Yes—if the existing concrete is structurally sound, properly prepared to a mechanical profile, and you use a compatible bonded overlay with a disciplined joint plan and curing. When the base concrete repair slab is weak, moving, or saturated, don’t overlay; remove and rebuild the slab with a corrected base. In Kansas City, success depends on water control, freeze–thaw durability, and jointing that matches the substrate. Request an overlay evaluation.
Overlay vs. Replacement: What Are We Really Talking About?
“Putting new concrete on top of old” usually means installing a bonded overlay—a new wearing course that adheres to the old slab and behaves as one system. A true overlay isn’t a cosmetic wipe; it is a carefully engineered resurface that demands sound substrate, mechanical surface preparation, correct mix design, and an overlay thickness that respects outdoor exposure.
- Bonded overlay: New material is bonded to old concrete. Great when the substrate is sound and you want to restore surface quality or elevation by 1½–2 inches or more (for exterior exposure). Unbonded topping: New slab placed over a bond breaker/slip sheet, acting like its own slab. Useful when you cannot guarantee substrate stability and you have elevation clearance for a thicker section (≈ 3–4"). Thin polymer-modified resurfacer: A millimeter-to-quarter-inch application for light scaling and cosmetic uniformity. Exterior longevity depends on prep, joint fidelity, and curing.
When a Bonded Overlay Works—and When It Will Fail
ConditionBonded Overlay?Notes Substrate is sound (no hollow areas, no rocking panels) Yes Proceed after mechanical prep to appropriate ICRI CSP Widespread delamination, dusting, or weak paste No Bond won’t last—remove/replace or consider unbonded topping Active movement or settlement (cracks open/close seasonally) No Overlay will reflect cracks; fix support (slabjacking/foam) or replace Persistent moisture from downspouts/grade trapping water No until corrected Control water first; overlays over saturated slabs scale and debond Limited elevation to thresholds/doors Maybe Confirm overlay thickness vs. clearances; thin resurfacers only if substrate is excellentKansas City Reality: Heat, Wind, Freeze–Thaw, and Clay
Kansas City’s summers are hot and often windy; winters bring freeze–thaw cycles plus deicers. Local subgrades frequently include expansive clays that swell when wet and shrink when dry, stressing slab edges and joints. A successful overlay project here requires water management, air-entrained exterior mixes, and curing discipline—otherwise the new surface may scale and crack within the first winter.
Step-by-Step: How Pros Build a Long-Lasting Overlay
Fix the cause first: Extend downspouts 6–10 ft, regrade soils to fall away ≈ 1/4" per ft, and clear landscaping that traps water at slab edges. If panels rock or joints pump, schedule slabjacking or foam lifting before overlay work. Soundness survey: Chain-drag or hammer-sound the slab to map hollow areas. Mark delaminations, spalls, and crack paths; define removal and patch zones. Mechanical surface preparation: Shot-blast or diamond grind to an ICRI Concrete Surface Profile (CSP) suitable for the chosen overlay—often CSP 3–5 for bonded overlays. Do not rely on acid etch. Crack and joint plan: Rout/clean cracks; treat static structural cracks (epoxy injection as appropriate); honor existing joints in the overlay—never bridge them without isolation. Bond strategy: Remove all laitance, sealers, paints, and contaminants. Use a polymer-modified bond coat or neat cement scrub coat per manufacturer guidance and place the overlay while the bond coat is tacky. Overlay placement: Use an exterior-rated, air-entrained mix at low water–cement ratio with a water reducer; set thickness to meet exposure needs (see table). Consolidate adequately; avoid over-troweling. Jointing on time: Space control joints ≈ 24–36× overlay thickness (inches) and align them directly above substrate joints. In hot weather, cut same day using early-entry saws. Curing and early-age protection: Apply a membrane-forming curing compound immediately after final finish and again after saw-cuts; protect from sun/wind in summer and from cold/dew in shoulder seasons. Sealing and first-winter discipline: After dry-back, apply a penetrating silane/siloxane sealer on broom finishes. Avoid deicers during the first winter; use sand for traction.How Thick Should a Bonded Overlay Be?
Overlay TypeTypical ThicknessWhere to UseNotes (KC-specific) Polymer-modified resurfacer 1/16"–1/4" Light scaling; cosmetic uniformity Demanding prep and curing; honor joints; expect periodic maintenance Bonded concrete overlay ≥ 1½–2" Driveways, patios, walks Air-entrainment; avoid feather edges; align joints; great balance of durability/elevation Unbonded topping ≈ 3–4"+ Questionable substrate stability Requires slip membrane; behaves like a new slab; needs elevation clearanceMix Design and Admixtures That Succeed Outdoors
- Exterior flatwork: 4,000–4,500 psi, air-entrained (≈ 5–7%), low water–cement ratio (≈ 0.40–0.48), water reducer mandatory. Hot weather: Light retarder widens the finishing window and reduces crusting in KC wind. Cold shoulder seasons: Non-chloride accelerator helps achieve early saw-cut strength and reduces overnight risk. Fibers: Useful for plastic-shrinkage control; do not replace reinforcement where crack-width control is required.
