Concrete makes for an extremely hard and durable building material. That same strength poses a difficulty when you have to cut through it for any reason. Most cutting methods involve breaking the concrete at the cutting point, which creates jagged cuts as well as messy cutting areas. Using a circular saw, however, allows you to cut right through the concrete, making a smooth straight cut where needed quickly and efficiently.
Mark your cutting line onto the concrete using a 2-by-4 as a straightedge to guide the chalk. Orange chalk works well as a marker because it shows up brightly against the concrete and washes away easily.
Attach a diamond blade to your circular saw for use on the concrete surface. Set the blade to the required cutting depth of up to 2 inches. Place masking tape along the bottom of the saw body where it runs against the concrete to prevent the bottom from scratching the concrete surface.
Place the saw onto the surface of the concrete with the blade aligned with the chalk line. Place the 2-by-4 straightedge against the base of the saw so that it runs parallel to the chalk line to serve as a guide for the saw to maintain the straightness of the cut. Begin the cut from the edge of the concrete or from the start of the chalk line. Turn on the blade and then plunge it into the concrete, cutting to your set depth.
Follow the chalk line closely with the blade of the saw. Use as little pressure on the motion of the saw as possible, allowing the saw to do most of the work for you. Use a low RPM setting when cutting through the concrete to keep the blade from overheating and throwing chunks of concrete from the surface of the slab.