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I think those joints aren't really expansion joints, rather, predetermined locations for cracking as the slab cured. I say, fill them in with Top-N-Bond or something similar after proper prep work.
John
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+1
Those are called "control joints". In theory, they provide a place for the concrete to crack if it is going to. I think you will have trouble getting any kind of mortar to stick without chipping or flaking. If you fill it with some elastomeric product, I think you will have better results. Otherwise, just sweep or vacuum it out.
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Clean them out really good, fill with Sikaflex.
It's in the concrete aisle at Home Depot. Check the expirarion date, it does have a shelf life.
Ed
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You might need to run a small bead of silicone in the bottom of the groove (where needed) to fill any cracks.
Sikaflex is self-leveling and will drain through any voids. Sawcuts must be filled with backer rod first.
Ed
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Like others said sikaflex or similar product. However buying it by the tube at depot will be extremely expensive. Check craigslist or find a company that does airports,hangars,shops and see what they would want for material as they buy it by the drum and it's way cheaper that way. There is a product specifically for airports and I forget the name but it's very good stuff.