Continuation deformed reinforcing bars across sawcut contraction joints 5

[hetgen]

The detail below is for a 9.0m x 9.0m [30 ft x 30 ft] slab-on-grade (slab on ground) panel, my question is won't that amount of steel across the saw-cut joint completely lock the contraction and risk a random crack to form elsewhere?

 

[Blackstar123]

Yes, if large percentages of deformed reinforcing bars is continued across a saw cut joint, it will restraint the joints from opening as the slab shrinks. This will increase the probability of out-of-joint random cracking.

 

[ChooChooBridgeEngineer]

I've typically seen this detail where the reinforcing amount is reduced by half at those locations. Whether that methodology is effective, I leave it up to others to debate that, right or wrong.

 

[r13]

The note shall say "Discontinue/Splice Alternate bars..." to meet the intent. Otherwise, what difference it makes from letting all bars continuous across, but to add labor costs.

 

[jayrod12]

wow 29m bars @ 250 bottom for a slab on grade. For a 350 thick slab on grade no less. That seems like an extreme amount of bottom reinforcing for a slab whose more critical tension face is the top usually.

 

[WARose]

Atypical compared to the detail(s) I am use the seeing for a slab on grade, saw cut joint. Typically I see a welded wire mesh and a note saying something like "cut at joint" or "cut every other wire at joint".

Of course there are a lot of options.

 

[Agent666]

For that sized pad, and that reinforcement. I wouldn't even bother with saw cuts if it was unrestrained at the slab edges.

 

[Gopher13]

No answers here......just more questions.

The case for continuing the reinforcing through the sawcut joint: If the slab thickness and reinforcing remains the same throughout the area and a sawcut is made, that section through the slab where the sawcut is would now be the weakest point, right? So the crack should form at the sawcut and not a some other random location where there is no sawcut.

(slab thickness "X" with reinforcing "Y") > (slab thickness "X - sawcut depth" with reinforcing "Y")

 

[Agent666]

That's usually the general idea, sawcut acts as a crack initiator.

 

[Gopher13]

The case for continuing the reinforcing through the sawcut joint: If the slab thickness and reinforcing remains the same throughout the area and a sawcut is made, that section through the slab where the sawcut is would now be the weakest point, right? So the crack should form at the sawcut and not a some other random location where there is no sawcut.

(slab thickness "X" with reinforcing "Y") > (slab thickness "X - sawcut depth" with reinforcing "Y")

I guess what I am asking is if everything else is the same, isn't it unnecessary to make some of the reinforcing bars discontinuous at the sawcut joint?

 

[ChooChooBridgeEngineer]

Concrete is a notably non-homogenous material. In a perfect world, you would be theoretically correct, but in reality, you have to do more to make sure that the concrete cracks through the saw cut joint.

 

[KootK]

I guess what I am asking is if everything else is the same, isn't it unnecessary to make some of the reinforcing bars discontinuous at the sawcut joint?

You're really aiming for two things with the detail:

1) Initiate the first crack where you want it. The sawcut alone, without the rebar reduction, serves this purpose.

2) Allow the first crack to strain enough that it prevents the concrete between joints from picking up enough tension to random crack. This is where the rebar reduction kicks in as less rebar means less resistance to the opening up of planned cracks.

All that said, it's not like we've all been calc'ing out how much rebar should be run through the joint to accomplish this and magically landing on 50% every time.

I favor Agent's no joint approach for this given that all cracks will likely be small and no joint detail is likely to make much of a difference.

 

[Gopher13]

Ah yes. Makes sense. Thanks for explaining!!!

 

[r13]

Fully agree with point 2 of KtooK's explanation, and the last sentiment. But in general, it is just an added insurance on a random event.

 

[novembertango88]

Technical Report 34: Concrete Industrial Ground Floors, has a good section on joints and says that for sawn restrained-movement joints typical steel area is 0.08% - 0.125% (cl 11.5.1) and that its assumed that the reinforcement across the joint will yield as the slab shrinks.

 

[hetgen]

Thank you all for your help.

@Agent I agree with you, why bother with saw cuts with ~1.0% reinforced slab on grade.... PCA recommends ~0.6-0.7% reinforcement for a 'continuously reinforced concrete pavement' with no joint.

The slab is for an abandoned aircraft hangar project which is now coming to life again, the design was done by a reputable consultant, and the drawings are well detailed. But I'm not sure why they proposed this detail. This detail doesn't even allow for sufficient saw-cut depth (i.e. 1/4 to 1/3 x thickness of the slab).

 

[dik]

For SOG construction I usually carry all steel through the sawcut... and cut the slab above the rebar... only for tunnels, etc. do I terminate half the reinforcing at the control joint. Time of sawcut is critical. Just noticed the bars are huge and I would likely sawcut through 1/2 the depth of the top bar.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[hokie66]

I concur with Agent666 that no control joint is needed in this case. And also that the joint detail is strange and illogical.

jayrod12 raised the question of why there is so much more reinforcement in the bottom than in the top. That is indeed atypical but not completely illogical. Perhaps the designer was just being cognizant of the fact that the main restraint to shrinkage, thus the applied force, is at the bottom.

 

[IDS]

Perhaps the designer was just being cognizant of the fact that the main restraint to shrinkage, thus the applied force, is at the bottom.

Or maybe the slab is a mining dump slab, or similar, subject to very high wheel loads, which can generate significant tension in the bottom face.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Tomfh]

Don’t bother with a saw cut in this case.

I’m somewhat skeptical of those “cut every 2nd bar” saw cut details.