1.1. The Concept of Fiber Reinforcement
Concrete is undoubtedly the most common building material in use today. There are several reasons that explain this issue: its component materials are relatively inexpensive and easy to produce; its manufacture is comparatively simple; as this is a product delivered in a semi-fluid state, it allows for great versatility terms of shaping; and it can be applied to a large number of buildings and civil engineering infrastructures. However, its use also encounters certain drawbacks, the main one being its brittleness, i.e., its low tensile strength compared to its compressive strength, its low deformation capacity until the appearance of the first crack, along with its rapid propagation.
The issue of brittleness of the materials used in construction is as old as the activity itself. Since ancient times, building materials, e.g., clay sun baked bricks, were reinforced with fibers such as horse-hair, straw, and other vegetable fibers to try to overcome this hindrance. The brittle matrix supports, surrounds and protects the fibers, providing compressive strength, while the fibers enhance the mechanical response of the composite by bridging the cracks that appear after tensile failure and providing a sort of ductile behavior post cracking.
The concept of fiber reinforcement was developed in modern times specifically for its use with cement-based materials. Asbestos cement products were introduced in the early 1900s and became widely used following the development of several mechanized manufacturing processes, such as the Hatschek, Magnani [
1], and Manville [
2] processes. Asbestos fibers were employed until about the 1970s when their effect on the development of lung cancer was proven.
The development of fibers specifically intended for concrete followed a different path from asbestos. Two different periods can be distinguished. The first one, up to the 1960s, can be considered a pioneering phase, with many ideas but few actual applications. It is a period of invention rather than scientific research, characterized by a growing number of patents. The first one is usually ascribed to A. Berard in 1874 [
3], in California, who proposed the addition of granular waste iron to concrete. Certain other relevant examples, already in the 20th century, are Weakley [
4], Meischkle-Smith [
5], and Constantinesco [
6]. All of them dealt with the use of steel fibers. The latter is particularly noteworthy since the fiber reinforcement parameters he recommends are somewhat similar to those of steel fiber reinforced concrete currently employed.
The second phase of the development of fiber-reinforced concrete can be considered as the period in which ascertaining the true influence of fibers in concrete properties from a scientific perspective was discovered. This phase started in the early 1960s with the research work of Romualdi et al. [
7,
8] in the US and Krenchel [
9] in Denmark, and was expanded upon with several studies undertaken over the next decade (Shah [
10], Naaman [
11], Swamy [
12], among others) that boosted the development of a multitude of fibers [
13] and materials [
14] that continues until the present as new discoveries and applications are identified [
15,
16].
Steel fibers are the most used type of fibers for concrete reinforcement and crack control. They have been chosen for a wide range of concrete elements such as pavements, slabs, bridge decks, beams, tunnel linings, foundations, and walls, with volume fraction rates standing at mostly below 2%. Their advantages are that they are widely available; relatively inexpensive when compared to other fibers; they present high strength, modulus, fracture toughness, and temperature resistance; and are deformable. On the downside, they are corrosion-prone, and their manufacturing cost is high if small diameters are wanted. In non-load bearing applications, such as pavements, the fiber content is usually under 0.5% by volume [
17]. A common figure, used later in this paper for concrete mix design, is the addition of 20 kg of steel fibers per m
3 of concrete (
Vf = 0.26%).
Polypropylene fibers were first used in concrete in the 1980s [
18,
19]. These are also widely available, their cost is low, and they are highly stable in a concrete matrix, still they have low modulus, medium to low strength, poor bonding capabilities and are difficult to mix in larger volumes. Consequently, their use has found a niche that could be considered as secondary reinforcement in non-structural applications, particularly to control plastic shrinkage cracking of concrete at an early age. In such applications, the volume fraction of fibers is less than 0.2% by volume, being often close to 0.1%. In this paper, an addition of polypropylene fibers of 1.0 kg/m
3 is considered (
Vf = 0.11%).
Most of the steel and polypropylene fibers used for concrete reinforcement are comprised of industrial products manufactured from raw materials. Over the last two decades, the concern for the sustainability of industrial processes has led to the undertaking of numerous research efforts looking for the replacement of industrial products by recycled fibers at low cost and with less environmental impact [
20,
21].
1.2. Recycled Steel Fibers from Tires
According to the 2019 statistics of the European Tire and Rubber Manufacturers Association (ETRMA), that accounts for 70% of the world tire industry turnover; in other words, 5.1 million tons of tires were produced throughout 2018 by its members [
22]. The EU Landfill Directive [
23], which came into full effect in July 2006, requires that virtually all end-of-life tires (ELTs) be recycled or re-used in some way. The adoption of this policy means that, according to ETRMA, 91% of ELTs were collected and treated for material recycling and energy recovery in 2018 [
24]. In the UK alone, over 40 million used tires are treated every year. Around 2 million tons (61.75% of total ELTs treated) were treated through material recovery, where secondary materials from ELTs are used in construction, automotive, and civil engineering applications.
