Add Concrete to Existing Concrete
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 add an additional concrete layer 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.
Steps
- Choose a concrete mix with the right aggregate for the job. Aggregate is an additive - usually sand or stone - added to the cement mix to make it cheaper. For a very thin coat, you should choose a small aggregate. Larger aggregates cannot be used in thin coats.
- Prepare the existing concrete. To prepare the existing concrete, clean and roughen the current surface before you begin any other steps. You can use a chemical concrete product that will accomplish both of these goals at the same time.
- Saturate the existing surface. Soaking the existing concrete will make it unable to absorb liquid from the new concrete. Failing to follow this step can create a poor bond between the new and existing concrete.
- Prime the concrete. Make a water concrete mix that is thinner than the package directions. A viscosity similar to paint will work well. Use a stiff brush to apply it to the existing concrete. This step is not a requirement, but it will help create a tighter bond between new and existing concrete. Do not let this slurry coat dry before applying the new concrete.
- Prepare the new concrete. Mix the concrete according to package directions. If you are using regular concrete, add a concrete bonding additive to help the old and new concrete join. If you are using a concrete patching product, do not add the bonding additive because it is part of the patching mix.
- Apply the first coat of new concrete. Pour your concrete mixture to a level slightly below what you want the final level to be. Do not use a trowel at this point. A rough surface on the first pour will help the final pour to stick better.
- Wait several hours for the cement mixed from the first pour to begin to set. You may begin the next step when the first pour is mostly hardened.
- Mix concrete for the second pour according to package directions. Add a concrete bonding additive, if necessary.
- Pour the final coat. Use a trowel to evenly spread the concrete in the second pour into the concrete from the first pour. This is the point at which you need to be careful to make the surface smooth and level.
- Protect your work. Cover the newly poured concrete with a plastic sheet or spray it with curing compound to ensure the concrete takes a long time to cure. The longer the curing process, the stronger the finished concrete repair will be.
Tips
- Concrete work is best done on cool, but not cold, dry, overcast days. While not always possible, try to choose a cooler, overcast day for your project.
Things You'll Need
- Concrete mix or concrete patching product
- Concrete bonding additive
- Concrete cleaning chemical product
- Wheelbarrow or bucket for mixing
- Trowel
- Curing compound
- Stiff brush
Related Articles
- Finish Concrete
- Choose Concrete for a Project
- Finish a Large Span of Concrete
- Make Concrete
- Use Prepackaged Concrete Mix
- Pour Concrete
