Surface Prep Precast Plank For Structural Topping 1

[BSVBD]

The precast plank provider states, in their manual, that the structural concrete topping "is usually a 3,000 psi minimum mix bonded to the top of the hollowcore plank using approved cleaning and bonding procedures." However, even when questioned over the phone, the plank provider would not provide any specifics to describe the "approved cleaning and bonding prodcedures."

Therefore, through my own research, to refine our general structural notes, i included the following:

The pre-engineered, precast hollowcore floor or roof plank shall be designed and manufactured by Spancrete or equal. Plank shall be factory prepped providing a roughened surface adequate to promote bonding of structural concrete topping. Concrete topping shall be structural concrete topping; a minimum 3,000 psi mix bonded to the top of the hollowcore plank using approved cleaning and bonding procedures to include removal of all dust, dirt, grease or any debris that would prevent bonding. Joints between plank units shall be similarly cleaned. Surface roughening and cleaning shall be adequate to provide a bond to act compositely with the precast plank to form a unified structural element. Bonding agents are not necessary. Structural concrete topping must be continuous from support to support, uninterrupted by walls, expansion joints, embedded conduits or any other obstruction.

I also read that "watering" the surface, without puddles or standing water, prior to applying topping, is recommended. However, that seems vague. does anyone have more specifics on "watering"?

Does anyone have any further specifications you would recommend?

Thank you!

 

[MotorCity]

It seems a little ridiculous that the precaster cannot provide an explanation of his own notes. Cleaning procedures approved by who?.....apparently not the precaster.

Typically when you are trying to achieve a bond between members cast at different times (i.e. a cold joint), you may roughen the surface (ACI recommends a 1/4" amplitude) and/or apply a bonding agent. If you are trying to achieve composite action, you may need to provide dowels or other reinforcing across the interface.

 

[Teguci]

- Always frown on named products on structural drawings. Use PCI criteria to remove precast riff-raff.
- Use raked finish to ICRI CSP9 roughness - Get rid of wordiness - any roughness promotes bonding.
- Concrete topping is spec'd and reviewed by you directly and not done with notes. I suggest a 5 ksi mix appropriate for your region per ACI 318.
- Keep the cleaning criteria and get rid of the bonding words. Whether it bonds or not is established by the surface roughness and cleaning process and not because you tell it to bond (ie either give them performance criteria or specified criteria, not both).
- What is the purpose of cleaning the joints?
- Add "Surface Saturated Dry" to the surface preparation. This pushes them towards power washing without getting into means and methods.
- Delete last 3 sentences. Walls, expansion joints, conduits will be per plans which the EOR will be directly responsible for.

Are you providing reinforcement in the topping (chord steel, WWR, etc..)?

 

[TehMightyEngineer]

Working for a precaster I get to see both sides of things. Agreed that they should provide explanation of their notes but I would be willing to bet money that note comes from something someone stuck on their fab drawings ages ago and they don't know where it really came from and thus have no explanation.

Overall I agree with teguci with a few other opinions.

I'd avoid specifying a specific CSP unless you 100% need precisely that CSP. For example, a smooth, glossy finished floor would be a good place to specify a CSP. For this the folks in the shop finishing the surface are not going to know what a CSP is let alone how to measure it. Give them instructions for a raked surface with an approximate amplitude that can be understood by a layman. This gives them a little flexibility on the acceptability of the finish. You could also specify an exposed aggregate surface, this is what we typically use when we need maximum bonding between pieces without a bonding agent.

I agree on the saturated surface dry requirement, but I'd also add a note to specifically restrict placing the topping if standing water is present in case they try and get lazy.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[BSVBD]

TME, as i stated in my opening sentence, the "approved cleaning and bonding procedures" note came directly from the providers (Design & Engineering) manual. Please see attached...

Thank you all for your replies.

 

[KootK]

I followed this recent thread with interest: Link. The consensus seemed to be that SSD was unspecifiable.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Teguci]

Good link - use "surface wetted with no standing water" instead of SSD. And for roughening, use raked surface to 1/4" amplitude.

 

[JLNJ]

The raked surface is effective but expensive. That said, there is some literature out there that shows the "as cast" condition produces adequate bond.

