Contraction joints - RC slab on ground

[n3jc]

Hi, Im designing contraction joints for RC slab on ground.

I have never done this before but I have found and read quite a few articles about it.

What Im asking is should I make square plates 8,75 x 7,40 m (between columns) or is that way too much?
I have read they shouldnt be more than 36 d (where d is thickness of slab) so if my slab is 200 mm thick that is 7,20 m which is pretty close I guess. But if I want my slab to be thiner - lets say 150 mm, that means I can only go as far as 36 x 150/1000 = 5,40 m

Id really like to make contraction joints between columns so Id get: 8 x (8,75 x 7,40 plates)
If I choose thinner slab I have to make more cont. joints which looks way too much: 16 x (4,375 x 3,70 m)(2nd image

What is you take on this?

Thank you

 

[jayrod12]

I would probably keep the 7.4 and 200 slab, up the reinforcing slightly in that direction. And put another line down the 8.25.

 

[n3jc]

tnx for reply but segments of the slab between contraction joints should be pretty much a square.

Hope for more opinions/advices.

 

[Ron]

Keep the segments as square as possible and in no case having an aspect ratio greater than 1.2:1. Keep the spacing of the control joints (they are control joints, not contraction joints) to between 24 and 30 times the thickness. Essentially for your 200mm thickness, or 4.8m to 6m. Even 6m is stretching it. Reduce accordingly if you reduce the slab thickness.

It is important that the saw cuts be made the same day as concrete placement. Make sure they are cut to at least 20 percent of the slab thickness (40mm) and preferably 25% of slab thickness (50mm).

 

[AskTooMuch]

Shouldn't there be diamond isolation joint around the column?

 

[jayrod12]

You're right they should be. Hence me recommending upping the reinforcement in the long segment direction.

But if you know the rules you should follow, why even ask the question. The appropriate way would be to maintain your slab thickness and provide a grillage of squares that don't exceed your maximums.