The fiber reinforced concrete (FCR) It is a particular type of concrete that contains inside special fibres which improve the overall mechanical characteristics, as well as reduce the amount of pollutants released during the production of this material. It is therefore a mix of innovative materials that aims to replace, at least in part, the reinforced concrete. But what types of fibers can be inserted inside it? And why is this material not to be confused with Eternit?
What is fiber reinforced concrete used for?
The concrete, obtained from the chemical reactions that the cement (binder) develops with the dough water and with the addition of elements aggregates (like sand), is a mixture of materials that over time acquires a solid conformation very similar to that of a rock. However, this conformation – being mainly linked to this cement matrix that acts as a glue between the aggregates – can be easily “broken” by the poor ability of the material to resist tensile stresses: the concrete, that is, yes slot easily under the action of external forces and if not “armed” is practically ineffective in its use as a construction material. Historically, the remedy for this serious deficiency was provided by steel reinforcement cages, which are still widely used on a daily basis. However, recent experiments seem to prove the effectiveness of alternative techniques to overcome this problem, including the path of fiber reinforcements.
What are fiber reinforcements and what are the types?
Technical research related to the development of new building materials attempts to eliminate steel reinforcement bars to produce equally resistant concretes, but whose production requires less carbon dioxide for its development. In this panorama the fiber-reinforced concrete: the aim is to insert fibres into the concrete mix that give it non-negligible tensile strength and, therefore, a minimisation of the possible triggering of crack patterns. The tensile strength that the material can now develop is called tenacity, and it depends essentially on the volumetric percentage of fibers inserted in the mixture, as well as by the physical-mechanical characteristics of the inserted fibres.
Types of fibers
The fibers are added to the mix without a specific order, unlike what is done for the reinforcing bars. However, the quantity of fibers must be homogeneous throughout the element created, minimizing the presence of voids that could locally compromise the resistance of the product. Among the fibers most used to compose the mix we find:
- Fibers of Steel: these have an “equivalent” diameter between 0.15 mm and 1.20 mm, with lengths ranging from 6 to 70 mm;
- Fibers polymeric: of acrylic, nylon or polyester (to name a few). In these cases we speak of microfibres (lengths in the order of millimetres) and macrofibres (up to 80 mm);
- Fibers of carbon: like the previous ones, they are classified into micro and macro fibres;
- Fibers of glass and fibers of organic nature.
The concrete recipe requires a specific identification of the quantities of cement, water and aggregates present to provide adequate characteristics in terms of mechanical resistance. In particular, the aggregates are selected in order to have a specific Granulometric assortment. In the case of fibre-reinforced concrete, it is common practice to increase the percentage of fine material present between the aggregates, in order to ensure good coupling between the fibers and a good workability. As regards the quantities of fibres, experimental evidence shows that a volumetric percentage of fibres between 2-8% significantly improves the expected mechanical behaviour of the cement matrix, significantly increasing the tensile strength of the conglomerate.
The regulatory framework
Fiber-reinforced concretes are one of the most up-to-date innovations Technical Standards for Construction (NTC 2018)which regulate the design rules and criteria. In this standard, a fibre-reinforced concrete is considered to all intents and purposes a building materiallike reinforced concrete, wood or steel. The standard also provides a clear boundary for the fibres that can be used to create the matrix, i.e. steel fibres or polymeric material, thus removing a whole series of products currently available from the range of use. For example, consider that even the hemp fibers they are tested and experimented to be candidates for fibre reinforcement of the cement matrix.
In addition to the NTC 2018, there are technical documents that regulate the design of fiber-reinforced concrete. It is worth mentioning the CNR Instructions of 2006 (CNR DT204, 2006). This document, in particular, contains useful information not only relating to the design phase but also to the execution and control phase over time.
Similarities with Eternit
Eternit is a patent that comes from the combination of cement with asbestos fibres, in fact it is also known as asbestos-cement. The role of asbestos in the mixture is the same as any other fiber reinforcement used today. In addition, asbestos gives the cement mix aexcellent heat resistance. However, after the discovery of its degree of danger linked to the carcinogenic effects caused by breathing its dust, the use of this type of product was officially banned, both as a material in itself (asbestos, often used as an insulating material) and in combination with cement. Even today, the discovery of Eternit products involves a series of non-trivial remediation procedures and significant problems related to its disposal.
