THE TECHNICAL PARAMETERS

Now we will talk about what are the most commonly found technical parameters on the technical data sheets of concrete fibers, in order to be able to understand their meaning, to know how to compare the fibers together.

Length

The most immediate, simple and representative of the technical parameters of a fiber is certainly represented by its length! Normally the product code of a fiber is very often also related to its length. Since the fiber, especially the polymers, are the product of a yarn cut at regular intervals, all the intrinsic characteristics of the fiber itself are derived directly from the starting yarn, while the length has direct effects on the behavior of the fiber in the concrete and on its application. Moreover, given that each producer from the same yarn can obtain different fiber models simply by cutting to different sizes, the length plays a fundamental role in distinguishing the different fibers of the same product portfolio.

The fibers for cement conglomerates that have appeared on the market are practically all included between 3 mm and 60 mm in length. No fibers are known at this time that go beyond this range. Normally short fibers (<25 mm) are destined almost exclusively to the segment of anti-cracking reinforcement; long fibers (> 30 mm) are almost always fibers intended for structural applications.

Having made this first great distinction, in the segment of structural fibers almost all models are concentrated between 38 mm and 54 mm. In structural fibers, however, the choice is very often dictated by the models available and by their declared performance (according to CEregulations), so it is not a choice deriving from the use, the thickness or the aggregate of the concrete, but precisely the length that the supplier found more effective in their laboratory tests.
Therefore the choice for the user is normally oriented towards the more performing fiber and length (except for specific and specific cases and needs).

However, this argument does not apply to auxiliary/anti-cracking fibers, for which the choice of length is not so much a consequence of technical performance, as much as the actual needs of the customer in relation to the final use and thickness of the product. For auxiliary fibers, the most common lengths are between 6 mm and 18 mm. The shorter ones are for mortars and plasters, while the longer ones are practically for all other cases.

Diameter

Even the diameter of a fiber immediately classifies its function: the thinnest fibers, with diameters smaller than a tenth of a millimeter, are exclusively auxiliary fibers, while the structural ones always have diameters greater than at least 0.3-0.4 mm.

The standard classification for synthetic fibers, given by EN 14889-2, distinguishes two categories:

• “Macro-fibers” – diameter> 0.3 mm
• “Micro-fibers” – diameter <0.3 mm

The “equivalent diameter” can also be found in the technical data sheet of a fiber. In this case we mean the diameter of the circle having the same area as the section of the fiber considered. For example, in the case of a rectangular section fiber the equivalent diameter is calculated as the diameter of a circle of equal area.

Dimensional relationship

The dimensional ratio is calculated as the proportion between the length of a fiber and its diameter (or equivalent diameter). It is a parameter that is often overlooked but which in reality is useful, in some types of fibers that we will see shortly, as a measure of the degree of “resistance to extraction”. The resistance to the extraction is, as the name suggests, the capacity of the fiber to oppose the extraction, in other words not to be removed from its seat after the concrete has hardened and incorporated it.

In the case of microfibres (which by definition have diameters less than 0.3 but in fact in the vast majority of cases have diameters of a few hundredths of a millimeter) the dimensional ratio has very high values, which often exceed 200-250. For these fibers the resistance to extraction is a completely marginal characteristic, given that their function is not structural.

Fibers with an equivalent diameter greater than 0.5 mm, therefore rather coarse yarns, are almost always subject to industrial processes which improve adhesion thanks to devices such as undulations, roughening and knurling, particular shapes, etc. In these cases the ratio dimensional is lower than 70-80 and its evaluation is secondary since the resistance to the extraction is mainly due to these processes.

The dimensional relationship is a particularly interesting parameter in the case of thin structural macrofibres, having diameters smaller than 0.5 mm. For these types of products the surface workability is reduced and less effective. The result is that yarn stranding is frequently used and work is being done to precisely optimize the dimensional relationship: by increasing the dimensional ratio a better resistance to extraction is obtained. For these types of fibers the size ratio is normally around 100-110.
It should be noted that from a simple evaluation of the dimensional relationship it is perfectly able to deduce the type of fiber and classify it according to its function.

