Using Concrete as Filler in a Tank Cell

[MrEngineerUS]

I have a client that would like to completely fill a 8'x8'x8' cell in an existing multicell tank and cap it with a reinforced concrete slab. The cell shares two walls with neighboring circular tanks on opposite walls. The other two opposing walls are straight. The intention is to fill the void and cap with a slab to prevent water infiltration into the unused cavity of the cell. The cell was previously filled about halfway with concrete that had formed channels in it to direct flow. The modification uses piping so these channels are no longer required.

The fill this client would like to use is concrete. When filling such a large void with concrete what sort of reinforcing is required, if any? Should the concrete fill have reinforcing bars drilled into the existing cell walls?

If they were to use compacted stone there wouldn't be reinforcing so why use it when filling with concrete? This just seems like a weird detail I was given so I was wondering if there were any engineers with experience in this type of modification to an existing tank structure. I would think that filling the void to the prior height then capping with a slab and post-installed waterstop would be the preferred method. This would maintain the prior design loads on the existing tank, more or less, and still prevent water infiltration.

In any case, I'm still curious on if in this situation any reinforcing or dowels into the existing structure would be necessary.

Thanks for any help!

 

[JStephen]

I can't think why you'd need to reinforce it.
Consider using flowable fill; supposedly, it can be dug out later if needed, instead of jackhammering it out.

 

[SlideRuleEra]

Do not use reinforcing steel or dowels. If fact, keep the fill concrete 28 day compressive strength low, say well under 3000 psi. Low strength concrete requires less cement, less cement means less shrinkage as the concrete cures.

Keep in mind that even 1000 psi concrete means an ultimate bearing pressure of 144 kips / ft[sup]2[/sup] (1000 psi x 144 in[sup]2[/sup]/ ft[sup]2[/sup]) - plenty of support for the structural slab.

I agree with JStephen on use of flowable fill.

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[JedClampett]

I don't like using structural concrete here either. Sure as shooting, the next modification will require the cell being empty. Getting out anything cementatious will be a major pain. I prefer adding a structural slab with an access opening. But I can see why your client doesn't want that. Of all the choices, CLSM seems the best.

 

[MrEngineerUS]

Thanks for the comments. I'm sticking with the CLSM detail they want to use but advising against putting in bars on the basis that 1) they aren't necessary, and 2) drilling a bunch of holes in a tank wall when they aren't required is asking for trouble and should be avoided, if possible.

I'll suggest capping with a slab on grade over the fill with some sort of joint seal or post installed waterstop around the edges.

Who knew a big chunk of concrete required so much thinking!

Cheers!

 

[cvg]

you might look into cellular concrete instead of normal CLSM. It will flow just as good if not better, less weight, possibly cheaper and less or no shrinkage