How to Fix a Crack in Concrete
Repair and seal concrete cracks using a hammer, chisel, and elastomeric filler to restore structural integrity and prevent water damage.
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🧰 Required Tools & Equipment
- ✓ Cold chisel
- ✓ Small sledgehammer or hand drilling hammer
- ✓ Wire brush
- ✓ Shop vacuum
- ✓ Caulking gun
- ✓ Margin trowel or putty knife
📦 Materials & Supplies
- ✓ Concrete crack sealant or polyurethane caulk
- ✓ Backer rod (for cracks deeper than 1/2 inch)
- ✓ Concrete cleaner or degreaser
⚠️ Safety Checkpoints & Warnings
- • Wear ANSI-approved safety glasses during chiseling to guard against flying concrete chips.
- • Put on heavy work gloves to protect hands from hammer strikes and sharp concrete edges.
- • Wear an N95 dust mask when brushing and vacuuming fine silica concrete dust.
📋 Step-by-Step Guide (5 Steps)
Chisel and Undercut the Crack
Position the cold chisel along the entire length of the crack at an angle. Strike firmly with the hammer to chip away loose concrete and slightly undercut the edges, creating a backward-angled channel that locks filler in place.
- • Crack edges form a distinct inverted 'V' or squared-off groove with no loose flakes.
- • Only skimming the surface without widening the base of the crack.
- • Chiseling too deeply and breaking the surrounding intact slab.
Scrub and Clean Debris
Vigorously scrub the inside and surrounding perimeter of the crack using a wire brush to remove moss, dirt, and loose grit. Follow up by thoroughly vacuuming all dust and debris out of the cavity.
- • The interior cavity shows bare, clean concrete without standing dust, dirt, or loose gravel.
- • Leaving fine concrete dust inside, preventing the sealant from bonding.
- • Blowing dust with a leaf blower instead of vacuuming, creating a safety hazard.
Insert Backer Rod for Deep Cracks
If the crack is deeper than 1/2 inch, press a foam backer rod into the crevice using your fingers or the tip of a putty knife. Ensure the foam sits approximately 1/4 to 1/2 inch below the concrete surface.
- • Backer rod is seated uniformly beneath the concrete surface with no bulging spots.
- • Skipping backer rod in deep voids, causing excessive sealant sagging and air pockets.
- • Pushing the rod too deep so the sealant layer becomes too thick to cure properly.
Apply Concrete Sealant
Cut the sealant tube nozzle at a 45-degree angle to match the crack width and puncture the inner seal. Insert the tube into the caulking gun, place the tip deep into the crack, and squeeze a steady bead along the gap, slightly overfilling it.
- • A solid bead of sealant fills the entire crack void, crowning slightly above the surface.
- • Moving the caulk gun too quickly, leaving voids under the top bead.
- • Cutting the nozzle opening too wide, resulting in messy overflow.
Tool and Smooth the Joint
Hold a margin trowel or putty knife at a low angle and scrape across the crack to level the sealant flush with the surrounding concrete. Wipe away excess material from adjacent surface edges with a clean rag.
- • The sealant surface is completely flat and level with the concrete slab with crisp, clean margins.
- • Gouging the filler by holding the blade at too steep of an angle.
- • Smearing excess sealant broadly over adjacent undamaged concrete.
Credit: inspired by an original tutorial by MrFixItDIY — watch the original on YouTube.
Don’t have everything?
- No Cold chisel? Stiff flathead screwdriver (demolition grade)
- No Shop vacuum? Stiff nylon hand broom and dustpan