Increasing Shear Capacity of Precast Hollowcore Plank
Increasing Shear Capacity of Precast Hollowcore Plank
(OP)
I've got some existing HC plank that needs a rather large upgrade with respect to shear capacity. I'm contemplating accomplishing that by breaking the planks open from the top and grouting them solid for a length. A substantial length really. This is similar to what precasters have done themselves on a few of my past projects. I've got a lot of question, however, and I'd welcome responses to any or all.
1) Where can I find the officially sanctioned method for doing this? You know, equations and stuff.
2) I'll be breaking open the tops of the plank to get the new concrete in rather than stuffing it into the ends. How does that change things? Is the new concrete just likely to lift away from the old?
3) In my heart of hearts, I kind of question the strategy altogether. I feel that vertical shear capacity and horizontal shear capacity are always and forever coupled. I envision these concrete plugs slipping around loose-ish in the core holes and therefore compromising the horizontal shear component of said coupling. On the other hand, a diagonal crack through the plank must pass through the concrete plug. So maybe it's okay? Smarter precast folks than I seem to think this checks out. Really, this is a fundamental problem that I've always had in my understanding of concrete shear. Am I worrying about nothing here?
4) There's already a concrete plug in the planks for bearing, per the detail below. So do I cast new concrete up against it and allow a cold joint between the two (disconcerting for shear)? Or do I try to remove the existing plug somehow and possibly cause issues with the bearing in the process? With the natural angle of repose of the existing plug, chances are that the cold joint would be oriented favorably. Of course I'm in no great hurry to place any real reliance on that.
1) Where can I find the officially sanctioned method for doing this? You know, equations and stuff.
2) I'll be breaking open the tops of the plank to get the new concrete in rather than stuffing it into the ends. How does that change things? Is the new concrete just likely to lift away from the old?
3) In my heart of hearts, I kind of question the strategy altogether. I feel that vertical shear capacity and horizontal shear capacity are always and forever coupled. I envision these concrete plugs slipping around loose-ish in the core holes and therefore compromising the horizontal shear component of said coupling. On the other hand, a diagonal crack through the plank must pass through the concrete plug. So maybe it's okay? Smarter precast folks than I seem to think this checks out. Really, this is a fundamental problem that I've always had in my understanding of concrete shear. Am I worrying about nothing here?
4) There's already a concrete plug in the planks for bearing, per the detail below. So do I cast new concrete up against it and allow a cold joint between the two (disconcerting for shear)? Or do I try to remove the existing plug somehow and possibly cause issues with the bearing in the process? With the natural angle of repose of the existing plug, chances are that the cold joint would be oriented favorably. Of course I'm in no great hurry to place any real reliance on that.
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.






RE: Increasing Shear Capacity of Precast Hollowcore Plank
RE: Increasing Shear Capacity of Precast Hollowcore Plank
2) I'd actually like this better than through the ends. Seems like through the ends leaves too much risk of pockets of air being trapped and creating voids. I doubt it will lift away unless you get a lot of shrinkage. The surfaces inside the cores shouldn't be buttery smooth so it should bond fairly well. I'd suspect that using a redimix concrete specifically formulated to be slightly expansive would be beneficial here.
3) I share your concerns. Doubly so in prestressed as you now have both diagonal shear and flexural shear cracking that should be evaluated. However, see my above comment about a slightly expansive cement which I suspect will help alleviate much of the concerns about getting the equivalent of a monolithic section.
4) I'd say open up a hole above the plugs as this will be the logical place to start pouring the new concrete and will allow you to inspect the plug. I agree that the angle of repose is in the strong direction so any cold-joint will be perpendicular to the shear crack. I'd check to make sure the surface of the grout plug is rough and that you can get a strong bond between the two pours. Might even be worth having your access hole wide enough for a drill and epoxy injection gun to get in there and put a dowel in the grout plug; probably more trouble than it's worth though.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Increasing Shear Capacity of Precast Hollowcore Plank
Nope. Cutting the span in half but that's another story. With hollow core, I often find that demonstrable shear capacity is a problem.
