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Re: Even more fibers

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Sorry - my thank you goes to Will Hanes.

This article really helps to understand the fiber issue - it is primarily used to reduce temperature cracking in slabs especially where the concrete sub-contractor or the GC is not taking precautions to keep the slab moist then the fiber mesh will help to control minor temperature cracking.
The fact is that unless you specify pulling up the wire fabric into the slab, most contractors place the fabric on "chairs" and do not pull up the mesh. This leaves the mesh below the slab except where the chairs occur and therefore has little effect on controlling cracking and separation. I am pretty particular about this.
I designed the home next door and the contractor substituted Fiber Mesh for the steel mesh I specified without my knowledge. When I found out it was too late and this is when I called Fiber Mesh and received confirmation of my opinion of its use. There was a 3-foot change in elevation from the living room to the hall and the slab was poured above a CMU retaining wall. During drying, a crack normal to the CMU wall occurred in the slab and it ultimately separated horizontally. There was no vertical separation that I could see, but thankfully, the builder was using carpet over this area. When the home was sold, no mention of the crack was made and the subsequent owners (there has been three) always used carpet in this area.

I was particular about the drying process on my own slab, but was not here (I lived in Los Angeles about 150 miles west when my home was built) to watch the drying process. The concrete sub used an accelerator and care was taken on the slab, but he did not pour supporting column footings for my Kitchen (raised roof) separately as I instructed. This caused a number of crack radiating from the columns even though I had a modified Proctor test done on the property after compaction.

The solution, since all my floors are Saltillo tiles (paver), they are subject to cracking. If a crack exists in the slab, it will radiate through the hard surface. Fortunately my local tile supplier (I actually had a friend send a flatbed to Mexico to pick up 2,100 square feet of tile because he wanted only 400 s.f. and it saved me almost 50% on the cost) had a membrane that used an elastimeric between two sheets of fiber. I laid this over ever crack at $1.50 a linear foot and it has been over 8 years now and not one crack radiated. I wrote a recommendation to homeowners on my Structuralist.Net Blog that discusses the difference between using a builders paper and a flexible membrane to cover cracks where hard surfaces are applied.This is on the Public side of my Structuralist website and lists a few manufacturers of membranes you can find in your area.

Of course this is not 100% effective if you are building on heavily expansive clay soils since the slab can rise and fall past the reasonable ability of a membrane to control cracking. I have not seen this in my area as most soil her is silty sand and decomposed granite on granite formations - very stable after properly compacted.

Hope this helps and than you for the article.

Dennis

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Dennis S. Wish, PE
California Professional Engineer
Structural Engineering Consultant

dennis.wish(--nospam--at)verizon.net


760.564.0884 (office - fax)

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