Clarifier Slab Construction Joint ACI 350
Clarifier Slab Construction Joint ACI 350
(OP)
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?
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?






RE: Clarifier Slab Construction Joint ACI 350
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.
RE: Clarifier Slab Construction Joint ACI 350
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).
RE: Clarifier Slab Construction Joint ACI 350
Steve
RE: Clarifier Slab Construction Joint ACI 350
What is the title of the PCA book?
Do the additional circumferential bars need added to the bottom of the membrane slab?
Thanks.
RE: Clarifier Slab Construction Joint ACI 350
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.
RE: Clarifier Slab Construction Joint ACI 350
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.