Any chance slightly expansive concrete would run the risk of busting apart what's already there? Or is this just paranoia on my part? It is a good solution to my concerns regarding non-mono-licity (sp?).
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
Maybe there's a hollowcore guy on here who knows the tricks to "demonstrate" that extra inherent shear capacity which must be present.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
Perhaps, but I wouldn't think so. The amount of expansion isn't that much of a percentage for a lot of the mixes I've used, mostly just enough to ensure it's not pulling away which is what you want here. That said, you could have some weird tensile stress combinations between the holes you core to allow the grout to be poured, the prestressing at the bottom, and the expansion forces from the concrete that could result in cracking. I'd say it's worth considering at least a little and running a few back-of-the-envelope calculations to confirm. Plus, you can always try one and evaluate it before going all out on the panels.
Wish that were I, we don't pour hollowcore at our plants. I do however have a PCI manual back at the office. If I get a minute I'll run through their equations for shear strength in hollowcore slabs.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Increasing Shear Capacity of Precast Hollowcore Plank
RE: Increasing Shear Capacity of Precast Hollowcore Plank
Agreed. The plan has been to sawcut the tops of the planks where grouting was required, including at the existing plug.
I'm not certain that this would work as the shear reinforcement would not make it out to the tension reinforcement (pre-stressing cables). Or are you contemplating new tension reinforcement higher in the section? This is another grey area for me. I've always assumed that the shear reinforcement needs to make it out to the tension reinforcement but I'm not entirely sure.
And once that is done, how is shear strength calculated? Simply as though it were a solid section?
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
RE: Increasing Shear Capacity of Precast Hollowcore Plank
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
If you aren't comfortable with the truss analogy without steel chords, maybe a sort of fabricated ladder would make sense. Most welded wire fabric manufacturers should be able to do it for you. Another brainstorm would be a steel web, using expanded metal.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
The Europeans have a code specifically for hollowcore, EN1168 "Precast Concrete Products - Hollow Core Slabs" Annex F, that includes a method for determining the extra shear resistance from the fill. This method also assumes that the additional shear strength of the fill is added to the shear strength of the unfilled prestressed plank (as defined in Eurocde 2) and is based on the additional shear width of the filled cores and the shear depth. The shear strength equation is Vcore = (phi 2/3 bcore dv fctd), where bcore is the total shear width of the filled cores, dv is the shear depth and fctd is the concrete strength assumed for design.
The Anderson method is generally more conservative.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
To go back to JLNJ's question about plain hollow core shear strength; PCI seems to just use the typical prestressed shear strength equations from ACI 318 (22.5.8.2 or 22.5.8.3 in ACI 318-14). They make no reduction for the effect of the cores.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Increasing Shear Capacity of Precast Hollowcore Plank
RE: Increasing Shear Capacity of Precast Hollowcore Plank
I let it ride. Thanks for the brainstorm suggestions. Some will be going into my schematic report.
Seconded. Thanks cooperDBM. And everybody else as well.
Good paper on the subject: Link
Yeah, I get the impression that this is kind of the source document. I've been trying like the devil to get my hands on it but, alas, no luck so far. I'm willing to pay. I found the CTA report attached that covers shear strength of hollowcore but doesn't speak to filled cores.
Jackpot! Thanks for that.
Got a better handle on what bothers me about this I think. If the plank and the plug don't act compositely for horizontal shear, then there's no reason to expect them to act compositely for vertical shear. As such, failure could go like this:
1) 100% of the shear goes to the plank, sans plug.
2) Plank develops shear crack if it can't take it all.
3) 100% of the shear goes to the plug, sans plank.
4) Plug develops shear crack if it can't take it all.
5) Shear failure.
Comments welcome.
