Concrete Crack Repair: Filler vs Caulk vs Self-Leveling Sealant
The repair has to match the crack. Rigid fillers for cracks that have stopped moving, flexible sealants for cracks and joints that haven't, and the test that tells you which one you have.
Last updated: June 10, 2026
Overview
Most failed concrete repairs fail the same way: a rigid product went into a crack that moves, or a flexible product went into a crack that needed structural repair. The product aisle calls everything "concrete crack filler," but the materials behave completely differently once cured.
There are three product families, and each has one job:
- Rigid crack fillers and patching compounds: cementitious repair mortars (ASTM C928) and epoxy injection systems. They restore the slab or wall to a hard, monolithic surface. Correct for dormant cracks, spalls, and surface defects that have stopped moving.
- Flexible concrete caulk: gun-grade polyurethane or hybrid sealants tested to ASTM C920, Use M (masonry). They stretch and compress with the crack. Correct for working cracks and any joint that opens and closes with temperature or load.
- Self-leveling (SL) sealants: pourable C920 Grade P sealants that flow flat into horizontal cracks and joints in slabs, driveways, and sidewalks. Same flexibility as gun-grade caulk, but they finish themselves on flatwork.
The one-question shortcut: does the crack move? Tape a piece of glass slide or a crack monitor across it for 30 days, or compare width on a hot afternoon vs a cold morning. Moving crack: flexible sealant. Dormant crack: rigid filler. When in doubt, treat it as moving. A flexible repair in a dormant crack still works; a rigid repair in a moving crack cracks again next to the patch.
Moving vs Dormant Cracks
Concrete cracks for a handful of reasons, and the cause tells you whether the crack is done moving:
| Crack cause | Typical pattern | Moving or dormant? | Repair family |
|---|---|---|---|
| Plastic shrinkage | Shallow map cracking, first 24 hours | Dormant | Rigid filler or surface treatment |
| Drying shrinkage | Straight cracks between joints, first year | Usually dormant after ~1 year | Rigid after movement stops |
| Thermal movement | Opens in cold, closes in heat | Moving, forever | Flexible sealant |
| Settlement / heave | Vertical offset between sides | Moving until cause fixed | Fix subgrade first, then flexible |
| Structural overload | Diagonal, flexural, or widening cracks | Active | Engineering evaluation, then epoxy injection |
| Control joints doing their job | Cracks inside sawn or tooled joints | Moving by design | Flexible or semi-rigid joint sealant |
Structural cracks are not a sealant problem. Diagonal cracks near columns, flexural cracks under load, cracks with vertical offset, or anything wider than 1/2 inch and growing belongs in front of an engineer before any product goes in it. ACI 224.1R covers crack evaluation; epoxy injection (ASTM C881 adhesives) restores structural continuity only after the cause of cracking is resolved.
Rigid Fillers & Patching Compounds
Rigid repairs restore a hard surface and, in the case of epoxy injection, structural strength. They have zero tolerance for movement.
Cementitious repair mortars are the workhorse for spalls, pop-outs, and dormant cracks wider than about 1/4 inch. Specify products qualified to ASTM C928, the standard for packaged rapid-hardening repair materials. C928 grades by strength gain speed: R1 (3 hours to 3,000 psi traffic strength is not required), R2 (3,000 psi at 3 hours), and R3 (3,000 psi at 90 minutes) for repairs that have to take traffic the same day.
Epoxy injection is for dormant structural cracks down to about 0.002 inches. Done correctly it welds the crack and restores monolithic behavior. It is the wrong product for any crack that still moves: the epoxy is stronger than the concrete, so movement just relocates the crack to the edge of the repair.
Semi-rigid polyurea and epoxy joint fillers are a special case for industrial floors. Saw-cut control joints in warehouses take hard wheels from forklifts; a soft sealant gouges out, and an unfilled joint chips at the edges (arris spalling). Semi-rigid fillers (Shore A 80 to 90) support the joint edges while allowing small movement. They are a floor-joint product, not a crack repair product.
