How do I fix crumbled concrete?
After power washing I noticed the concrete was loose and crumbled under garage door. Best way to deal with a section like this
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Quik-crete should work well for this. You'll need to remove all loose concrete, first. Wash with Dawn to remove dirt/grease. Rinse and dry thoroughly. Wet the areas needing repair before adding the quik crete. Mix up no more than you will use in 15 minutes, and keep it covered in the mixing pail.
There are epoxy fillers they use to repair on the highways, even had sand to look like concrete. More expensive and can be purchased in gallons, check for other cement type fillers or if area is failing jusevbust out and fill in. The joint of new version old, old, adhesion and expansion. Go and get opinions of several suppliers. There is also it is very worth while just to pay someone to do it that knows what they are doing. It sounds like the slab may have been too thin, mixed wrong or not enough mesh-rebar etc. may be best to remove bad slab and replace that area. Go ask questions is the suppliers, more than one.