Clarifier Slab Construction Joint ACI 350

[ATSE]

For a round clarifier, 140' diameter, I am using a flexible membrane slab (approx 7" thick), which dowels into the integral wall-footing (inverted T shape). Some rotation at this point is expected, with top cracking as a result.
In addition, there is a 2" thick grout topping over the slab, continuous between the hub and the inside radius of the ring wall. No expansion joints, just construction joints. See pdf attached.
1. Is it prudent to place V-groove + joint sealant (such as Sikaflex 2C) at this construction joint?
2. Even at other construction joints ("cold joints" are in a pie-shaped pattern) where rotation is not expected, how to keep the concrete cracks from telegraphing thru the grout?
3. If groove + joint sealant is used in the concrete, it seems like it must be used in the topping grout as well (aligned above)?
4. Is it ever a good idea to use groove joint + sealant below the water table at construction joints? My concern is about long term (15 years) maintenance & replacement.
5. Use fibermesh in grout?

 

[hokie66]

The grout will crack at or near the joints if the grout is not jointed. There is no way to prevent this cracking, so I think the grout needs to be jointed at the slab joints. Sealant joints should not be in a V shape, but rather proportioned according to the manufacturer's recommendations for the particular sealant used. Joint profile, backing, priming, and sealant placement are all critical in ensuring long term performance, but nothing lasts forever.

There will be random shrinkage cracking in the grout between joints due to restraint by the base slab. I doubt that fibermesh would do much to prevent this type cracking.

 

[JedClampett]

This is the second time in as many days I've seen this detail. Why don't you skip the joint and just keep the bottom at a constant elevation until the slab thickness is "T"? Provide radial joints at approximately 40 ft. (12 joints at 30 degrees) spacing at the outside wall.
Besides the crack that's going to form at the joint and transmit through the grout, do you have enough resistance to the fixed moment at the wall-slab joint?
Don't forget to add circumferential bars to resist the shear at the base (very poorly explained by the PCA Book on this).

 

[SteveWag]

Where is the water table in reference to the 7-inch slab?

Steve

 

[NoName1]

JedClampett,

What is the title of the PCA book?

Do the additional circumferential bars need added to the bottom of the membrane slab?

Thanks.

 

[JedClampett]

"Circular Concrete Tanks Without Prestressing"
The circumferential bars only need to be in the area adjacent to the wall joint. When I reduce the footing as I suggested above, I put the circumferential bars only in the thickened portion.
Once you get the reference, go to Table A-12. Multiply the shear per foot times the radius the tank. When you run the numbers, you'll end up with a total tension in the footing. You divide that by your allowable (working stress or ultimate), get an area of steel required and provide that in the footing. Then you use your judgement and provide a size of reinforcing and number to satisfy the area. I'm guessing you'll need about 20 square inches of steel in the footing for this.

 

[ATSE]

JedClampett,
Thanks for the helpful comments.

Regarding circumferential steel at the base, I wouldn't bother using the PCA tables for this, but instead ignore the beneficial cantilever action (due to fixed wall-ftg base) and also the beneficial membrane action, and calc hoop demand based on tank statics only (T=pressure*radius). Not overly conservative once you consider creep.