Pour a Floating Concrete Floor

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About: Technical Editor for two magazines. Software tester for the computer controlled electronic brakes of Locomotives.

We had a building that had an existing concrete floor that was poured directly on the ground. Over the years it had cracked. Normally you would jackhammer the old concrete out and do it right. However the building supports were placed directly on top of the concrete pad.

This presented a problem of how to remove the concrete pad and still hold up the roof! After much deliberation, we decided to top the cracked concrete with a new "floating" slab. The idea being, let the original concrete slab crack and do what it wants because the upper slab will float on top, undisturbed.

Supplies:

Step 1: Patch the Existing Concrete Slab.

We used Quikcrete's Vinyl Concrete Patcher to fill the cracks and to smooth the floor as much as possible.

Step 2: Creating the "Floating" Layer

We covered the floor with Home Depot's HDX 6 mil Black plastic sheet and then laid wire re-mesh sheet (also from Home Depot) over top of the plastic. The re-mesh strengthens the cement that you will lay in the next step.

Note: Keep the re-mesh sheet up about an inch above the black plastic so the re-mesh is centered in the 2" pour to strengthen the cement.

Step 3: Mixing the Concrete

You have to decide just how you intend to mix the concrete. You can use the Black plastic mixing tub or use a mixer. With 28 bags to mix, we chose to rent a mixer.

Some may ask why not just order up a truck of cement? With the new slab only being a proposed 2" thick, you can't use conventional cement. It will crack. Conventional cement is designed for 4" thick or thicker.

Step 4: Pour the New "Floating" Concrete Slab

You need a specialty concrete mix for this "Floating" concrete floor to work properly. We used Sand/Topping Mix from Quikcrete in 60 lb. bags. This mix is pre-mixed, just add water. This Sand/Topping Mix is specially designed for 2" or less thickness. We poured a 2" slab.

Step 5: Finishing the Slab

There are two methods for finishing a concrete slab, floating it smooth or brooming. We chose brooming because we wanted a non-slip floor, even in the wintertime. A wet coarse broom is lightly dragged over the partially set up cement.

Step 6: Ready for Use

After a few days, the slab was dry and hard.
Note, this "Floating" concrete slab is now 5 yrs old and has withstood -35 deg F below zero and heaving in the spring thaw with zero cracks!

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    14 Discussions

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    tytower

    5 months ago

    7 years in January actually . Good job . Only bit I would add is if you kept it wet with a layer of wetted sand for 28 days it would gain maximum strength . Twice what you have now.

    4 replies
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    gabe_lucitytower

    Reply 5 months ago

    That's an often-forgotten point! Cement doesn't "dry", it "sets" - a chemical reaction happens between the cement and water. So you want to keep it moist until all the chemical reactions have completed.

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    tytowergabe_luci

    Reply 5 months ago

    In fact crystals keep growing for 28 days whilever it is kept wet . Growth stops if it drys out . The growing crystals squeeze the aggregate they encapsulate .

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    Pa1963tytower

    Reply 5 months ago

    Concrete reaches 98% of its strength in 28 days, the remaining 2% in 28 years. Covering it with wet material was only done in very hot conditions. Additives are added now.

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    tytowerPa1963

    Reply 5 months ago

    When you have built a few ferro cement boats you can start telling me . Quoting from books does not work with me . Additives that strengthen are almost non existent . Fly ash at 3% is added to waterproof and rice water was added for the great wall of china and they still don't understand exactly why it works so well.The water used was the discarded cooking water after boiling rice and it contained very fine rice flour particles.

    In fact the only way to do this without wetting it down is to paint a layer over the top to seal the moisture in or to steam it which speeds the growth process and does the same thing as water.

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    OmarJ3

    5 months ago

    Reminds me of a garage floor from a house I once owned. If I presume correctly, this garage is not used for car shelter but rather for light to medium household storage and/or woodwork shop. In other words no heavy concentrated loads?

    1 reply
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    TechEditorOmarJ3

    Reply 5 months ago

    Correct. Storage and a generator room.

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    GregS278

    5 months ago

    Nice job I thought concrete needs to be 4 inches so this is good to know!
    I give it - you a thumbs up!

    1 reply
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    TechEditorGregS278

    Reply 5 months ago

    Normally that is correct. But this special mix is rated for only up to 2" thick

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    enginepaul

    5 months ago

    In some areas, you can order a truck that mixes the concrete at the site using any formula that you may want. Nice job; I just did something similar myself because breaking out the old floor would notmhave been cost-effective.

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    LindseyM53

    5 months ago on Step 6

    VERY GOOD JOB!!!! My husband owns a concrete finishing company and you were right about everything you did!!

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    RyanD192

    5 months ago

    Nice and conscise. From the thumbnail it appeared as though the starting point was the final product. Perhaps switching photos may be more enticing for folks.

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    joseph.campo.35

    5 months ago

    Nice job. Nice to hear that there are no heaves with -35F. Wow.

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    BobH160

    5 months ago

    I have laid many floors and if you want a nice smooth flat floor I use self-levelling compound as the last layer. Paint the dried layer you have previously poured with PVA to provide a bond between the two layers. Mix as per the directions and pour, it may need a little moving about but the thicker the layer the less it needs, it dries in about 2 hours depending on which manufacturer you use. Once it is fully dry you can paint it or lay any material you want on it.