Q & A: How do I repair a deep crack in my concrete slab and add flexibility to prevent the crack from growing?

Repair a crack in your concrete slab:

Q: Hello Mr. Maxwell;
Thanks for previous help re Lichen & Roof problems. Now have found that I have a crack some 6″ deep x approx. 2″ wide x 5 feet long where, over 30+ years slab outside   basement door has moved. Looks like the internal floor & the external pad were poured as one as crack is irregular under the door but does not seem to have been “attached to the foundation for one foot either side as the crack has vertical straight face there. I believe I need something flexible or the repair will break again. An expansion joint was suggested but crack is not straight enough. Most concrete repair mixes state that they not be used if thickness more than about an inch or two – some say ½ inch max. And their flexibility is in question. Wondering if you you can advise best method/methods to effect a lasting repair? -Regards, Charles

Hi Charles!
Thanks for your note. I think I have something to suggest.
A 2″ crack is quite wide, but the movement is probably happening slowly if it has taken 30 years. Without seeing the situation, I can’t tell if the problem needs to be stabilized or not, but assuming it doesn’t, here’s what I’d do.
How about cleaning the irregular crack out as best you can, fastening some flexible foam (about 1/2″ thick) to one side of the crack with double-sided tape, then fill the remaining space with concrete. When that’s hard, remove the foam, then fill the gap with polyurethane caulking. If I understand your problem, this approach should keep things sealed while still allowing some small movement.
Does this help? Please let me know.


Posted on October 4th, 2014