Reasons to Use Fiber Over Rebar for Concrete ...
Reasons to Use Fiber Over Rebar for Concrete ...
The tensile strength of concrete is only one-tenth of the material’s compressive strength, but using fiber-reinforced concrete can increase tensile strength and prevent larger cracks from forming, said expert presenters during a World of Concrete educational session recently.
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Tyler Ley, PE, a professor of civil engineering and construction at Oklahoma State University, spoke with Jerry Holland, PE, vice president and director of design services at Structural Services Inc., about some of the advantages of using fiber-reinforced concrete.
“Fibers do the exact same thing as rebar,” Ley said, but added rebar is not as good at stopping cracks. Conversely, when compared with rebar-reinforced concrete, Ley said fiber-reinforced material is not as good at carrying loads. “Rebar carries load after concrete cracks,” Ley said. “Think of rebar like a bone in your arm.”
Ley compared fibers to stitches and said they can prevent small “microcracks” from becoming larger.
Holland said he worked on one of the first steel-fiber slabs in the U.S. nearly 45 years ago. In the decades since, he said fiber-reinforced characteristics have improved to a “tremendous degree” with both user friendliness and crack control.
Holland thought that fibers would revolutionize the industry, but he said that isn’t the case. Nevertheless, Ley said fiber-reinforced concrete has revolutionized certain types of projects, like industrial-grade slabs.
Ley said microsynthetic, macrosynthetic and steel fibers are still underutilized in crack-sensitive projects. He said many builders don’t think to use the materials, despite microsynthetic and macrosynthetic fibers having the added benefits of being able to provide reinforcement without the risk of corrosion that comes with steel.
According to ASTM C1116, there are four types of material from which fibers to reinforce concrete can be produced: Type I—Steel Fiber-Reinforced Concrete; Type II—Glass Fiber-Reinforced Concrete; Type III—Synthetic Fiber-Reinforced Concrete; and Type IV—Natural Fiber-Reinforced Concrete.
The Pennsylvania Aggregates and Concrete Association’s “Things to Consider About Fiber-Reinforced Concrete,” an information guide published on its website, said synthetic fibers are the most used and may be characterized as macrofibers and microfibers.
“Microfibers are used primarily to minimize cracking due to plastic shrinkage or plastic settlement in concrete. They provide much needed reinforcement when early age concrete has limited tensile strength and may be manufactured from polypropylene, polyethylene, polyester, nylon or other synthetic materials such as carbon, aramid and other acrylics.”
PACA’s information guide about fiber-reinforced concrete also touches on the characteristics of macrofibers.
“Macrofibers usually have lengths greater than 1.5 inches and increase the post-crack tensile strength and ductility of concrete. They are a proven cost-effective alternative to traditional welded-wire or rebar reinforcement where temperature and shrinkage reinforcement (secondary reinforcement) are a consideration.”
Fiber concrete or steel in concrete
tradesman said: Click to expand...
There are a number of threads that get into the technicalities of this, but:
- You are fine as-is. Your lift instructions don't tell you that you need rebar. They also don't tell you that you need fibers. The engineers who work for the lift manufacturer know something. (So you don't need to sweat too much or worry about analyzing too much. BTW - There are some members here who have done a great job with their slabs. There are also some members who have made some very poor choices with good intentions. Concrete is a heterogeneous material and does not behave like wood or steel.)
- As pointed out below, fiber and rebar - in the type of slab we are talking about here in a garage - are just for different types of crack width control. This time of year in PA, you don't need to worry about the type that fiber goes after. At 6" of depth, you are making a heavy investment in concrete and the slab will be very strong. So, you really don't "need" the rebar, either - if you do your control joints properly. (And you have a properly prepared base.)
So, rebar would just be a shrinkage crack insurance policy for you OR it would allow you to space your control joints further apart. If your concrete finisher has a rebar "formula" for a 4" slab, you would need to beef up the quantity of steel by 50% of you go to 6", etc. (The need is proportional to the thickness.) There are also engineering formulas for this if you want to get exotic on the spacing of the joints.
In the end, your time is most worthwhile to be spent on figuring out what your control joint cut plan is going to be. You'll want to work this out with your finisher. ALL slabs crack. (Generally, even when rebar is used.) The trick is to get the cracking to happen in your control joints.
So, you will want these control joints spaced properly away from your posts. (Your lift instructions will give you parameters for how far you need the posts to be away from joints or cracks.) This is the most important thing!
There are a number of threads that get into the technicalities of this, but:- You are fine as-is. Your lift instructions don't tell you that you need rebar. They also don't tell you that you need fibers. The engineers who work for the lift manufacturer know something. (So you don't need to sweat too much or worry about analyzing too much. BTW - There are some members here who have done a great job with their slabs. There are also some members who have made some very poor choices with good intentions. Concrete is a heterogeneous material and does not behave like wood or steel.)- As pointed out below, fiber and rebar - in the type of slab we are talking about here in a garage - are just for different types of crack width control. This time of year in PA, you don't need to worry about the type that fiber goes after. At 6" of depth, you are making a heavy investment in concrete and the slab will be very strong. So, you really don't "need" the rebar, either - if you do your control joints properly. (And you have a properly prepared base.)So, rebar would just be a shrinkage crack insurance policy for you OR it would allow you to space your control joints further apart. If your concrete finisher has a rebar "formula" for a 4" slab, you would need to beef up the quantity of steel by 50% of you go to 6", etc. (The need is proportional to the thickness.) There are also engineering formulas for this if you want to get exotic on the spacing of the joints.In the end, your time is most worthwhile to be spent on figuring out what your control joint cut plan is going to be. You'll want to work this out with your finisher.(Generally, even when rebar is used.) The trick is to get the cracking to happen in your control joints.So, you will want these control joints spaced properly away from your posts. (Your lift instructions will give you parameters for how far you need the posts to be away from joints or cracks.) This is the most important thing!
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