[Heel n Toe]

rnln, I get the impression from your OP that the concrete is already there.

If so, you should tie into that somehow, otherwise in a couple of years (or sooner), that brick and the concrete under it will probably begin to pull away somewhat... beginning with a crack right where it joins, then more over time because of people stepping on just the brick.

You could use a hammerdrill and drill laterally into the slab an inch or two every couple of feet, then put a piece of rebar in the hole, bent at a right angle and tied into rebar running alongside that you imbed in the concrete you're gonna pour to set the brick in/on.

Also, be sure to wet the edge of the slab down real good (as well as the bricks, like Zeke said) before you pour that new concrete and set the bricks, then cover about 2 to 3 feet of the slab and the new concrete/bricks/mortar with plastic for a week or more while it cures. You want it to dry/cure slowly for maximum strength.