Circular Tank Control Joints 1

[DCBII]

Our office has designed a 75' diameter circular concrete tank to ACI 350-06. In the mat foundation slab we have called for a circular control joint with a diameter of 32'. (8) radial control joints start at this circular joint and extend to the perimeter of the foundation. The contractor is saying it is difficult to install a waterstop on a radius. Has anyone out there run into this issue? How else would you arrange control joints in a circular slab/tank?

 

[SlideRuleEra]

I agree with the contractor, if the waterstop specified is the traditional PVC type.

One option would be to change from PVC waterstop to flexible hydrophilic waterstop, such as these Sika products: Link

Another option that may minimize design changes is to replace the 32' circular control joint with a 32' regular octagon control joint with any type waterstop, including PVC.
An added benefit - an octagon is easier to form accurately than a circle.

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[hokie66]

Why would you want control joints in a 75' diameter "mat foundation slab"? Why not just cast it all in one piece?

 

[DCBII]

hokie66,

That was one option the contractor suggested, but I'm hesitant to pursue it. The control joint spacing has been reduced to prevent cracking for water-tight structures. The reinforcement ratio determines the joint spacing per ACI 350-06 Table 7.12.2.1.

 

[hokie66]

Then did you ask the contractor if he would prefer to use more reinforcement to eliminate the joints? I would think that would be a benefit to both the owner and contractor.

 

[DCBII]

hokie 66,

I don't doubt that the original engineer probably added more control joints than were necessary as an "insurance plan" against cracking.

I think increasing the reinforcement ratio would be a good solution if we were still in the design phase, but the rebar shop drawings have already come through and were approved about 2 months ago. Changing the reinforcement this late in the game would likely cause a delay and a change order.

 

[SlideRuleEra]

hokie - Sounds like the following is not an issue on this project, but making the concrete placement in increments improves concrete screeding accuracy. Important for the rotating rake clearance in clarifiers and thickeners. Most have bottoms sloped towards the central underflow outlet, but some are flat-bottomed.

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[hokie66]

Fair enough, but my experience is different. In buildings, say a suspended floor, a pour of this size would almost always be done in one piece, and with competent screeding assisted by laser levels, floor flatness is not an issue. Neither is shrinkage cracking, if the appropriate amount of reinforcement is used.

Joints are cracks, just regular ones.

 

[DCBII]

This particular structure does not have a rake mechanism in it (although other tanks on this project do). The engineer who designed this is retired and has a lot of experience in water-containing structures, so I'm inclined to follow his lead on the control joint locations, even if they are a little excessive. I am however surprised that he hadn't run into the circular control joint issue before though.

Joints are cracks, but they do have waterstops in them. Also, if you wait between pours, you can cast the next pour against a pre-shrunk section and reduce the size of the cracks.

 

[DaveAtkins]

My experience with this type of tank is that the base slab is a single pour, but the walls are poured in lengths (curved lengths, in this case) of no more than 60'.

If your reinforcing steel is at least 0.5% of the area of concrete, you don't need any control joints.

DaveAtkins

 

[Sawbux]

I've done tanks with diameters of 75 ft. without joints - 80 ft. across octagon foundation on piles. No cracking. It had heavy top & bottom rebar mats and low W/Cm ratio.

Also have seen prestress water tank slabs with thickened edges, 5" center thickness. They used post-tensioning at 5 ft. oc. to induce about 250 psi compression in slab to prevent cracks opening up.

 

[JedClampett]

I've done a lot of these. I try to pour the center well (these are usually clarifiers or sedimentation basins) in the shape of an octagon (see below) or something like that out 15 feet or so. Then I divide the floor into an even number of segments such that the outside is less than 40 feet long (Diameter times pi/40ft; round up to an even number). And the wall joints match.
All construction joints (reinforcing through the joint), not control joints.