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How to Repair a Cracked Concrete Patio

Bob Jackson
Last Updated on
by Bob Jackson

This project explains how to repair a cracked concrete patio with QUIKRETE® Concrete Repair (No. 8620) caulk.

My concrete patio slab developed a 13 foot long crack that varied from a hairline next to the house, widening to 3/8 inch at the edge of the patio. I wanted to repair the crack before winter prevent water from freezing in the crack, causing it to get worse.

Freezing water expands with tremendous force that will widen a crack by shifting the concrete slab and/or creating new cracks to relieve the stress. Repeated freeze/thaw cycles can do a lot of damage to concrete.

Cracked Concrete Patio Slab

Update: Why the Concrete Patio Slab Cracked

I later understood why the patio slab cracked during my sagging wood deck repair project; the concrete slab lacked a proper footer set at least 12 inches below grade (ground level) and had no rebar or reinforcing wire. The prior homeowner’s didn’t like building permits because they thought it would increase their property taxes – which it may by a very minor amount. A building permit would have required a footer inspection by the building inspector to ensure the footer is below the frost line (12 inches below grade in my area per local building code) and didn’t know to ask the contractor to set rebar and remesh in the slab to resist tension (pulling apart) forces. Rebar and remesh is inexpensive and an inexcusable oversight.

The result was a low-bid contractor who simply set a wood form and poured the slab directly on the ground. This resulted in frost heaving and allowed chipmunks to burrow under the slab undermining the ground support. The crack developed as the unsupported slab sagged and started to slide downhill. Rebar and remesh would have provided tension strength to limit the crack widening, however nothing can make up for a non-existent footer. My solution when replacing the 6×6 wood deck post was to dig and pour a 38 inch deep by 2 foot square concrete pier with a rebar cage to pin the slab in place.

Concrete Patio Crack Repair Options

There are many products and methods to go about patching or repairing a cracked concrete patio slab. The main options are:

  • Caulks & Sealers – inexpensive, easy, non-structural (i.e. has no strength), surface patch of uncertain long-term durability. Only for cracks less than 1/2″ wide.
  • Concrete – inexpensive, semi-structural, repair to full depth of the crack. Best for cracks 1/2″ or wider due to difficulty of forcing concrete into a narrow crack all the way down in a 6 inch thick concrete slab. For best results, the crack should be undercut with a chisel to form an inverted V notch for maximum holding power at the “cold” joint between new and old concrete. Concrete is relatively weak in tension and can’t prevent slab movement if the foundation is less than 100% stable.
  • Polymer Resins – expensive, deep penetrating, structural repair which forms a permanent chemical bond with the concrete slab, gluing the slabs together. Compressive- and tension (i.e. “pull apart”) strength of 4,000+ psi exceeds that of concrete to prevent the slab from moving. Slightly less rigid than concrete for stress relief. Epoxy and Polyurea adhesives are example materials.

This table summarizes several QUIKRETE® concrete slab crack repair products that are readily available at home improvement stores. Click on the table for a full size view.

Concrete Slab Crack Repair – Product Comparison Table

For this repair, I chose the QUIKRETE® Concrete Repair (No. 8620-10) sanded acrylic latex caulk for the following reasons:

  • It is designed for concrete crack repair
  • Sanded formula to resemble concrete
  • The slab crack was less than 1/2″ wide

I decided against the following products for this application because:

QUIKRETE® Concrete Repair Caulk

I purchased a QUIKRETE® Concrete Repair (No. 8620-10) 10oz caulk tube and a bag of 3/8″ x 20′ closed cell polyethylene backer rod. The backer rod is a filler for cracks 1/4″ or wider.

QUIKRETE® Concrete Repair (No. 8620-10) and Backer Rod

The QUIKRETE 8620 caulk is suitable for cracks 1/8″ to 1/2″ wide. The caulk should be applied no more than about 3/8″ thick, using backer rod or sand to fill deep cracks.

The maximum crack width on my patio was about 3/8″ wide to the full 6″ or 8″ depth of the slab.

3/8″ Wide Crack in Concrete Patio Slab

I hope the builder installed wire reinforcing mesh when the concrete slab was poured, but I couldn’t see well enough into the crack to confirm. If wire mesh is in there, it has broken as the slab cracked in two.

Backer Rod Installation and Caulking

The foam backer rod is pressed into the wide sections of the crack to serve as a filler before caulking. Press the backer rod into the crack with your fingers.

Concrete Patio Crack Repair – Backer Rod Installation

Then seat the backer rod about 3/8″ deep with a screw driver.

Seating the Backer Rod in the Concrete Crack

Fill the crack with the QUIKRETE 8620 caulk. Note the gray color of the concrete in contrast to the white concrete stain on my patio. The QUIKRETE 8620 caulk is paintable and will be covered when I stain the deck.

QUIKRETE Concrete Repair No. 8620 Sanded Caulk

This project is continued in How to Repair a Cracked Concrete Patio – Part 2.

Thanks for reading,

Bob Jackson

backer rodcaulkconcrete cold jointconcrete patio crack repairconcrete stress
Bob Jackson
Bob Jackson
Technology product manager by day and a prolific handyman in the evenings and over the weekends. Bob was the founder of the original Handyman How To website and that tradition continues on this site with excellent new handyman content into the future.
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