You're standing in the caulk aisle looking at 40 tubes that say "concrete." One says self-leveling, one says paintable, one says crack filler, and none of them tell you which one survives a Green Bay February.
The brand matters a lot less than the type of sealant and a few codes printed on the label. Here's how to sort it out.
First: Does Your Crack Even Need Caulk?
Caulk is for cracks and joints that move a little with the seasons, on slabs that are still sitting where they should be. It is not a fix for:
- A crack where one side is higher than the other. That's a settled slab. Sealant won't level it.
- Crumbling, flaking, or broken concrete. That needs patching or replacement, not a bead of caulk.
- A slab tipped toward your house. Lift it first, or you're sealing it in the wrong position. More on that in caulking where concrete meets the house.
If you're not sure what kind of crack you're looking at, our guide to concrete crack types will help you sort it out first.
Concrete Crack and Joint Sealants Compared
| Type | Flexibility | What it's good at | Watch out for | Best use outdoors in Wisconsin |
|---|---|---|---|---|
| Polyurethane | High | Grips concrete hard, tough under foot and tire traffic, paintable | Needs a clean, dry joint. Not naturally UV resistant, so it relies on additives to hold up in sun. | Driveway, walk, and patio joints. Moving cracks. The joint between slab and house. |
| Silicone (concrete or pavement grade) | Very high | Excellent weathering and UV resistance, handles a lot of movement | Not paintable. General-purpose silicone can stain porous concrete and masonry, and old silicone residue makes resealing harder. | Joints that move a lot, if the product is made for concrete. Never tub-and-tile silicone. |
| Hybrid polymer | High | Combines polyurethane's toughness and paintability with silicone's UV resistance | Wide quality range. Check for the C920 rating and traffic use, not just the word "hybrid." | All-around exterior joints, especially in full sun |
| Acrylic / latex (including "siliconized") | Low to moderate | Cheap, easy soap-and-water cleanup, paintable | Many meet only the less demanding latex sealant spec. Don't apply below 40°F or before rain. Not built for joints that move much. | Interior trim. Not outdoor concrete joints. |
| Epoxy, cement patch, rigid "crack fillers" | None | Strong bond, fills stable cracks solid | Rigid. If the slab moves, it cracks again beside or through the repair. | Stable interior cracks, like garage floors before a coating. Not exterior joints. |
Rigid fillers do have a real job: we fill garage floor cracks as part of coating prep, because a coating needs a solid, flat base. Outdoors, slabs swell, shrink, and shift every winter, and a rigid filler can't move with them.
Self-Leveling or Gun-Grade?
Pick a sealant type from the table above, and you still have to choose how runny you want it. Most come in a pourable version and a thick version, and grabbing the wrong one ruins a lot of weekend jobs.
| Self-leveling (pourable) | Gun-grade (non-sag) | |
|---|---|---|
| How it behaves | Pours out thin, a lot like pancake batter, and spreads flat across the joint | Comes out thick, like peanut butter, and holds its shape on a slope |
| Best for | Flat, horizontal joints: driveways, patios, garage approaches | Sloped joints, steps, vertical surfaces, the slab-to-foundation joint when it isn't level |
| Tooling | Usually none needed. It levels on its own. | Has to be shaped by hand before it skins over, usually with a dampened tooling blade |
| Common failure | Runs downhill on a slope, or drains away through an unsealed gap below | Left lumpy or proud of the surface, where plows and tires catch it |
| Label code | Grade P (pourable) | Grade NS (non-sag) |
Self-leveling products usually list a slope limit. Believe it.
Read the Label Like a Contractor
You don't need to know the brand. You need to know what these codes mean.
- ASTM C920. The standard spec for flexible joint sealants, tested through heat aging, water soaking, and repeated stretching on concrete. If it's not on the tube, check the maker's data sheet.
- Class 25 (or 35, 50, 100/50). How far the sealant can stretch and squeeze as a percentage of the joint width. Class 25 means plus or minus 25 percent. Higher is more forgiving. Watch out for marketing that says "25% total movement," which is only half as much.
