Concrete Control Joints

[mrpid]

I am reviewing a set of pit drawings, and I noticed there are not any control joints. The pit is 120'x15'x6'deep. I discuss with the designer and he mentioned that he has never used control joints. Then I ask of crack scenerios and the reply was something to the affect that yes it cracks, but he is not concerned. I am curious as to other thoughts of omitting control joints. I am under the impression it is not a good idea. And certainly not recommended by the ACI.

Thanks in advance.

 

[frv]

By pit, I assume you mean something like a slab on grade.
I don't think joints are strictly necessary from a structural capacity perspective. However, the slab will crack randomly instead of in a controlled manner with CJ.

I don't know if they are required by ACI, but as you mentioned, they strongly suggest their usage for every 225 ft^2 of slab area (15x15) max.

Just seems like poor engineering practice not to provide for them.

 

[lkjh345]

What kind of pit is this?

With enough steel in any concrete, cracks will be kept small. If hairline cracks are not a concern, and there is sufficient rebar in the concrete, this can work.

 

[hokie66]

mrpid,

I imagine you are concerned about the lack of control joints in the walls, not the slab. There are a lot of reasons for including control joints, and probably the same number for omitting them. It is impossible to comment about the designer's decision without knowing more about the structure's use, reinforcing quantities, etc. Is this a water retaining structure?

 

[mrpid]

Thanks! Something I needed to clarify as I was considering the vertical CJs in the pit wall. The walls are 6' tall(12" thick...overkill, I know) and I propse a WCJ every 20'.

 

[mrpid]

It is a truck scale access pit, water not a major concern (Low water Table), the scale is supported independent of the pit, so the wall is only acting as a retaining wall. Originally #4 at 12" hor/ver on inside face only...I suggested EF (or at least the face with tension).

 

[frv]

My post becomes void. Never mind.

 

[hokie66]

I think more than suggestions are required if there was no reinforcing specified on the tension side. If you are not the responsible designer, you should insist on someone who knows what they are doing to do the design and resubmit.

 

[mrpid]

Thanks, actually I was trying to avoid a long story (and believe me it is long) as I am now the responsible engineer; however, the original designer (because he is the supplier of the scale and will be the contractor) still has the ear of the client, so my use of control joints has come into question. My reponse has been simply "I highly recommend them, but if you choose not to these are the consequences (primarily unpredictable cracking); however, since it is not a structural integrity issue, I can be flexible on the issue, but cannot be held reponsible for uncontrolled cracking." I was using this forum to hear other comments on the WCJs, without getting to involved in the politics of my situation.

 

[MarcbSE]

FWIW ACI requires two mats of steel whenever the wall is greater than 10" thick.

 

[tngolfer]

If noone will see the cracks or if the randomness of them is not a concern I would not be too worried about CJs. I would ensure there is enough steel (ACI minimum) at least to make sure the crack width does not become very large.


BTW: #4 @ 12" (one face only) in your walls does not meet minimum As per ACI (vertical or horizontal).

 

[mrpid]

Thanks for the comments.

There is now (2) layers of #4@12". (the original designer had one layer)

 

[civilperson]

ACI does not give guidance on the frequency or necessity of contraction joints in ACI 318.

 

[mrpid]

ACI committee Report 224.3 has "recommendations" which are useful.

 

[civilperson]

Report 224.3 is for slabs on grade.

 

[DaveAtkins]

The CRSI Handbook says that if you use 0.005bh for area of horizontal steel in a wall, you can go an unlimited length without control joints. As others have mentioned, the wall will crack randomly, but the cracks will be tight.

If you use the ACI minimum of 0.002bh for horizontal steel, without control joints, the wall will again crack randomly, but I imagine the cracks will be wider.

I just talked to a contractor yesterday who thinks control joints don't work very well. He says he has put crack inducers on both sides of a concrete wall, and has still seen the wall crack a foot away from the crack inducers.

DaveAtkins

 

[hokie66]

I would agree with the contractor that crack inducers in wall are unreliable. But full depth joints work.

Cracks will form in restrained walls at about 3 metre centres, so if control joints are not spaced that close together some cracking will occur.

The Australian code varies required horizontal reinforcement according to exposure and required degree of control: .0025bh for minor degree of control, .0035bh for moderate, and .0060 for strong control.

 

[mrpid]

Thanks again as all this information is useful.

Civil Person,

ACI 224.3 R-95 Chapter 8 covers walls. I do not know if this is the latest and greatest version, but it is the one I have access.

Dave/Hokie,

Thanks for the additional references. Good arguments for the owner "double the steel or add a couple of strips inside the form (and maybe a waterstop depending on function)"

From my field experiences and general everday observations (as an engineer you begin looking around at the details of other construction), I have seen it work effectly and other times not so effectively. Based on this discussion there are several items that can improve their effectiveness. Not only spacing but location seems to be critical in my observations. I prefer to place the joints at locations a cracked would occur naturally (wall pentrations, openings, near piers that are integrated with the wall, and near corners). I do not know if this is the best approach, but it seems to help.