SOG joints and reinforcing continuity 1

[HeavyCivil]

I got great feedback here yesterday but have additional questions about reinforced SOG's:

1. When using a considerable amount of reinforcing I understand that control joint spacing can be reduced or even eliminated. In applications where cracks are highly undesirable and both code RHO values (.0018-.003) are used in addition to moderate joint spacing (s' ~= d"*2) is it permissible to allow any bar or wire to be continuous across joints? How much?

2. Where it is reasonably certain the slab will not carry significant flexural stresses the best placement of reinforcing for crack control would be in the top third, correct?

3. Where a concrete topping is used to acheive multi-planed slopes, and joint locations are matched, is reinforcing still necessary? The reality of final WWM location is well documented when it is intended at crack control and further more the finishing and grading could be impeded by bar close to the top.

4. Will a construction joint still serve dual purpose as a control joint if reinforcing is continuous across it?

5. In instances of slab haunches, pipe (or sleeve) penetrations and other unintential 'restraints' are isolation joints necessary? Ringo and Anderson allude to isolating all restraint but that only makes sense to me in plain concrete applications so I think that is what they were refering to. What are the implications of placing isolation joints (or control joints for that matter) in a location that may experience high shear stress (relatively high- not approaching capacity though) that would be otherwise distributed across the joint?

6. Where slabs are not rellied on to brace walls or take any other loads besides local compression, the additional restraint from a ridgid connection with foundation walls would do more harm than good - correct?

Your professional input is greatly appreciated. Thanks!

-VT

 

[Ron]

It is acceptable for wire or rebar to be continuous across joints. Reinforcement does not stop cracking, even a lot of reinforcing. Put the control joints in anyway...you'll stop most of the closely spaced, tight cracks you get with reinforced slabs.

A construction joint will serve as a control joint, even if wire or rebar is continuous across the joint.

For toppings, I would use steel fiber enhancement of the concrete, match the control joints, and not add reinforcement, assuming you want a bonded topping. For unbonded toppings, reinforcement should be used.

 

[miecz]

I've inspected a few slabs with many cracks near and parallel to the control joints. In all these cases, the reinforcing was run through the joint.

 

[dik]

miecz... I've encountered similar problems. Often the cracking is due to delaying the sawcut.

Dik

 

[youngstructural]

1. There is a formula available to calculate the required amount of reinforcing to prevent a given size of cracking: IE: You can add reinforcement to effectively limit cracks to a certain size.

However; Here's the major caveat: Even when bandied about for all sorts of purposes, that formula was created for slabs on grade (thus concrete restrained one side) and has it's limits, unless you like reinforcing things to greater than 0.025, etc). And this will only ever reduce the size of your cracks, not elliminate them. It was also never meant to do away with construction and/or control joints, but to extend the distance between them.

Furthermore, as has oft been said on this forum, there are two kinds of concrete: The concrete that has cracked, and the concrete that will crack.

2. Yes, for thick slabs, but generally the ideal placement is as high as possible (ie: get your cover, then have the steel). This applies for most slabs up to 190mm, afterwhich I apply the 1/3 rule (and the steel gets much bigger!).

3. Not reinforcing a topping is a recipe for all sorts of nasty effects, including but certainly not limited to structurally insignificant pattern cracking (alligator, etc) all the way through to pop-outs. Reinforce, or make provisions for using a different technique. It's always going to be a matter of probabilities, but when pouring toppings onto a cast slab without reo you're playing roulette with your insurance policy IMHO.

4. Yes; As per Ron's answer, EXCEPT when very long and at a change in slab pattern/shape. Then you need something that will accomodate relative moments, such as a diamond dowel. See

http://www.unicon.ca/store/brochures/PNA Diamond Dowel System Info Sheet v0107.pdf

5. I think I see your question, but a sketch would be helpful to be sure. In my opinion large, particularly thickened areas, of slabs are a horrible restraint against movement, which will induce cracks. So it isn't a matter of whether there will be a crack, but of whether or not you want to have a good chance at controlling the crack location. Always isolate anything where the penetration is stiff, or you're going to find the concrete cracks. Something gives, and steel pipe vs. concrete normally results in the concrete loosing out.

6. Often, but not always. There is always something to be said to gaining extra strength when you can with some jobs, but as a general rule I separate (and properly isolate) where possible/practical.

Good luck with it,
Cheers,

YS

P.S. I agree with Ron, particularly about the construction joints. Even though many people (including *cough* sales agents *cough*) will tell you that you can add this bonding agent, or use that procedure/admix/product/etc to make your construction joints structurally "invisible", they won't. If you buy into that sort of thing I have shares available in a fine assortment of bridges and momuments you can purchase!

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

 

[HeavyCivil]

Thanks for the input- I appreciate it.

On the subject of varried opinions on the topping, one additional issue is that there are two filter 'trains' on top of it which weigh 96kips each (three tanks per train at 32kips each). The 8" slab can deal with this fine, but I am uncertain whether or not shear will develop entirely in the topping (probably causing fracture as the topping would be 3" thick at that point). Or will distrubute to the slab bellow.



