How to Add Concrete to Existing Concrete

No matter how strong a slab of concrete seems, it will wear out over time. Imperfections form when concrete hardens or sinks into the ground. Adding fresh concrete is a common way to level out old slabs and patch damage. If you plan on pouring a lot of concrete, build a wood and mesh barrier first to ensure your new slab is strong. Finish the work by priming the surface and pouring a mix over it, giving your concrete foundation a fresh, new coat.

Part 1

Part 1 of 4:

Cleaning Old Concrete

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1
Sweep dirt and debris off the concrete. The old concrete needs to be completely clean or else anything you pour over it won’t bond to it. The first order of business is to remove gravel, leaves, sand, and dirt. Get as much of it off the concrete as you can. Push the debris off the concrete surface or collect it in a bag to throw away.
- Use a stiff-bristled broom to knock as much debris out of cracks as possible.
2
Use a pressure washer to clean out the remaining debris. You won’t get all of the hidden debris at first, so you will need to give the concrete a more thorough cleaning. Use a fan tip on the pressure washer with a PSI of about 3,000 and hold it about 6 in (15 cm) above the concrete. Sweep the nozzle slowly over the concrete, making sure you hit every area.^{[1]XResearch source}
- If you don’t own a pressure washer, check home improvement stores in your area. You may be able to rent a washer from them.
- You can add a liquid detergent or degreaser to the water to ensure you get all stubborn spots, including those from mildew and algae.
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3
Scrub difficult spots with chemical products. Purchase a commercial cleaner formulated for use on concrete. Pour it onto tough stains you can’t otherwise remove, then work the cleaner in with a stiff-bristled scrub brush. When you’re done, rinse the area with a hose. This can help remove stains from oil and sap, which are very difficult to get rid of once they set in.^{[2]XResearch source}
- Most home improvement stores sell these cleaners. Look for ones that are effective against oil.
- Another option is to use trisodium phosphate (TSP). Follow the manufacturer’s instructions to mix the TSP powder with water into a paste. Try mixing about 1 fl oz (30 mL) per .125 fl oz (3.7 mL) of water.
- If commercial cleaners and TSP don’t work, you may need to get muriatic acid. The acid is strong, so dilute it first, mixing 1 part acid in 3 parts water. Wear protective gear, including a respirator mask.
4
Saturate the existing surface with water. Right before you get to work mixing and pouring new concrete, spray the old surface with a hose. Get the old concrete slab completely wet, making sure any cleaning chemicals you used get rinsed off in the process. Keep spraying it until the moisture runs off the sides instead of being absorbed. Dry up any water pooled over the concrete before continuing.^{[3]XResearch source}
- Concrete is porous, so it can absorb liquid. If it absorbs moisture from the new concrete, you will get a dry top slab that didn’t bond well with the old concrete.

Part 2

Part 2 of 4:

Setting up a Slab Perimeter

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1
Measure out the space you wish to fill with concrete. Taking measurements is necessary if you plan on filling a large space. Use measuring tape to figure out the length and width your new slab needs to be. Write your measurements down so you know what supplies to get to pour a smooth, consistent slab later.^{[4]XResearch source}
- If you are only going to pour a thin slab or patch up existing concrete, make a general estimate of the area’s size and how much concrete you need for it. You can usually mix and pour a small batch without setting up a perimeter.
2
Mark the height you want the concrete to reach. Slab thickness is important and depends on your plans for your home. If you want the slab to reach up to your doorstep, for instance, you need to measure from the existing slab to the bottom of the doorstep. Use chalk to mark the height the slab should reach.^{[5]XResearch source}
- Take your time measuring. The ground beneath the existing concrete may not be level, so measure on all sides.
3
Use the measurements to calculate how much concrete you will need. Measure the length, width, and depth of the area you wish to fill. Multiply these numbers together to get a general estimate of the amount of concrete you need. Add an extra 10% onto your total estimate to account for spillage.^{[6]XResearch source}
- The calculation can be an imperfect estimate, especially for areas that aren’t perfect squares or rectangles. Always get more concrete than you need to ensure you have enough.
4
Install braces around where you will pour the concrete. Using your measurements, cut some boards to help hold the liquid concrete in place. You can get wood boards that will reach the depth marks you made in chalk earlier. Set the braces up around the perimeter of your old concrete slab.^{[7]XResearch source}
- These braces serve as a mold. When you pour the concrete into it, you no longer have to worry about it spilling off the old slab and making a mess. It gives you a better chance at creating a strong, level slab.
- You can cut the wood yourself with a circular saw. Wear protective gear, including a dust mask and eye protection, if you do this. Alternatively, get the wood cut at a home improvement store.
5
Prop up the braces with wooden stakes. Dig out the soil around the braces, then stand the stakes in them. You can use 2 in × 4 in (5.1 cm × 10.2 cm) wood for most projects. Place the stakes approximately every 12 in (30 cm), then drill them to the braces with 3 in (7.6 cm) wood screws.^{[8]XResearch source}
- Make sure the stakes are planted firmly in the dirt so the braces don’t fall as you work.
- If your slab won’t be very high, you may be able to lay the wood braces on the ground and screw them together without using stakes.
6
Test the evenness of the boards by laying a bubble level on top of them. The braces should be even before you begin pouring the concrete. Set the level on top of each board 1 at a time. Watch the liquid in the center of the level to make sure the bubble stays in the center. If the bubble moves to 1 side, that side is lower than the other side and needs to be adjusted.
- Another way to test the boards is to run string behind them. The string should be the same distance from the board at all times. If 1 end is closer to the string than another end, the brace isn’t straight and should be adjusted.
7
Lay wire mesh between the braces. Wire mesh provides extra strength to thick concrete slabs. It can also prevent cracking and settling.^{[9]XExpert SourceGerber Ortiz-Vega
Masonry Specialist & Founder, GO Masonry LLCExpert Interview. 10 March 2020.} You can purchase a roll of welded wire mesh at a home improvement store, then simply spread it out in a single layer over the old concrete. Press it down so it is flat and level before you add the new concrete.^{[10]XTrustworthy SourceScience DirectOnline archive of peer-reviewed research on scientific, technical and medical topicsGo to source}
- The new concrete you pour binds to the mesh. Although it makes the concrete stronger, it won’t prevent it from cracking.
- Another option is to get rebar and lay it out in a grid pattern similar to the mesh. Place rebar chairs underneath the rebar to hold them in place.

