how do I remove adhesive from a concret patio (from outdoor carpeting that was torn off)

I am sorry to say that, Ginger, but you're facing a tough job. This is as tough a cleanup job as they ever get.

Basically, you can use solvent to soften the glue and then scrape it off. You're going to need a special solvent that is formulated for the type of glue you're removing. Sentinel Products made a name for themselves by making various types of solvents for these types of jobs. Here is their adhesive removal products page:

Check out their product list here where they recommend which solvent to use for which situation . I think you'll need their "747" solvent but please do verify it by the type of glue, maybe you need the "626" instead. Regardless of the name all of these are pretty nasty stuff that not only smells awful but also tends to stain pretty much everything they touch. You'll have to wash and scrub really hard with soapy water and a stiff brush after the solvent did its job and you finished scraping.

You  can also soften the glue with a heat gun. The key here is not to make it too soft so it does not spread around. You want to soften it just enough to be able to pick it up and scrape it off with a joint knife of a putty knife.   Try it on a small patch, see if you get a hang of it. If it does not go well, you can always go back to the solvents.

Good luck!