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We will be doing a concrete pour on an existing basement floor / slab to even it out. I have seen a contractor use concrete screws to put in the forms. It is a time consuming process that has the potential of slight damage to the existing concrete. Is there a simpler/less destructive way to attach the forms to the existing slab - given the attachment is temporary and has minimal structural requirements?

Update More info was requested. This is a fix to the existing basement slab and the pour will be all the way up to the walls/drywall. Whether or not to put "protection" (e.g. a shim) between the drywall and the new pour is a separate question.

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  • A photo of the work and where you want to use screws would be helpful, i don’t usually use screws to existing concrete and this sounds unusual to me.
    – Ed Beal
    Mar 13 at 20:19
  • @EdBeal There's nothing special about the existing concrete: so how do you typically attach forms to concrete? It does seem strange to use screws seems way too overkill. Mar 13 at 20:56
  • I may not be following how you are doing the job, I always use concrete steaks the round rod with holes that alternate by 90deg. These hold my forms or screed hight boards and in some pours the steaks are even in the pour until rough screed , pulled then a bull float to level things out. The only time I use fasteners in the old slab or footing is when adding to the side. I will drill holes and usually use nails in the holes with the nail longer than the hole so when the sidewalk or additional concrete work is cured it is tied and then the small slab work won’t shift in 5-10 years.
    – Ed Beal
    Mar 13 at 23:10
  • how are the stakes attached to the slab? Mar 13 at 23:48
  • The steaks are driven into the ground adjacent to the pour location.
    – Ed Beal
    Mar 14 at 18:36

2 Answers 2

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When I do overlays in 1 pour, I have never screwed anything to the floor.

On pours to the walls I purchase fur veneer (I live close to several plywood plants).

I put veneer strips at the Sheetrock so it won’t suck up the water from the pour. the thin strips can be pulled or cut off and the base boards will hide them.

I use a bull float to flatten it out if I need screed boards I hang them up on the wall(s) and will have one set loose in the middle pour both sides and the center last and float it out

I see no reason to screw to the slab if I only do a section. I would use the wall for support with horizontal bracing, I don’t like drilling basement slabs because of leaks that can be caused.

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Wet concrete is a force to be reckoned with - formwork is hardly "structurally minimal" and if a contractor could save time by not bothering to screw, they would - which tells you that the mess you have to fix when the forms blow out in the middle of a pour is far more hassle and wasted time/money than bothering to screw them in place.

If you are doing a very shallow pour you might be able to get away with attaching the formwork to concrete blocks set around the outside.

If pouring to the edge of the existing slab, support with stakes in the ground as usual for a completely new pour. It sounds like you are doing something not right to the edge, on top of the old slab.

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