Joints required for concrete slab?
Joints required for concrete slab?
(OP)
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.
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.





RE: Joints required for concrete slab?
RE: Joints required for concrete slab?
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
RE: Joints required for concrete slab?
RE: Joints required for concrete slab?
RE: Joints required for concrete slab?
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.
RE: Joints required for concrete slab?
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.
RE: Joints required for concrete slab?
RE: Joints required for concrete slab?
RE: Joints required for concrete slab?
RE: Joints required for concrete slab?
RE: Joints required for concrete slab?
Can you recommend a good reference that I can find online? I don't have time to order one.
RE: Joints required for concrete 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.
RE: Joints required for concrete slab?
As civilperson noted, the grade beams create a restraint condition, so put the joints in between them.
RE: Joints required for concrete slab?
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.
RE: Joints required for concrete slab?
RE: Joints required for concrete slab?
RE: Joints required for concrete slab?
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).
RE: Joints required for concrete slab?
The other option is to split the 120' length in half or to introduce a pour strip and 2 cold joints.
RE: Joints required for concrete slab?
RE: Joints required for concrete slab?
Dik
RE: Joints required for concrete slab?
Dik
RE: Joints required for concrete slab?