Concept of reinforcement of concrete through fibers
The presence of fibers having adequate tensile strength, and being homogeneously distributed within concrete, builds a micro-scaffolding that, on the one side, demonstrates itself being efficient in counteracting the known phenomenon leading to crack formation due to shrinkage, and, on the other side, leads the concrete’s ductility(1) to become increasingly relevant with increasing strength of the fibers.
This provides the concrete with a high toughness (2) as well. As it is known, in the vast majority of currently applied calculation and verification rules, the concrete’s tensile strength is generally neglected in the calculation route, given concrete’s brittle behaviour. The use of a fiber-reinforced matrix makes it possible to stabilize tensile properties. In this way, the tensile strength can be now be exploited as well between other mechanical properties in the design phase. T
Given operative difficulties, tensile tests are generally not realized onto the concrete directly. The evaluation of tensile properties, as well as of ductility and toughness, are carried out indi-rectly through bending tests on beams or sheets, as it will be reported in details in the following sections.
Graphic 2.1 shows qualitatively the possible outcomes which can be obtained from bending tests on fiber-reinforced concrete elements through graphs reporting load vs. crack formation or load vs. deflection.
- Curve I depicts the behaviour of a traditional concrete without reinforcement. Since the structure is isostatic (the beam is simply supported at both ends), it collapses immediately after first-crack loading is reached, such as typically occurring for brittle materials.
- Curve II shows some ability of the (fiber-reinforced) concrete to absorb, departing from the first-crack point, a certain although low load (A-B) through a progressive slower collapse (degrading behaviour).
- Curve III, in contrast, is typical of a ductile material, and shows a concrete able to sustain, departing from the first-crack point, a considerable deflection (A-B) under constant load, still before the even slower occurrence of collapse (plastic behaviour).
- Curve IV finally highlights even an increment in the tolerable load under a wide deflection (A-B) after the first-crack point (hardening behaviour).
It is evident that all these possible behaviours, or different ductility and toughness levels acquired by the concrete, depend both from the quantity of the present fibers as well as from their mechanical, and geometrical characteristics.
Considering the influence of the fiber geometry on the behaviour of FRC(3) and of SFRC(4), although any aspect is relevant, it is the relationship between the fiber length and equivalent diameter (L/D named aspect ratio or slenderness ratio) which is considered as the most characterising element, since ductility and toughness of a fiber-reinforced concrete depend in large measure on its value (Graphic 2.2).
Equally important are the mechanical characteristics of fibers, and, between them, tensile strength essentially. Tensile strength plays a fundamental role on the behaviour of FRC and of SFRC as the hindered pull out, which is due to the real and the forced adherence between fiber and concrete (Graphic 2.3), can lead to fiber breakage caused by insufficient tensile strength (Graphic 2.4).
Finally, dosage, which is the effective quantity of fibers embedded in the concrete (kg/m3 or%Vf 5), certainly impacts considerably on the ductility and toughness levels acquired by the fiber-reinforced concrete (Graphic 2.6), together with the already mentioned geometric and mechanical characteristics of the fibers. It is interesting to observe that through an increase of the aspect ratio (L/D), the quantity of fibers (dosage) necessary to achieve a given result decreases within certain limits (Graphic 2.5). This is due to the fact that, statistically, the tensile strength is increased as a direct consequence of the statistic increase of the fiber length to be pulled-out.
It is important to point out that it is definitely the whole set of reported characteristics which collectively contribute to determine the behaviour of the fiber-reinforced concrete, and that an optimal output is always depending from an adequate combination of all these factors together since each factor alone is capable to impact on the final behaviour up to a certain limit, after which its influence would become useless if not even damaging, as it is highlighted by Graphic 2.6 as far as the influence of dosage is concerned.
The initial part of the curve shows how a very small dosage has practically no effect (degrading behaviour) because, if only few fibers are dispersed into the mixture, their relative distance is as high that no resistance is produced.
The second part shows how, by increasing the number of fibers, which is reducing the volume of influence of each fiber, configurations of static superimposition of fibers between themselves, with high possibility to interact, are achieved (plastic behaviour). As such, an increase in the concrete’s ductility is produced, which is directly depending on the effective dosage.
Finally, the third part shows how, by overcoming a certain dosage (hardening behaviour), the increase in ductility is insignificant, although evident. Thereby, it becomes in contrast increasingly difficult to obtain a uniform and fluid mixture.
To conclude, the following quantitative considerations can be given regarding the quality and quantity of fibers to be embedded into a SFRC:
- Fibers should have very high mechanical properties, whereby their tensile strength should be in the order of 1100 MPa.
- Their aspect ratio should be sufficiently high as well, ranging between 45 and 70.
- Dosage should not be lower than 25 kg/m3, whereby it could reach 40 or 80 kg/m3 for particularly demanding applications.
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