Excessive sidewalk cracks indicate that a slab may be beyond repair. Replacing broken concrete eliminates a dangerous trip hazard and prevents the damage from spreading. Removing concrete is a labor-intensive process made easier with power tools that can help break down the broken section without damaging the rest of the sidewalk. Once the old concrete is removed, plan a full day to form and pour new concrete, and four days for the slab to set.
Place the tip of a jackhammer in the center of a crack and drill down at an angle. Follow the line of the crack with the jackhammer to cut out large chunks of the slab. Crush the chunks into smaller pieces with a sledgehammer and throw them in a bucket or dumpster. Pry out remaining pieces with a pry bar until all of the damaged concrete is removed. Call a waste disposal company to collect the broken concrete.
Lay a sheet of plastic by the site and scoop out the old gravel base with a shovel. Measure the depth of the site once you reach subsoil and continue digging until the site is 10 inches deep. Tamp the subsoil beneath the old gravel with the end of a hand tamper or 4-by-4-inch board.
Fill the site with two 3-inch layers of 3/4-inch angular gravel. Pound the flat end of the tamper down hard against the gravel to stabilize the base.
Measure two strips of expansion joints to fit across the width of the sidewalk between the new slab and the two existing ends. Cut the strips with a utility knife and stand them against the two inner walls adjacent to existing concrete. Made from cork or another semi-flexible material, expansion joints are 1/4- to 1-inch thick. They will help prevent future cracks by absorbing tension as the slab shrinks and shifts with temperature fluctuations.
Place two 2-by-6-inch boards against the remaining sides of the site. Hammer wooden stakes every few feet behind the boards to brace them against wet concrete.
Add small amounts of water at a time and mix it with concrete in a mixer. Scoop the concrete into the site with a shovel and shift it into corners with a gauge rake. Drag a 5-foot screed board back and forth over the top to remove any excess and smooth the slab. Push a squeegee over the surface to remove any marks left by the screed board.
Cover the new slab with damp, breathable fabric so the new slab doesn’t dry too fast. Keep it covered and spray the fabric with water for four days.