Part 3

Part 3 of 4:

Pouring a Primer Coat

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1
Purchase concrete with a fine aggregate for small repairs. Aggregates are additives that make up most of a concrete mix. Fine aggregates are typically sand or crushed stone. This type of mix is best for pouring a very thin coat of concrete, such as when topping off or leveling out an existing slab.^{[11]XResearch source}
- The aggregate in a concrete mix is listed on the label. You can also see it when you open the bag. Fine concrete looks smooth or has very small stones.
- Concrete is generally a mixture of cement, sand, gravel, and water.^{[12]XExpert SourceGerber Ortiz-Vega
  Masonry Specialist & Founder, GO Masonry LLCExpert Interview. 10 March 2020.}
2
Choose a coarse aggregate concrete when pouring thicker slabs. Coarse concrete has gravel or larger stones as an additive. This type of concrete is stronger but less dense, so you can safely pour a large slab onto existing concrete. It is a good option if you need to fill a large area with something thick that will last a long time.^{[13]XResearch source}
- Larger aggregates cannot be used to make thin coats, so make sure you get the type of concrete mix best suited for your project.
Concrete can splash up as you mix and pour it, and that isn’t anything you want in your eyes or on your skin. Wear protective goggles or safety glasses, a safety mask with ventilation, and long jeans.^{[14]XExpert SourceGerber Ortiz-Vega
Masonry Specialist & Founder, GO Masonry LLCExpert Interview. 10 March 2020.} Also consider wearing work gloves to protect your skin from concrete splatter.^{[15]XResearch source}
4
Mix up a scratch coat consisting of concrete and water. The scratch coat is a layer of wet concrete mixed to a liquid consistency similar to paint. You will need a big plastic mixing bucket. Combine the concrete in a ratio of about 1 part water to 7 parts concrete, then mix it with a wooden mixing stick or electric paddle mixer until it reaches a uniform liquid consistency.^{[16]XResearch source}
- Use the same concrete you plan on using for the final layer. Have the area estimate available for the space you wish to cover, then follow the manufacturer’s instructions for an estimate on how much to mix.
- Less is usually more with the scratch coat. Remember that is liquid, so it will spread out over the existing slab.
5
Spread the liquid mix over the existing concrete. Dump it all out onto the existing concrete, then begin spreading it around using a hand trowel or paver. Press down hard on the concrete to work the liquid mix into any cracks as you smooth out the scratch coat layer. The layer of wet concrete doesn’t need to be thick. A layer about ¹⁄₈ in (0.32 cm) thick, about the thickness of a credit card, is enough.^{[17]XResearch source}
- You can also use a rag or a gloved hand to spread the scratch coat. This can work well when priming small areas.
- The wet mix is there to help the new concrete bond to the old concrete, so you don’t need a lot of it.

