Concrete Control Joints
Concrete Control Joints
(OP)
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.
Thanks in advance.






RE: Concrete Control Joints
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.
RE: Concrete Control Joints
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.
RE: Concrete Control Joints
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?
RE: Concrete Control Joints
RE: Concrete Control Joints
RE: Concrete Control Joints
RE: Concrete Control Joints
RE: Concrete Control Joints
RE: Concrete Control Joints
RE: Concrete Control Joints
BTW: #4 @ 12" (one face only) in your walls does not meet minimum As per ACI (vertical or horizontal).
RE: Concrete Control Joints
There is now (2) layers of #4@12". (the original designer had one layer)
RE: Concrete Control Joints
RE: Concrete Control Joints
RE: Concrete Control Joints
RE: Concrete Control Joints
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
RE: Concrete Control Joints
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.
RE: Concrete Control Joints
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.