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About PVA Fibers : Frequently Asked Questions
What is PVA?
What is PVA? PVA is polyvinyl alcohol, an organic material that we use to make concrete reinforcement fibers, among many other uses. It is made from carbon, hydrogen, and oxygen. How are PVA fibers different from polypropylene fibers? They differ in many ways, but as reinforcement for concrete, these differences are especially notable. Polypropylene, nylon and the other synthetic fibers used in concrete serve only one purpose: to restrain plastic shrinkage during the first 24 hours after concrete is poured. Because of their high elongation, or "stretchiness", these fibers are essentially useless for the drying shrinkage and other sorts of cracking that all concrete suffers from. These cracks, in addition to plastic cracking, is what PVA is designed to handle. In that regard it is a structural fiber like steel and AR glass. How much should I use? That depends on your application. You might use 1.5 pounds per yard (0.9kg/m3) for crack control, or you might go as high as 1.4% by weight of the concrete for replacement of steel reinforcement. Give us a call and we'll help you sort out the ideal fiber type and dosage for your application. What is a structural fiber? A structural fiber is one that is less stretchy than the concrete it is reinforcing. This is measured by the Young's Modulus of Elasticity. To be considered structural, generally you must have a Young's ME greater than 25. There are sophisticated measures, like the ability to strain-harden, that can be considered, but the ME is a good basic parameter for separating the stiff fibers from the stretchy ones. How do PVA fibers compare with steel fibers in tensile strength? Steel fibers have greater tensile strength than PVA fibers, but this strength is rarely used. Steel fibers, like rebar, are passive reinforcement, meaning they have no reinforcing effect until the concrete cracks. The problem then is that steel fibers tend to pull out rather than hold the concrete together. Many techniques have been used to increase the mechanical bond of steel fibers--hooks and waffles are among the most popular--but even so the steel fibers pull out easily. Contrast this with PVA fibers which create a molecular bond with the cement during hydration. When you try to pull a PVA fiber out, it holds on firmly, taking full advantage of PVA's tensile strength. Can PVA replace steel reinforcement for concrete? Yes, in many cases. Or it can be used to reduce the steel needed, or the concrete required to protect the steel. Please call us to discuss your application. What is PVA-ECC? PVA-ECC is a micromechanically-designed cementitious matrix that achieves dramatic ductility and tensile strength, up to 500 times that of other fiber-reinforced mortars. ECC stands for Engineered Cementitious Composite. PVA-ECC was designed by Dr. Victor C. Li, professor in the Department of Civil and Environmental Engineering at the University of Michigan, Ann Arbor, and director of the Advanced Civil Engineering Material Research Laboratories. Read an introduction to ECC. Can I use PVA fibers with my steel reinforcement? Absolutely. PVA fibers can be used to enhance the strength and ductility of your existing design simply by adding fibers while mixing the concrete. Can I add PVA fibers to my existing mix? Yes. PVA can be used in all types of mortar and concrete and can be used to enhance all types of reinforcement. Why would I want to add PVA to my existing mix? Greater flexural strength and impact resistance in particular. Does PVA handle drying shrinkage cracking or just plastic shrinkage cracking? Unlike other fibers, PVA handles both plastic shrinkage and drying shrinkage cracking. Can I replace my glass fibers with PVA? Yes. PVA has been used extensively to replace GFRC, producing a less expensive, lighter and more durable product. Are PVA fibers expensive? PVA fiber is about the same price as glass, but you use 1/3 as much, so it ends up being much less expensive. It is a little more expensive than polypropylene and nylon, but it offers far greater performance. Does PVA break down in concrete? No. Even the best AR glass degrades in strength dramatically over time, up to 80%. Field studies have shown that PVA degrades about 1% over 20 years. What about ultraviolet rays from sunshine? PVA is highly resistant to UV radiation. Will PVA fibers break down in mixing like glass fiber or carbon? No. PVA is extremely tough and durable. You can bend PVA fibers over a hundred times before they break. No other concrete reinforcement fiber is so tough. I need a road repair fiber. Is PVA abrasion resistant? PVA fibers are highly abrasion resistant. Can I use PVA fibers for concrete countertops? Yes. PVA fibers work well with very thin concrete elements. Even if there are cracks, PVA's hydrophilic bond helps keep the concrete in place. Is PVA safe to use around food? Yes. PVA is FDA approved. It is an organic fiber and is non-toxic. Can I use PVA fibers to build a pool? Yes. PVA fibers can be sprayed with gunite/shotcrete, cast, or applied by hand. Will I get "hairy" concrete with PVA fibers? All lightweight fibers can produce "hairy" concrete. The fibers that float, like polypropylene, suffer this problem to a greater extent. PVA has a density of 1.3, so the fibers do not float. However, you must finish the concrete properly or some fibers will be vertically aligned. Do I need special equipment to mix PVA fibers? No. Just follow the instructions. Can I overmix PVA fibers? No. Unlike glass and carbon fibers, PVA cannot be overmixed. Overmixing can cause other problems, including too much air entrainment and loss of superplasticizer effectiveness. What about earthquakes and PVA fibers? No, no fiber can do that. Concrete can be made "waterproof" by paying close attention to mix design, especially maintaining a water to cement ratio below 0.40. But cracks play a large part in allowing water to penetrate the concrete. PVA fibers provide excellent control of all types of cracking, including plastic shrinkage, drying shrinkage, autogenous and map cracking, differential curing cracking, settlement and structural cracking. |
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