Part 4

Part 4 of 4:

Adding the Top Layer

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1
Mix a batch of concrete according to the manufacturer’s instructions. Read the concrete packaging to find out how to mix it. The ratio is usually about 1 part water to 3 parts concrete. Add the water and concrete to a mixing container, then use a wooden stirrer or electric paddle mixer to combine them into a thick liquid.^{[18]XResearch source}
- The mixing ratio may vary depending on what product you own, so check the manufacturer’s recommendations to get the concrete to the proper consistency.
2
Add a bonding adhesive if you are using regular concrete. If you got a bag of concrete and mixed it normally, you should use a concrete bonding adhesive to ensure the layers of concrete stick to one another. The adhesive is a liquid that comes in a plastic jug and you pour it directly into the concrete mixture. Check the manufacturer’s instructions to add the product at the recommended ratio.
- You can get the additive online or at most home improvement stores.
- If you are using a concrete patching product, you most likely do not need the adhesive. It is often included as part of the patching mix. Check the product’s label to make sure.
3
Apply the new concrete over the primer. Pour concrete onto the existing slab until it is around where you want the final level to be. Make sure you have enough concrete to fill the entire area to the depth you desire. Add all of the concrete right away so none of the mixture has a chance to dry out.^{[19]XResearch source}
- You do not need to wait for the scratch coat layer to dry completely. It will have dried enough while you mixed this batch of concrete.
- You can try to fill in a slab with multiple pours of concrete, but this usually isn’t advisable. You won’t get a single, uniform slab, which can lead to bonding issues in the layers.
4
Use a trowel to smooth out the concrete. The concrete needs to be leveled out before it solidifies. You can use a trowel to smooth over small areas or a screed board and bull float for large areas. Work back and forth, passing over a little more of the rough concrete each time. Make multiple passes over the entire surface to ensure it is smooth.
- On hot days, the concrete can dry out very quickly, so don’t waste any time after pouring it.
- Make sure there are no marks left in the concrete when you are done smoothing it.
5
Protect your work by spraying the concrete with a curing compound. A good curing compound is the best way to preserve wet concrete. Mix the curing compound according to the manufacturer’s instructions, add it to a garden sprayer, then spray it directly onto the concrete. This should be done right after you finish pouring the concrete. Concrete takes about 7 days to fully cure.^{[20]XResearch source}
- Another way to do this is to spray the concrete thoroughly with water from a garden hose, then lay a polyethylene sheet or concrete curing insulation blanket over it. Make sure the plastic is flat against the concrete or else it will cure unevenly, causing discolorations. Remove the sheet to moisten the concrete again every day for a week.^{[21]XResearch source}
- The longer the concrete is allowed to cure, the stronger it will be. Make sure no one is able to step on the concrete until it has finished curing.

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Question
How do I clean up my tools after pouring concrete?
Gerber Ortiz-Vega
Masonry Specialist & Founder, GO Masonry LLC
Gerber Ortiz-Vega is a Masonry Specialist and the Founder of GO Masonry LLC, a masonry company based in Northern Virginia. Gerber specializes in providing brick and stone laying services, concrete installations, and masonry repairs. Gerber has over four years of experience running GO Masonry and over ten years of general masonry work experience. He earned a BA in Marketing from the University of Mary Washington in 2017.
Gerber Ortiz-Vega
Masonry Specialist & Founder, GO Masonry LLC
Expert Answer
If you're done using your tools, clean everything off with water and a wire brush. Put your excess concrete in a contractor grade trash bag and then dispose of it. Make sure you clean everything immediately when you're done with it so the concrete doesn't dry.
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What PPE should be worn when pouring concrete?
Gerber Ortiz-Vega
Masonry Specialist & Founder, GO Masonry LLC
Gerber Ortiz-Vega is a Masonry Specialist and the Founder of GO Masonry LLC, a masonry company based in Northern Virginia. Gerber specializes in providing brick and stone laying services, concrete installations, and masonry repairs. Gerber has over four years of experience running GO Masonry and over ten years of general masonry work experience. He earned a BA in Marketing from the University of Mary Washington in 2017.
Gerber Ortiz-Vega
Masonry Specialist & Founder, GO Masonry LLC
Expert Answer
Wear safety goggles, a protective mask, and long jeans when mixing and pouring concrete. The small particles can harm you if you don't protect your eyes, nose, and mouth.
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Can a concrete slab be poured over an existing 6-inch, heavily cracked concrete porch?
Community Answer
It sounds like the existing porch had an unstable footing for the soil movement (freeze/thaw) and/or was attached to a home with a lot of movement. If the slab underneath continues to move, the upper layer also eventually will move.
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Tips

You can always cut out old concrete and replace it. Use a chisel or jackhammer to remove damaged portions.
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If your concrete is badly cracked, it may not be salvageable. Consult a professional to determine what needs to be done.
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Oil and sap must be removed before new concrete can be poured. If you can’t remove it, chisel it out or cover it with a sealant.
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