When a spalled garage floor, a bolt pocket, or a golf-ball-sized void in a slab needs a repair that will not pop out in a year, using epoxy to fill holes in concrete is the professional’s answer. Unlike patching compounds that shrink and crack, structural epoxy bonds molecule-tight to the surrounding concrete and cures harder than the slab itself, so it carries load and shrugs off traffic.
The catch is that epoxy is unforgiving of poor prep and sloppy mixing. Get those two things right and the repair outlasts the surrounding surface; get them wrong and it delaminates. This guide walks through choosing the right product, prepping the hole, and filling it so the patch becomes a permanent part of the floor.
Why Choose Epoxy to Fill Holes in Concrete
Concrete repair products fall into a few families, and each has a job. Standard portland-based patch and hydraulic cement are cheap and fine for cosmetic cracks, but they shrink slightly as they dry and bond only mechanically. Epoxy is different: it is a two-part thermosetting resin that chemically fuses to clean concrete, cures with essentially zero shrinkage, and reaches compressive strengths of 8,000 to 12,000 psi — far stronger than the 3,000 to 4,000 psi of typical residential concrete.
That strength and adhesion make epoxy the right call for holes that see weight or movement: garage and warehouse floors, machine bases, anchor-bolt pockets, spalled areas near expansion joints, and holes deeper than a quarter inch. It also resists oil, gasoline, and many chemicals, which is exactly why it belongs on a garage or shop floor where lesser patches get eaten away.
Choosing the Right Epoxy Product
Not every epoxy in the paint aisle is built for filling holes. Match the product to the size and location of the void.
- Epoxy mortar / patching kits: These come with resin, hardener, and a graded aggregate you blend in for holes deeper than 1/2 inch. Products like a two-part concrete repair epoxy fill volume economically. Expect $30 to $60 per kit covering a few small repairs.
- Fast-set structural epoxy gel: A thicker, non-sag formula for vertical or overhead holes and bolt pockets. It stays put while it cures rather than running out.
- Self-leveling epoxy: A thinner pourable resin ideal for holes and low spots on a flat floor where you want the patch to flow flush with the surface.
- Injection epoxy: For hairline structural cracks rather than open holes, delivered through ports; a different job from filling a visible void.
For a hole wider than about an inch and deeper than a half inch, always bulk out the epoxy with clean, dry silica sand or the aggregate supplied in the kit. Pure resin in a large void generates excess heat as it cures and wastes expensive material.
Tools and Materials You Will Need
Gather everything before you mix, because epoxy has a limited working time and will not wait while you hunt for a tool.
- Two-part concrete repair epoxy and, for deep holes, clean silica sand or the kit aggregate
- Angle grinder with a diamond cup wheel or a wire brush for cleaning the hole
- Shop vacuum to remove every trace of dust
- Chisel and hammer to undercut loose edges
- Margin trowel or putty knife, plus mixing sticks and a disposable mixing container
- Nitrile gloves, safety glasses, and a respirator — epoxy fumes and dust are no joke
- Acetone for cleanup of uncured resin
Preparing the Hole for a Lasting Bond
Preparation is 80 percent of a successful epoxy repair. The resin can only grip clean, sound, dry concrete, so this stage decides the outcome.
Start by chiseling away any loose, crumbling, or spalled concrete until you reach solid material. Where possible, undercut the edges slightly so the hole is wider at the bottom than the top — this mechanical key locks the cured patch in place. Grind or wire-brush the interior to expose fresh aggregate and roughen the surface, then vacuum out every particle of dust. A dusty hole is the number-one cause of epoxy failure because dust prevents the resin from touching the concrete.
The surface must also be dry. Epoxy and standing water do not mix; moisture in the hole blocks the chemical bond. If the slab is damp, let it dry or use a moisture-tolerant epoxy specifically rated for that condition. Temperature matters too — most epoxies want a surface and air temperature between 50°F and 90°F to cure properly.
Mixing and Applying the Epoxy
Now the clock starts. Once you combine the two parts, you typically have 15 to 40 minutes of working time depending on the product and temperature.
- Measure precisely. Epoxy cures by chemical reaction, so the resin-to-hardener ratio is not a suggestion. Follow the kit ratio exactly; too much or too little hardener leaves a soft or brittle patch.
- Mix thoroughly. Blend for the full time stated — usually two to three minutes — scraping the sides and bottom of the container. Streaky, under-mixed epoxy will not fully cure.
- Prime the hole. For deep repairs, brush a thin coat of neat (unfilled) mixed epoxy onto the clean concrete first. This wets out the surface and guarantees adhesion.
- Blend in aggregate for depth. Fold clean sand or the supplied aggregate into the remaining epoxy to make a stiff mortar, then pack it into the primed hole in lifts, working out air pockets with the trowel.
- Overfill slightly and screed. Fill just proud of the surface and strike it off flush with the trowel. Epoxy does not shrink, so what you leave is what you get.
- Feather and clean. Smooth the edges level with the slab and wipe any smears with acetone before they cure.
Curing, Finishing, and Aftercare
Most repair epoxies are hard to the touch in 2 to 8 hours and accept foot traffic within 24 hours, but full chemical cure and maximum strength take 3 to 7 days. Do not park a vehicle on the patch or subject it to heavy load until it has fully cured — the label will give an exact schedule for your product.
Once cured, the patch can be ground flush, sanded, and even coated. If you plan to apply an epoxy floor coating or paint over the whole slab, a fully cured epoxy patch takes it beautifully and disappears into the finish. Keep the repair clean of oil until it cures, and if you filled a large area, protect it from direct sun in the first hours to prevent surface bubbling from rapid heat gain.
Final Thoughts on Using Epoxy to Fill Holes in Concrete
Choosing epoxy to fill holes in concrete is the difference between a repair you do once and a patch you redo every season. The material is genuinely permanent, but it demands respect: sound clean edges, a dust-free dry hole, exact mixing, and enough aggregate to bulk out deep voids. Nail those steps and the cured resin becomes stronger than the concrete around it.
For a first-time repair, start with a small kit and a modest hole to learn the working time before you tackle a large spalled area. Wear the respirator and gloves, respect the cure schedule, and you will end up with a floor that reads as one solid surface — and a skill that saves you the cost of a full slab replacement down the road.
