Shellac stains on concrete pad

How about a power washer???

Wear over many years. Concrete is a porous material, lots of places for something to get into. Alcohol is the right solvent, but you are going to dissolve it, spread it, and pick up some, but not all of it. Repeat that 50 times and there will still be some residue.

May be easier to just shellac the entire floor.

Lacquer thinner should do the trick. BTW, is that cut and grouted concrete, or tlle

No, it won't for shellac. Alcohol

Soak a paper towel in alcohol, place it over the stain, put some plastic wrap over the paper towel (so it won't evaporate so quickly), wait 30 minutes, then wipe off the shellac. You may need to repeat the procedure if the shellac is thick.

Try this on one tile: soak paper towel with denatured alcohol, lay it on the tile, cover right away with tin foil weighed down so it doesn't lift up. Let it soak an hour or so, mop up whatever liquifies with a clean rag (mebbe a touch of d.a. on the rag). It should dissolve in alcohol, and scrubbing doesn't do much if it hasn't dissolved.

I fixed a neighbor's wood-stain-on-concrete problem with acryllic craft paints. I had already tried paint remover on the same concrete deck and it didn't touch it. It certainly disguised it, but doesn't get much weather exposure.

If you look on the back of the can of shellac, you'll see that it recommends denatured alcohol or household ammonia. It is my experience that the ammonia works better than the alcohol for cleaning up shellac, as amazing as denatured alcohol is.

The trick with denatured alcohol, BTW, is to use a lot of it at once.