Slab on grade crack 2

[EngineeringNgma]

Hi everyone!
I have casted Fiber reinforced concrete slab on grade. I had construction joint at two sides with dowels. After drying shrinkage, now crack is appearing to adjacent slab. I am presuming that its because of slab shrinkage which has caused the dowels movement in perpendicular direction to the adjacent slab and caused cracks.
Kindly can anybody guide me whether my assumption is correct.

 

[rowingengineer]

Normally it is because the saw cut was too late, however I can't see the saw it in you photo

 

[EngineeringNgma]

Saw cut provided already within 24 hours of casting.
This crack has appeared after 2 two months of casting the slab

 

[EngineeringNgma]

Also crack is perpendicular to the saw cut joint

 

[hokie66]

Yes, the dowels restrain the slab, which has to contract. Thus the slab must crack. It is possible to lessen that potential cracking by using square dowels with compressible material on the sides, or diamond plate dowels.

 

[Ron]

Your detail does not show the dowels to be greased on one side. As hokie66 noted, your cracks are related to dowel restraint. In addition to a compressible material at one end of a greased dowel, the dowel must be smooth, not deformed. If you used rebar for the dowels, the "floating" side should have been wrapped with tape and greased to prevent bonding to the concrete.

 

[hetgen]

Is the detail shown Construction Join (C.J) or Saw-cut Contraction joint (S.C.J)?

If it is a Construction Joint you normally coat the existing slab edge with bitumen emulsion. You don't need to saw cut the joint except for a small cut as a sealant reservoir where require.

 

[EngineeringNgma]

This is a construction joint.

 

[hetgen]

I think the design detail is wrong. Construction joints are detailed as shown below, no saw-cut is required.
The dowel should be large in diameter and does not need to be developed on both ends.

Φ12mm bar @ 200mm c/c shown in your detail is a tie-bar, it is not a dowel-bar. Tiebars by themselves are not designed to act as load-transfer devices. Load transfer across a construction joint having tiebars is provided by a keyway as shown in the second clip below. But no keyway is shown in your detail so the tiebars are just there to cause the unwanted crack.

 

[hokie66]

Dowels like these can assist with controlling cracking perpendicular to joints.

https://danley.com.au/product/dowelmaster

https://danley.com.au/product/diamond-dowel

 

[Harbringer]

Does the picture actually show a crack? I can't make it out.

 

[dik]

24 hours is way too long... within 6 or 8 hours of finishing. If you wait 24 hours, you will not likely catch shrinkage cracks... leave and route them as they form and add caulking... won't be as pretty. How thick was the slab, and what spacing of sawcut joint? Also no rebar, only fibres? That with a 4" slab would require joints at 8' or 10' on centres, or thereabouts....

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik