Placement Of Control Joints

beeson76

What are the "rules" for placement of control joints. I know concrete cracks diagonally where 2 slabs come (ex. think of a pour in the shape of the T) Why wouldn't you cut diagonally from where the 2 intersect instead of across? Always wondered that. Also, does anyone know of a good book that talks about placement of control joints.

TERM101

Depending on the type or quantity of construction I try to keep the rule of thumb "2 1/2 times the depth = length between joints." As for the placement you just have to use your own judgement and experience as in how they meet etc. No worries, you will get a knack for it as you do more of them.

Anti-wingnut

Would this be better stated as 2.5 depth in inches = dictance in feet?

Hoven

I usually try to get good depth on cuts but not so deep that your cutting through the wire reinforcement. I usually place a 1 foot piece of #4 steel across the point of an inside corner or I will take my 4 in diamond blade on my grinder and plunge cut at the finish grade line and hit it with a hammer below the cut. It will usually break off easy and it gets rid of the inside corner. Then I pour the floor just and 1/8 below the cut.

B.K

When pouring slabs, you should have a bond breaking material placed between the slab and footing.

For foundation walls, control joints should be be provided in foundation walls more than 25m long at intervals no more than 15m.

I can't remember what its called, but when we poured slabs, we would put a 2"x4"x8' flexible material around the whole perimeter of the building, placed right at the top of the slab. The concrete wouldn't bond to it, making it extremely hard for the concrete to crack at the joint of the slab and wall.

For garages and basements we would put a control joint 1/3 of the depth of the slab, right down the middle, on both sides.


-Bill

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scrapecc

control joints should be 25% the depth of the thickness of the slab. any 90 degree outside corners should have a 45 taken off from them, and the distance between them can vary greatly. Go to PCA (portland cement association) on the web. then are the biggest source of concrete information in the country. They will definately have any information you may need.

many names

When doing 4" slabs I never run over 10' stones (10'x10').I prefer smaller stones to help control cracking.I often get creative with type finish and shine.The 25% depth is correct,concrete cracks tend to come straight of 90's,not wrote in stone either.Pre existing soil conditions/compaction also contribute to this.

Fiber in your concrete is a good crack deterent but sometimes I will use wire mesh in areas also,just depends.