Concrete furniture — tables, benches and planters

A concrete table is a slab with manners. A bench is a wall you sit on. A planter is a vessel that must fail at drainage before it fails at the rim. None of these objects is a miniature building, and none is a ceramic. They sit in an awkward middle: heavy enough to need design, small enough that people will drag them across a patio when the afternoon light moves.

This journal likes that awkwardness. Furniture is where architectural concrete meets the hand, and where a mix that was specified for a floor is suddenly asked to be thirty millimetres thick and still survive a winter in the ranges.

A mix for objects, not for slabs

Thin sections and fibre

Ordinary structural concrete is a poor furniture mix. It is heavy, it wants thickness, and it is graded for a pump and a poker rather than for a mould on a bench. The useful idea — without turning it into a product name — is a richer, often fibre-reinforced mix that can be cast thin: glass-fibre reinforced concrete in the broad sense, or a high-binder mix with fine aggregate and alkali-resistant fibre, designed so a table top can be twenty to thirty millimetres and a planter wall can be fifteen to twenty-five.

Fibre is not magic. It helps a thin section survive demoulding, transport and the first knock against a door frame. It does not replace a rib, an upstand or a steel frame under a long dining table. A two-metre top that is a pure biscuit will crack. A two-metre top with a downturned edge, or with a hidden chassis, is furniture.

Decorative mix design still applies. Water-cement ratio, oxide loading and aggregate colour decide whether the piece looks like stone soup or like a considered grey. Because the section is thin, the face is almost all paste. A coarse twenty-millimetre aggregate that looks handsome in a driveway will look like an accident in a table edge. Fine, well-graded stone and a disciplined oxide are the usual honesty.

Pigment in furniture also shows every fingerprint of the mould. A glossy mould makes a glossy face that then has to be sealed or honed. A timber mould prints grain that will collect oil from plates. Choose the mould as if it were veneer.

Weight, movement and the lie of permanence

Concrete furniture is sold as permanence and then asked to move. People rearrange courtyards. Renters take a planter when they leave. A house is restumped and the bench has to come off the verandah. Weight is not a virtue if two people cannot lift the piece without a crack or a crushed finger.

Design for the move you will actually make.

• A dining table should come apart: top off base, or a top in two pieces with a designed joint.
• A bench can be a seat slab on separate legs, so the heaviest part is a length you can tilt.
• A planter should be sized for wet soil, not for the empty photograph. Soil and water can double the load.

Cast-in handles, hidden sockets for a lifting pin, or a base that accepts a trolley — these are not inelegant. They are how the object stays intact.

A piece that can only be moved by a crane is not furniture. It is a small precast panel that wandered into the garden.

Indoor pieces have a different weight problem: floors. A polished concrete living room will take a table. A first-floor timber cottage, a floating timber floor, or a balcony with a published load limit may not take four planters against one balustrade. Ask. The answer is sometimes “two planters, not six”.

Frost, heat and the Australian year

Australia is not a single climate, and furniture is more exposed than a wall. Alpine New South Wales, the A.C.T. and Tasmania still freeze. A saturated rim, a saucer with no overflow, or a mix that was more water than anyone admitted will spall when the ice expands. The remedy is ordinary: a dense mix, air if the specifier knows why they are asking for it, drainage that actually empties, and a winter in which you do not leave a planter as a birdbath.

Heat is the more widespread test. Inland courtyards and west-facing balconies will take a table to temperatures that make metal bases unwise to touch. Concrete handles that heat better than many people expect, then radiates it into the evening — pleasant in a highland autumn, less so under a Queensland deck at six o’clock. Colour matters. A dark oxide top in full sun is a griddle. A lighter face, or a table that lives under a pergola, is a comfort decision as much as an aesthetic one.

Sudden wetting of a bone-dry planter — a hose at midday in a forty-degree week — is a thermal shock the mix did not need. Water in the morning. This is not preciousness. It is how thin sections avoid hairline maps.

Bushfire-prone gardens add a further, quieter argument. Concrete furniture is non-combustible, which is useful near a house that has a BAL rating and a timber bench that would have to go. The plants inside the planter are still fuel. The furniture is not a fire strategy. It is one less thing to burn.

Drainage in planters

A planter without drainage is a pot of wet concrete and dead roots. The hole is not enough. You need:

• a drainage hole, or several, at the true low point
• a void or a coarse layer so the hole does not clog on day three
• a way for water to leave the patio without staining it
• an overflow if a saucer is part of the design

The underside of a planter is a finish. Water will find it. A raw, dusty base will leave a ring on sandstone and on a sealed slab. A closed, slightly raised base, with feet that let the hole drip into a tray you can empty, is the difference between an object and a stain.

Soil chemistry is a long argument. Some potting mixes are acidic; concrete is alkaline. Over years the inner face can powder or the outer face can show damp maps. An inert liner — a pot in a pot — is unromantic and kind to both the plant and the concrete. If you want the inner face exposed, accept that roots and fertiliser are a weathering system.

Indoor versus outdoor, and honest wear

Indoor furniture can be finer. A honed table, a film sealer you are willing to refresh, felt pads, and a life spent away from frost and ultraviolet light. It will still etch under vinegar and still mark under a dropped mug. The ethics are the same as a kitchen bench: test the acids on a sample, and do not sell the piece as marble.

Outdoor furniture should be specified for the sealer you will not maintain every spring. A penetrating treatment, a colour that looks like itself when wet and when chalked, and edges that can take a knock from a bike. Australian ultraviolet light will chalk a cheap film and will yellow some coatings that looked clear in the workshop. If you want a wet look on a garden bench, be prepared to recoat, or choose another material.

Honest wear is the point of concrete furniture. Hairlines at a corner, a softened arris, a darker ring where a pot sat for a summer — these are not defects if the piece was designed to take them. They are defects if the piece was sold as a showroom cube. Commission the wear you can like. Leave the rest to timber, steel or stone.

A sample for furniture should be the thickness of the job, finished both on the face and on an edge, and left outside for a fortnight if the piece will live outside. Bring it in, wet it, put a lemon on it, drag a plate across it. If that feels excessive for a stool, the stool may be more serious than the brief admitted.