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Yea, that's a bitch. The suggested rubber may do it. Or just a flat piece of rubber or maybe carpet under them. If it's really slick you could use a piece of rope, carpet, strap, or something under the ramp that's long enough to reach the back tires, or front tires if you're trying to back a front wheel drive car up them. Beware that anything narrow or flexible you put under the ramps might create a high spot of load on them. Probably a non-issue if you'r'e not loading them heavily, but do consider that. Another option is a piece of wood or pipe or something from the front of each ramp to a wall, or drill a hole in the concrete (if not post tensioned) and drop in a bolt as a stopper. The wood ramps I made work on concrete because they're stepped instead of a ramp so there's a small climb, then another so there's less push on them plus they have more traction surface to hold them in place. You can just use pieces of wood without nailing them together and stack as many as you need for the desired altitude. I nailed mine together because I carry them in my van for mobile service and wanted the convenience of one piece 'ramps'. Other than really low cars I find that two 2x6 is generally enough, but I'm thin & flexible. When I need a little more I put a 4x6 past them as a third bump up. For anything that doesn't require tire removal ramps are way better than a jack. In cases of low cars I sometimes use the wood ramps to gain space for getting a jack under a control arm or other front lift point instead of lifting by the body/frame.