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We're doing a 1 3/4” concrete leveling layer over an existing slab as prep for LVP flooring. Does the tar need to be removed completely for new concrete to bond? And do I need to seal existing cracks on floor or will overlay be enough? Thanks

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    So you are pouring 1 3/4 inches of concrete over a wood floor! Have calculations been done to verify the flooring structure can support that weight? . If in fact the structure can support the weight and the live load, the bonding is moot as the weight of the concrete will keep it from moving. Any open cracks larger than about 1/8 inch should be sealed and can be done with something simple as tape. I am still concerned about why the decision to pour concrete. Have other options been ruled out? Is this concrete over concrete? Cement should not be on wood
    – RMDman
    Feb 3 at 15:04
  • We’re not pouring new concrete over wood; we removed old damaged wood floors so new concrete would go on existing slab. But slab has cracks and I wanted to know if it’s necessary to seal them prior to pouring new concrete and also does the tar has to be removed? Thanks
    – Ann
    Feb 3 at 15:25
  • I edited your question to add that fact.. The tar is moot and does not need to be removed unless it is hugh chunks. The cracks are no issue either unless they are big also and are the result of the slab shifting. ( We don't know that because we cannot see what you see.) Post some pics for better answers. Also important to know is what the finished flooring is going to be. Tile could need more work to insure no cracks transfer, where as LVP or wood does not have that issue.
    – RMDman
    Feb 3 at 15:32
  • I appreciate it! We’re going with lvp.
    – Ann
    Feb 3 at 15:43
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    then as said unless the cracks show slab movement or heaving, they are not an issue.
    – RMDman
    Feb 3 at 15:57

1 Answer 1

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Don't worry about doing anything special with the existing cracks.

Tar on the floor suggests oily contamination that could prevent the new concrete from bonding with the old concrete even after the tar has been removed. For a 2" thick bonded topping (and down to 3/4" thickness under the American Concrete Institute [ACI] 302.1R's class 3 category--the ACI is the highest authority on concrete in the US), the tar must be removed at a minimum. If the floor is contaminated with oily residue, concrete sealer, etc., then mechanical scarification or shotblasting is called for. A bonding agent gets applied to improve bond between the clean, old slab and the new concrete topping. I would be tempted to put down fresh tiler's thinset as my bonding agent, but there are certainly other options. All of this prep is also required for a self leveling product, where I would use the self leveling stuff unless there's a lot of thickness that needs filling to level things out.

Leaving the tar alone and supplementing it with a debonding layer everywhere, the unbonded topping prescriptions of ACI 302.1R call for a minimum 3" thickness, but I'm skeptical. Lightweight concrete used for soundproofing apartment floors is typically installed at 2" thickness and I assume without bonding. The ACI spec is technically for actual walking surfaces instead of mere underlayment, where I suspect that explains the discrepancy. Putzing around the internet I found a couple lightweight concrete recipes with 2" floor thicknesses, but nothing under the ACI's or PCA's (Portland Cement Association's) letterhead. Anything from the PCI (Precast/Prestressed Concrete Institute) would be for bonded topping slabs.

Alternatively, searching the TCNA Handbook for the phrase "unbonded mortar" pulls up a bunch of options. They get as thin as 1-1/4", but always with lathe reinforcement, where that stuff can get expensive for larger areas. There's a break even floor area in there somewhere, with unbonded mortar cheapest on the smaller area side of the threshold and scarification/shotblasting cheapest on the larger side of the threshold.

The unbonded mortar is the better quality option. It's also relatively labor intensive compared to a bonded self leveling option, where the application step for a bonded product can involve as little as hand-spraying some bond agent on the floor and then squeegeeing around the self leveling product.

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