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
That makes perfect sense and I agree is an issue.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Increasing Shear Capacity of Precast Hollowcore Plank
Yeah, it'll be slightly expansive concrete for me on this one.
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
I was actually thinking of mocking up a few scaled-down hollowcore slabs and stealing some of the concrete from our shop. I could then construct a few with filled, roughened cores and a few with filled cores with a bond-breaker. I could then put them in our concrete cylinder press and test the shear strength. Something tells me though that by the time I got around to this your project would be done. Plus I'm not sure how to simulate the prestressing force; maybe some threaded rods?
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Increasing Shear Capacity of Precast Hollowcore Plank
RE: Increasing Shear Capacity of Precast Hollowcore Plank
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
Would using a latex polymer-modified grout to fill these cores alleviate your concerns any regarding the lack of composite action?
Dave
Thaidavid
RE: Increasing Shear Capacity of Precast Hollowcore Plank
RE: Increasing Shear Capacity of Precast Hollowcore Plank
I'd have to do some research to know David. I'm not very familiar with that technology. Thanks for the recommendation.
Based on the information that I've collected from Ingenuity and others, I'm not all that concerned about the composite action at this time. My real concerns are as follows:
1) I would want to remove the existing grout plugs in order to replace them with new stuff. How difficult is that demolition?
2) I need to leave the last 4" of the existing grout plugs in place as they are providing bearing resistance for walls stacked above the precast. With there being a cold joint between the remaining grout plugs and the new grout that I will have installed for shear resistance, is that a problem with respect to shear capacity? I'm considering bolting a bearing angle on to the wall to move the critical shear plane outward a bit for this reason. Perhaps I'm just kidding myself with regard to the effectiveness of that however.
Got any thoughts on those things?
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
Agreed on both counts. The precast protects the deck above from a fire in the occupied space below. Check. But then I've had to get into fire proofing and sprinkling some weird "compartments" on occasion. Sounds as though you've been there yourself.
I'm also considering installing a network of trusses above the precast and then dropping some hanger rods down to pick up the precast near mid-span. All that would have to be fire-rated I believe. And, philosophically, that kinda throws me off. it's an example where you're required to fire rate structure that is not a part of an adjacent occupancy. Quite like the steel beams under my proposed platform.
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
Sometimes I think too rationally for architects and code reviewers apparently. I'm not sure how you'd fire rate that area without either:
RE: Increasing Shear Capacity of Precast Hollowcore Plank
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
And I hate to break it to you, but Calgary's AHJ was one of the worst I've had (albeit my experience with them hasn't been a lot of jobs, and the last time I dealt with them was a long(ish) time ago).
RE: Increasing Shear Capacity of Precast Hollowcore Plank
This sounds quite difficult if it's well-bonded. Having helped demo and repair a damaged 5'x5' area of precast slab last year it's much harder to have to demolish parts of a component using rotary hammers with chisel bits than just being allowed to go all out with the jack hammer. I'd suspect quite a bill to do what I assume is a large number of cores. Plus, you risk damaging the surface of the core you'll then bond to; perhaps resulting in less favorable bond performance than you expect? Even worse, what happens when (not if) they punch through a core or expose a prestressing strand?
I'd say only demo if you suspect the quality of the grout is not sufficient for your needs when you actually open up the cores.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Increasing Shear Capacity of Precast Hollowcore Plank
That's the thing. I assume that the original grout cores, being merely for bearing, were installed by shoving some loose material into the cores to block flow and having at 'er with some generic grout packing. The grouting wouldn't have been designed or executed with composite shear transfer in mind and I worry that at the tail end, the grout just kind of devolves into a lumpy heap with goose feathers and McDonald's wrappers stuck to it. I'm gong to be looking at core grouting up to the 1/4 points of these planks. I'd hate to go to all that trouble and then wind up barfing in my mouth when I see that, at the most critical location, my "cold joint" is really just a hot mess at an angle of repose of 15 degrees etc.
