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Re: To Steel or Not to Steel

From: info@builderswebsource.com
Category: Concrete
Remote Name: 63.198.181.43
Date: 12 Mar 2001
Time: 10:42 PM

Comments

Thank you for your question on slab reinforcement. Builders Websource will give you yet another opinion, but we will document our assumptions so you can be confident in our reasoning. Keep in mind our assumptions are based on the limited facts provided in your post.

1) Since the proposed slab will also be an exposed visible surface (stained), we recommend a dual approach to reinforcement: structural steel coupled with fibermesh secondary reinforcement.

2) Fibermesh is NOT a substitute for primary steel reinforcement, either for thermal expansion or structural requirements. In general, you should add approximately 1.5 lbs (range is 0.75 - 3.0) of fiber per cubic yard of concrete. Since the floor will be exposed, we recommend a product such as Stealth Fiber (see http://www.fibermesh.com/ for additional technical information). This product is ideally suited for exposed concrete surfaces where aesthetic appearance is important (you don't want a bunch of little fibers sticking up).

3) As to the use of remesh (welded wire mesh) versus rebar, either one can be used to help keep the concrete intact while providing reinforcement for thermal expansion and contraction. However, wire mesh tends to get matted down during the actual pour. Studies of WWM show that it does little good in practice unless extreme care is given to "pull it up" into the slab as the wet concrete is poured. Cross-sectional views after the fact show that most slabs have the WWM near the bottom of the slab.

As a result, we generally recommend #3 or #4 bars at 12" or 16" centers each way, respectively. This spacing is closer than one of your proposals to use #3 bars on 30" centers.

Let's do the math. The building code and general concrete reinforcement guidelines suggests that the cross-sectional amount of steel in a slab should be no less than 0.25% of the cross-sectional area of the concrete slab itself (each way). Suppose your slab is 4" thick, which is considered minimum for a structural floor slab. Some slabs are even thicker, depending on the soils and freeze line. However, in your case, you make no mention of a frost line, so we will assume it is not a factor. Furthermore, your soil is compacted and non-expansive, so upheaval is negligible.

For a 4" slab, each 1-foot section will have a cross-sectional area of 48 sq-in (4" thick x 12" wide). 0.25% of 48 sq-in is approximately 0.12 sq-in of rebar required. It so happens that the cross-sectional area of #3 bars is roughly 0.11 sq-in and the cross-section of #4 bars is 0.19 sq-in. Therefore, this says that roughly 1 #3 bar is required in each direction for each 12" width of 4" thick slab. If you use #4 bars, technically you could place these closer to 20" on centers...but 16" o.c. is a common practice.

This is the minimum recommended reinforcement for thermal expansion, not accounting for additional loads to which the slab may be subject. Reinforcement along the footing is critical to carry the required live and dead loads.

Now, this may sound excessive, but consider this: at retail prices, a 20-foot length of #3 rebar is about $2.60 each. Therefore, for well under $200 in steel (excluding labor), you can cover a 1,000 square feet of slab on 12" centers each way. This is cheap compared to fixing your slab later.

When laying the rebar, you should make sure there is a minimum 6-mil approved vapor barrier underneath...and that the rebar is suspended in the center of the slab with small dobies or little rebar "chairs".

As s system, the rebar provides primary thermal and structural reinforcement while the fiber provides secondary tensile strength for improved crack performance due to plastic shrinkage. According to Fibermesh FAQs:

Is fiber reinforcement as good as traditional reinforcement?

"Fibers and traditional reinforcement represent two different approaches to concrete reinforcement. Traditional reinforcement is utilized to hold concrete together after it cracks. Fiber reinforcement provides an internal network of reinforcement to protect concrete from cracking in the first place."

We appreciate that you have received different advice from reputable sources. It's possible that in each case, there are different operating assumptions. However, the advice above is based on sound engineering practices, building codes, as well as manufacturer recommendations. It's possible you could get by with less and experience no problem. But why cut corners on something as permanent as your foundation? Do it right, it's worth a few hundred dollars more in material and labor.

For additional information on foundation reinforcement, read our chapter on Foundations in our online book, Building a Dream at:

http://www.builderswebsource.com/book/foundations.htm

Builders Websource

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