Building Concrete Furniture

Concrete is one of the most demanding materials an architect or designer can specify for furniture — and one of the most rewarding. It is fluid before it sets, rigid and nearly permanent once cured, and formally expressive in ways no other material quite matches. What follows is an updated guide to building concrete furniture: the construction sequence, the structural logic behind rebar placement, and the range of surface finishes that make each piece unique.

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01 — The Construction Sequence

It is necessary to first conceive of the piece as a whole — its form, its structural demands, its intended surface — before a single board of formwork is cut. The drawings and details must anticipate the pour. Improvising in concrete is expensive; improvising in the formwork is free.

Step One: build formwork and embed rebar to floor

Step Two: add network of rebar that will provide tensile support

Step Three: finalize formwork and rebar for pour

Step Four: pour concrete and finish surfaces as necessary

Step Five: secondary pour for seat

Step Six: enjoy the furniture — concrete bar by Duncan Nicholson

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02 — Rebar Placement and Structural Logic

Concrete is extraordinarily strong in compression — it resists being squeezed. It is comparatively weak in tension — it resists being pulled apart. Steel rebar does the opposite: it is excellent in tension, and that complementary relationship is the basis of reinforced concrete. The art is in knowing where the tension lives inside any given form.

In a cantilever — a slab or counter projecting from a single support with no support at the free end — the top fiber of the concrete is in tension. The element bends downward under its own weight and any imposed load, stretching the top face and compressing the bottom. Rebar must therefore be located near the top of the section, close to the tension zone, where it can actually engage and resist those forces. Placing rebar at the bottom of a cantilever slab is a common and serious error; it is sitting in the compression zone where concrete is already doing its best work, and provides little structural benefit.

In a simply-supported span — a slab resting on supports at both ends, like a bridge or a shelf between two pedestals — the bottom fiber is in tension. The element sags under load, compressing the top and stretching the bottom. Here, rebar belongs near the bottom of the section.

In either case, the rebar cannot be placed flush against the form surface. A minimum concrete cover — typically 3/4” to 1-1/2” for interior furniture applications — is required to bond the steel into the matrix and protect it from corrosion and fire. Small plastic chairs or wire spacers hold rebar at the correct depth during the pour.

For furniture and architectural elements with complex or irregular geometries — curved forms, multiple cantilevers, integrated seating that spans in two directions — intuition about tension zones breaks down quickly. This is where 3D structural modeling using finite element analysis (FEA) becomes invaluable. Software can model the actual forces moving through a complex concrete solid, producing stress maps that show precisely where tension accumulates and therefore where reinforcement is needed. What looks like a sculptural object from the outside is, structurally, a network of internal force paths — and the rebar layout should follow those paths, not some generic grid.

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03 — Surface Finishes

The surface of concrete is not a default — it is a choice made at two points: before the pour, through the nature of the form; and after the pour, through mechanical or chemical treatment of the hardened surface. Each approach produces a fundamentally different material expression.

Board Form

Among the most architecturally expressive finishes available to concrete. The form is built from rough-sawn lumber — typically 1x4, 1x6, or similar boards laid in a pattern — and when the concrete cures and the form is stripped, the grain, the saw marks, the joints between boards, and even the knots are transferred faithfully to the concrete face. The result is a surface that reads simultaneously as stone and wood: mineral permanence with an unmistakable memory of the tree. Board form was used to great effect by Ando, by Wright in certain late works, and throughout the Brutalist tradition precisely because it acknowledges the labor of making while producing a result no tool applied after the fact can replicate.

Smooth Float Finish

Achieved by forming against a smooth surface — melamine-coated plywood, GFRC-grade form liner, or glass — and carefully controlling release agent application. The result is a dense, near-gloss surface that shows aggregate shadow beneath a tight cement paste skin. Any bug holes or honeycombing from inadequate vibration will be immediately visible. Smooth form finishes reward precise workmanship and punish haste.

Sandblast

Applied after the concrete has cured. Abrasive media is projected at the surface under air pressure, cutting away the cement paste matrix and exposing the aggregate below. Light sandblasting produces a fine matte texture; heavier passes begin to reveal the pebble or stone aggregate in the mix, producing a rougher, more naturalistic result. Sandblasted concrete has excellent anti-slip properties and a visual warmth that smooth finishes lack. It is also highly effective at unifying a surface that has form-release marks or minor color variation.

Wet Sand / Grind and Polish

Wet grinding with diamond tooling removes the surface paste and progressively reveals aggregate, much like finishing stone. The process moves through a series of grits from coarse to fine, and can be taken all the way to a high polish if the mix design includes attractive aggregate — river pebble, marble chip, glass cullet. The result at its best resembles terrazzo and can be stunning in furniture applications: a countertop with the visual complexity of a geological section. This finish requires the longest post-cure time and the most labor of any option.

Bush Hammer

A mechanical tool with a grid of pyramidal points is driven across the hardened surface, fracturing the cement paste and aggregate in a uniform pattern of small craters. Bush hammering produces a deeply textured surface that reads as stone quarried and dressed — rugged, honest, and highly tactile. It is rarely used for furniture but is powerful in large architectural elements where scale allows the texture to read properly.

Acid Etch

A dilute acid solution — typically muriatic or phosphoric — is applied to the cured surface and reacts with the calcium carbonate in the cement paste, dissolving the outermost layer and opening the pores. The result is a matte, slightly textured surface that takes sealer more readily than an untreated face. Acid etching is a common preparation step before staining or sealing, and can also produce mottled color variation that gives concrete countertops an aged, organic quality.

Integral Color

Pigment is added directly to the mix before the pour, dispersing color throughout the full depth of the concrete rather than as a surface coating. When the piece is ground or chipped, the color is consistent throughout. Integral pigments are typically iron oxide-based (earthy tones: reds, ochres, blacks, greens) and are extremely lightfast. This is the most honest form of color in concrete: it cannot peel or wear away because it is the material itself.

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04 — Sealing and Maintenance

Concrete's liability as a furniture material is its porosity. Untreated, it will absorb water, wine, oil, and acids with alarming speed — any of which can stain the surface permanently. Sealing is not optional for countertop or dining applications; it is a structural requirement of the design.

Penetrating sealers (silane, siloxane, or reactive epoxy penetrants) soak into the pore structure and react chemically with the concrete, making the pores hydrophobic without changing the surface appearance. They are invisible and do not alter sheen. Topical sealers (acrylic, epoxy, or polyurethane) form a film over the surface, offering higher stain resistance and a controllable gloss level, from matte to high-gloss. They are more visible and will eventually wear, requiring reapplication every few years in high-use areas. For most furniture applications, a combination approach — penetrating sealer first, topical coat over it — gives the best long-term performance.

Avoid harsh alkaline cleaners, which attack both sealers and the cement matrix. A pH-neutral soap and periodic re-sealing are all that is required to maintain a concrete surface indefinitely.

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Concrete is a material of commitment. It is labor intensive from the first board of formwork to the final coat of sealer, and it does not forgive indifference at any stage. But its benefits are proportional to that investment — a concrete surface, properly executed, is capable of lasting thousands of years. Few materials used in furniture can make that claim. All images are works by Nicholson Architects with Andrew Nasser, Structural Engineer, and Ostermann Construction. All rights reserved.

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