- Use T. Rated for traffic. If people walk or drive on it, you want this.
- Grade P or NS. Pourable or non-sag, as covered above.
- Application temperature. Most concrete joint sealants list 40°F as the minimum, on a dry, frost-free surface. In northeast Wisconsin, that rules out a good chunk of the year.
- Tack-free and cure time. These vary widely. On the product data sheets we checked, tack-free ran from about an hour to roughly a day, and full cure from a few days to two weeks. Traffic time is on the label. Follow it.
Don't Skip the Backer Rod
Backer rod is a round foam rope you push into the joint before you caulk. It's cheap, and it does three jobs:
- 1Sets the depthMost sealant makers want the bead about half as deep as the joint is wide, and generally between 1/4" and 1/2" deep. Deeper isn't stronger. A thick plug can't stretch as well as a properly shaped bead.
- 2Stops three-sided adhesionSealant should grip the two sides of the joint and nothing else. If it also bonds to the bottom, it can't stretch, and it tears when the slab moves.
- 3Saves sealantWithout it, you're filling a deep joint with expensive caulk that does nothing.
Sizing: Use closed-cell rod outdoors (open-cell soaks up water). Pick a diameter slightly bigger than the gap so it compresses into place, roughly 25 percent larger, or about 1/8" over the width on narrower joints. A 1/2" joint typically takes 5/8" rod.
Handling: Don't stab it, stretch it, or jam it in with a screwdriver. A punctured closed-cell rod can release gas into curing sealant and leave bubbles. A blunt tool or a wooden paint stirrer works.
For shallow cracks where rod won't fit, some sealant makers call for bond-breaker tape instead.
Prep Steps That Decide Whether It Lasts
- 1Pick your day. At least 40°F (or whatever the label says), dry concrete, and no rain in the forecast before it skins over.
- 2Dig it out. Remove old caulk, rotted filler strip, sand, and weeds. A utility knife and stiff brush handle most of it.
- 3Blow or vacuum the dust. Sealant bonds to clean concrete. Dust is a release agent.
- 4Set the backer rod to the right depth along the whole joint, in one long piece where you can.
- 5Apply the sealant from the bottom up, without trapping air. Let self-leveling flow. Tool gun-grade smooth.
- 6Keep it slightly below the surface. Some sealant makers call for the finished bead to sit about 1/8" to 1/4" below the slab surface, depending on joint width. That keeps it away from tires, shovels, and plow blades.
- 7Fence it off until the label says traffic is OK.
Common DIY Mistakes
- Sealing a damp joint after rain, dew, or a hose rinse. It won't bond.
- Caulking on a 38-degree October afternoon because it's the last nice-ish day.
- Using self-leveling on a slope. It ends up in a puddle at the low end.
- Skipping backer rod in a wide joint, then wondering why the caulk split down the middle.
- Leaving the bead proud. The first plow pass peels it right out.
- Filling a joint with mortar or patch. It pops loose and can chip the slab edge.
- Caulking over the old stuff. New sealant on failed sealant fails with it.
- Sealing an offset crack. The trip edge stays, and the low side is usually low because there's a void under it.
When DIY Is Fine, and When to Call Us
Do it yourself if the slabs are level, the cracks aren't offset, and you'll do the prep right. A few tubes, backer rod, and an afternoon handle most walks and patios.
Call us if:
- One side of a crack or joint is higher than the other. We lift the low slab back into position with polyurethane foam, then seal the crack or joint once it's level. Our concrete lifting page explains how that works.
- The slab is tipped toward your house or holds water against the foundation.
- You've got a lot of footage (a long driveway, a big patio, a pool deck) and you'd rather have it done once.
- It's your garage floor and you're thinking about a coating. Crack repair is part of our coating prep. See garage floor cracks.
On our jobs, we use a flexible polyurethane joint sealant, self-leveling on flat joints, with closed-cell backer rod wherever the joint is deep enough to need it.
For the step-by-step on repairing specific cracks, see how to fix concrete cracks.