Why Is My Epoxy Floor Cracking?
Troubleshooting
Most cracks in an epoxy floor are not cracks in the epoxy. They are cracks in the concrete showing through it. Epoxy is a thin, rigid layer bonded tightly to a slab, so when the slab moves, the coating has no choice but to move with it and break where the concrete breaks. That is why the first and most important question is not how to fill the crack but what is underneath it and whether it is still moving.
General guidance, not a diagnosis of your floor · Authorized dealer for every brand we carry · Counter in Pompano Beach · We speak English, Spanish and Portuguese
What you are looking at
Three kinds of crack
Reflective cracking
An existing slab crack telegraphing up through the coating. The line in the epoxy follows the line in the concrete exactly, wanders like concrete cracks wander, and often runs off the edge of the floor or into a joint.
Joint movement
A straight, deliberate line: a control joint, a saw cut or a construction joint that was coated over instead of being honored. Dead straight and regularly spaced is the tell.
Cracking in the coating itself
Fine crazing, shrinkage cracks or brittle fracture that does not correspond to anything in the slab. Much less common, and it points at the material or the way it was applied rather than the concrete.
To tell reflective from coating-only, chip out a small section at the crack and look at the concrete. If the slab is cracked underneath, the coating is reporting on the slab. If the concrete is intact, the problem is in the film.
Causes, most common first
Why it happened
1. An existing slab crack that was coated over
The most common cause. A hairline crack that was invisible on grey concrete is very visible under gloss, and a crack that carries any movement will reopen through whatever is over it. Coating does not bridge a moving crack. Nothing at coating thickness does.
2. Control joints filled with the wrong thing, or not honored
Control joints exist to tell the slab where to crack. Fill one rigidly, or coat straight over it, and the slab still moves. It simply cracks somewhere else, usually right beside the joint. Joints either get honored, which means the coating is terminated and the joint is filled with a material that can move, or they get filled with a semi-rigid filler where the traffic demands a continuous surface, and even then some movement can show.
3. New concrete still shrinking
Concrete keeps shrinking for months as it dries. Coating a young slab locks a rigid film onto something that is still changing dimension. Cracks that appear within the first year on a new build are frequently this, and they are the slab doing what slabs do rather than a coating fault.
4. Structural movement, settlement or subgrade problems
Cracks that are widening, that have a vertical step across them so one side sits higher, or that run diagonally across a corner, are telling you about the building and not about the floor. This is not a coating repair. It needs somebody qualified to look at the slab and the ground under it before anything else happens.
5. Thermal movement
Slabs expand and contract. Where that movement is large and repeated, a cold room, a washdown area cycling between hot cleaning and chilled storage, or an exterior slab in direct sun, a rigid epoxy is the wrong body for the job. Urethane cement systems exist specifically for this, because they move closer to the rate that concrete does.
6. Brittle or over-thick coating
Cracking in the film itself usually means an over-thick section, a deep fill that cured hot and shrank, or a system too rigid for the service. Crazing across a whole floor can also come from a topcoat applied over a build coat that was not ready.
Narrowing it down
How to tell which one you have
| What you see | Most likely cause | Check this |
|---|---|---|
| Irregular wandering line, concrete cracked underneath | Reflective crack | Was the crack repaired and detailed before coating? |
| Dead straight, evenly spaced across the floor | Control joint | Were joints honored or coated over? |
| Appeared in the first year of a new building | Shrinkage in young concrete | Slab age at the time of coating. |
| Widening over time, or one side higher than the other | Structural movement | Stop and get the slab assessed before recoating. |
| Cracks in cold rooms or washdown areas only | Thermal cycling | Whether the system suits the temperature range. |
| Fine crazing, concrete intact underneath | Coating brittleness or film build | Film thickness and recoat timing. |
A simple movement check: bridge the crack with a rigid marker or a strip of plaster and leave it through a full seasonal cycle. If it breaks, the crack is live and no filler will hold it closed permanently.
The repair
Can it be fixed in place?
Static cracks
Yes. Chase the crack out into a clean V or square section, remove dust, fill with a rigid epoxy crack filler, level it flush, then abrade and recoat the area or the floor. A properly chased and filled static crack does not usually come back.
Moving cracks
Here we have to be straight with you: a moving crack cannot be permanently hidden by a coating. You can repair it, and it will look right for a while, and then it will reopen on the same line. The honest options are to treat it as a joint, which means filling it with a material designed to move and accepting a visible line, or to cut a proper joint there so the movement has somewhere to go.
Anybody who tells you a coating will permanently bridge a live crack is selling you a repeat visit.
Joints
Do not try to make a joint disappear. Honor it, fill it appropriately and let it stay visible. A visible joint in a good floor looks far better than a ragged crack beside a joint you tried to hide.
Structural cracks
Do not coat over them. Get the slab looked at first. Putting a floor over a moving structural crack wastes the material and hides the evidence somebody needs to see.
Crazing in the coating
Abrade and recoat with attention to film build, and check that the recoat timing matches the data sheet. If the section is over-thick, it may need grinding back before it is rebuilt.
Next time
The prep that prevents it
Walk the slab before you quote it
Find every crack and every joint while the concrete is still grey and easy to read. Mark them. They are much harder to see and much more expensive to deal with after the floor is down.
Chase and fill before coating, not after
Open each static crack out, fill it properly and grind it flush as part of prep. A crack repaired under the system disappears. A crack repaired through a finished floor always shows.
Decide what happens at every joint
Honor it or fill it, but decide deliberately and tell the customer what they will see. Surprises at joints are the most common complaint on an otherwise good floor.
Let new concrete dry
Young slabs are still shrinking and still wet. Moisture testing tells you whether the slab is ready, and it protects you from more than just cracking. The three tests worth running take an afternoon.
Match the system to the movement
Where thermal cycling is part of daily service, specify a body that can handle it. Urethane cement is built for thermal shock and washdown. Tell us the temperature swing and we will spec it.
Related symptoms
Often confused with
Talk to somebody who has stood on a slab
Tell us what you are seeing
This page is general guidance. Cracking has several possible causes and a web page cannot see your slab. Tell us whether the line is straight or wandering, whether the concrete under it is cracked, and whether the crack is getting wider, and we can usually tell you quickly whether this is a fill or something that needs a structural opinion first.
Latux Epoxy
2201 N Andrews Ave, Suite 108
Pompano Beach, FL 33069
(800) 413-1427
Monday to Friday, 8:00 AM to 5:00 PM.
Saturday by appointment.
Sunday closed.