2.5m thick concrete raft - poured in stages

[neil254]

I am currently designing a piled raft which for a number of reasons will be poured in 2 stages. An initial 1.5m thick pour and then a second 0.9m thick pour on top. I would expect there will be a time gap of 4 weeks or so between the lower and upper pour. The top of the lower pour will be roughened prior to the placing of the upper pour. I have carried out a check for thermal cracking for the lower pour and provided the required area of reinforcement. I am concerned about the thermal contraction of the upper pour (which will be restrained by the lower section of raft) causing thermal cracking. I was therefore wondering if anyone had ever had to pour a raft slab in stages such as this and if so, how you ensured that the sections could be considered monolithic? Did you provide vertical bars between pours and if so on what basis did you size what is required? or some other reinforcement arrangement?

Any pointers would be much appreciated.

Thanks,
Neil.

 

[hokie66]

I have never done such a thing, but agree with you that it is problematic. Because the upper slab is heavily restrained, it will require more reinforcement if shrinkage cracking is to be controlled.

 

[slickdeals]

This is not very uncommon. There are situations where the contractors like to pour the bottom of the lift pit first and then do the second pour above it. You can check for the composite action (horizontal shear transfer) and if it's not adequate based on a roughened section, you can provide additional vertical reinforcement.

Are you providing additional top reinforcement at the top of the 1.5 m pour? What is the basis for your thermal cracking calculations and how did you calculate the reinforcement required? I am curious.