Horizontal Cold Joint

[slickdeals]

Recently I was having a discussion with a contractor on horizontal cold joints in a raft foundation. There is a raft 3m thick, with 2.4m deep lift pits, creating a 5.4m section in some areas. They were toying around with the idea of pouring the first 3m below the lift pits, form the lift pits and pour the 2.4m thickness the next day or two.

Structurally, I don't see much issue with doing this as long as long as there is adequate shear transfer between the two pours, to transfer the horizontal shear resulting from base shear and longitudinal shear from flexure. There should already be plenty of "standees" that should be tying these pours together and any additional bars can be designed, as required.

Is it taboo to have such a joint, and what is the normal construction practice when building such thick rafts?

 

[JedClampett]

I'm not sure there is any standard practice for mats that thick. Is it that thick to provide mass for overturning? if so, I'd feel better if there was some bar(s) crossing the cold joint. You probably don't technically need it, but to develop shear friction, bars are required.
The problem to me is that you're likely to have a layer of laitance or other crap on the top of the first pour that would inhibit bond. It should be cleaned off, but it's bound to be a little weak. Hence the bars.

 

[hokie66]

I imagine that there are already bars crossing the joint. I would want to leave the joint intentionally rough, then make sure it is clean before casting the upper slab.

I would want to cast the entire 3 m thick main slab monolithically, so the rough construction joint at the side of the pits would be sloped 600 over the width of the joint.