pour strips in in-situ slab on grade

[App1907]

hi everyone,
I'm working with a contractor on a G+4 level residential project with a basement. The basement parking is slab on grade with a raft slab thickness of 350mm. The area is around 160m X 65m with only construction joints and no expansion or control joints. The consultant is asking for pour strips even though it is cast insitu and not PT. Is there any specification in code books related to the requirement of pour strips? As contractors we strictly want to avoid this due to the delay in work. Are pour strips necessary in a building unit like this? Any code book reference would also be really helpful so we can hold our stand. Also would like to mention the construction site is a desert area in the middle east with high temperatures during summer.

 

[BAretired]

Sounds like a pretty large slab to pour without control joints or construction joints. The pour strips will reduce shrinkage cracking. I don't know of any building code which specifies the use of pour strips but offhand, it seems like a good idea to me.

BA

 

[JAE]

Agree with BAretired - unless you have some super-magic concrete mix design you will get shrinkage cracks over that distance and I could imagine some of them being large.

There are ways to add reinforcement, post-tensioning, shrinkage reducing admixtures, etc. to minimize the number of joints, pour strips or staging.

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[KootK]

Is the system a slab on grade over a raft slab? Or is the raft slab also the slab on grade? Does the raft slab extend over the entire building footprint? You might be able to work something out if you're intelligent about your pour sequence.

I also know of no code requirement for pour strips. But then, if designers limited themselves to only what the code specifically told them to do, we'd have lot of performance issues cropping up in our building stock.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[App1907]

Thank you so much for your inputs. The raft slab is monolithic with pile caps with the top of the pile cap at the same level as the top of raft slab. And it does extend over the entire building. WE are giving construction joints but since it is a basement floor and hence underground, we are avoiding expansion or control joints. And this is NOT a pre-stressed slab, I';m sorry if that wasn't clear from my post. Which is why I wanted to confirm if using pour strips was a general practice for normal RCC slabs.

 

[KootK]

@App1907: since this is your thread, I'm going to do my best to help you make your case here. And I believe that there's a case to be made. If I'm out to lunch, BA and JAE will set me straight.

1) The piles and caps are going to "hang up" slab movement in a way that columns in a suspended slab would not. This will reduce the effectiveness of the delay strips.

2) Unless delay strips are also cast into the basement walls, which is rare, the basement walls are also going to "hang up" slab movement. Depending on how fast you'll have the walls up, this may also limit delay strip effectiveness.

3) A suspended slab on grade will have 2-5X the reinforcement of a conventional slab on grade. This will help with shrinkage cracking.

4) Can you pour your concrete in transverse strips alternating from one side of the building then the other? This might gain you some of the benefits of of a delay strip without actually having one.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[App1907]

@KootK,
Thanks a lot. That was very helpful. Yes we argued that since this was resting on soil and not suspended above, shrinkage will be greatly reduced. We are using precast hollow core slabs for the ground floor level.
And they want continuous pour strips through the walls as well, another idea which we are not so thrilled about.
We have 5 pour sequences proposed with the first strip being divided into two for easier execution. I can talk to the project manager about your idea of alternate pouring. The gap between each pour is almost 7 days. If it is alright with the PM to execute on site then that would be greatly beneficial.

 

[App1907]

Hey guys,
The verdict just came in. The municipality has asked for pour strips so we have to stick with that now. Both in slabs and continuing through the walls. Now I have to design and provide detailing for the strip areas. Could you please give me tips on how a pour strip is designed?(width, total number of strips required, location and how the reinforcement varies there). I searched online but all results on pour strip design are related to PT slabs and not RCC

 

[hokie66]

If you are being directed to use pour strips, the details should be provided by the designer. I tend to agree with others that crack control in this type slab is provided solely by the amount of reinforcement, which will probably be around 0.5 to 0.6%Ag. Shrinkage will not be "greatly reduced", and cracking will occur, but the more reinforcement, the smaller the cracks.