Slab on Graded Smooth Doweled Control Joints

[southard2]

I have a monolithic poured metal building slab/foundation measuring 32 ft x 24 ft. I've decided to place one control joint in each direction turning the slab into four 16 ft x 12 ft section. Because there will be heavy forklift type loads I designed to use sawn control joints with 3/4" x 14" smooth dowels coated with grease on at least half the dowel length. I have to rely on the contractor getting the dowels in straight and level for them to work properly. But it has occurred to me that because the slabs are restrained on the edges by the turned down slabs at the perimeter that the slab will also be shrinking perpendicular to the doweled joints as well.

I'm just curious as to how other engineers are handling this situation. It will occur even with slabs that don't have restraint at the edge as all slabs technically will shrink in to directions. Am I just over thinking this issue or can I only place dowels along the joints running north south and not the joints going east to west?

I thought about have the smooth dowels place in sleeves to provide for a little more play in both directions but this would defeat the whole purpose of having the dowels.

John Southard, M.S., P.E.

http://www.pdhlibrary.com/

 

[JAE]

There are the proprietary products developed for this such as the Diamond Dowel system
(

http://www.pna-inc.com/products/diamond_dowel/)

(

http://www.interconex.us/pdfs/sales/pna/Diamond_Dowel_System.pdf)

...I'm not affiliated with them.

Also check out this link - good article:

http://www.acpa.org/Downloads/RT/RT 10.01 - Plate Dowels.pdf

 

[hokie66]

Another solution is square dowels, with a sleeve to allow movement parallel to the joint. I prefer these to plate dowels, as I worry about getting concrete under the plates.

http://www.lesasystems.com.au/products/lesa_dowel_range/lesa_square_dowels.aspx

 

[TXStructural]

You will see the result of what you postulate near the corners of roadway and slab segments. Doweling in two directions especially when the dowels are crooked or too near the corners frequently results in restraint cracking.
Seems that it would be less complicated to either reinforce the slab to restrain it fully, or to modify the mix to reduce shrinkage. Largest practical aggregate size (1-1/2" minimum), lowest paste fraction needed, and low water cement ratio with admixtures to keep it workable and able to be finished.

Also, cut contraction joints as soon as you can get on the slab. Just a few minutes too late, and cracks beyond the dowels will have initiated.

 

[hokie66]

I agree with what I think TX means about more reinforcement for crack control and eliminating the joints, particularly in such a small building. The slab will still crack, but the width of the cracking will be controlled. Where forklifts operate on ground slabs, joints are usually where problems occur.

 

[Ron]

Agree with JAE on the plate dowels. Use plenty of grease.

You can also mitigate the intersection cracking from shrinkage by holding the dowels back in one direction from the intersection. Usually about 3 or 4 dowels back in each direction will help. For your section, you will have more shrinkage in the long direction, of course.