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Question DetailsAsked on 7/25/2016

Can I chemically etch an old slab floor before pouring a leveling coat of concrete? The foundation has settled.

And part of the uneven slab is under a bathroom shiny tile floor. This is where the etching would be needed most. I'm intending to install rebar and steel mesh to strengthen the leveling layer which might approach 3 inches at the deepest edge. The near edge feathers to blend at grade. Or what about cracking each 8 x 8 tile with a hammer before etching or adding bonding agent before the pour?

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Voted Best Answer

Etching with anything short of hydroflouric acid will do little to ceramic tile - that is a fused ceramic surface or porcelain tile in most cases, hence relatively inert and not affected significantly by normal etching acid. Plus putting an overlay over tile, especially if part tile and part exposed concrete, just sounds like a poor idea to me.

I would say first you need help from a Geotechnical (soils and foundations) or Structural Engineer for the floor settlemetn and overlay issue, or a Concrete Repair specialist for this project, I would say, for following reasons:

1) just overlaying a settling slab is not "fix" for the problem - will likely keep on settling unless something has been done to solve this - foundation underpinning, mud-jacking, etc - or totally removing and then replacing the slab after the subgrade issues causing the settling has been solved. Also, any cracks in the slab will almost "reflect" or come through the overlay in short order - and a 3 inch differential settlement is almost certain to be a significant soils issue or erosion of material from under the slab or significant soil loss due to broken pipe, not just settlement of the fill put within the foundation after it was placed, so normally one would dig up the slab to find the source of the problem to solve that first, THEN replace the slab after properly compacted structural fill is placed under it. So - I would advise against a bandaid which might be money thrown away a year or few down the road if it continues to settle (as is likely if it has gone 3 inches so far).

2) 3 inches thickness is VERY marginal for reinforcing bar and that is only at the thick end - that thin a slab is almost never rebar reinforced due to lack of cover on the rebar - mesh is the normal means until you get under about 1 to 2 inch thickness, then generally nothing or maybe fiber-reinforced concrete in structural cases.

3) Putting a levelling pour over tile is just a bad idea - very poor bond no matter what you do to it unless you seriously power sand the tile through the glaze with carbide sander - and cracking it with a hammer before placing levelling layer will undoubtedly cause a poorer not better bond, because if you hit it hard enough to crack the tile you will be normally be popping a good deal of it loose at the same time, meaning NO bond under those points - leaving an unbonded, drummy overlay.

4) You don't say what flooring you are putting over this levelling coat, but if something other than a carpet or floating floor like laminate or engineered wood (putting aside any moisture transmission issues) where the subfloor quality is not critical, a levelling coat that goes from zero to 3 inches is not likely to give a very good final product for a hard-surface floor. Usually, in that case (like with new stone or tile) the old tile would be stripped off (not that big a thing using power tools like an extension air hammer with wide chisel blade unless it was put down with liquid epoxy glue), the surface of the concrete "bush hammered" or rough ground to fresh rough concrete, surface vacuumed and washed to provide a rough fresh concrete surface, then a nailed-down metal grid or mesh reinforced concrete mortar "mud coat" put down under the tile, which can vary from about 1/4 inch + to several inches thick as necessary to level the surface for subsequent thinset and tile or stone placement.

5) As noted above - etching is normally used for prep for paints and epoxy and such - not for bonding levelling concrete, just not enough grip there to be trusted because etching only locally removes some cement paste - it does not provide a "rough" surface to adhere to - bush hammering is the norm for that. This is what a bush hammer bit and rotary concrete surface roughening tools look like - available in hand-held, impact drill, hand-held rotary bush hammer disc tool, and jack hammer sizes depending on the area to be done -

Another alternative is diamond or carbide floor tooth prep disc on a concrete polishing machine - looks much like a floor polisher or mini Zamboni ice rink machine but designed to surface prep, remove paint, or polish concrete - for levelling course bonding uses a toothed blade that cuts small grooves the surface.

Also about the etching - nasty acid fumes from that that are not only dangerous to work around but can also damage fixtures and piping and wiring in the room where it is being done unless excellent ventilation is provided - many hundreds of cfm, not just a bathroom ceiling fan.

Answered 3 years ago by LCD

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