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Control joints in slab on grade

Control joints in slab on grade

Control joints in slab on grade

I have a slab on grade in an industrial structure, and I am trying to figure out where to put my control joints.

The slab is 9" thick, and has two layers of reinforcing, each with #5 bars on a 12in grid. The slab is continuous with the strip footing around the perimeter of the building for overturning reasons, so no control joint can be placed around the perimeter. The building is long, and ~40ft wide.

In order to avoid putting control joints underneath machinery, I would like to run a single control joint down the middle of my building making the joint 20ft away from the strip footing. The slab can not shrink away from the strip footing, so the effective length of slab doubles to become 40ft.

Is this design feasible? Do I need to do something special to make the single control joint system work?

RE: Control joints in slab on grade

The recommendation for unreinforced panels is generally 4.5m and yours is almost three times this. However with appropriate reinforcement you could make it work. Smaller reinforcement at smaller spacings is more effective in distributing shrinkage cracking. However you only speak of a joint in one direction. You will also need them in the other direction and at the edges in both directions you will have to cut through your footing which is going to be a tricky detail. You will need some kind of dowel detail to prevent differential settlement across the joint.

A useful formula from PCI for reinforcement required to control cracking is

p(percentage reinforcement)=1.77L(length between edges)/S(allowable steel stress).

The most important thing in a concrete floor slab and the hardest to control is the quality of workmanship including mixing, placing, vibrating, curing, floating.

Carl Bauer

RE: Control joints in slab on grade


The current May-June 2002 issue of the ACI Structural Journa has a technical paper, No. 99-S37,titled "Shrinkage-Based Analysis for Control-Joint Spacing in Slabs-on-Ground".  The article covers the subject very well and has a list of 13 references.  It does not carry the weight of a code provision yet but, it does give a way of computing the necessary crack control.  Table 1 lists the PCA recommended joint spacing for a 9" thick unreinforced slab with a maximum course aggregate size less that 3/4" as 18'.  You should also note carlbauer's comment, joints are required in BOTH directions.

RE: Control joints in slab on grade

Thank you carl and jheidt for your advice.

Just to clarify, I do have joints in both directions. The joints in the short direction are easy so I have not mentioned them.

Carl, you said that I need joints along the edges. I would like to be able to mobilise the moment connection between the slab and foundation - in fact I have seen this detail in other projects - but placing a joint around the perimeter would kill the idea. Why do I need perimeter joints? Also, what are the units of your slab reinforcement formula?

Jheidt, it is interesting to know that the slab will go 18' with no rebar...my firm is mostly a bridge design firm and I don't have immediate access to the ACI Structural Journal. Does the article mention how much extra distance you get by introducing rebar?

Another question: I may have to put my slab joints under the machinery. The machinery in question is for an electrical substation. It is switching boards and transformers mostly, and is quite heavy. I am keen to ensure that the joint does not cause any problems for the machinery...Will a normal dowel joint be acceptable?


PS: carl: I noted from your little CV that you worked with Arup in Botswana. Did you ever meet Eugene De Souza? He was quite senior in the Arup Zimbabwe office before all the violence errupted there, and he moved to America. I worked with him briefly in the Arup Detroit office in 2000 (I'm now with another firm in New York).

RE: Control joints in slab on grade

Sorry of course you need the units. L is in metres and S is in MPa. The formula assumes a coefficient of friction of 1.5 under the slab.

What I meant about the joints was that they must go through the edge (perpendicular to the edge) not along the edge. Because your slab is joined to your footing this means the joint must go through the footing too (also perpendicular to it). My concern with that is that you then have to dowel the footing.

I heard of him but never met him.

Carl Bauer

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