I understand the topping's abillity to resist or transmit shear is unrelated to reinforcing but on that secondary note, to cast it at just over 1/8" per foot slope will be pretty difficult work, especially if there is wire or bar chaired to 1 clear of the top.

Does anyone have experience with sloping thick slabs??? I feel that pouring the slab with the slope monolithically would be probitively difficult as well, but I have no experience pouring anything sloped except side walks so I do not know.

 

[youngstructural]

You can get a fairly aggressive slope on concrete, and I think you might be surprised. It's a function of the viscosity (slump) of the mix, as well as a number of other factors (admix, equipment used, experience of the contractor, etc). I've never found a satisfactory answer regarding just how aggressive you can get, but unless you're talking VERY high slopes (like 8 deg or over), I'd talk to the contractor about what is possible.

Oh, and when dealing with thick, sloping, slabs watch out for the minimum thickness to be respected as well as remember to consider the increased possibility of differential settlement due to the differing self weight of the distributed slab.

Cheers,

YS

P.S. If anyone can point to a reference regarding single pour sloped slabs I'd be very keen to hear about it!

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

 

[HeavyCivil]

When do you use smooth dowels for construction joints and when do you simply allow deformed reinforcing to be continuous through the joint?

Even if the restraint would cause a crack, won't the location of the 'crack' fall at the joint, and therefore not really occur?

 

[youngstructural]

VTEIT: You're asking good questions, but I think that to fully satisfy yourself you're going to need to sit down with someone at your office. That said, I'm always pleased to help where I can...

I think of restraint in three fundamentally separate ways:

1. Roughness restraint: The far side (underside/side against ground or compacted fill) of the slab does not want to move with shrinkage. This is a distributed shrinkage which is commonly minimized by use of a plastic sheet underlay (often called a Damp Proof Cover or Membrane, so DPC or DPM) and getting the compacted fill as smooth as possible.

2. Thickening/Slopping/Transition restraint: The positions in your slab where the thickness changes are very effective physical restraints. They prevent the slab from shrinking smoothly, even if there a DPC.

3. Protrution restraint: Large pipes/fitting, etc penetrating into the slab and protruding into the surrounding founding materials will act to hold the slab from moving laterrally, even if only slightly so.

On to your two part question about continuous reinforcing and smooth dowels:

- If you don't cut at least every second bar/wire, you don't actually cause a control joint, but rather have a less effective against shrinking construction joint. This still doesn't guarentee that your cracks will appear there, however you are increasing the chances that it will. The reason for using smooth dowels is actually do that you can debond (grease wrap, tape, etc) one end (ie: Make it fully smooth with minimal friction) so that two slabs can pull apart, but will still transfer shear loads across the joint and *cough* ideally *cough* not have relative vertical movement between them.

- The choices between simple saw cuts, saw cuts with every second bar cut, construction joints with continuous deformed reo, construction joints with smooth dowels (remember to de-bond one end!), or a custom diamond dowel system is a function of judgement, experience, and cost.

Slabs on grade are not easy, nor are they the trivial challenge they at first seem.

And one last piece of advice: At some point, someone (or some article), some where, will tell you that you can make your slabs even cheaper by elliminating even more reinforcing, or doing away with reinforcement accross joints entirely. They'll talk about "mechanical interlock" or "Aggregate Interlock", and you will likely wind up in court. MI/AI works in the lab. After 18 simulated months of ageing. I have yet to see an in-service slab which relied on this technique which did not experience some form of serviceability failure (edge curl, relative movement, etc).

Hope that helps,
Good luck,

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

 

[HeavyCivil]

YS- thanks, that helps a lot.

On asking questions in-house: I'm the only structural designer at my firm and am pretty much self-taught so I spend a lot of time reading code and papers and responses like yours!

 

[youngstructural]

VTEIT: That's a very tough approach, and I have to say that I respect your courage in doing it. You will get a lot of help on this forum, from some very experienced guys and gals, but it's too bad you won't be getting the personal touch. Just talking through a problem can often help in nearly intangible ways...

So how long have you been practicing? Did you do anything pre-grad, or perhaps before you went to get the degree?

Happy to help you where I can,
Cheers,

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

 

[HeavyCivil]

YS-

I have a good background in all phases of construction ( residential, commercial, excavation/site, trans, surveying etc) but its all labor/forman experience, not design.

I graduated in May so I've been at this for a few months and it can be challenging but I really enjoy it - especially concrete design, its really an art.

 

[rowingengineer]

While YS has got this subject more than covered, I have posted a rule of thumb FAQ for SOG's for your information, While the rules of thumb section is still under development for the grad manual. I thought you may be interested in this particular section so put posted it for your information, I will be adding to this rule of thumb FAQ in the future.

When in doubt, just take the next small step.