Driveway Unbonded Concrete Overlay: Structural Restoration

What a Driveway Unbonded Concrete Overlay Actually Is

A driveway unbonded concrete overlay is a brand-new, 4- to 6-inch concrete slab poured on top of your existing drive, but separated by a thin slip-sheet (usually plastic or building paper). Because the two slabs are NOT structurally bonded, the old concrete acts only as a stable “platform,” while the new slab carries the traffic loads. Think of it as a fresh start without the mess, cost, and landfill fees of full removal.

Homeowners choose this method when the original drive is cracked, settled, or spalled but still intact enough to support a new layer. Done right, an unbonded overlay gives you 25–30 years of life at roughly 30–40 % less cost than a complete tear-out and repour.

Key Benefits of an Unbonded Overlay Over Other Fixes

True Structural Restoration

Unlike thin resurfacers or coatings (which are only ⅛–¼ in. thick), a 4- to 6-in. unbonded overlay is engineered to carry vehicles, trailers, and even light RVs. It’s a full-strength concrete slab, not a cosmetic band-aid.

Lower Cost & Faster Timeline

Skipping demolition saves on labor, dump fees, and heavy equipment. Most 2-car driveways (600 ft²) are finished in three days: day 1 prep, day 2 pour, day 3 saw-cut joints and seal.

Sustainable Choice

Keeping the old slab out of the landfill reduces CO₂ by roughly 1,200 lb per 100 ft². Ask your contractor if they use supplementary cementitious materials (fly ash or slag) for an even greener mix.

Design Flexibility

Because you start fresh on top, you can integrate integral color, stamped borders, or exposed-aggregate finishes without disturbing the base below.

When an Unbonded Overlay Makes Sense—and When It Doesn’t

Good Candidates

• Moderate to severe surface spalling or scaling, but no major heaving
• Stable, well-compacted subgrade (no “bounce” or pumping water)
• Cracks are dormant—width hasn’t changed in 12 months
• Driveway elevation allows +4 in. height at garage threshold and sidewalk

Red Flags—Choose Full Replacement Instead

• Deep vertical displacement (>½ in.) between slabs
• Evidence of subgrade wash-out or sinkholes
• Garage floor is already level with existing drive—overlay will block door
• City sidewalk compliance limits (some codes cap total thickness)

Step-by-Step Homeowner Guide to a Successful Overlay

1. Initial Inspection & Measurements

Mark all expansion joints with chalk. Measure elevation at the garage door, walk, and street gutter. A quick string-line test reveals high/low spots that will telegraph upward if not corrected.

2. Hire the Right Contractor

Look for ACI-certified flatwork finishers and request at least three local references older than five years. Verify they carry concrete-specific insurance (not just general liability).

3. Surface Prep Weekend (DIY Savings)

1. Pressure-wash at 3,000 psi to remove oil, paint, and loose chips.
2. Fill any active cracks >¼ in. with backer rod and elastomeric sealant—this prevents mirroring in the new slab.
3. Install 6-mil poly sheeting or 15-lb roofing felt as the slip-sheet; overlap seams 12 in. and tape them.

4. Set Forms & Reinforcement

Use 2×4 or 2×6 forms depending on desired thickness. Add #3 rebar 24 in. on-center or 6×6-10/10 wire mesh, plus 1½-in. chairs so steel sits in the middle of the new slab—not on the plastic.

5. Concrete Mix Specs to Request

• Min. 4,000 psi compressive strength at 28 days
• 5–7 % air entrainment for freeze-thaw zones
• 4–5 in. slump with mid-range water reducer (avoids excess water)
• Fiber mesh (micro-poly) for secondary shrinkage control

6. Pour Day Checklist for Homeowners

• Notify neighbors—concrete trucks need street access.
• Have spare tarps ready for sudden rain.
• Keep plastic sheeting off the delivery chute—contaminants weaken the mix.
• Ask for a printed ticket to verify mix design.

7. Finishing & Curing

Order a “stampable” or “class-A” finish if you want decorative borders. Start curing immediately: apply white pigmented curing compound or 4-mil plastic for 7 days. Mist the edges twice daily in hot weather.

8. Jointing Strategy

Saw-cut control joints ¼ the slab depth (1 in. for a 4-in. overlay) within 6–12 hours. Match existing joint pattern to minimize random cracking. Seal joints 30 days later with self-leveling polyurethane.

Typical Cost Breakdown (600 ft² Driveway)

Compare that to full removal and replacement at $6,500–$9,000 for the same area and the savings are clear.

Long-Term Care: Protect Your Investment

Year 1

• No rock salt—use calcium magnesium acetate for ice.
• Wait 30 days before first vehicle traffic; 7 days for foot traffic.

Annual Routine

• Re-seal expansion joints every 2–3 years.
• Apply penetrating silane/siloxane sealer every 5 years to reduce water ingress.
• Keep gutters directed away to prevent edge erosion.

Frequently Asked Questions