A while back I got a question from a subscriber about how to fix a big crack in a concrete slab. There’s a lot of misunderstanding about this, a lot of bad products that won’t work long term, and limited knowledge about the best option for repair:
Q: How can I fix a crack that’s six inches deep by about two inches wide and five feet long in a 30-year-old concrete slab? Most concrete repair mixes state that they must not be used if thickness is more than about an inch or two; some say ½-inch max. And their flexibility is in question. Wondering if you can advise of the best method to affect a lasting repair?
A: A two-inch crack is quite wide, but the movement is probably happening slowly if it has taken 30 years to develop. Without seeing the situation, I can’t tell if the problem needs to be stabilized or not. Regardless of the reality though, here’s what I’d do.
Start by cleaning the crack to remove everything loose on either face. You’ll need a shop vac to do this properly.
Next, get familiar with one of the new carbon fibre reinforcement systems. I’ve used ones made by DRICORE and Rhino and both work amazingly well. They come with a system for filling cracks, but also some carbon fibre cloth and epoxy resin for bonding this fabric to the concrete. This combination completely stabilizes concrete cracks and, according to my tests, the result is amazingly strong. More on that later.
DRICORE comes with an injectable polyurethane foam for filling cracks, and this would be better for you than the epoxy filler that’s part of the Rhino system. The foam won’t seep away into the soil under your slab like the liquid epoxy might.
With the crack filled, level the area, then apply a coat of anchoring epoxy, lay down the carbon fibre cloth, then apply more epoxy on top. When this has hardened, you’ll have a repair that can actually resist further movement.
The video below shows this system in action on a wall mock-up I made for the video. The same process applies to floor cracks.
And this video below shows how I tested the strength of a carbon fibre repair. Very impressive, I think.
