Waffle Slab - does CRSI Handbook specify mesh for waffle slab? How thick a topping is required?

[ajk1]

Question #1:
My 1963 CRSI Handbook says to use 6"x6" 6/6 welded steel wire mesh in the 2½" slab of waffle slab systems. I don't have the latest CRSI Handbook here at home. Does the CRSI Handbook still say that?

Question #2:
The existing waffle slab garage that we are going to repair does not have any mesh in it. Am planning on putting mesh in the new bonded 50 mm thick structural topping (which will replace the old unbonded topping that we are going to remove). If we lay the new mesh directly on the top of the existing waffle slab, can we get 40 mm concrete cover to it, even at the laps of the mesh? I realize that alternate sheets of mesh can be inverted so that there are only 3 wires of thickness there instead of 4, but does that work in both directions? I suppose I should sketch it up to help visualize it.

 

[KootK]

I don't have the latest CRSI Handbook here at home. Does the CRSI Handbook still say that?

It does. See the snippets below taken from the 2008 10th edition.

can we get 40 mm concrete cover to it, even at the laps of the mesh? I realize that alternate sheets of mesh can be inverted so that there are only 3 wires of thickness there instead of 4, but does that work in both directions?

Yes and yes. That said, I would not install reinforcing in this application. Consider:

1) The point of the reinforcing would be to restrain the size of shrinkage cracks. The continuous bond to the non-shrinking substrate will accomplish this same goal. If you were installing a 4" topping and could get the WWM up near the top surface, I'd be on board. With the WWM at the bottom of the topping, however, I think that it's benefit will be nil.

2) Laying the WWM right against the existing concrete causes two problems in my estimation. Firstly, your bond on the underside of the fabric seems questionable. Secondly, the WWM will introduce a stress raising discontinutity at the very plane where you're attempting to keep things together.

If anything, I might use some fibermesh in the topping. I'd consult with both the manufacturer and your client regarding suitability for your application however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

A contrary view. I would put the mesh in. It will be more or less centrally located in the topping/existing top slab. It will provide some crack control, although not much. The reason the topping will tend to crack is the restraint, and the restraint does not in turn control the size of the cracks. As to fibremesh, if you mean the plastic stuff, it is useless, IMHO.

 

[KootK]

I've been using the attached PCA document as my goto for a while now. The snippet below is an excerpt. Obviously, it is just one source and it has a regional bias embedded within it. It's also a little old.

The reason the topping will tend to crack is the restraint, and the restraint does not in turn control the size of the cracks.

I'd like to further discuss this point as I believe that the restraint, when that restraint is continuous, does indeed control the size of cracks. Is that not precisely how rebar controls crack size? By restraining the development of crack growth on a continuous basis?

As a thought experiment, consider a precast sandwich wall panel where the insulation "meat" was somehow replaced by fresh concrete after the "bread" had long hardened. Would you expect a few large cracks in the new concrete or a bunch of distributed small ones? I would argue for the latter.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

One of my British references has the blurb below to say about "forbidden" topping thicknesses and curling. That said, I've done a lot of 50 mm toppings successfully over precast, albeit with wire mesh.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ajk1]

To Kootk -

As always, your comments are interesting and thoughtful.

Can you tell me from which edition (year of publication) that CRSI excerpt is?

Regarding point #2, I had similar thought about the mesh introducing a discontinuity that would perhaps not be good for the bond of the new topping to the old substrate. Note though we are using a proprietary bonding agent that has produced very good bond strength results (>1.5 MPa) in the past, albeit without the mesh present.

Regarding the bond of the concrete to the mesh, would you not expect that there is enough of the perimeter of the mesh that will be bonded to the new concrete that the mesh could still be developed, and also that if the topping is bonded to the substrate that the mesh would be clamped together and that would also contribute to developing the mesh?

My concerns with omitting the mesh are:

a)Must rely solely on unreinforced slab to take the stresses from wheels on the 2.5" slab, which is not permitted by Code for suspended members. I don't think that we can rely on arching action (20" span / 2.5" slab = 8), can we?
b)There is nothing to restrain the existing substrate cracks telegraphing thru the new topping (which I am sure they will do, as they will move when vehicle wheels pass over them). Although the mew waterproofing membrane will be doubly reinforced, it is still better not to have cracks trying to tear it apart, or at least to have narrower cracks. It may be that the mesh will not help much in this regard, but is may help a little.
c) We will be in violation of criteria published by a recognized authority (CRSI). Another engineer can then come along and say we did not conform to recognized practice.

To Hokie66 - Thanks for your comment. I share the sentiment.


I am left undecided. I think perhaps that it is worth placing the mesh in the first of the next several pours and then do some bond testing at 7 days. If it is good (>1.5 MPa) my inclination would be to continue with the mesh; otherwise not. What do you think? I would be interested in your further comments.

 

[KootK]

Can you tell me from which edition (year of publication) that CRSI excerpt is?

10th edition, 2008.

Regarding point #2, I had similar thought about the mesh introducing a discontinuity that would perhaps not be good for the bond of the new topping to the old substrate. Note though we are using a proprietary bonding agent that has produced very good bond strength results (>1.5 MPa) in the past, albeit without the mesh present.

Regarding the bond of the concrete to the mesh, would you not expect that there is enough of the perimeter of the mesh that will be bonded to the new concrete that the mesh could still be developed, and also that if the topping is bonded to the substrate that the mesh would be clamped together and that would also contribute to developing the mesh?

I stand by both of my original points but consider them to be relatively inconsequential. As I mentioned, I've 50 mm topped plenty of precast with the mesh probably at the bottom of the topping. And, to my knowledge, nothing has gone awry.

a)Must rely solely on unreinforced slab to take the stresses from wheels on the 2.5" slab, which is not permitted by Code for suspended members. I don't think that we can rely on arching action (20" span / 2.5" slab = 8), can we?

If you need the reinforcing for flexural strength that is, of course, another matter. However:

1) I wouldn't expect the WWM to be very effective installed above the centroid of the slab.
2) After the topping is installed will your slab depth not be 2.5" + 2" = 4.5"? Span/depth = 4.4? If so, that's suitable arching territory.

b)There is nothing to restrain the existing substrate cracks telegraphing thru the new topping (which I am sure they will do, as they will move when vehicle wheels pass over them). ...It may be that the mesh will not help much in this regard, but is may help a little.

In my opinion, with the mesh installed at the bottom of the topping, you're going to get telegraph cracks either way. If you feel that the mesh improves mattesr, go with your gut and Hokie's recommendation, and have it installed.

c) We will be in violation of criteria published by a recognized authority (CRSI). Another engineer can then come along and say we did not conform to recognized practice.

The mesh provides a definite crack control benefit in a new install that is non-existant/debatable in your scenario. It's apples and oranges.

For me, it would come down to this:

1) I don't feel that the mesh would provide meaningful flexural capacity improvement when installed at the bottom of the topping.

2) I don't feel that the mesh would provide meaningful crack control when installed at the bottom of the topping.

3) I think that the prime concern here is preventing future corrosion problems. Given that you're likely to get telegraph cracks and the like, I worry that moisture may find its way into those cracks and corrode the mesh. That corrosion might generate expansive rust and that might produce a delamination and a frustrated client. Use galvanized mesh of course.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ajk1]

Darn it Kootk, you make good points. I am almost persuaded not to install the mesh. I will give it a little more thought.