What do you think of all that? Am I being too harsh? Once I get fixated on these things, it's sometimes hard for me to back myself out to a more reasonable perspective. I know you don't do plank but this is kinda your wheelhouse.
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
Jesus, how are you dealing with the lacking moment capacity then? that's a significant required shear increase.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
This has been my exact experience with partially grouted hollowcore planks. Grout usually consists of flowable concrete with 3/8" max aggregate in my area. Newspaper stuffed into cells of hollowcore.
I have personally been on the 'working-end' of many percussion-chipping hammers and diamond-core machines where a GC required to drill vertical through-holes for services close to supporting beams where the cells were partially grouted. 99% of the time when a core bit penetrates into the hollowcore of the plank where partial grouting was done, the grout easily separates from the upper and lower 'flange' of the plank. Literally breaks into three segments, and often if you piece them back together there is a sizable void/gap at the underside the upper flange. Not to mention boney, honeycombed and rock pockets within the grout.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
No, I don't think you're being too harsh. I'm simply saying that avoiding demo if at all possible seems to be the best approach IMO. Both for cost and to avoid headaches of damaged panels. Precast panels will not take kindly to the powah' tool wielding 200 pound gorilla who has only one more core to do before quitting time. But, if there's garbage grout in there then it is what it is.
If it were me I'd require inspection of an appropriate number of cores before getting too far into the project and/or have them demolish the McDonalds wrapper grout away to establish a rough feel of how well it was bonded to the hollow core and how deep the crap actually is (assuming it gets better as you work toward the end of the panel). I'd have it in the specification that at your direction (or even an engineer hired by the contractor willing to sign off on it) any unsuitable grout found is to be demolished back until suitable material is found. I'd have this done on a time & materials basis at a pre-established rate given during the project bidding. You could get as relaxed or as crazy as you want on the requirements for what qualifies as "suitable material" in the project specifications.
I like this because it's easy for the contractor to bid and understand what they're getting into. It establishes the intent of why this should be done to both the owner and contractor. It avoid unnecessary demolition, hopefully limiting cost, panel damage, and repairs. It provides for on-site inspection by an engineer which is always a good idea. Finally, it should satisfy your needs of structural performance while still allowing for the contractor to not feel that they're demolishing "good" grout only to be putting it back again.
What say you?
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Increasing Shear Capacity of Precast Hollowcore Plank
What I'm basically saying is don't have them demolish grout that doesn't want to come quietly.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Increasing Shear Capacity of Precast Hollowcore Plank
I have another recent thread going about that very thing. Your recommendation there is actually the first recommendation made at the beginning of my feasibility report: replace the darn planks. In the other thread, theonlynamenottaken gave me what turned out to be some very apt advice: watch our for shear. With lots of $$$ to spend and an appetite for a system depth increase, I can do some moderately reasonable things to hit the target moment capacity. The plank end condition is a killer though. Gotta consider:
1) Bearing capacity for walls above.
2) Restoring in-plane shear connection to walls below.
3) Diagonal tension shear.
4) Flexural shear.
5) Accidental rotational restraint.
A lot there to go wrong really.
I say that's some quality advice TME. I'll go that route if this thing makes it past the feasibility study stage.
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
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.
RE: Increasing Shear Capacity of Precast Hollowcore Plank
We didn't have any pre-grouted cores like you do here, so I can't give you any better advice there than you already have. We drilled underneath the planks at one end, and epoxied in a threaded sleeve for a pressure fitting. We then drilled an overflow/relief hole in the top at the other end of the plank. We then pressure grouted slowly, until we got a consistent flow from the relief hole on the other end. Once that set up a few hours, we came back and low-pressure grouted the overflow area to try and touch up to ensure complete filling of the core there. This was done to retrofit the floor planks as a part of the SFRS on a lab building in Charleston, SC (a high-seismic area).
Dave
Thaidavid