Joints required for concrete slab? 2

[langeng]

I am a civil engineer and have recently received a contractors submittal for a slab on grade, which I am concerned about and would like some input.
The slab is: 120' long X 40' wide with 1' X 1' grade beams every 20' across the width and also down the center running lengthways of the slab. It also has 18" wide X 24" deep grade beam around perimeter. It is 6" thick and contains #3 bars @ 16" O.C. both ways. he is proposing the use of normal weight 3000 psi concrete with no joints being cut.
My question is, won't the concrete crack excesively without the use of control joints. I have designed plenty of pavement but not many slabs so I am a bit green in this area.
I appreciate any input.

 

[Ron]

Yes it will crack. Even though reinforced, it will still crack, so joints should be cut.

 

[langeng]

Thanks Ron.
I have been doing some research trying to educate myself before marking up the plans for re-submition. Can you recomend any reference in particular?
Thanks

 

[hokie66]

It just depends on whether you want straight cracks or crooked ones.

 

[langeng]

I really don't care what they they look like as long as they not noticed without close examination. We know that concrete will crack. And I am use to dealing with it in a parking lot or a road, but this slab is tricky and I am not sure on the best way to approach it. It is up to the contractor to submit an exceptable design, but I would like to be able to talk it over with him and not come across like I don't know what I'm talking about. The suport columns line up with the grade beams and I know that the columns should be isolated from the slab and that the joints should line up with the columns, but I am having a hard time picturing in my head how this will prevent cracks if the slab is 1.5' where the joints are cut. Maybe I can suggest that he not make the grade beams monolithic with the slab?

 

[hokie66]

It would help to know the function of the building, and also the soil conditions.

With the stiffened slab proposal, it is likely that the contractor intends erecting the columns on the slab rather than through it.

Did he give a reason for the thickenings? Have you asked?

Is the contractor an engineer or does he have an engineer doing the slab design.

 

[langeng]

The building is a pre-engineered metal building being constructed for the Air National Guard and is a storage building at a Crash Rescue facility.
The top 5' of soil is soft CL clay w/ PI = 16 (the contractor is proposing the removal and replacement of this with an engineered fill Class 9, group C w/ PI between 8 & 12)
The submittal did not come on an engineers letterhead but it is a requirement that it is stamped by one before final approval.
I have not asked about the grade beams as the contract has already been awarded and the lack of joints is the first thing that jumped out at me. I have ACI 360R-06 Design of Slabs on ground reference at work but it is not very difinetive with regards to joint requirements.

 

[civilperson]

If you must use joints: Put the crack control joints, (T/2), at the center of the panels with the beams at the perimeter, (maximum spacing of 24T). The fewer the joints, the better the building. A patterned pour of alternate strips of concrete will help prevent early shrinkage cracks. The 16" spacing of panel rebar should limit the size of cracks to unnoticed.

 

[langeng]

Thanks civilperson. I get the impression that you think that the joints are unnecessary. Do they have to use joints at all? Some of the literature I have read dances around the notion that you don't have to and I feel like with it being a storage facility (nothing covering the floor),that it would look better without them. I don't want a faulty product for my client either.

 

[rapt]

#3 bars at 16" centres in a 6" slab will not give crack control in my opinion!

 

[civilperson]

The closer the spacing on the rebars, the smaller the cracks will be, (average width). 12" spacing of #3's in the top 1/2 of slab would be my recommendation with a good placement and cure process, (flooded concrete cure or heavy cure compound).

 

[langeng]

Thanks for the advise!
Can you recommend a good reference that I can find online? I don't have time to order one.

 

[beton1]

Being from a northern climate, I am assuming that this building is in the southern U.S.. This appears to be an attempt at creating a monolithic 'structural' footing/slab.
Would top steel help in the slab at the grade beam locations? You might then just pour in 3 sections (or two ends, then middle)making a 40 x 40 grid. (Then if you wanted to, you could saw-cut the balance, creating 20 x 20 square panels), or live with some cracking, this is the debate. But definitely, as mentioned, some good curing practices would be mandatory.The clay soils would concern me.

 

[Ron]

While I agree with most of what civilperson has stated, I disagree with the patterned pour recommendation. First, a 120 x 40 foot slab should be placed in one pour. Second, patterned pouring does nothing to mitigate early shrinkage cracks. Proper placement, finishing, control jointing, and curing mitigate early cracks.

As civilperson noted, the grade beams create a restraint condition, so put the joints in between them.

 

[JAE]

When I worked in the southern US we used this type of systems quite a bit for residential housing to deal with expansive clays.

This situation (with a relatively low PI and overexcavation, doesn't seem to fit the expansive clay situation, but the structural system, called a "stiffened slab-on-grade" was used quite a bit. We even once used it on a three story motel building - quite long like yours.

I would agree with many of the above posts - that the concrete might crack a bit, but in our practice, we didn't cut the joints at 3x or 4x the slab thickness like a non-stiffened slab. We usually tried to insist on #4 @ 18" o.c. each way and also sometimes introduced pure expansion joints to keep the slab sizes relatively rectalinear and of moderate size (perhaps 40 feet).

In most of these cases, the cracked slab occurred, but if the use of the building was such that the concrete was not exposed to view in the final product (i.e. carpet, tile, etc covered the floor) then we allowed the cracks and kept in the documents a provision to rout out and patch or fill the cracks once they occurred.

The use of joints is there simply to control the cracking into discrete, controllable lines that don't interfere with an exposed concrete surface in a warehouse or other such usage. In the case of hotels, etc. with floor coverings, this isn't as big an issue. You still need to keep vapor from coming up through the cracks - thus the patching, but having nice straight "cracks" in covered slab wasn't seen as that essential to us.

 

[beton1]

JAE: Good summary. Stiffened slab-on-grade is the 'term'.

 

[hokie66]

Is the same contractor also responsible for the metal building? I agree with others that the stiffened raft is probably not for dealing with an expansive soil, as you are replacing 5 feet of soil. Your comment about isolating the building columns is not appropriate for this type footing system, as the slab and footings are monolithic. If it were mine, I would saw a joint about 35' in from each end, leaving the centre 50' in one piece, with no sawcut in the longitudinal direction. I would stop half of the reinforcement in the slab at the joints, and would stop all bars in the thickenings, using smooth dowels with caps.

 

[langeng]

Thank you all for the good advise!
The stiffened slab is for differential settlement. Although we are replacing the top 5', the underlying soil is of the same character but is stiff. CL clay does not have a very high swell potential, but it does shrink (consolidate).

 

[csd72]

I have used similar construction before with no joints but we had more than twice as much reinforcement.

The other option is to split the 120' length in half or to introduce a pour strip and 2 cold joints.

 

[Ron]

langeng...check the FAQ in this section...it gives some additional info for this.