Cracked Concrete Patio: Repair It or Replace It?

Look at whether the two sides of the crack are level. That one observation decides most cases. A crack with both sides level and not growing is cosmetic. A crack where one side sits higher than the other means the ground under the slab is moving the two halves independently, and nothing done to the surface will fix that.

Concrete cracks. That is not a defect in itself. The National Ready Mixed Concrete Association's guidance says all concrete has a tendency to crack and it is not possible to produce completely crack-free concrete, and that most random cracks that appear early, though unsightly, rarely affect structural integrity or service life.

Run a straight edge across the crack. Vertical displacement is the signal that matters. Then mark the ends of the crack with a pencil and a date, and look again after a season. A crack that has grown past the marks is moving.

Concrete shrinks as it cures and dries, and it expands and contracts with moisture and temperature. It is strong in compression and weak in tension, so when shrinkage is restrained, it cracks. NRMCA lists the usual culprits as missing or poor contraction and isolation joints, poor subgrade preparation, too much water in the mix, poor finishing, rapid drying and inadequate curing.

Control joints are how good concrete work manages this. Their guidance is joints cut or tooled to about a quarter to a third of the slab's thickness, spaced at 24 to 36 times the slab thickness, with a maximum of about fifteen feet often recommended, and panels close to square. A patio poured as one big panel with no joints will choose its own crack lines.

Wire mesh does not prevent cracking. NRMCA is explicit that reinforcement can reduce crack width when placed properly, but cannot stop cracks forming.

The subgrade is the usual reason a crack becomes a displacement. On the Springfield Plateau, soft spots in red clay, washouts from downspouts, and settling foundation backfill next to the house do it. Over the Fayetteville shale, which swells wet and shrinks dry, a slab is carried up and down with the seasons.

Freeze-thaw adds surface damage. Outdoor concrete exposed to freezing should be air-entrained, per NRMCA, and should never be placed on a frozen subgrade. A slab without air entrainment, or one that holds water because it has no fall, will scale and flake over the winters.

An overlay is a thin layer bonded to the slab. If the slab moves, the overlay cracks in the same place, usually within a couple of seasons, and often looks worse than before because the crack now runs through a decorative finish.

Overlays are a legitimate product on a sound slab that is simply tired or stained. They are not a structural fix. Anyone selling one as a cure for movement is selling you the second problem.

The same goes for filling a displaced crack with sealant and hoping. The sealant keeps some water out, which helps, but the two halves will keep moving.

For fine cracks on a stable, level slab, a flexible crack sealant keeps water out of the crack and the subgrade, which slows further damage. For a tired, stained but sound slab, cleaning and resealing, or an overlay, can give it years.

Where a slab has settled as a single unit and is otherwise intact, slab lifting from below exists as an option. It deals with the level, not with the reason it sank, so the water has to be fixed as well.

NRMCA's guidance sorts common cracks into a handful of types, and they point to different causes:

• Plastic shrinkage cracks: short, often parallel cracks from the surface drying too fast while the concrete was still fresh. Usually shallow and cosmetic.
• Cracks from improper jointing: random cracks across a panel that was too big or joints cut too late or too shallow.
• Cracks from missing isolation joints: where the slab meets the house, a post or a step and was not separated from it.
• Craze cracks: a fine web of surface cracks, usually from finishing and curing, cosmetic.
• Settlement cracks: the ground under the slab has moved. These are the ones that matter.
• D-cracks: freeze-thaw deterioration of some porous aggregates, which NRMCA notes start at joints near the bottom of exterior slabs and appear at later ages.

Only settlement and active movement make the difference between repair and replace. Most of the others are about appearance.

Stamped concrete is a slab with a pattern and colour on top, so everything above applies, with two extra complications. A crack through a stamped pattern is more visible than one through plain concrete, and patching a stamped and coloured slab so the repair disappears is very difficult. Colour and texture rarely match.

Stamped surfaces are also usually sealed, and the sealer needs renewing. A tired, faded stamped patio that is structurally sound can often be cleaned and resealed. One that is cracked and displaced is a replacement, and it is worth asking then whether pavers would suit the ground better.

Look at enough older patios around here and a pattern appears: the slab cracks and drops next to the house, or at the corner under a downspout, or along the edge that meets the lawn. Those are the places where backfill was loose, where roof water concentrates, and where the lawn keeps the ground wet. They are water and subgrade problems first and concrete problems second.

The ground makes it worse. The red clay of the Springfield Plateau softens when it is saturated. The Fayetteville shale, as University of Arkansas geologist Walter Manger put it, expands and contracts with the clay's weathering. A slab poured on either with a thin base and no plan for the water will crack in exactly those places.

So when a slab is replaced, the concrete is not really the fix. The fix is the subgrade, the base, the fall and the downspouts, with the concrete poured on top of all that.

Sometimes. If the damage is confined to one panel between control joints, that panel can be cut out and replaced, and the failed ground under it fixed. The new panel will not match the old concrete's colour exactly, which matters less on plain grey and more on stamped or coloured work.

If several panels have moved, partial replacement tends to become a patchwork, and the cause, usually water or a poor subgrade, is under the whole slab.

A slab poured tight against the house foundation, a post base or a step, with nothing to separate them, is restrained at that edge. As the slab shrinks and moves with temperature, it cracks near the restraint. NRMCA's guidance is for isolation joints through the full thickness of the slab wherever movement would otherwise be restricted. If your cracks radiate from a corner of the house or a post, that is the likely story, and it is a detail to get right on the replacement.

For a stable crack, yes, it is worth it. A flexible sealant keeps water out of the crack and out of the ground under the slab, which is exactly where water does the damage: freezing in the crack, softening the subgrade, washing out fines. Rigid patching compounds tend to crack out again because concrete keeps moving slightly with temperature.

For a crack with displacement, sealing is a holding measure while you decide what to do, not a repair. Keep an eye on the marks you made at its ends.

• Are the two sides of the crack level, and is it growing?
• What caused it, and how do you know?
• If you are proposing an overlay, what happens if the slab moves?
• If you are proposing replacement, what changes in the base and drainage?
• Where will the control joints go, and how deep?

• Dig out soft spots and topsoil, and compact the subgrade properly.
• Build a proper base, compacted in lifts.
• Give it a fall away from the house, and take downspouts to an outlet.
• Joint it properly, and isolate it from the house and posts.
• Use concrete suited to freezing weather and cure it properly.
• Run conduit for lighting, power or gas while the ground is open.

It is also the moment to ask whether a slab is right for the ground at all. Over the Fayetteville shale in particular, a paver surface that can flex and be reset has a real advantage over a slab that can only crack. See our pavers versus stamped concrete guide for the trade-offs.