×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Circular Tank Control Joints

Circular Tank Control Joints

Circular Tank Control Joints

(OP)
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?

RE: Circular Tank Control Joints

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Circular Tank Control Joints

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

RE: Circular Tank Control Joints

(OP)
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.

RE: Circular Tank Control Joints

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.

RE: Circular Tank Control Joints

(OP)
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.


RE: Circular Tank Control Joints

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Circular Tank Control Joints

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.

RE: Circular Tank Control Joints

(OP)
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.

RE: Circular Tank Control Joints

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

RE: Circular Tank Control Joints

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.

RE: Circular Tank Control Joints

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources