Water Tank Slab Joints

[TonyES]

I am looking for some assistance on slab / foundation joints for use in a water tank.
The tank wall has 12” cast-in-place concrete walls with an ID of 30’. I currently show a thickened continuous ftg that is 6’-6” wide x 1’-0” thk transitioning at a 3H to 1 V slope to a 6” slab (all poured at the same time). The slab is reinforced with #5’s @ 12” oc EW. I currently show (2) tooled/saw cut joints running each direction at 15’-0” apart. I show the rebar running thru the joint with a cast-in waterstop.
It seems like there are varying details…
- To run the rebar through the joint, to stop all of the rebar short of the joint, or to run half of the rebar through the joint
- Some don’t provide a waterstop, but it seems like ACI 350 is pretty clear that it is required
- It seems like a good idea to provide a joint at the thin point of the ftg to slab transition, but that would be a circular joint. I’m not sure that the specified waterstop can bend at that radius or how easy that will be to cut/tool in.
- Because the slab meets the min. reinforcing of ACI 350 for joints at 37'-6" (max diameter including the footing), does that mean that no joints are required as there is enough steel to hold the cracks tight? Any opportunity to skip joints seems like a good choice as those are typically the problem areas.
- Is there a better alternative altogether? All of my previous tanks were much larger (105’ ID) and used a post-tensioned slab with no joints. Because this tank is so much smaller, it didn’t seem worth going to post-tensioning.

 

[JedClampett]

I'd pour the slab in one pour and skip the joints. Make sure you have at least .005 reinforcing ratio. Use a starter wall at the slab/wall joint if there's any chance the contractor will do it right. Otherwise use a thicker slab and lower the top reinforcing to miss the waterstop. Do not use any tooled joints. You don't want to encourage any cracking at non waterstopped joints.
But the circumference of the wall is over 90 ft. I'd add at least two joints in the wall or even three. Make sure the waterstop is welded to the slab wall horizontal waterstop.

 

[Splitrings]

We use as few cold joints in our tank slab as possible. If it is practical we will do it in a single placement. Appendix H of ACI 350-06 has some good information.

Its going to be pretty tought to get adequate clearance for the waterstop if the reinforcing is continued through the joint. If the reinforcing is placed at the midheight or slightly above than you don't have 2" of cover. We discontinue reinforcing at the joints.

We always cast the footing and slab separate. We design our footings and walls assuming the footing is allowed to rotate. We most always use a bentonite waterstop but we are storing agricultural waste and not water. I have read about concerns with bentonite waterstops not functioning properly in water tanks. We have not had any issues but the waste likely has some sealing (plugging) properties unlike water.

We used to use the old subgrade drag theory to select the reinforcing for our tank slabs. Now that it has been pulled from ACI 350 and 360 we are using the new recommendations in 350. The fact that the table only requires 0.005% for lengths greater than 40' is bothersome to me. Tough to understand why its adequate for 40' joints and also 225' joints? Obviously the shrinkage stresses in a 225' joint spacing are considerably different than with a 40' joint spacing. Then if you cast your footing and slab monolithically, the footing and wall add significant restraint that is not accounted for.

 

[hokie66]

Splitrings,
It is .005Ag, not .005%. My preferred number is .006, but .005 should work reasonably well. Why do you have a problem with longer lengths? The concrete will crack at about 10' intervals, so the overall length doesn't matter.

 

[dik]

If for water, consider using bentonite 'rod' in lieu of a PVC waterstop.

Dik

 

[Splitrings]

Hokie,
It is 0.5% as you mention. I was typing one thing and thinking another. Why do you think slabs crack at 10' intervals? In order for the concrete to crack, its tensile strength must be exceeded. A slab on grade with proper support and little load causing bending should only be stressed by shrinkage and temperature. A long slab will have more restraint to shrinkage from the subgrade than a short slab.

dik,
You mention bentonite "rod". We have used the half round sections of bentonite, about 3/4" in width. I believe I have seen sections that look more like small diameter rope too. Is this what you are referring to?

 

[hokie66]

Splitrings,
The 10' interval is just a personal observation,could be 12 or 15 depending on a lot of variables. But if you construct a slab on ground with joints at 30', and cracks occur at 10', then cracks in a long slab constructed the same way will occur at roughly 10' as well. The restraint force stops at each crack or joint.

 

[Splitrings]

Hokie66,
My intial assumption (which is incorrect) was that the intent of the 0.5% was to prevent cracking. This is why I couldn't understand why the table stopped at 40' spacings. So, obviously the ratios they have choosen are based on a max assummed crack spacing in order for the steel to remain in the elastic range. Thanks for your thoughts.