Talking about fiber geometry often doesn’t make much sense except in the case of polymer fibers. Steel fibers are exclusively made up of single filaments, possibly “attached” in bundles of a few dozen strands to facilitate transport and handling. But beyond some small variations as to the shape (hooked, undulating, straight, etc.) on the whole they are all conceptually identical! The same goes for example for glass fibers, for which it is even easier to simplify as they are always more or less straight and more or less long straight wires.

Let us therefore see the main differences regarding polymeric fibers:

Monofilament fibers

Are fibers consisting of a single wire, more or less straight, generally significantly larger than the multifilament, worked to increase its adherence to the cement matrix (knurling, side hooks, undulations, zigzag shapes, etc.). They appear in the bag as a multitude of individual “needles”, randomly scattered among them.

Monofilament fibers often receive mechanical treatments after extrusion (to improve their characteristics) such as, for example, ironing to orient polymer chains and increase strength, or treatments to increase surface roughness and therefore anchoring to concrete.

The monofilaments are almost exclusively (if not always!) fibers of “large” dimensions, belonging to the macro-fiber category, and with lengths between 30 and 60 mm.

Multifilament fibers

Fibers produced by the extrusion of a multitude of small straight filaments, then cut to size and bagged. Are fibers that are not normally treated after extrusion. In the bag they appear as “tufts” of tiny, soft and flexible fibers.

Are generally small-sized fibers, micro-fibers, with very small diameters (often less than a tenth of a millimeter!) and between 3 and 30 mm long.

Stranded fibers

Fibers produced by single filaments (therefore basic are simple monofilaments) twisted together in bundles, called precisely “strands”. Before being cut to the final length, they are very similar to a rope. This category includes for example the READYMESH PF-540.

Fibrillated fibers

The fibrillated fibers are technically produced not by the extrusion of one or more filaments (such as the mono and the multi-filaments) but rather by a thin and continuous film of polypropylene, which is then separated into smaller strips and subsequently “fibrillated” for realize the classic “net” shape when the fiber is “dilated”. Fibrillation is therefore a procedure for producing low-cost polypropylene yarns, often used in packaging. The fibrillated fibers tend to flake in single filaments during mixing thanks to the mechanical action of the movement of the aggregates present in the mixture.

The resistance measures the capacity of the fiber to withstand longitudinal tensile stresses, normally measured with the test described by the EN 10002-1 standard (see below).

The value of “tensile strength” (or even ” breakdown strength”) found in the datasheets is representative of information regarding the quality of the fiber before it is inserted into the concrete. It does not therefore represent a measure of the fiber’s ability to alter the behavior of the fiber-reinforced concrete structure subjected to mechanical stresses (for this reason there are specific tests whose results often do not appear in the technical data sheet, such as that of the standard EN 14651). From which it can be deduced that a comparison of the structural fibers on the basis of the simple evaluation of the resistance is by no means true about its actual structural capabilities.

Theoretically, the strength of the fiber itself has a direct relationship with the strength of fiber-reinforced concrete subjected to bending, but the high resistance alone is by no means sufficient to guarantee also excellent performance of the fiber-concrete system! The structural fiber, by definition, must oppose the increase in the gap opening. To achieve this objective it must obviously be able to resist itself, but it is also very important the number of fibers that are collaborating on the same objective, how homogeneously they are distributed, and how much their shape or surface finish opposes the extraction of the fiber from its seat.

The tensile strength of the single fiber is an almost useless parameter in the case of auxiliary fibers, in fact they do not exercise their function by traction and therefore a high resistance would not be of any benefit.

The density of the fiber, sometimes also referred to as specific weight, is a measure that derives directly from the material of which it is made, usually expressed in kg/dm³ or kg/m³. Steel fibers usually have a density of 7-8 kg/dm³, typical of steel, while synthetic fibers have a density of about 0.9 kg/dm³.

The density is indicative of the tendency to segregate the fiber in the concrete: heavier fibers than concrete (which has a density of about 2.4 kg/dm³) will tend to deposit on the bottom while light fibers will have the natural tendency to move upwards. In fact, this phenomenon is not very relevant since the concrete is a viscous compound, with many solid parts inside it and the “migration” of the fibers inside it is very difficult even in the presence of fluid concretes.