concrete slab on ground joints 4

[rowingengineer]

I am told that the sawn joint detail with the reinforcement continuous is out of date (first detail in the PFD). Apparently the second detail should be adopted as the standard.

I am told everyone does the second detail in the PDF these days. However I have been using the first detail for years and never had any more problems with it then I do with the second detail. I have also worked at four practices that have used the first detail as standard.

So what I would like to know is what your standard sawn joint or tool joint detail. Do you still use number one or are you using number two and WHY?


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[ishvaaag]

It doesn't surprises me that ratios of failures are as bad in one or the other detail. The second detail being more complicated puts in place a number of causes for misbehaviour, just out of irrgularity; I prefer the simplicity of the first detail. Yet I do not practice frequently these "thin" slabs on the ground. Here we either build true "structural" mats and even when for road pavements etc they are taking the practice of making them fully jointed in the whole depth, so more opinions welcome.

 

[Ron]

RE...there are reasons to use either; however, I would not dowel every joint because of the restraint that is caused at intersecting joints.

We generally do no use flat plate dowels. When dowels are necessary, we use typical AASHTO dowels.

My typical joint detail is similar to your No. 1, except that we specify the width of the joint and filling with closed cell foam backer rod and sealant to get proper sealant profile.

 

[asixth]

Our typical sawn joint detail just calls for every second wire crossing the joint to be cut.

 

[hokie66]

In thin slabs, say less than 150 mm, I would use the first type, just sawn without dowels, for control joints other than construction joints. For construction joints, I use dowels. My preference is the Danley square dowel rather than the diamond plates, due to the difficulty of getting adequate consolidation under the plate.

For thicker slabs, I use dowels in both directions, again the square Danley dowels with compressible material on the sides.

 

[RHTPE]

I was recently involved with a project(8"/200mm slab)that had a detail similar to the 2nd, only using smooth round dowels. The contractor was using an early-entry saw as soon as possible after finishing (within hour or so). Yet the slab cracked about 12-24 inches (300-600mm) from the intended joint. I'm not clear if the cracked occured before or after the saw-cutting, though it may have been there before and was not yet visible at the surface. We were at a loss to determine why, other than a possible incorrect installation of the reinforcing. My preference is that the saw-cutting must occur as soon as the slab will permit.


Ralph
Structures Consulting
Northeast USA

 

[cntw1953]

Ralph:

Have you ever suspected that the crack was due to saw-cutting too early - made prior to most volume change has taken place, and the crust of the concrete is still weak to resist stress induced after the cut?

 

[RHTPE]

cntw1953 In a word - no. I can't reason out the mechanics of how too early would make the slab crack away from the joint. Early cutting has typically created problems with the joint appearance (raveling, etc.), but I'm not aware of how it would contribute to a crack occuring parallel with the saw cut.

In the case I mentioned, the crack kinda followed a line connecting the embedded ends of the dowels. I wondered if the slab reinforcing stopped short of the joint location.

Ralph
Structures Consulting
Northeast USA

 

[cntw1953]

My thinking is after the cutting, the gap of rate of shrinkage of the interior and the crust remained high, without support, the saw-cut groove became weak spot.
Just a guess here. I wouldn't doubt there are other contributing factors.

 

[cntw1953]

I shall have said "the side walls of the groove became weak spot".

 

[rowingengineer]

RHTPE
If the reo mesh near the edge of the cradle gets too close to the surface it will act as a saw joint. This happens often with detail two if they put the mesh on top of the cradle. The reo causes restraint during plastic shrinkage and a crack develops. There is an article somewhere that I read very recently, can't remember where at the moment when i do i will try and post.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[rowingengineer]

Ron, Hokie and Asixth,
Thankyou for confirming that detail one is still used out there, I thought I was going crazy.

Ron,
I would seal an external joint but not an internal joint.

Hokie,
I have never given much thought to the possibility that you couldn’t get adequate consolidation under a plate dowel. But now you say it, it makes sense since you have this problem with cast-in plates.

Asixth,
I have given lots of thought to the cut every second wire to not cutting every second wire at this joint. If you cut every second wire you are not leaving a lot of steel to ensure the crack will still be able to transfer load. Alternatively if you don’t cut the steel how do you ensure the crack will open at all???? As you can see I am still undecided about the best path.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[hokie66]

RHTPE,

You can get that type cracking if the dowels are not normal to the joint, the bond is not broken on one side, the ends are burred, or a combination of these factors. I doubt if it has anything to do with the sawcutting.

 

[RHTPE]

Hokie66,

This was an 8" slab reinforced with a mat of #4s EW, T&B. The causes you suggest are valid, but with that kind of reinforcing, IF properly installed, leads me to believe there was more to contribute to the cracking than the contractor's crew would admit to.

I was asked to support the concrete contractor's side that they did nothing wrong. I couldn't do it, since I do not have x-ray vision. If I did, I may still not have been able to take his side.


Ralph
Structures Consulting
Northeast USA

 

[hokie66]

Yes, difficult without photos taken during construction or demolishing a bit of the slab.

 

[OzEng80]

RE
Depending on the spacing of joints you don't need any steel crossing the sawcut. The more steel you have crossing this point the less effective the joint will be. If you keep the joints at similar limits as that for plain concrete (about 6m max from memory) then aggregate interlock is sufficient to take the shear across the joint (I am talking about your typical 100th lightly loaded slab).

The key to sawcutting is making sure the concrete is cut as soon as it is hard enough to not have aggregate chipping. We specify within 12-24 hours of pouring slab.

 

[apsix]

You're probably right OzEng, but I specify 50% of the steel to continue across the joint and cut early, and haven't had a problem that I can recall.

 

[southard2]

If the bars aren't perfectly perpendicular that can cause problems also

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

http://www.pdhlibrary.com

 

[cntw1953]

"If joints are cut too earlier..."

The attached PDF discuss early/late cut of joints in pavement. Have a look, for those interested, but I am not positive about credential of the source of publication.

 

[dik]

Ralph... ever consider that the sawcut location was off by a foot or so? <G>

Dik