Can you pour concrete over concrete?

Old slab condition

sound and stableprerequisite for any overlay

Surface profile

CSP scaleICRI surface preparation guidance

Bond verification

pull-off testASTM C1583

Unbonded overlay

bond breaker requiredACPA overlay practice

Old cracks

expect them to reflectACI 546 repair guidance

Short version

• The old slab is the foundation now. If it moves, the new concrete moves and cracks with it.
• Bonded overlays go thin and depend entirely on surface preparation — cleaning, profiling, and a sound substrate.
• Unbonded overlays go thicker and ride on a bond breaker, so the old slab’s cracks stay below.
• Cracks in the base telegraph up through bonded work. Plan joints over them or expect them to reappear.
• Adding height changes door swings, step risers, and drainage. Check those before you order concrete.

When is the old slab good enough to pour over?

A slab qualifies when it is structurally sound, well supported, and no longer moving. Sound means the concrete rings solid under a hammer rather than sounding hollow, the surface is not scaling or delaminating, and there is no spalling that exposes rebar. Well supported means the subgrade under it has not washed out or settled — tap around the edges and look for voids at cracks.

Not moving is the part people skip. Run a straightedge across every crack and check whether the two sides sit at different heights, then check again after a wet week and a freeze. Vertical offset, widening, or seasonal movement means the slab is still working, and no overlay survives a base that shifts underneath it.

ACI’s repair guidance frames the whole decision this way: repair or resurfacing addresses the surface, but the cause of the deterioration has to be corrected first. A slab that failed from a drainage problem or a bad subgrade will fail again with a new top on it.

General information, not engineering advice. Structural work should be designed by a licensed engineer.

What is the difference between a bonded and an unbonded overlay?

Bonded and unbonded overlays differ in whether the new concrete is meant to act with the old slab or independently of it. A bonded overlay is fused to the substrate, so the two behave as one thicker section. That takes an aggressively prepared surface and a substrate strong enough to carry the stress at the interface, but it lets the overlay be thin.

An unbonded overlay is deliberately separated from the old slab by a bond breaker — plastic sheeting, sand, or a separation layer — so the old concrete becomes a base course rather than part of the structure. The American Concrete Pavement Association uses the same distinction for pavement overlays, where the choice between bonded and unbonded turns on the condition of the existing surface. Unbonded costs more thickness and more elevation, and buys tolerance of a cracked base.

How do you prepare the old surface so the new concrete sticks?

Bond is made or lost during surface preparation, not during placement. The old surface has to be clean, sound, and roughened. Remove every trace of paint, sealer, curing compound, oil, and laitance, because a bonding agent applied over contamination bonds to the contamination. Then profile the surface mechanically — shot blasting, scarifying, or grinding — to open the pores and expose sound aggregate.

ICRI publishes the concrete surface profile scale that specifiers use to name how rough that finish must be, matched to the type and thickness of the overlay being placed. Thin toppings need a lighter profile; heavier repair mortars need an aggressive one.

Verify rather than assume. ASTM C1583 covers the direct-tension pull-off method used to measure the bond strength of an overlay to its substrate, and it is the standard test cited when bond has to be proven rather than trusted. Dampen the substrate to a saturated surface-dry condition before placement so the old concrete does not pull mixing water out of the new.

What happens to the cracks and joints in the old slab?

Existing cracks and joints keep working after you cover them. A crack in the base slab concentrates movement at that line, and in a bonded overlay that movement transfers straight up into the new concrete — reflective cracking, usually within the first year. Joints do the same thing, because they are cracks you planned.

Handle them one of three ways. Place a joint in the new surface directly over every joint and every active crack in the base, so the movement lands where you want it. Use an unbonded overlay so the separation layer absorbs the differential. Or remove the cracked section and rebuild it. What does not work is filling the crack and pouring over it — the filler is not stronger than the concrete around it, and the movement does not stop.

Common mistakes

• Pouring over a slab that is still moving — settlement or heave keeps working and cracks the new concrete on the same lines.
• Skipping mechanical profiling — a bonding agent over a smooth, sealed, or dirty surface produces a debonded layer that hollows out and breaks up.
• Feathering an overlay to zero at the edges — thin edges have no strength, so they crumble under the first traffic.
• Ignoring cracks and joints in the base — they reflect through bonded work and show up as cracks in a brand-new surface.
• Forgetting the new elevation — added height fouls door clearances, step risers, thresholds, and the slope that used to drain the slab.