Patching Compounds That Actually Bond
The will-call window at our plant sells bagged product to whoever walks up, and I work that counter phone between dispatching loads. A guy bought two pails of resurfacer in the spring for a spalled patch by his garage door and was back seven weeks later asking what was wrong with it. Nothing was. It had come off in one clean sheet, and the underside of that sheet was gray dust. I see that sheet constantly.
Patch material almost never fails because somebody bought the wrong bag. It fails because the concrete underneath was never made ready to accept it.
Bond is a prep question before it is a product question
A repair holds when it is mechanically keyed into sound concrete and chemically stuck to a clean surface. Both are on you, not the manufacturer.
- Sawcut the perimeter 1/2 to 1 inch deep with vertical edges. Never featheredge. A patch tapered to zero has no thickness to develop strength, so it curls and peels from the thin side inward. That mistake alone accounts for most of what walks back to my counter.
- Chip out everything unsound. Tap around the repair with a hammer. Where it sounds hollow instead of sharp, the concrete is already delaminated and the patch will just ride on it.
- Roughen the base. Coarse-sandpaper texture at minimum, from a diamond cup wheel or a needle scaler. Smooth troweled concrete is a poor bonding surface.
- Get the dust out. Vacuum, then vacuum again. Sawing and grinding leave a fine film that acts as a release agent.
- Condition the moisture. Cementitious repairs want the substrate saturated but surface dry: soak several hours, then squeegee off standing water. Dry concrete drinks the mix water out of the patch and starves the cement. Epoxies want the opposite, a dry surface.
Repair mortars have published standards behind them, including ASTM C928 for packaged rapid-hardening materials and C882 for bond strength. Serious manufacturers cite them on the data sheet, and ASTM is where those specifications live. A bag with no test data on the label has none behind it either.
Four families, and what each is actually for
Polymer-modified cementitious mortar
The everyday workhorse, $28 to $55 for a 50-lb bag. Acrylic or latex polymer raises bond strength into the 1,500 to 2,500 psi range and lets it go thinner than plain mortar, generally 1/4 inch and up, some resurfacers to 1/8 inch. Good for spalls, chipped edges and worn surfaces. Keep it damp a couple of days.
Rapid-set calcium sulfoaluminate
Different cement chemistry, not just an accelerator. Sets in 15 to 30 minutes and takes traffic in one to three hours, which is why road crews use it. Figure $22 to $45 per bag. Excellent where downtime costs more than material, unforgiving if you mix more than you can place.
Epoxy mortar
Two-part resin with graded aggregate, $95 to $220 per gallon kit. Strongest bond available and effectively waterproof, so it belongs on thin high-value repairs, joint edges and equipment bases. It will not tolerate a damp substrate, and being rigid it does not belong across a slab that still moves seasonally.
Polyurethane and polyurea joint fillers
Not patches, and misused constantly. These stay flexible on purpose and belong in control joints and moving cracks, which is a different problem from a hole. Fill a working joint with rigid mortar and the crack reappears a few inches to one side.
When the honest answer is a small load of real concrete
There is a size past which no bag is the right call. If the damage is full depth, bigger than about 4 square feet, or in a wheel path where a truck tire lands, patch material over failing concrete just buys a delay. Cut the panel out and replace it.
That is where my side of the counter gets interesting. Plant minimums run 8 to 10 yards, and anything under carries a short-load fee of $40 to $100 per yard short, so one yard delivered often lands between $350 and $650 once fee, fuel surcharge and delivery are on the ticket. That number barely moves between 1 yard and 3, worth knowing before you order. For small volumes a volumetric mixer batches on site and meters what you use, usually a $200 to $400 minimum, and bagged mix runs 45 eighty-pound bags per cubic yard. Our piece on resurfacing over old concrete covers where the line sits.
Frequently asked questions
Do I need a separate bonding agent under a patch? Usually not with a modern polymer-modified mortar on a properly prepped, saturated surface-dry substrate. A scrub coat of the same material brushed in is often enough, and a bonding agent allowed to dry before placement can become a bond breaker.
Why did my patch come off in one piece? Almost always featheredged, dusty or placed on dry concrete. All three produce the same failure: a thin shell that releases in a sheet with powder on the back of it. Getting that dust off is its own job once the floor is any size — an industrial cleaning contractor in the Twin Cities will vacuum and scrub a slab back to bare, sound concrete before any patching starts.
What is the smallest amount of ready-mix I can order? Most plants will send one cubic yard, but you pay the short-load charge on everything below the minimum. Under about half a yard, bagged material or a volumetric truck is usually better value.
Trade body: American Society of Concrete Contractors.