The addition of concrete block to an existing concrete pad is a situation that will most commonly come up during a remodeling project. Though this is a task that is more complicated than simply laying new block on the slab, as it requires a higher level of thought and preparation. It is much easier than one might expect. This is a job that can be completed in a short time by just about anyone with any experience with traditional masonry techniques, depending on the project size.
Lay out the path of the block, measuring and marking its corners using your tape measure and lumber crayon. Then snap reference lines connecting these points using the chalk line.
Stretch the tape measure across the first space, making marks with the lumber crayon at 8 inches from the start, followed by marks at 16 inches, which will lay in the hollow cells of the concrete block.
Wrap a small piece of duct tape around the drill bit 3 inches from the tip. Most concrete pads are about 4 inches thick; this tape will indicate how deep to stop drilling.
Hold the drill above the mark and pull the trigger to start cutting. Push gently without forcing the bit into the concrete--it will cut as fast as possible without your pushing--until the bit has drilled deep enough to where the tape is touching the floor, then carefully pull the drill from the hole. This will be easier if you keep holding the trigger in the on position.
Repeat Step 4 on any remaining marks.
Measure and cut the rebar at about 8 inches in length--plus or minus an inch will not matter. Cut enough pieces to plug all the holes you drilled.
Fill the holes with epoxy. The epoxy containers will most commonly be like any other glue container in which you can simply squeeze the bottle to apply, but some may come in a gun form like caulk. In this case, follow the application instructions on the packaging.
Set the rebar pieces in the epoxy-filled holes and leave to dry. Refer to the epoxy packaging for drying times. It is not important for the rebar to be perfectly plumb (straight up and down).
Set the first course of block along the marked lines from earlier once the epoxy has had time to dry.
Fill the block cells (empty cavities) with concrete up to the top of the first course and trim with a trowel. Once this concrete has cured, it will have created a bond between the block, rebar, and concrete slab--forming a single structural unit--and the rest of the blocks can be laid as normal.