How to roughen the existing concrete slab surface around a column

[ajk1]

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?

 

[TehMightyEngineer]

4. I would expect less "bruising" (but still some) could be obtained if using an electric hammer with a wide chisel bit to get a 5 mm amplitude. I would suspect that the limited area of "bruising" around just the column would be acceptable for bonding.

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

http://www.americanconcrete.com

 

[Duwe6]

Yes, a needle gun will produce a thin 'bruised layer'. Thin. Bush hammering has a proven track record, so I would be reluctant to refuse to use it. But for small areas, a needle gun would make better sense.

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.

 

[ajk1]

To TehMightyEngineer (Structural)and Duwe6 (Industrial): thanks for these ideas. Sounds like some field testing would be beneficial. Thanks for the help. I guess I should contact Blastrac to see how close to the column they can come.

 

[Duwe6]

Wouldn't be surprised if they can almost 'kiss it' after wrapping columns in carpet [for protection].

 

[Ingenuity]

Back in the late 90's BLASTRAC made a hand-held portable shot blaster that was powered by a Makita cordless grinder, modified, with a captive shot bin and an attachment for a vacuum for dust control. You were able to get within 1/2" of any adjacent obstructions. Not sure if they still make it.

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

 

[ajk1]

to Duwe6 -

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?

 

[Duwe6]

"Kiss it" = approach within small fractions of an inch.

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.

 

[canwesteng]

Found a link here

https://www.usbr.gov/research/projects/download_product.cfm?id=446

shownig bush hammering delivers lower bond strength. Although CSA A23 only allows 0.5 MPa for shear for concrete placed against roughened surfaces, and experiments showed 2 MPa and 1 MPa bond strength in shear, so I'm not terribly concerned about bush hammering.