Shrinkage, WWM & Synthetic fibres

[PG35]

I have a 4" slab-on-grade with in-floor heating tubes for an elementary school. 6x6-10/10 WWM is placed at the bottom of the slab so that the tubes can be held in-place.

If the WWM is lifted 1/2" from the bottom will it be somewhat effective in controlling temperature/drying shrinkage? The WWM has to be located in the upper 1/3rd of the slab to be effective correct?

Basically I have to add a second layer of mesh or use synthetic fibres/ The second layer will be hard to accomodate so I don't want to go with that.

I know that many new schools in our area have been constructed with only polypropylene fibres (fibremesh) in the slab-on-grade (NO WWM except at bottom). This does nothing to control temperature shrinkage. I don't think this is good practise. Should I use a better fibre product to control long term temperature change effects?

What do you suggest.
Thanks,
P

 

[Lion06]

We had a rather lengthy discussion on this very topic some time ago and came to the conclusion that the synthetic fibers are NOT a replacement for WWF. I would provide a second layer of WWF at the top.

 

[hokie66]

My opinion is that WWF located near the bottom of a slab on grade is more effective in controlling shrinkage cracking than if located higher in the slab. The force which causes shrinkage cracking is subgrade drag, and if the steel is closer to the force, the stress in the steel, and thence the width of the cracks, will be less.

I know this is not the conventional view, but it is not mine alone. The best and most compelling argument I have seen for this theory is a paper by Cesar Kiamco in the July 1997 Concrete International magazine, entitled "A Structural Look at Slabs on Grade".

I don't think you can get 2 layers of mesh and heating tubes in a 4" slab. If you decide you must have the top layer, may have to increase thickness to say 5".

 

[miecz]

I was going to provide a link to the long discussion on the use of fibers as reinforcing, but I can't find it. Is it gone? As I remember it, some posts provided evidence that steel fibers were effective in controlling shrinkage.

I agree with hokie that reinforcing located near the bottom of the slab is effective. I would use a lot more than 6x6-10/10 though. In fact, I would use #4@18" E.W.

 

[JAE]

Please go check out these three related threads:

thread507-144777

thread507-126594

thread592-152445

 

[PG35]

JAE thank-you for the links. I found some good info.

My sawcut spacing will be 10 feet o.c. governed by the geometry of the classrooms.

I plan to use fibremesh 300 and specify that the WWM be placed 1/2" from the bottom.

Next job I will try a 4 1/2" slab with 2 layers of mesh.

 

[JKW05]

How big are the heating tubes, and what is the floor finish?
You may want to consider where control joints are in the finish, unless it's carpet.

JW