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Contraction joints in concrete retaining wall

Contraction joints in concrete retaining wall

Contraction joints in concrete retaining wall

Hi Engi dudes
In a tennis court construction project I am working on, it calls for a 240mm wide by 300mm high concrete plinth surrounding the court hard surface. The length of the concrete spans of the plinth are 100 x 50 feet (the outside dimensions of the court).  Penetrating the center line of the plinth are the fence posts which have a 700mm embedment into sub surface and these are spaced every 3.2 meters.  

How often do I need to place contraction joints in the plinth span and where should these be located (amidships between the fence posts or at the fence post)?

How should the contraction joints be best constructed (cutting the two reinforcment bars (placed within the plinth) or routing a groove in the concrete during curing?

How do you groove the surface that will be covered by the boxing form?

Appreciate any comments.  

RE: Contraction joints in concrete retaining wall

Hopefully, I'm envisioning this correctly. Is any portion of the plinth (I'd call it a curb)above grade? If not, put a bondbreaker around the fence posts and then tool a joint in the top concrete surface at each fence post. This way the cracks form at the posts. You could also use a premolder filler material around the posts, which have to be installed before you do the curb.

Is your contraction joint actually a control joint? People have different definitions of contraction joints. Usually with a contraction joint the bars don't pass through it, unlike a construction joint, or a control joint, which is intended to provide a location where a crack can form without being easily seen.

If the curb is above grade you should have vertical control joints in addition to a tooled joint across the top. The control joint is created with a piece of wood - triangular cross-section- placed in the form. I would put one at each post. I wouldn't tie the corners together.


RE: Contraction joints in concrete retaining wall

Agree with BB...put the joints centered on the posts...it will crack there anyway because of the reduced cross-section.

You can form the joint by putting a small strip of wood or styrofoam adhered to the form.  When the form is removed, you can remove any wood or foam that is stuck to the concrete.  One simple way to do this is to use closed cell foam backer rod (3/4-inch diameter, cut in half).  Glue it to the form (as straight as possible, then tool a corresponding profile in the concrete at the top, unformed surface.  Dragging your finger through the concrete along a straightedge will suffice, since it will be almost the same profile as the 1/2 backer rod.

When you pull the forms, you'll likely see a fine crack develop at the formed profile.  You can then fill the profile with sealant to protect and hide the crack.

I know it is not the desired profile for a sealant joint, but it will work in this "non-critical" application.

RE: Contraction joints in concrete retaining wall

Thanks BB and Ron.  

To clarify two of the four perimeter spans will be above grade and two below, since it is a cut and fill job.  

Nice advice on the form suggestions to make a groove in these (control) joints. Sounds like they don't have to be very deep.

BB you say do not tie the corners together.  I think maybe it could be necessary on the above grade corners since i don't want any possibility of forward slippage (down gradient) of curb on this perimeter.  I was intending to run my reinforcement bar continuous through the corners.  Is there a reason this would be a bad idea?

Cheers Isotope  

RE: Contraction joints in concrete retaining wall

You can tie them together with "L" shaped bars. Again, it had to do with the way I pictured it.

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