If it's all under one contract, you might be able to assign the composite/non-composite decision (and surface prep) to the contractor's team. If you are not right at the edge of a given plank's capacity, the precaster can decide if it's worth adding strands and making the topping non-composite OR counting on composite action of the topping and reducing the strand count. Make them responsible for specifying the surface prep, be it scarifying, or wetting, or a portland cement wash "bonding agent". I have done this on a couple of occasions and I believe the contractor/precaster chose non-composite both times.

This also eliminates the rather serious question of whether or not the composite section depth in the middle of the span is reduced due to camber.

 

[BSVBD]

Yes - Helpful link!

A question (provoked by link) remains... to bond agent or not to bond agent? Or is it - to prime or not to prime?

TME, in the link, you suggested a "primer". What is the purpose of the primer you suggest? Is the primer the same thing as a bonding agent?

I've heard differing opinions regarding bonding agents. I've been warned that bonding agents may actually act as a bond breaker if they set to early.

 

[TehMightyEngineer]

BSVBD: I think you've misread my post in your first reply.

TME, as i stated in my opening sentence, the "approved cleaning and bonding procedures" note came directly from the providers (Design & Engineering) manual. Please see attached...

I know, I'm stating that they should be more descriptive about this and should know why they're doing it. I suspect this note was added into their documents some time ago and the people responsible for it are no longer around. That happened a lot at the precast company I work at until I came on board; typically with us it was some engineer 10 years ago told them to put some note on a drawing, which turned into that note shows up on all their drawings.

TME, in the link, you suggested a "primer". What is the purpose of the primer you suggest? Is the primer the same thing as a bonding agent?

You're right "primer" is not the right word; "bonding agent" is what I meant (though I suppose that's really what a primer is).

As for using them I've only specified them when I had to bond to concrete that was never meant to have a secondary pour. Typically damaged areas and their repairs. Otherwise I believe that properly preparing the surface to be a more cost-effective and stronger approach. However, over a large enough job it might make more sense to just leave the concrete surface unfinished and use a bonding agent. As suggested by others in this thread bonding agents come with their own concerns and if it's not critical that I get a perfect bond I usually just use a scrub coat of cement slurry on "SSD" concrete and call it a day.

KootK: I followed that as well with interest but honestly I think SSD is fine. It's very specific but I believe most people just treat it as "make concrete wet, remove standing water, pour new concrete" which seems fine to me.

JLNJ: I would disagree about being expensive. If it's entirely to promote bonding the precaster need not be concerned with appearance and can use randomized directions of the rake and it will likely be just as fast as floating and troweling. We wouldn't raise our cost for this unless someone asked for a specific CSP or otherwise wanted very precise controls on the surface profile.

I do agree about the pushing the design to the precaster. They're (usually) on top of this sort of thing and it's becoming more the industry standard to have precasters engineer their own product similar to how prefabricated roof trusses are engineered by the truss manufacturer. Gives me a job and saves some time for the engineer, but this will raise the cost of the precast as now you're shifting engineering from the EOR to the contractor/manufacturer and this isn't free.



Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[Teguci]

I do agree about the pushing the design to the precaster.

Just talking -
To push this on the precaster would require a performance spec. Further, it would require any warrantee items to be wrapped up between the precaster, the site concreter and probably the sealant installer. If you want the precaster responsible for the finish, then don't detail a field topping system. A plant finish is often superior to any site cast finish. For hollowcore, the finish will probably be covered with a membrane or carpet anyhow since you wouldn't use hollowcore in an exposed caustic environment - good for interior floors. The field topping on hollowcore is then relegated to just a non-structural leveling system (1/2" thick at the midspan) and bonding wouldn't be that critical.

Another topping guide with ACI reference -

http://www.strescore.com/

http://www.strescore.com/Technical_Data_files/Topping Information.pdf

And the ACI reference -

http://www.bpesol.com/bachphuong/media/images/book/3021r_96.pdf

 

[TehMightyEngineer]

If you want the precaster responsible for the finish, then don't detail a field topping system. A plant finish is often superior to any site cast finish.

Teguci: By finish I was mostly talking about the raked finish between the precast panel and the topping. Not sure if this was clear as the precaster being responsible for the finishing of the topping is what it sounded like you were talking about in the quote above.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com