Concrete cracks because of settling in the earth under the concrete or because of a problem with the application or mixture of the concrete before it hardened. Filling a concrete crack is crucial to prevent the spreading of the crack. When water enters the crack, freezes and thaws, it puts immense stress on the concrete and causes the crack to worsen. Once a crack has been mended, it may be noticeable or discolored in comparison to the existing concrete.
Brush away any loose concrete from the crack with a wire brush. Remove any dirt from the crack with a nonacid concrete cleaner.
Fill small cracks or wall cracks with silicone-modified acrylic cement caulk or urethane-based cement caulk. Pour a ½ to ¾ inch layer of sand into deep cracks. Fill the deep crack a ¼ full with a cement crack sealer. Let the cement sealer dry, then fill a ¼ of the remaining space each time until the crack is filled.
Widen large cracks with a hammer drill by shaping the crack so that the bottom is wider than the top. Check for weak spots and chisel them out. Mix a vinyl concrete patch to the manufacturer's instructions for cracks up to ½ inch deep. Mix sand into the vinyl concrete patch mix for cracks deeper than ½ inch. Fill the crack ¼ full with the mixed vinyl concrete patch using a trowel. Let dry and fill a ¼ of the remaining space each time until flush with the existing concrete.
Let the patch's shine wear off for a few days. Finish the patched concrete's surface by running a wooden float over the area to blend the area with a textured concrete. Blend a patch to a smooth surface concrete by running a metal finishing trowel.