Raft Foundation on Grade Expension Joints ??? 1

[TekEngr]

Do I need to provide expansion joint in on grade raft foundation, if the area of slab is approximately 10000 sqm (100m x 100m) and thickness of slab foundation is 225mm. this slab foundation is for double storey light gauge steel hospital building.

However the maximum pouring per day is limited to 200 cum which mean there will be total 11 construction joints in whole foundation slab.

The second question is that, there is 100 mm finishing concrete on raft foundation so do I need to provide the controlled joint in finishing concrete too at same location ???

 

[Structurecrg]

You need control joints or construction joints to control cracking. For a single, square placement of 200 cubic meters, the side lengths would be (200 m3/0.225 m3)^0.5 = 29.8 m. This is too long to not provide any control joints. Rule of thumb is the joints spacing = 24x(slab thickness) up to 36x(slab thickness).

36(0.225 m) = 8.1 m. Thus you want control joints spaced between the construction joints by no more than 8m.

Lay out the pours first, then lay out the control joints. Make sure to have a joint connecting into any re-entrant corners. Also, make sure you have sufficient time between pours for the concrete to set and to prepare the joint surface. You will need control joints in the topping slab also, though probably not at the same locations.

 

[JAE]

You don't use control joints in a raft/mat foundation. These types of foundations have much more reinforcement in them compared to think slabs-on-grade and thus cracking is usually controlled by the reinforcement.

In addition, the main reason to provide a control joint is to "control" the location of the cracks in a straight, controlled line to allow joint fillers, etc. for maintenance and appearance reasons.
None of that is necessary in a mat foundation.



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[spieng89]

I second JAE...no control joints required given reinforcement will be present in a mat foundation

 

[Structurecrg]

Having reinforcement doesnt preclude the need for control joints in any case.. But I admit I was treating as a slab on grade.. The mat is going to be 100m x 100m x 225 mm (330' x 330' x 9"), with some added thickness from the topping slab - not a typical mat to me. Regardless, I would provide some saw-cuts in the topping slab to control cracking, even if just visible during construction.. I would also try to place the construction joints away from areas of max negative moment in the mat.

 

[JAE]

carrg1 - I missed the 9" thickness due to my lack of metric familiarity - and I didn't read the numbers.

Even so, with a "mat" foundation with reinforcement top and bottom, despite any thickness, the need for a CJ is only there to control the location of the cracks - not to prevent them or somehow make the mat function better.

If the mat is buried and never seen, I see no reason to worry about a diagonal crack vs. a straight crack - both of which are kept closed with the rebar.

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[hokie66]

JAE is correct. But a 225 slab is not much of a "mat" foundation. Hopefully, the OP will have sufficient reinforcement, about 0.6% Ag, to control the cracks.

 

[TekEngr]

Thanks you all for your guidance , but one part of my question is still remain what about the expansion joint for such kind of large area on grade slab/raft footing i should provide the expansion joint (however there is no expansion joint in my light gauge super structure) if answer is yes then what should be location/spacing of expansion joints however as i mentioned above 11 construction joints already will be there in whole slab.

i am using the minimum area of steel for my raft footing is 0.18% top reinforcement only (which mean 8mm dia @ 125 mm spacing c/c)and bottom reinforcement i am only placing under the load bearing wall in 1 m span as shown below (bottom reinforced calculated as strip footing method).

 

[BigH]

As some have mentioned - 9 inches (225 mm) is quite thin for a mat foundation for something of this size. You might wish to provide a thicker section at the column lines presuming you have columns at typical spacings. Then you might get away with the slab thickness. I would think that for a slab like this - you'd be looking at a minimum of 500 mm.

Question - why are you limited to only 200 m3 per day? Is this a restriction due to "noise" or due to permitted working hours or availability of equipment and or batching plant capability? I would think that you should be able to do a continuous placement with enough pumps and proper sequencing of work - if using 500 mm thick slab - it would only be about 5,000 m3.

 

[hokie66]

1) I see no need for expansion joints. Concrete shrinks, it doesn't expand except due to temperature.

2) That small amount of reinforcement will do little to control shrinkage cracks, but perhaps the cracks don't matter.

 

[mronlinetutor]

I am a newbie. i would put in control joints even though there is extra work. it looks like the job is more probably done. i see a lot of building have foundation cracks and the owner who is not a structural engineer might think that there is a building failure.

 

[JAE]

It still cracks at the joints.

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