Concrete crack filler: how to seal cracks so they stay sealed

First, decide if the crack is moving

The filler that works depends on one question: does the crack still open and close? ACI's crack repair guide starts there. If cracking came from drying shrinkage, the cracks are likely to stabilize over time. If it came from ongoing soil or foundation movement, they will keep moving^[1].

All concrete tends to crack, and NRMCA says crack-free concrete is not possible^[7]. Most flatwork cracks are shrinkage cracks. A crack that seals once and stays sealed is usually dormant. A crack that reopens every winter is active, and it needs a flexible product.

Quick ways to tell:

• Mark the ends with a pencil and the date, and check again in a few months.
• Watch for offset. If one side is higher than the other, the slab has moved. NAHB guidance says cracks with significant vertical displacement may indicate a structural concern^[8].
• Note the season. Cracks tend to be wider in cold weather, when concrete shrinks.

If one side keeps dropping, filling is not the fix. Read why concrete sinks first.

Which filler for which crack

Crack	Moving?	What the pros use	Source notes
Hairline to about 0.08 in (2 mm), many fine cracks	No	Low-viscosity resin (methacrylate, urethane or epoxy) poured and broomed in	ACI gravity filling range is 0.001 to 0.08 in[1]
Narrow to medium, opens in winter	Yes	Rout a groove, set a bond breaker, fill with flexible sealant	ACI routing and sealing[1]
Wide, dormant, clean edges	No	Dry-pack mortar in an undercut slot about 1 in wide and 1 in deep	ACI says dry-pack is not advisable for active cracks[1]
Crack that must carry load again	No	Epoxy injection, which can bond cracks as narrow as 0.002 in	ACI says it takes a high degree of skill[1]
Saw-cut or tooled joint	Yes, by design	Joint sealant, not crack filler	Joints are planned cracks[6]

For joints, see control joint repair and caulking. Joints are meant to move, so they need a sealant that can stretch with them^[6].

How routing and sealing works

This is the standard repair for a crack in a driveway, walk, patio or garage floor. ACI calls it a common, relatively simple method, much easier than epoxy injection^[1]. The steps:

1. Cut a groove. A concrete saw or grinder opens the top of the crack into a vertical-walled groove, typically 1/4 to 1 inch deep^[1].
2. Clean and dry it. ACI lists air blasting, sandblasting or water blasting, then drying^[1]. Sealant does not stick to dust or damp concrete.
3. Add a bond breaker at the bottom of the groove on active cracks. This lets the sealant stretch from side to side instead of tearing^[1]. Foam backer rod is the common way to do this.
4. Fill with a flexible sealant. ACI wants the groove width to be at least about twice its depth so the sealant can respond to movement^[1].
5. Tool the surface so it sits flush or slightly below the slab.

Cutting and grinding concrete creates fine silica dust. OSHA links it to silicosis, lung cancer, COPD and kidney disease^[9]. Use a saw or grinder with water or a vacuum shroud, and wear a fitted respirator.

What to look for on the label

• ASTM C920. This is the US specification for elastomeric joint sealants. It rates products by type, grade (P for pourable or self-leveling, NS for non-sag), movement class and use. Use "T" means rated for traffic areas^[2].
• Pourable vs non-sag. Pourable sealant flows level on flat slabs. Non-sag (gun grade) is for vertical faces and steps^[2].
• Traffic rating. On a driveway or garage floor, pick a traffic-rated product. ACI notes that on floors, a sealant must be rigid enough for the traffic, or loads can make rigid sealants debond^[1].

Myths to ignore

• "A crack over 1/8 inch (or 1/4 inch) is structural." Some cost guides say this. HomeGuide uses 1/4 inch as a trigger for an engineer^[3] and HomeAdvisor uses 1/8 inch^[5]. No engineering standard sets one width as the line. Offset, pattern and change over time matter more^[8]. See the crack width guide.
• "Mortar is the strongest filler." In a moving crack, rigid mortar cracks again. ACI says to avoid cement grouts for routing and sealing^[1].
• "Epoxy fixes everything." Epoxy can bond a crack, but if the cause is not corrected, new cracks will probably form nearby^[1].

What it costs

Source (date)	Price	Note
HomeGuide (Dec 2025)	$0.50 to $3 per linear foot	Minimum fees of $100 to $250 apply[3]
Fixr (Jan 2025)	$2.00 to $2.50 per sq ft of cracked area	About $1,200 to $1,400 for a 500 sq ft repair, filler plus sealant[4]
HomeAdvisor (2025 data)	$250 to $800 to fix cracks	Same dataset family as Angi[5]
DIY (HomeGuide)	$5 to $10 per bottle of filler	Covers about 24 linear feet of cracks under 1/4 inch[3]

The sources price the job differently (per foot, per area, per job), so compare quotes on the same basis. Small jobs usually hit the minimum fee. Prices also vary by region with labor rates and access.

For more repair pricing, see concrete repair cost.

When to call a pro

• One side of the crack is higher than the other, or the offset is growing.
• A crack you sealed reopened within a season or two, which suggests the slab is still moving.
• Cracks near the house that appear with sticking doors or new drywall cracks inside: start with signs of slab foundation problems and a structural engineer.
• Wide cracks that need routing on a large driveway, where a contractor in your area has the saw, dust control and traffic-rated sealant.