it is the difference between a bench that sheds a rain and a bench that holds a puddle exactly where someone's back goes. Ease every arris with a half-inch to three-quarter-inch radius or a small chamfer. A sharp concrete edge chips at the corner, telegraphs every form imperfection, and is unpleasant against the back of a leg.
One more consideration that belongs in Southern California specifically: thermal mass cuts both ways. A dark, dense, steel-troweled seat in full afternoon sun becomes genuinely unusable by two o'clock. Lighter cement and lighter aggregate help, an exposed texture helps more by reducing contact area, and shade helps most. Put the bench where the building already casts a shadow at the hour people will use it.
The Material
Weather, and the Cracks You Plan For
Concrete is among the most weather-resistant materials available for outdoor use. It does not rot. It does not require painting, staining, or oiling. Properly mixed, placed, and cured, it handles sun, rain, frost, and heat without complaint, which is a great deal more than teak or powder-coated steel will do over the same twenty years.
What it will do, and what most writing on the subject quietly omits, is rust. Reinforced concrete contains steel, and steel corrodes. Every built-in bench is on a clock from the day it is placed, and the decisions covered further down are what set the length of that clock.
Color is the honest exception. Integral pigments and film-forming sealers do shift under sustained UV, some of them noticeably within a few years. If long-term color matters, put it in the aggregate and the cement rather than in a coating, and accept that the surface will lighten and settle into a patina. That patina is usually an improvement, but it should be a decision rather than a surprise.
The second thing to plan for is cracking. Concrete shrinks as it dries, roughly a sixteenth of an inch over ten feet, and it will relieve that shrinkage somewhere. Reinforcing steel does not prevent cracks. It holds them tight and distributes them, which is a different and more useful thing. The design move is to decide where the crack goes: a formed reveal aligned with a board joint, a saw cut hidden at a change of plane, or a deliberate joint where the bench meets the wall. Reentrant corners crack, without exception, so either detail a joint there or add diagonal steel and accept a hairline.
The Long Term
The Steel Is on a Clock
Fresh concrete has a strongly alkaline pore solution, somewhere around pH 13, and that alkalinity grows a passive oxide film on the reinforcing that stops corrosion cold. Two things destroy that protection. Carbonation, as atmospheric carbon dioxide works into the surface and drops the pH. And chlorides, which arrive from marine air, from salt-chlorinated pools, and from anything used to melt ice. Once the passive film is gone and oxygen and moisture are present, the bar corrodes, and the corrosion product occupies several times the volume of the steel it came from. That expansion is what cracks and spalls the cover off from the inside. The rust stain on the face is not the problem. It is the receipt.
Built-in furniture is a harder case than a wall, and the cantilever is the reason. Tension steel in a cantilevered bench sits near the top, which is the same face that takes the rain, the sprinklers, the spilled drinks, and the cleaning. The bar doing the structural work is the bar closest to the weather. Then a bush-hammered or ground finish takes a quarter inch off the cover that was protecting it. Thin section, shallow cover, exposed face: that is the exact recipe for corrosion, and it is why sealing a concrete bench is not housekeeping. It is structural maintenance.
The first defense is the concrete itself, and it is the cheapest one available. Corrosion is governed by permeability far more than by compressive strength, so a water-cement ratio at or below 0.40, thorough consolidation, and a genuine seven-day moist cure will outperform any product bolted onto a sloppy mix. Fly ash, slag, or silica fume cut chloride diffusion substantially and cost close to nothing.
Crystalline waterproofing admixtures are the next step, and they suit this application unusually well. Xypex Admix and its equivalents are batched into the mix at roughly two to three percent by weight of cement, and their reactive chemistry grows crystals in the capillary pores, cutting water transport and re-sealing hairline cracks up to roughly four tenths of a millimeter whenever moisture is present. Two things make it the right call here. It is integral rather than topical, so grinding, sandblasting, or bush hammering the face does not remove it, which is precisely the flaw in relying on a coating for a finish you intend to abrade. And it works on the cracks the previous section told you to plan for. Corrosion-inhibiting admixtures, calcium nitrite or the migrating amine types, do a different job by protecting the bar chemically rather than tightening the matrix, and on aggressive coastal sites the two are commonly specified together.
Then there is the bar itself. Epoxy-coated reinforcing performs well when the coating is intact and disappoints when it is not, because a nick at a bend or an unpatched cut end concentrates corrosion at that one point rather than distributing it. Specify patching and careful handling, and expect the yard to be casual about both. Galvanized bar tolerates handling far better. Stainless, 316 or duplex, is the honest answer and the one worth reaching for on furniture, because the objection that kills it on a bridge deck does not apply here. A bridge deck carries tons of steel. A built-in bench carries a few hundred dollars of it, against formwork, finishing, and design time that dwarf the bar cost entirely. Glass-fiber reinforced polymer bar is immune to corrosion outright, though its low stiffness means deflection rather than strength will govern the cantilever, and it cannot be bent in the field.
Last comes the sealer, the only item on this list that depends on someone doing something in year four. A penetrating silane or siloxane keeps liquid water and dissolved chlorides out of the pore structure, resists staining from wine and oil and citrus, and leaves the surface looking like concrete rather than like plastic. Reapply every two to five years depending on exposure. Film-forming sealers look wetter on day one and fail visibly later. Write the interval into the closeout documents, because the client will not remember the conversation.
None of this is optional if the piece is meant to outlast the people who commissioned it. The cantilever puts the steel where the weather is. Everything above is how you keep it there.
The Idea
Not Placed on the Site, Grown from It
The virtue I care about most in built-in concrete is integration. A concrete bench is not a piece of furniture sitting on a patio. When it is designed well, it is the patio: grown out of a retaining wall, wrapping a fire pit, defining the edge of a terrace. You cannot imagine the outdoor space without it, because the furniture is not in the space. It is part of the space.
This is what working at Sheats-Goldstein teaches at close range. Lautner did not detail seating as an object to be moved in after the certificate of occupancy. Concrete runs from wall to ledge to terrace edge without ever announcing a transition, and the same pour that holds up the building also holds up the person looking out of it. Once you have measured, patched, and drawn those conditions in the field, it becomes very difficult to go back to designing furniture and architecture as separate problems.
The same principle runs through all the work of the organic tradition: materials used honestly, forms grown from the logic of structure, and no meaningful distinction between the building and the things inside it. A concrete bench that grows from a concrete wall that grows from the land beneath it is a small version of the same idea, and it is available to any project with a budget for formwork and the patience to draw it properly.
Build the form carefully. Get the reinforcing right. Pour it once.
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| Concrete furniture designed and photographed by Conner & Perry Architects |
Architectoid · Material Studies
Sullivan → Wright → Lautner
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