How to roughen the existing concrete slab surface around a column
How to roughen the existing concrete slab surface around a column
(OP)
We want to bond a new structural reinforced topping to an existing concrete floor slab so that the composite slab can support 250 psf live load. We are using a water based epoxy bonding agent with a 24 hour window. We have used this product on a number if projects and generally get 2 MPa (290 psi) bond strength.
Questions
1. How would you roughen to 5 mm amplitude an existing floor slab surface surrounding a column, where the shotblast machine cannot reach?
2. Are there smaller shotblast machines that can get closer to the column?
3. How close can they get?
4. I expect bush hammering should not be used to achieve the 5 mm amplitude because it produces a "bruised layer". Do you agree that it should not be used.
5. Assuming that high pressure water blast would make too much of a mess, would a heavy sandblast be the way to go?
6. Would a needle gun produce a bruised layer?






RE: How to roughen the existing concrete slab surface around a column
6. I've heard of people using a needle gun but don't know about how "bruised" the resulting surface is. I would suspect it is similar to my above idea of a hammer and chisel bit setup and likely better.
Can you try each method on an area of the floor and create a field test of the three methods (bush hammer, chisel, and needle gun) and compare the bond strengths?
Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries
www.americanconcrete.com
RE: How to roughen the existing concrete slab surface around a column
There are indeed small 'detail' nozzles for shot blasting, just like sand/abrasive blasting. But using shot would still require recovery of the media. Personally, I would get as close to the columns as feasible with shot, then finish with a needle gun. I would remove that slight bruised layer with an acid etch. Etching is proven to provide a good surface to give coatings excellent adherence.
Like using any new method, prototype it and then test the adhesion.
RE: How to roughen the existing concrete slab surface around a column
RE: How to roughen the existing concrete slab surface around a column
RE: How to roughen the existing concrete slab surface around a column
With regards to bush-hammering, I seem to recall the ICRI (or some other related trade-industry group) issuing a position-statement on NOT using bush-hammering to achieve a 1/4" amplitude. But I cannot find the reference, so I may be making it up
RE: How to roughen the existing concrete slab surface around a column
1) Can you elaborate on what you mean by "kiss it"?
2) When you say that bush hammering has "a proven track record" I assume that you are referring to surface prep for structurally bonded topping. If so, that would be news to me. Can you refer me to any paper or study that shows that bush hammering does not cause a bruised layer and reduction in bond strength when compare to other methods of surface prep such as high pressure waterblasting or aggressive sandblast?
RE: How to roughen the existing concrete slab surface around a column
Bruised layer depends on the size of the tooling used to bush-hammer the concrete. Thus adhesion is highly dependent on the actual operator performing the work.
RE: How to roughen the existing concrete slab surface around a column