ACI 350 Membrane Slabs 1

[Splitrings]

Is anyone installing membrane slabs? ACI 350 seems to encourage thinner slabs as they are more flexible. We have been using 5" slabs for years for our agricultural tanks but using the subgrade drag theory for the amount of reinforcing required. This theory has been removed from ACI 360. This typically amounted to about .2% or .25% steel to gross concrete. ACI 350 requires 0.5% with no control joints. My question is about waterproofing control joints. In the past I have used a stepped control joint such as the attached picture. These slabs are not heavily loaded but I am wondering if there is a better detail. Is upper piece of concrete going to crack and break off. ACI seems to recommend a vertical joint with a groove to accept the sealant before the second placement. Any thoughts? Obviously thinner slabs will be less costly. I am cuurently working on a 225' diameter tank, so there is no way to place the slab in one placement.

 

[JedClampett]

For one, I wouldn't use a slab that thin. For water containing, we typically don't go any thinner than 8 inches, no matter what ACI 350 says. But irregardless, for the joints, I would locally thicken the slab to 12 inches, reinforce it top and bottom, and place the waterstop in the center.