Crack and joint repair before you coat a concrete floor

Before a floor is coated, every crack and joint gets one of three treatments: chase and fill it so it disappears under the coating, fill it and accept that it may show again, or keep it open and seal it after the coating. Which one depends on a single question: does it move? A coating bridges a stable crack, but it can’t stop a slab from moving, and a moving crack or joint will crack the coating above it.

The repairs go in after the moisture test and the grind, and before the primer; concrete surface profile covers the grinding.

Read the cracks before you quote

Walk the floor with a flashlight held low and mark every crack with chalk or paint. Note each one’s length, width and pattern, and whether one side sits higher than the other. Most cracks fall into a few types:

Then decide whether each crack is still moving. Signs that it is:

• Vertical offset. If a coin or fingernail catches on one side, the slab has moved and may move again.
• Fresh, sharp or crumbling edges. Something is still working the crack.
• An old fill that has split or pulled away. The crack reopened after it was filled.
• A young slab. Concrete does most of its shrinking in its first year, especially the first two or three months, so cracks in new concrete are still opening.

On a commercial job with time to spare, pencil a line and a date across a crack, or fit a crack monitor, and check it over a few weeks. Otherwise, treat every crack and joint as moving until you have reason to think it isn’t. All concrete moves a little with temperature and vibration; the question is how much.

Chasing and filling a crack

A crack filled as it is fails at the edges: dust and loose concrete line its walls, so the filler has nothing solid to grip.

1. Grind the floor first. Prep the whole slab, then work the cracks, so you’re cutting into sound concrete and can see the real crack edges.
2. Chase the crack. Use a crack-chasing blade or a narrow diamond wheel on a hand grinder with a dust shroud and vacuum. Cut a V-shaped groove at least three times the crack’s width and no less than about 3/8 inch wide.
3. Vacuum, don’t blow. Vacuum the groove clean. Compressed air only moves the dust around and packs some of it back into the crack.
4. Fill. Use a semi-rigid epoxy or polyurea crack and joint filler, or a 100% solids epoxy thickened with fumed silica. Very deep cracks can be partly filled with dry silica sand first if the filler’s data sheet allows it. Overfill slightly, because the filler sinks as it soaks in.
5. Shave it flush. Once it has cured, cut or grind the filler flush with the floor. Fast polyurea fillers can be ground in under half an hour; epoxy fillers take hours.
6. Prime, then look again. Primer shows pinholes and low spots you couldn’t see on bare concrete. Patch them before the base coat goes down.

Crack chasing is some of the dustiest work on the job, and the silica rules cover hand grinders as much as the big machine. Dust extractors and silica rules covers the controls.

Control joints: fill or honor?

Joints are planned cracks, and the kinds need different treatment:

• Control joints (contraction joints) are sawn or tooled grooves that make the slab crack in a straight line. In an older slab they’ve done most of their moving. See control joint.
• Construction joints are where one pour met the next. They usually behave like control joints.
• Isolation and expansion joints separate the slab from walls, columns, drains and other slabs so each can move on its own. They’re built to move. See isolation joint.

To fill a joint, vacuum it out to bare concrete, removing old filler, dirt, saw slurry and sealer, then fill it full depth with a 100% solids semi-rigid epoxy or polyurea and shave it flush once cured. Joints that carry hard-wheeled traffic, like forklifts and pallet jacks, need a semi-rigid filler with a hardness of at least Shore A 80. A soft sealant in those joints lets the wheels break down the joint edges.

Timing matters on newer slabs. Put off filling joints as long as the schedule allows, because a stiff filler can’t stretch: as the joint keeps opening, the filler pulls away from one wall or splits down the middle. That isn’t a product failure. The gap is cleaned out and refilled on a return visit, so agree in writing up front who pays for that visit.

Fill joints with the slab near its normal service temperature, or in the cooler part of its range if it swings. A cooler or freezer should run at its operating temperature for at least two days before joints are filled, and for two weeks if it runs below 0°F.

To honor a joint, keep the coating from bridging it. After the coating has cured, cut along the joint through the coating, clean it, press in a closed-cell backer rod so the sealant ends up no deeper than it is wide, and fill with a flexible sealant. Where joint edges are already broken and traffic is heavy, rebuild the edges with epoxy mortar first, then re-cut the joint through the repair and fill it.

Spalls, pitting and bug holes

A spall is a piece of concrete broken out of the surface, often along a joint. Pitting, popouts and bug holes are smaller voids. All of them have to be filled flush, or they show through the coating as dips and craters.

• Spalls. Cut back to sound concrete. Saw-cut the edges straight down about 1/2 inch deep and keep the shape simple, a square or rectangle, so the patch never thins out to a feather edge that cracks and lifts. Vacuum, prime the cavity with straight epoxy, fill with epoxy mortar (epoxy mixed with dry, graded silica sand) and trowel it flush. Build deep patches in the lifts the data sheet allows, because a thick mass of epoxy heats up as it cures (see exotherm).
• Broken joint edges. Rebuild with epoxy mortar, then re-cut the joint through the repair and fill it.
• Pitting, popouts and bug holes. Fill with epoxy paste, 100% solids epoxy thickened with fumed silica, pressed in with a squeegee or trowel. On a badly pitted floor, a skim coat of paste over the whole area is faster than spot filling.
• Cement-based patches. Fine for deep or large repairs and for building slope to a drain, but the patch must cure and dry to the coating’s moisture limit before it’s coated. Its data sheet gives the wait.

Grind any high spots flush once the patches cure, and vacuum again before priming.

Which product for which repair

Where you can, use fillers from the same maker as the coating system, or confirm on the data sheet that the coating can go over them. The filler becomes part of the floor.

What a coating can’t fix

• Structural and settlement cracks. Cracks that keep widening, have one side higher than the other or line up with cracks in the walls point to a slab or foundation problem. That’s for a structural engineer or a concrete contractor. Coat over it and the crack comes back through.
• Slabs that rock, heave or sink. They have to be stabilized, lifted or replaced first.
• Water coming up through cracks. Damp cracks, white salt deposits along them (efflorescence) or water seeping up point to slab moisture or hydrostatic pressure. Test before you quote (concrete moisture testing); a coating alone won’t hold water back.
• Weak or contaminated concrete. Scaling, a crumbling topping or oil soaked deep into the slab has to come out down to sound concrete. If an old coating is in the way, see removing old floor coatings.
• Joints built to move. Bridge an expansion joint and you’ve built in a crack. Honor it.

If the customer won’t pay to fix the cause, write the exclusion into the contract or pass on the job. A floor that cracks over a moving slab turns into your callback, whatever caused it. Why epoxy floors fail covers the other failures that start in the slab.

Price it and put it in writing

Crack and joint work is priced on top of the square-foot price, usually per linear foot of crack or joint, and per patch or per square foot for spalls and pitting. Measure it on the site visit (how to measure a floor for an epoxy estimate), list each repair as its own line in the estimate template and run the totals through the job pricing calculator.

Then write down what the customer should expect: which joints will be filled and which kept open, that moving cracks and filled joints may show as hairlines later, and that slab movement isn’t covered. The scope of work template and the warranty template have places for both.

Learning the prep

Crack and joint work is easier to learn by watching it done. The Epoxy Playbook is recorded hands-on installation training, full installations you can replay as often as you need, plus the business side, and it’s required before the hands-on class. If you’re still building the business around the work, start with how to start an epoxy flooring business.