Surfacing Options for Precast Hollowcore Plank

[KootK]

I'm helping a precaster out with the design of some hollowcore plank and some confusion has arisen regarding the required surface treatment required for the top of the planks. They could be left as cast, they could be intentionally roughened, or they they could be polished super smooth. As cast is obviously the cheapest and, as far as I can tell, there's no need for anything exceptional regarding finish.

There are two kinds of plank here:

1) Floor planks that get a 1.5" gypcrete topping only. No special treatment was specified in the contract documents.

2) Roof planks that receive no topping but only a roof membrane. Here the contract documents call for a smooth troweled finish.

So my questions are:

1) Could anyone point me to any photos of the various surface conditions? This would help the design team.

2) How would one describe the as cast finish on hollowcore planks? Would it qualify as smooth troweled?

3) Is there any justification for a roughened surface when using gypcrete topping? It's not composite, of course, but it should stay put.

4) At membrane conditions, is some degree of smoothness required in order to not damage the membrane?

When queried, the response that I got from the design team was "not smooth". I think that means as cast but I find it a bit ambiguous. All parties here --
including me -- are operating at a relatively modest degree of sophistication on this so just asking questions isn't getting me where I need to be.

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.

 

[BAretired]

Gypcrete topping is sometimes used on top of a plywood deck, so I don't think bonding is necessary. I would expect the precaster to provide a non troweled surface but would not lose any sleep over a steel troweled surface.

BA

 

[dik]

In these environs, HC slabs are extruded and have a fairly smooth surface... like a steel trowel. Bonding is not normally an issue, check with the gypcrete supplier to see if a primer is required.

For roof, is there a layer of insulation between the HC and the membrane? Also common to use 1/2" protection board over the HC slabs.

Dik

 

[KootK]

Thanks for the advice BA & dik.

check with the gypcrete supplier to see if a primer is required.

In progress.

For roof, is there a layer of insulation between the HC and the membrane? Also common to use 1/2" protection board over the HC slabs.

There is. Great point. I guess some degree of smoothness is necessary for system adhesion to the plank.

Hot off the presses, I have pics of machine cast, broom finished, and raked finished.

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.

 

[dik]

KootK

Typical HC in this area is extruded and has a smooth almost steel trowel finish... Never seen it with those surfaces, but, will check. After it comes from the behind the extrusion machine, you can walk on it... they use a zero slump concrete... it is steam cured and then sawcut to length...

Dik

 

[KootK]

Interesting. For what it's worth, this IS my plank. Not google pics etc.

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]

For surface roughness, I've always fallen back on the ICRI roughness cards. I would expect most precasters to have a set. These are especially appropriate for issues with surface prep for toppings.

http://etcmeasuringinstruments.com/portfolio/icri-concrete-surface-finish-comparators/

 

[TehMightyEngineer]

1) Agree with Teguci about the ICRI cards.

2) I would definitely say that it does not qualify as "smooth troweled".

3) In my experience a raked finish is only needed with composite topping without reinforcement at the interface. For other surfaces that require bonding but not composite action we use as-cast with moistening the plank to SSD and a scrub coat (or perhaps a bonding agent coat). You could also use powder actuated fasteners as shear reinforcement but that seems overkill to me and I'd just do a raked finish before I went that route.

4) Never had any issue with a membrane on troweled surfaces, but never really given this much thought either. Beyond just "no holes or pointy spots, please" I don't believe you'll have a problem.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[dik]

If you're looking at putting any type of topping on it, the roughened surface will not be a detriment. I'll check and see if roughened slab is readily available in these environs...

Dik

 

[JLNJ]

I would think the difference caused by differential camber between adjacent planks would be a bigger issue than the as-cast smoothness. Troweling seems wholly unnecessary.

There is some literature out there that says the as-cast condition is appropriate for composite topping action. I'm inclined to agree with this (except maybe for severe conditions such as a parking garage).

I have taken to requiring the precaster specify whether or not the topping is composite and, if composite, he specifies the topping surface treatment prior to topping placement. This also forces him to consider the reduced topping thinkness where the camber is max and the corresponding reduced section modulus. I think usually they find it easiest set aside the surface prep/camber/section thickness questions and just design the planks as non-composite.

 

[KootK]

Thanks all for your help on this. It's mostly resolved but I thought it prudent to report back on a few items.

Gypcrete topping is sometimes used on top of a plywood deck, so I don't think bonding is necessary

Indeed that is correct. I contacted the Gypcrete folks and they confirmed that they have no requirement for the surface at all. Their spec for hollow-core is attached if anyone is curious. It doesn't speak to this particular issue however.

For surface roughness, I've always fallen back on the ICRI roughness cards

New to me. Thanks for bringing these to my attention.

Never had any issue with a membrane on troweled surfaces, but never really given this much thought either. Beyond just "no holes or pointy spots, please" I don't believe you'll have a problem.

I agree, I would not have thought this a big deal. This aspect is still being debated on this project, however, as the architect is so far sticking to his guns wrt to the trowel surface. There doesn't seem to be anything for me to reference here so I've stepped back and am letting the architect and precaster sort it out themselves.

I would think the difference caused by differential camber between adjacent planks would be a bigger issue than the as-cast smoothness. Troweling seems wholly unnecessary.

Also a great point that I hadn't considered.

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.

 

[damorim]

The issue with the smoothness and membrane has become a VERY hot topic on a project we are currently working on. It is for the rehabilitation of a potable water reservoir that was originally constructed with a hollowcore slab roof. The roof was never insulated (left open to Manitoba seasonal conditions) and as such, deteriorated quickly. We identified many badly deteriorated slabs for replacement. The new design calls for an EPDM roofing membrane to be applied to the hollowcore roof as part of the new "roof envelope". We were receiving some very rough, jagged, slabs of simply extruded hollowcore which were all rejected due to concerns with puncturing the membrane. The protrusions had to be ground down on site to obtain a desirable smoothness. Although we finally got the surface finish to a quality we are happy with for applying the EPDM membrane, we couldn't find any true specifications out there for what is an acceptable surface finish. I agree with using the ICRI roughness levels as a specification if you know exactly what you want...