×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

How to roughen the existing concrete slab surface around a column

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

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
www.americanconcrete.com

RE: How to roughen the existing concrete slab surface around a column

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.

RE: How to roughen the existing concrete slab surface around a column

(OP)
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.

RE: How to roughen the existing concrete slab surface around a column

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

RE: How to roughen the existing concrete slab surface around a column

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 smile

RE: How to roughen the existing concrete slab surface around a column

(OP)
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?

RE: How to roughen the existing concrete slab surface around a column

"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.

RE: How to roughen the existing concrete slab surface around a column

Found a link here https://www.usbr.gov/research/projects/download_pr... 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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources