To make a new opening in an existing concrete wall, you will need to first install a lintel. Lintels are typically made of a strip of angled steel that is sufficiently strong to support and divert the weight of the structure above, around the opening in the wall and into the foundation and the ground. Lintels are installed just above an opening, and extend outward on either side to allow for a redirection of downward force into the remaining structure. The process of making a lintel opening in an existing concrete wall requires some masonry skill and a few specialized tools.
Mark the location of your steel lintel onto the outside face of the concrete wall. Use your tape measure to mark the wall so that the lintel slot will extend about 4 inches out from the edge of any window, door, air-condition vent or other opening you plan on installing. Place a strip of masking tape on the wall along the lintel installation location to serve as the cut line marker.
Cut a slot in the existing concrete wall in which to mount the steel lintel. Use your electric circular saw and a concrete or masonry cutting blade to create the slot. Start at one end of the tape marker and cut your way to the other. Ideally the lintel should be placed into an existing mortar groove, then the framework of the opening can be built out from there. This makes the cut easier and keeps the structural integrity of the wall and its concrete block components intact.
Repeat the cut until the slot is about 1/2-inch wide. Place your steel lintel into the slot and use your small sledgehammer to drive it into position. Apply mortar mix to both extended ends of the slot with your trowel and allow it to dry according the specifications of the manufacturer. The blocks beneath the steel lintel can then be removed as needed.