A typical tire consists mainly of rubber (47–48% by weight) and black carbon (22%), though an important part is comprised of steel wires and cords (15–17%) that provide stiffness and resistance to the tire [
25]. The remainder are fabrics and other minor additives. Steel wires and cords can be recovered and transformed into fibers. The extraction of the fibers from the tires is carried out mainly by three methods: shredding, cryogenic, and pyrolysis processes. The first two processes, called mechanical recycling, are the most commonly used. Mechanical processes damage the cables superficially to a certain extent, but they still retain the ability to transfer stresses, due to their slenderness ratio and their irregular shape, thus favoring anchorage [
26].
According to these figures, ELTs suggest themselves as the potential source of more than 300,000 tons of recycled steel fibers per year. Owing to this sizeable potential, a sizable number of research works have been performed over recent decades regarding reinforcement of concrete with steel fibers recycled from tires. First attempts were carried out by Wu et al. in the US in the 1990s [
27]. Their work focused on the shrinkage performance measured on ring-type specimens, as well as the flexural load vs. deflection behavior, since those are the main enhanced properties that can be expected from the use of fibers in concrete, as has been mentioned before. These works were continued both in the US [
28] and in the UK at the University of Sheffield [
26,
29], and during the last decade, more than 150 papers have been published on recycled steel fiber reinforced cement-based composites [
20].
The existing literature shows that most of the fibers used were obtained through the shredding process. A scant number of the works have focused on the geometric characterization of the fibers, reaching the conclusion that the main geometric characteristics, diameter and length, depend on the type of tire and the extraction process [
30], meaning that to obtain an effective characterization it should be necessary to perform a statistical analysis [
26]. The most thorough studies in this regard are those carried out in Italy by a group of researchers from the University of Salento [
31,
32,
33]. Usual diameter and fiber length ranges are 0.15–0.26 mm and 25–40 mm, respectively [
20].
With regard to industrial steel fibers, studies show that the addition of recycled steel fibers reduces the workability of concrete mixtures, being mainly affected by the type and content of fiber [
34,
35]. The variety of sizes and shapes of recycled fibers makes them susceptible to the “balling” effect (
Figure 1), which has caused countless issues in the applications of fibers in concrete. To avoid this problem, Aiello et al. [
31] recommends to limit the volume content under 0.46%. Grunewald et al. even reduce this limit to 0.25% [
36]. However, other references have used higher fiber contents without detecting any ball formation [
37,
38].
Regarding compressive strength, the studies show that, as with industrial steel fibers, a small addition of recycled fibers does not have a significant bearing on compressive strength [
33,
39], yet, after a certain threshold, increasing the fiber content decreases the compressive mechanical response [
40]. This threshold would depend on the fiber and concrete types.
Finally, the state-of-the-art includes a good number of works that investigate the influence of the addition of recycled fibers on the residual strength of concrete under flexural and shear loads, once the first cracking has occurred [
41,
42,
43,
44]. Results show that recycled steel fibers provide a similar energy absorption capacity and residual strength after first cracking upon flexural loading to that of industrial fibers, when used in similar volume contents.
However, although the review of the state-of-the-art shows that it is possible to obtain a sustainable and eco-friendly fiber-reinforced concrete through the use of recycled fibers from tires, its use is not widely implemented at present in the construction sector, despite the large potential source derived from the large number of ELTs processed each year. Most of the studies point out the distrust of the users when considering a recycled product, due to the fear of a decrease in properties compared to an industrial product [
21], as one of the reasons for this slow introduction. Furthermore, although most of the works have focused on the fibers’ structural behavior, especially with regard to the compression and flexural behavior of fiber-reinforced concrete, as well as aspects related to workability, other relevant properties such as durability, shrinkage, or creep require further investigation in the future, with the amount of data regarding these properties remaining limited. Interestingly, the first study on the use of recycled fibers focused on the characterization of the improving resistance to shrinkage cracking, which has hardly been studied since. Likewise, the geometric characterization of the fibers, of major significance from their application viewpoint, lacks unification when defining and measuring the main geometrical characteristics of the fibers, taking into account that the extraction process can generate great variability in these properties.
To overcome these drawbacks, the purpose of this work is to increase the knowledge on the behavior of recycled fibers from tires compared to the behavior of two widely accepted and trusted types of industrial fibers, steel and polypropylene. The first type is the most common material used for concrete reinforcement from the structural outlook, while polypropylene fibers are a common solution to protect concrete from the appearance of cracks due to shrinkage. This final aspect has hardly been considered in previous works dealing with recycled fibers. Likewise, a procedure for characterizing the two main geometric properties of the fibers, diameter and effective length, is proposed, in order to be able to proceed to their classification.