Of the materials that have been used in combination with concrete, of the many that have been tested, only 2 have successfully passed the test of time and the market: steel and polypropylene.
Steel is probably the material that best matches with concrete: its remarkable tensile strength combines perfectly with the excellent compressive strength of concrete. Moreover the latter, thanks to its basicity, constitutes a favorable environment for the conservation of steel over time. No wonder steel fibers have been used in concrete for many decades now.
Steel fibers are used almost exclusively for structural applications, for two reasons: the first is that to produce extremely thin and short steel fibers has a considerable cost and therefore typically, in applications such as concrete fiber, for the product to have economic sense are produced with a diameter and a length that allows to reach a few thousand filaments per kg of product. Anti-cracking applications need a higher number of filaments for one or two orders of magnitude. The second reason is that their high tensile strength, which can reach 1200-1400 MPa, goes well with this type of application.
Their use is rather simple from the design point of view as the European and Italian regulations on the subject have long given precise references to designers, who can prescribe them in the specifications without too many problems.
Steel fibers have the disadvantage of being subject to oxidation; this aspect is accentuated by the fact that the fibers by their nature cannot be controlled as to their position in the cement matrix (the concrete cover distance cannot be respected). In the long run, therefore, oxidation and corrosion can compromise the good condition of the most exposed fibers. There are also special steels which can be used in the field of concrete fiber market precisely to counter these negative aspects: the galvanized steel fibers and stainless steel fibers.
The galvanized steel fibers are normal steel fibers whose performances are similar to those of the corresponding carbon steel ones, the only significant difference is represented by a surface galvanizing. This galvanizing surface protects the steel from oxidation better and longer than the normal version. Compared to normal steel fibers, galvanized ones cost around twice as much. The galvanized fibers are suitable for applications in brackish environments and in the case of painted prefabricated buildings, for which the aesthetic function is very often primary compared to the structural one and therefore the presence of superficial oxidation spots would be very harmful for the aesthetics of the building.
Stainless steel fibers, on the other hand, are the definitive answer to the problems of oxidation in steel fibers. There are various types depending on the quality of the steel used and the degree of rust resistance. Here we list them from the least performing to the most powerful:
- AISI 430 – ferritic stainless steel
- AISI 304 – austenitic stainless steel
- AISI 310 – austenitic stainless steel
These fibers are mainly used (not to say almost exclusively) in the market of producers of refractory products, which require a very high melting temperature that only stainless steel can guarantee. The prices of stainless fibers are doubles of those for normal steel fibers; at the time of writing, it goes from 4.50 €/kg for ferritic ones to reach about 9.00 €/kg for those in austenitic steel 310. It is completely superfluous to comment that in “normal” and daily applications this type of fiber does not have economic sense.
Steel fibers are classified by the reference standard (EN 14889-1) based on one of the following production methods from which they are obtained:
- obtained from steel wire (“from wire”)
- obtained by cutting the flat (“cut sheet”)
- obtained from hot extrusion (“melt extracted”)
- obtained from cold drawing (“cold drum wire”)
The first two types are the most widespread in the market. For some time also the type 4 have been sold, qualitatively worse than the former, also due to the strong price competition that has been affecting this market for some time.
The first two types are easily distinguishable from each other since the wire ones have a circular section, while the flat ones are square in section. The specialist literature recognizes yarn fibers (type 1) as qualitatively superior to other types.
The polymer fibers are fibers obtained from synthetic materials consisting of polymer chains of the same plastic common family. The most widespread family, in the concrete fiber market, are the “polyolefinic” fibers. However, there are also producers that have offered other types of polymers to the market, mainly polyester and polyacrylonitrile.
The detractors of synthetic fibers (generally the producers of steel fibers) accuse these models to having a viscous plastic behavior (also called “creep”) when subjected to constant and prolonged tensions over time. The origin of this accusation, certainly founded, lies in the fact that many polymers used in the manufacture of fibers for concrete naturally exhibit this behavior and therefore, they maintain, even the fibers will repeat the same phenomenon. It is true that structural polymeric fibers are almost always obtained from pre-stressed and oriented polymers, to increase their resistance. This will greatly reduce the creep phenomenon. However, the legislation on the subject is still completely absent, just as specific regulations are absent to test and measure this phenomenon. The only information in this regard is due to tests commissioned by individual producers or carried out by research institutes, interested in investigating the phenomenon and the implications from the point of view of the structure as a whole. They are tests that last at least 2 years and at the moment there are no certain and public data (also because there is no legislation on the matter that homogenizes the methodology, any result would be completely arbitrary and not very comparable with others).
Let us now look at the main materials with which synthetic fibers are produced.
With the technical term “polyolefin” a set of materials is defined in the chemistry of plastics, of which the most famous are polypropylene (PP) and polyethylene (PE).
Virgin polypropylene, among those of the polyolefin family, is the polymer generally used in the manufacture of fibers. Non-oriented polypropylene normally has tensile strengths of around 30-50 MPa. In the case of yarns, that is with oriented chains, the tensile strengths easily reach 250-400 MPa! In the case of auxiliary fibers this value is more than sufficient.
For structural applications, however, higher strengths are required and therefore mixtures of polyolefin polymers more performance than virgin polypropylene are often used, or processing techniques such as yarn “ironing”. These additional devices allow to reach maximum strengths that can even exceed 600 MPa.
It should be noted that the custom has long been established in the market, now commonly accepted by producers and technicians in the sector, to define the least performing fibers as “polypropylene” (auxiliary) and “polyolefin” or “polyolefinic” fibers. This definition is not entirely true, given that saying polyolefin does not exclude that the fiber is virgin polypropylene, but it is an established custom.
Polyacrylonitrile (PAN) fibers, also called simply “acrylics”, are synthetic fibers obtained from a polymer chain different from polyolefins. The main things to know about acrylics are:
- Generally low quality fibers, obtained from scraps of other processes. Their commercial strength lies solely in the price.
- They are normally present in small flakes which on their own would find it difficult to disperse adequately in the cement matrix, therefore are “dressed” to increase the wetting index. In certain applications this scale caused the creation of foam in the concrete during mixing, with drastic reductions in performance.
- They are fibers that in many cases tend to incorporate more air than other fibers of similar shape and size.
Only auxiliary fibers of this type are known on the market. At the time of writing, there are no acrylic structural fibers.
Polyester is a synthetic yarn that is often used in the textile industry. It is a tough and resistant polymer, with a higher elastic modulus than polypropylene (more rigid). Despite the fact that the polyester has the “ALL THE NUMBERS” to potentially constitute a good material from which to make fibers for concrete, in fact almost no manufacturer has adopted it and those that have done it have achieved little satisfaction in terms of performance in concrete and counterpart market!
In principle, glass yarn is a material with very high tensile strength: it can reach values of up to 1700 MPa, well above the best steels, if the yarn is sufficiently thin (a thinner glass thread is more statistically free from microfractures that enhance the incredible fragility, for this very thin glass wires in the laboratory reach the strengths of the material that are even greater than steel). The problem as everyone knows is its inherent fragility, which therefore in everyday applications does not allow to reach these performances.
Glass is therefore mainly used for anti-cracking applications, with which glass fibers behave very well (Azichem mortars make extensive use of glass fibers in fact) even if they have a higher cost than the more widespread polypropylene cousins.
The fundamental thing for a fiberglass to be used in concrete is that it is alkaline resistant (AR), or when inserted into concrete it is resistant to the highly basic environment of cementitious or lime based mixtures. There are also non-AR fibers on the market which are clearly much cheaper than the AR ones but which after a while dissolve in the mixture.