Do large Mat foundations require expansion joints? 1

[Robert-32]

I have a Mat of about (70m * 28m * 1.2m) size. It will be designed as Raft on Piles. Should I introduce an expansion/ Contraction joint? If the soil is homogeneous, do we need a settlement joint? The location is in a hot climate of 45 Celcius in summer and the depth is about 7m but during construction, it will be exposed.

 

[JedClampett]

Kind of depends on what you're supporting. Expansion joints create two independent slabs. The structure above the slab will need to correspond to the jointing system of the slab. I'd avoid them, but 70 meters is marginal.

 

[Robert-32]

Thanks a lot Jedclampett, The raft is supporting one underground 6m heigh basement, and four floors on top of a part of it.

 

[dik]

That's a substantial foundation for a four storey building. You may have thermal issues with a slab that thick.

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So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[hokie66]

Agree with Jed. Avoiding movement jointing is best, but casting that amount of concrete in one go will be challenging. In your location, will you have adequate production and placement capacity for 2300 m^3 at one time? Probably not, so you will have construction joints. Managing fresh concrete temperature in the hot weather conditions will be critical.

 

[Just Some Nerd]

The thickness of the raft seems...extreme? Is there a particular loading on the basement, large pile spacing, or water pressures being resisted that necessitate a slab that deep? Having an expansion joint is not a great idea if you need to keep everything watertight in the latter case

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Why yes, I do in fact have no idea what I'm talking about

 

[Robert-32]

Correction: Future vertical expansion is possible up to Basement+6. Also the Raft thicknesses is 1.2m at max punch and 0.6m everywhere else.

 

[JoshPlumSE]

Correction: Future vertical expansion is possible up to Basement+6. Also the Raft thicknesses is 1.2m at max punch and 0.6m everywhere else.

Ahhhh. That makes perfect sense now.

My thoughts on this subject:
a) I don't think I've specified this for a truly structural slab. But, rather than a true expansion joint, can't you just add a "crack control" joint to force the cracking to occur in certain areas. I see this all the time for slabs on grade. Something like a 2" saw cut every 10 meters or so?

b) Alternatively, you mentioned settlement joints. Why not pour the structural support foundations for columns and walls separately from the slab. Then pour a slab on grade on top of them (with diamond cutouts and such to allow for settlement of the foundations without cracking the slab on grade). Or do you really need the main slab to carry structural loading?

c) The reason why I mention item (b) is that you are almost doing this already with all the thickened areas of slab. You MIGHT even save some cost on concrete, forming and excavation.

 

[canwesteng]

Is there an expansion joint in the building? I wouldn't include one in the mat if not. I think much beyond 6m, you are going to get shrinkage cracking either way, you need the rebar to restrain it regardless.

I just poured a 90m long footing without any expansion joints, but went with .6% T+S bar. After curing there is repeating minor cracking as expected, but no real issues. I would be surprised if a control joint achieved anything on a mat 600 thick, but the superficial cracking isn't a concern as long as it isn't exposed to view.