Flexible Concrete Caulk (Gun-Grade)
For working cracks and moving joints in vertical or sloped concrete, the correct product is an elastomeric sealant tested to ASTM C920 with Use M (masonry) in its classification. The C920 movement class tells you how much the cured sealant can stretch and compress:
| C920 class | Movement capability | Typical concrete use |
|---|---|---|
| Class 25 | +/- 25% | Standard polyurethane concrete caulk, most working cracks |
| Class 35 | +/- 35% | Hybrid (MS polymer) sealants, wider service range |
| Class 50 | +/- 50% | High-movement expansion joints, silicone or premium hybrid |
Polyurethane is the default chemistry on concrete: it bonds aggressively, tools well, takes paint, and tolerates abrasion. Hybrids (MS polymer) trade a little abrasion resistance for easier tooling, no isocyanates, and better UV stability. Acetoxy silicones from the bath aisle do not belong on concrete: the acetic acid cure reacts with alkaline masonry and adhesion suffers. Chemistry-by-chemistry detail is in the Sealant & Caulking Selection Guide.
Self-Leveling Sealants for Slabs & Flatwork
On horizontal concrete (driveways, sidewalks, garage slabs, plaza decks), a pourable self-leveling sealant does what gun-grade caulk does, but flows flat without tooling. In C920 terms that is Grade P (pourable) instead of Grade NS (non-sag). The same movement classes apply.
SL polyurethanes dominate residential and light-commercial flatwork. For highway, airfield, and DOT pavement joints the specs change: hot-applied asphaltic sealants run under ASTM D6690 (Types I through IV by climate), and cold-applied silicone pavement sealants under ASTM D5893. If the joint carries vehicle traffic at speed, start from the highway joint sealant guide instead of a building-sealant aisle.
Slope check before pouring: self-leveling sealant obeys gravity. Above about a 3% slope it flows downhill out of the joint before it cures. On sloped driveways and ramps, use a non-sag (Grade NS) sealant and tool it instead.
Decision Table
Match the crack or joint to the product family by width, movement, and orientation:
| Situation | Width | Product | Governing spec |
|---|---|---|---|
| Hairline surface cracks, dormant | Under 1/16" | Leave, or penetrating epoxy/sealer treatment | ACI 224.1R evaluation |
| Dormant crack or spall, slab or wall | 1/4" and up | Cementitious repair mortar | ASTM C928 (R1/R2/R3) |
| Dormant structural crack | 0.002" to 1/4" | Epoxy injection | ASTM C881 adhesive |
| Working crack, vertical or sloped | 1/4" to 1" | Gun-grade PU or hybrid, backer rod | C920 Grade NS, Class 25+, Use M |
| Working crack or joint, horizontal | 1/4" to 1" | Self-leveling PU, backer rod | C920 Grade P, Class 25+, Use M |
| Industrial floor control joint, hard wheels | 1/8" to 1/4" | Semi-rigid polyurea/epoxy filler, full depth | Shore A 80-90, ACI 302.1R |
| Highway / airfield pavement joint | Per joint design | Hot-pour asphaltic or cold silicone | ASTM D6690 / D5893 |
| Crack wider than 1/2" or offset faces | Any | Engineering evaluation first | ACI 224.1R / 362.2R |
Joint Design & Sizing
Flexible repairs live or die on geometry. Three numbers matter:
- 2:1 width to depth. Sealant depth should be half the joint width, with a 1/4 inch minimum depth and 1/2 inch maximum. A 1/2 inch wide crack gets 1/4 inch of sealant depth over backer rod.
- Backer rod 25% oversized. Closed-cell foam backer rod compressed into the joint sets the sealant depth and prevents three-sided adhesion. Pick rod about 25% wider than the joint so it stays put.
