Hairline Cracks in Concrete Furniture: Normal or Not

Cast concrete is a stone-like material made in a mould, and like stone it carries marks of how it was made. Some of those marks worry people unnecessarily. Others are genuinely worth acting on. Telling them apart takes a fingernail and a torch.

Crazing, which is normal

A network of very fine lines, often only visible at a low angle or when the surface is drying after a wipe. These form in the top fraction of a millimetre as the cement paste at the surface shrinks slightly during curing. They do not go deeper, they do not spread, and they do not weaken anything.

Crazing becomes more visible over time as the lines pick up a trace of dirt, in the same way that grout darkens. On a sealed piece that happens slowly. People who like concrete tend to like this. People expecting a plastic surface do not.

Shrinkage cracks

Wider than crazing, often one or two lines rather than a network, usually running from a corner, a hole or a change of section. These form during curing when one part of a casting dries faster than another or when a shape restrains the shrinkage.

They are stable. They stopped moving before the piece left the workshop. They are only a problem if they are wide enough to collect dirt or to let water in, and the answer to both is sealing.

Structural cracks, which are not normal

These go through a section rather than across a face. Signs:

• Your fingernail catches in it.
• It continues over an edge onto another face.
• The two sides are not flush, so there is a step you can feel.
• It moves or sounds hollow when you press either side.
• It appeared after an event you remember, which is almost always a drop, a knock or a bad lift.

The four causes

1. Impact. Concrete is strong in compression and weak in tension. A dropped corner concentrates a great deal of force into a small area.
2. Bad support. A slab bridging two supports with a gap in the middle, or a bench resting on three feet of four because the floor is uneven, puts the material into bending, which it hates. Packing a wobble is not cosmetic.
3. Bad lifting. Carrying a long piece from the ends, or by its top rather than its base, loads it in exactly the wrong direction. The method is in moving concrete furniture.
4. Freeze-thaw. Outdoors, water enters a fine crack, freezes, expands and levers the crack open. Each cycle does a little more. This is the one that turns a hairline into a failure over a few winters.

What to do

Crazing: nothing. Keep the piece sealed so the lines stay clean. See sealing cast concrete furniture.

A stable hairline: clean it with a soft brush and water, dry it fully, and work a penetrating sealer into the line. The aim is to stop water and dirt entering. Where it needs to be less visible, a colour-matched cementitious filler can be pressed in and the surface resealed, accepting that a repair in matte concrete is always a little different in sheen. If the piece has a matte finish, read maintaining a matte sealer on concrete first.

A structural crack: stop using the piece as a seat or a shelf and look at why. Fix the support first. A crack through a section is a workshop repair involving resin injection or recasting, and on a small piece recasting is often the sensible answer.

Prevention

• Level the piece properly, with a shim under any foot that does not touch.
• Support a long piece along its length when it is off its feet, never from both ends.
• Lift from the base, with two people on anything heavy. The Drum Table is a hollow casting at 22 kg and still gets lifted from the body rather than the rim.
• Do not drag across a doorstep or a tile lip.
• Outdoors, make sure water drains away rather than standing against the piece.

Reality check

• A network of fine lines is the material, not a fault.
• If your nail catches, look harder.
• A crack that crosses an edge is structural until proven otherwise.
• Cracks start at supports and at corners, and both are things you control.
• Outdoors, an open hairline will eventually become a problem. Seal it before winter.