Expansion Joint Filler: How to Repair and Refill Concrete Joints

What do you need to repair an expansion joint?

An expansion joint filler, something to pull out the old strip, and a way to clean and dry the gap. Which filler depends on how wide the joint is once it’s cleaned out.

Which expansion joint filler should you use?

Match the filler to the width of the joint. The polyurea joint kit’s coverage chart lists joints from 1/8 to 1/4 inch wide and from 3/4 to 2 inches deep. Many driveway expansion joints are wider than that once the old strip is out, and those get backer rod and a flexible concrete joint sealant.

The difference is how each one holds. The Federal Highway Administration explains that a joint filler goes directly into a narrow sawcut (opens in a new tab) and doesn’t rely on gripping the sides. A sealant does rely on that grip, so it needs backer rod under it to set its depth and keep it off the bottom.

The joint kit’s data sheet describes it as a hard, rubber-like polyurea with a Shore A hardness of 85 to 90 that stretches 170 percent before it breaks. It says the material meets the floor joint filler guidelines in ACI 302 and ACI 360, and calls out driveways, walks and patios for keeping out grass and weeds.

How do you replace a wood expansion joint?

Pry or chisel the old wood out, clean the gap, then refill it. To replace wood expansion joint strips, you don’t need to get every splinter out, only enough to give the new filler the depth it needs on a solid, dry base.

1. Pry the wood up

   Drive the thin end of a pry bar down beside the strip and lever it up. Rest the bar on a second bar or a block, not the concrete edge, so you don’t chip it.

2. Chisel out what won't lift

   If nails or tight packing hold the strip in, split the top of the wood with a chisel or an old screwdriver and pull it out in pieces. Go deep enough for the filler you’re using.

3. Scrape the joint walls

   Scrape both concrete faces down to bare concrete. Fillers and sealants grip the concrete, not dirt or rotted wood.

4. Vacuum and let it dry

   Vacuum out the debris and let the joint dry fully. If wood stays at the bottom, it needs to be solid and dry too.

5. Set the bottom of the fill

   For a sealant, press in backer rod at the depth its label calls for. For the polyurea kit, read the section below on deep joints first.

6. Fill and let it cure

   Fill the joint flush with the slab, then keep traffic off for the cure time on the label.

Shows prying wood strips out with a pry bar braced on a second bar, and chiseling out the top of strips held by nails.

How to fill expansion joints in concrete with the polyurea kit

Clean and dry the joint, mix the filler through the cartridge’s static mixer, and pump it in slowly until it’s flush. The steps below follow the joint kit’s instructions.

The kit’s limits come first. The concrete has to be at least 50°F, dry, and free of dirt, oil and debris, according to its data sheet. The product page adds that new concrete should be at least 28 days old.

1. Warm the cartridges

   The data sheet says to store them between 60°F and 90°F, and 70°F or warmer works best. Cold cartridges are hard to pump.

2. Shake the cartridge

   Shake it hard for 30 to 60 seconds.

3. Fit the mixer

   Take off the locking nut and the caps, snap on the flow restrictor, set the static mixer over the nozzle and screw the nut back on.

4. Purge the air

   Load it in a caulk gun, point it up and squeeze slowly to push out the air. Then tip it down and shoot a little into a disposable cup until it comes out mixed.

5. Fill the joint

   Pump slowly and evenly, keeping the cartridge pointed down. Fill the joint to flush with the slab.

6. Purge during pauses

   If you stop, the data sheet says to purge a little into the cup every 1.5 minutes so the mixer doesn’t clog. Never reuse a static mixer once the filler has set in it.

What if the joint is wider or deeper than the kit covers?

Switch to backer rod and a flexible concrete joint sealant. The joint kit’s coverage chart stops at 1/4 inch wide and 2 inches deep, and its instructions don’t mention backer rod.

A full-depth expansion joint with the strip pulled out is deeper than 2 inches, so something has to hold the filler near the top. The kit’s data sheet doesn’t say what should go under it. Before setting backer rod or anything else under the polyurea, check with the seller that it will work.

The sealant route is the one the Federal Highway Administration describes for wider joints. Its guide says backer rod should be about 25 percent wider than the joint (opens in a new tab), closed-cell, and pressed in snug at the depth the sealant needs. The backer rod guide covers sizing, depth and setting it.

How long before you can walk or drive on the joint?

About an hour with the polyurea kit in mild weather. Its data sheet says it’s tack free in about 5 minutes, ready for light traffic in 30 minutes and full traffic in 60 minutes at 70°F. Sealants take longer, so go by the label.

Cold slows every cure. Keep water and traffic off the joint until it's ready.

What mistakes make expansion joint repairs fail?

Dirt, water and the wrong material cause most failures. A filler poured over damp wood or dusty concrete lets go, and a rigid one cracks out the first time the slabs move.

• Filling over rotted wood. Soft wood keeps rotting and drops the filler with it. Dig down to solid wood or remove it all.
• Filling a damp joint. Both products need dry concrete. Wait out rain and morning dew.
• Using mortar or a crack patch. Expansion joints have to move. A rigid fill pushes against the slab edges and chips them.
• Using the polyurea kit in a wide joint. Its chart stops at 1/4 inch wide. Wider joints get backer rod and sealant.
• Letting sealant stick to the bottom. Sealant bonded to the bottom of the joint can’t stretch and tears. Backer rod prevents that.
• Filling a joint where a slab has moved. If one side is higher or sinking, the joint opens again. Fix the movement first.

When does an expansion joint need more than a filler?

When the slabs on either side have moved. An edge that sits higher than the next slab, a slab that tips toward the house, or a joint that opens wider every year means the soil under it is settling or washing out.

A joint filler keeps water out but won’t lift a slab or stop it sinking. Lifting, grinding the high edge or replacing the section are jobs for a concrete contractor. If the moving slab is against the house and the foundation wall is cracking too, have a structural engineer look first.

The joints hub explains control and expansion joints if you’re not sure which one you have.