Concrete slab crack repair

How would you recommend I seal these floor cracks?



The previous owner tried to seal them with something (red and white layers)... obviously didn't work.

 

Why do you want to seal them? Hoping it might reduce moisture levels? If you have moisture issues, sealing the cracks will not do much of anything. In the photos the cracks look pretty much what you could expect in a house as old as yours. If you only want to improve the apearance of the surface you could always consider applying a thin layer of self levelling compound after all the old tile surface is cleaned off.

 

I installed a french drain around the interior perimeter of the basement. I wanted to seal these cracks just so no water comes up from them.
I'm going to put Platon and a subfloor and all that on top of them, just figured I should seal them to make sure the water gets to the french drain.

 

There are concrete fillers/sealant caulking products available to do what you are requiring. Companies such as DAP provide these kinds of products for instance as do others.

 

If you're serious about sealing them (and all of them, so you WILL be pulling all of the floor tile up, yes?), proper prep work should include routing everything out, probably at least 3/4" deep and 1/4" wide at the top. Vacuum out the dust/dirt, flush, dry and then install a good low-modulus, 2-part epoxy that's good for damp conditions. Use the gravity flow method of working the stuff into the Vee'd out openings, with lots of plunge moves using a narrow putty knife. If you really want to go overboard, install plastic injection ports every foot or so using a gel, and pump a low viscosity epoxy in through the ports. Using a low-modulus material will ensure that even if the cracks are moving (slightly), it will adhere to both sides while having enough "give" to not fracture. Do an internet search to find what's available in your area--Sika, Master Builders, Euclid, all come to mind, and there are others.

It's a tremendous amount of work, and the epoxy will run at least $65 a gallon. Good luck.