Pour A Concrete Slab Over A Concrete Slab, #3 Porch Conversion Video

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How to Add Concrete to Existing Concrete

Four Parts:

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.


Cleaning Old Concrete

  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.
    • 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.
  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.
    • 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.
    • 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.

Setting up a Slab Perimeter

  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.
    • 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.
    • 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.
    • 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.
    • 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.
    • 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. 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.
    • 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.

Pouring a Primer Coat

  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.
    • 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.
  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.
    • Larger aggregates cannot be used to make thin coats, so make sure you get the type of concrete mix best suited for your project.
  3. Wear eye goggles to protect yourself while mixing concrete.Concrete can splash up as you mix and pour it, and that isn’t anything you want in your eyes. Wear protective goggles or safety glasses with polycarbonate lenses at all times. Also consider wearing work gloves and long-sleeved clothing to protect your skin from concrete splatter.
  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.
    • 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  in (0.32 cm) thick, about the thickness of a credit card, is enough.
    • 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.

Adding the Top Layer

  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.
    • 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.
    • 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.
    • 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.
    • 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.

Community Q&A

  • Question
    Is it acceptable to pour concrete for a grade beam without lean concrete?
    wikiHow Staff Editor
    Staff Answer
    It usually is but the concrete needs to be thicker than normal. About 3 inches, or 75 mm, of concrete will do. The lean concrete adds some extra anchoring strength if you have it there first.
  • Question
    Can I pour a 6 inch slab over a portion of a cracked patio in order to build an enclosed room on it?
    wikiHow Staff Editor
    Staff Answer
    If you are sure the slab is stable, you can usually do this. Be aware that the crack will appear in the new slab. There's no way to avoid it, but you may be able to patch it once it forms.
  • Question
    Can I add enough concrete to level out a large slab?
    wikiHow Staff Editor
    Staff Answer
    You usually can level out a large slab as long as it is stable. If it doesn't have massive cracks or show signs of sinking into the ground, it's worth a try. Otherwise, you can always break the existing slab and replace it.
  • Question
    How do I remove cracked tile from around my in-ground pool and pour a quarter inch of concrete in an 8 x 8 area?
    wikiHow Staff Editor
    Staff Answer
    You should be able to chip off the tile with a chisel, although you can get a jackhammer if you want to destroy it. If you want the concrete to pour neatly into a small area to reach that exact height, create a mold out of wood or other materials. You can also pour the concrete directly and smooth it out.
  • Question
    I repaired my driveway with cement that isn't level now. Can I pour new cement on top to make it level?
    wikiHow Staff Editor
    Staff Answer
    You can try, and if you measure it out and level it carefully it may work. Layering the cement can cause problems, so you may wish to break off the old cement, level out the area, then pour the new cement.
  • Question
    I need to cover a parking area where there will be lots of traffic. What thickness of concrete do I need? The concrete below is not cracked but it has a rough surface.
    wikiHow Staff Editor
    Staff Answer
    Concrete parking lots need a thickness of about 6 inches or more. Making it a little shallower than this may still be safe thanks to the underlying layer.
  • Question
    What is the difference between liquid and powder concrete additives?
    wikiHow Staff Editor
    Staff Answer
    Powder additives can be a little messy, but it can also add color and texture to concrete. Liquid additives work more immediately so you know what effect they have on concrete, but they can also leave wet spots. They can also be tough to use on sloped surfaces. They are basically the same thing and are often mixed together to get the best of both worlds.
  • Question
    If I pour concrete on a existing slab, do I need a moisture barrier between them if there is a moisture barrier on the existing slab?
    wikiHow Staff Editor
    Staff Answer
    You shouldn't need to add another barrier. The old barrier should do well enough, unless it's in bad shape.
  • Question
    How do I know if I should go with an extension or a brand new porch?
    wikiHow Staff Editor
    Staff Answer
    There are a few factors. The most important is how the old porch looks. Is the old floor badly cracked or otherwise damaged? If it isn't level or easy to work with, you may need to redo it before you can expand it.
  • Question
    How do I make new mortar stick to old mortar in a shower pan?
    wikiHow Staff Editor
    Staff Answer
    Smooth the old mortar out with a floor grinder before adding the new mortar over it. It may not stick well if it isn't smooth.
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Quick Summary

To lay new concrete, sweep and clean the old concrete, and saturate the old concrete with water. Then, build a perimeter, or a wood brace, that is the same height as you want the concrete to be. Make sure the braces are level, and lay down a barrier of wire mesh over the existing concrete inside of the barrier. Pour and spread a thin later of scratch coat, and then add a bond layer on top of the coat. Mix and pour the concrete according to the package instructions, and spread the concrete with a trowel before spraying it with a curing compound.

Did this summary help you?
  • Concrete work is best done on cool, dry, overcast days. If this isn’t possible, you can work on concrete during warm days, but be quick so the concrete doesn’t have a chance to dry out.
  • You can always cut out old concrete and replace it. Use a chisel or jackhammer to remove damaged portions.
  • 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.
  • If your concrete is badly cracked, it may not be salvageable. Consult a professional to determine what needs to be done.

Things You'll Need

  • Concrete mix or concrete patching product
  • Concrete bonding additive
  • Concrete cleaning chemical product
  • Wheelbarrow or bucket for mixing
  • Mixing stick or paddle
  • Trowel
  • Curing compound
  • Stiff scrub brush
  • Eye protection
  • Long clothing
  • Wood for large projects
  • Electric drill
  • Wood stakes
  • Chalk
  • Wire mesh or rebar
  • Level

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