Jointing, Edges, and Thresholds: Where Overlays Win or Lose
- Align joints: Overlay joints must be directly above substrate joints and cracks you chose to honor. Edge support: Maintain full base support to the perimeter; do not feather underlayment near forms. Corners without support chip first. Thresholds and tie-ins: Where overlays meet garage slabs or stoops, consider dowels for load transfer if elevations allow and the detail is engineered.
Costs: Overlay vs. Remove-and-Replace (Typical Tendencies)
ScopeWhat You GetKC Cost Tendency*When It Makes Sense Polymer-modified resurfacer Thin cosmetic upgrade; light wear Lower than full overlay; price varies by brand/area Substrate is excellent; you need appearance uniformity Bonded concrete overlay (≥ 1½–2") New wearing surface, durable exterior mix Mid-range; depends on prep area and thickness Sound base; want longevity without full demo Remove and replace slab New base, new slab, full reset Highest; includes demo, base rebuild, new concrete Substrate is weak/moving; elevations permit rebuild*Exact prices vary with access, area, demolition needs, saw-cutting, and restoration. Request line-item bids with prep, jointing, curing, and sealing explicitly included.
Common Failure Modes—and How to Avoid Them
SymptomLikely CausePrevention Overlay debonds in patches Poor mechanical prep; residual sealer/paint; dusty surface Shot-blast/grind to CSP; vacuum; verify bond coat timing Random cracks telegraph through Moved to bridge existing cracks; late saw-cuts Honor/isolate cracks; align and cut joints on time Surface scaling after first winter Under-cured paste; deicers; trapped moisture Immediate curing; first-winter deicer ban; later penetrating sealer Edge chipping at corners Feathered base; turning loads; no load transfer Uniform base; consider dowels at thresholds; keep full overlay thicknessKC Weather Playbooks
Summer (Hot + Windy)
- Schedule sunrise placements; set up shade/windbreaks; fog the air (not the surface). Use a water reducer instead of adding water; light retarder to prevent crusting. Cut joints early with an early-entry saw; cure immediately after finishing and after cuts.
Shoulder Seasons (October/April)
- Place on non-frozen substrate; request warm mix water and consider a non-chloride accelerator. Protect overnight with insulated blankets if lows approach 40°F; watch for dew before sealing.
Case Studies (Metro Examples)
1) Driveway Overlay That Lasted
Problem: Cosmetic scaling with sound base. Solution: Shot-blast to CSP 4, crack routing and static epoxy injection, polymer bond coat, 2" air-entrained overlay, joints aligned and cut same day, immediate curing, penetrating sealer after dry-back. Result: After three winters, broom texture and edges remained crisp.
2) Patio Overlay That Failed Early
Problem: Owner overlaid to hide cracks without addressing downspouts dumping at the edge. Outcome: Debonding and scaling within the first winter. Lesson: Water control first; prep and curing are not optional.
3) Garage Apron Rebuild Instead of Overlay
Problem: Threshold chipping and rocking panel. Action: Replacement with base rebuild and dowelled tie-in to the garage slab. Result: Turning loads no longer chipped the edge; joint remained tight.
DIY vs. Professional
- DIY-friendly: Cleaning, minor cosmetic resurfacing kits over excellent substrate; small crack sealing. Hire a pro: Shot-blasting/grinding to CSP, structural crack injection, bonded overlays ≥ 1½" outdoors, joint mapping and early-entry saw-cutting, cold-weather or hot-weather placements, and any work exposed to deicers. See our overlay services.
Pre-Bid Checklist (So Your Quotes Are Apples-to-Apples)
Confirm substrate soundness with chain-drag/tap testing; mark hollow areas. Document water sources and specify downspout/grade corrections. Specify mechanical prep method and target ICRI CSP. Require joint alignment plan (overlay joints above substrate joints) and same-day cutting strategy in hot weather. Call out mix and admixtures ( air-entrained exterior mix, low w/c, water reducer; retarder/accelerator by season). Include curing compound application after finishing and post-cuts, and a penetrating sealer after dry-back. Spell out restoration (landscaping, edges, thresholds) and first-winter care (no deicers).Frequently Asked Questions
- Will an overlay hide all cracks? Only if cracks are static and honored in the joint plan. Moving cracks will reflect through. Is higher PSI the answer? Not by itself. Low water–cement ratio, air-entrainment for exterior, joint timing, and curing matter more than nominal PSI. Can I pour a very thin layer to avoid raising thresholds? Thin resurfacers work only on excellent substrates and demand strict prep/curing. Outdoors in KC, durability improves markedly once you reach ≈ 1½–2" bonded overlays. Do I need reinforcement in an overlay? Many overlays rely on thickness + mix + jointing. Fibers help with plastic shrinkage; some details benefit from steel/dowels at thresholds—follow engineered plans.
Authority Reference
For fundamentals on overlays, proportioning, jointing, and exterior durability, see the Portland Cement Association.
Bottom Line
You can successfully place new concrete over old when the base slab is sound, water is controlled, the surface is mechanically prepared to the right CSP, and the overlay uses an exterior-rated, air-entrained mix with on-time jointing and immediate curing. If the slab is weak or moving, skip the overlay and rebuild. In Kansas City’s climate, that decision is the difference between a one-and-done fix and a redo after the first winter. Book a site visit.
Kansas City Concrete Contractors
6041 Walrond Ave
Kansas City, MO 64130
Phone: (816) 408-3461
https://kcityconcretecontractors.com