- Joint width 4x expected movement. A Class 25 sealant in a 1/2 inch joint accommodates 1/8 inch of total movement. If the joint moves more than the class allows for its width, widen the joint or move up a class.
Estimating material: a 10.1 oz cartridge covers about 12 linear feet at a 1/2 x 1/4 inch bead. The sealant yield calculator converts joint dimensions and length into cartridges, sausages, or pails, and the ASTM C920 page covers bulk formats and case quantities for larger jobs.
Common Failures (and What Caused Them)
| Symptom | Root cause | Fix |
|---|---|---|
| Crack reappears beside a hard patch | Rigid filler in a moving crack | Rout and seal with C920 flexible sealant |
| Sealant split down the middle | Three-sided adhesion (no backer rod or bond breaker) | Cut out, install backer rod, reseal |
| Sealant pulled off one face cleanly | Dust, laitance, or moisture at application | Grind/wire-brush clean, dry, prime if specified |
| Latex caulk crumbled or washed out | Painter's acrylic used outdoors in a working joint | Replace with C920 polyurethane or hybrid |
| Bubbles in self-leveling sealant | Moisture outgassing or damp substrate | Apply to dry concrete, in falling temperatures |
| Joint edges chipping in warehouse aisles | Soft sealant where wheels need edge support | Semi-rigid polyurea filler, full joint depth |
Why big-box latex "concrete caulk" fails: painter's acrylic and siliconized latex products cure by evaporation, shrink as they dry, and typically handle 7 to 12% joint movement. Outdoor concrete joints routinely cycle more than that, and water plus freeze-thaw finishes the job. If the tube doesn't say ASTM C920, it is an interior trim product, whatever the label art shows.
Frequently Asked Questions
What is the difference between concrete crack filler and concrete caulk?
Crack filler is rigid: cementitious or epoxy products that cure hard and restore a solid surface in cracks that have stopped moving. Concrete caulk is flexible: an elastomeric ASTM C920 sealant that stretches and compresses with cracks and joints that still move. The crack's movement, not its appearance, decides which one you need.
When should I use self-leveling concrete sealant?
On horizontal concrete with less than about 3% slope: driveways, sidewalks, garage slabs, and plaza joints. Self-leveling sealant (ASTM C920 Grade P) flows flat into the joint without tooling. On vertical surfaces or sloped ramps it runs out of the joint, so use a non-sag Grade NS sealant there instead.
Can I use regular caulk on concrete?
Not outdoors. Painter's acrylic and siliconized latex caulks handle roughly 7 to 12% joint movement and degrade under UV, water, and freeze-thaw. Exterior concrete joints need a sealant tested to ASTM C920 with Use M in the classification, which means at least +/- 25% movement capability and tested adhesion to masonry.
How wide a crack can I repair with sealant?
Gun-grade and self-leveling sealants work in cracks and joints from about 1/4 inch to 1 inch wide, with backer rod setting the depth at half the width. Below 1/4 inch, rout the crack out to 1/4 inch so the sealant has enough cross-section to stretch. Above 1/2 inch with offset faces or signs of growth, get an engineering evaluation before sealing anything.
Why did my concrete caulk crack down the middle?
Three-sided adhesion. Without backer rod or bond-breaker tape, the sealant bonds to the bottom of the joint as well as both sides, so it cannot stretch and tears instead. Cut it out, compress closed-cell backer rod into the joint, and reseal at a 2:1 width to depth ratio.
Do warehouse floor control joints need filler or sealant?
Where hard forklift wheels run, saw-cut control joints need a semi-rigid polyurea or epoxy joint filler (Shore A 80 to 90) installed full depth. A soft elastomeric sealant gouges out under wheel traffic, and an unfilled joint spalls at the edges. Save the flexible sealants for the building's perimeter and isolation joints.
Sealing joints or cracks across a whole facility?
We source US-made C920 concrete sealants, self-leveling polyurethanes, and repair materials by the case or pail. Tell us the scope and we will be in touch with options.
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