Over time, concrete hardens and becomes less porous. Hardening can cause imperfections. Additionally, concrete can sink into the underlying ground. Hardening or sinking can make a concrete slab unlevel, in which case water may puddle on concrete. A common method of resolving this issue is to pour concrete over concrete over the top of the current surface. If you are experiencing an unlevel concrete patio, driveway, or sidewalk, you may wonder how to add concrete to existing concrete. For a small patch, you can use a thinner coat. A complete recover job requires a thicker coat.
Step 1: Clean the surface thoroughly. If the surface is flaking or spalled, scrape it with a spade to dislodge as much loose concrete as possible, then sweep the surface clean.
Step 2: Dig a 6\” wide trench around the surface on all sides to create room for 2*4 forms.
Step 3: Stake 2*4 forms flush against the sides of the concrete slabs, 1\” to 2\”
above the concrete surface (make sure height is even). Drive stakes every 3 ft. and at every joint in the forms. Mark control joint locations onto the outside of the forms, directly above existing control joints. Coat the inside edges of the forms with vegetable oil (so the forms will strip off the newly poured concrete once it has hardened).
Step 4: Apply a thin layer of bonding adhesive over the entire surface. Follow the directions on the bonding adhesive product carefully. Instructions for similar products may differ slightly.
Step 5: Mix concrete using sand-mix concrete. Make the mixture slightly stiffer (drier) than normal concrete. Spread the concrete, then press down on the concrete with 2*4 or shovel to pack the mixture into the forms. Smooth the surface with a screed board. Calculate how much material you will need with theconcrete calculator.
Step 6: Float the concrete with a wood float, then tool with an edger. Recreate any surface treatment, like brooming, used on the original surface. Cut control joints in the original locations. Let the surface cure for one week, covered with plastic.
This procedure is best used on walkways or other small areas.