**3. HPC and UHPC Containing Fibers: Composite Materials**

High-performance fiber reinforced concrete (HPRFC) or ultra-high-performance fiber reinforced concrete (UHPFRC), emerged as a need to improve ductility and tensile strength properties in HPC and UHPC [25,70]. Research with this type of material is relatively recent, starting from the 1990s, about 30 years ago [187]. The researchers observed that, although the mechanical behavior related to compression was increasing, the same did not happen with tension, which compromised the concept of high performance [188].

In this context, a new perspective of HPC and UHPC emerged: using the concepts of composites applied to concrete materials. Composites are materials that use two different components to obtain a material with superior properties, these two components are the matrix phase, the majority phase, responsible for involving and protecting the second phase, which is the reinforcement, dispersed throughout the matrix. The matrix of a composite can be metallic, polymeric, or ceramic. In the case of high-performance concrete, the matrix used is cementitious, essentially composed of HPC and UHPC [13,188,189]. Despite the great potential of using fibers to reinforce HPC and UHPC, few standards were developed to date, such as the French standard [37].

This concrete, used as a matrix, has the same components highlighted in Section 2, that is, OPC, mineral additions, preferably pozzolanic of high fineness, such as silica fume and fly ash, low w/c, requiring the use of superplasticizer chemical additives, and fine and coarse aggregates following the same specification detailed in Section 2.2.

The reinforcement phase of a composite can be in the form of filler particles or in the form of fibers. In the case of HPC and UHPC discussed in this section, the reinforcement phase is composed of synthetic fibers that can be metallic, polymeric, and ceramic, or natural fibers, of mineral, animal, and vegetable origin [12,13,190]. Figure 5 shows the behavior of tenacity and elongation at the break of the main fibers used for application in HPC and UHPC, while Table 5 shows the main properties of these fibers. The main advantages of applying fibers in HPC and UHPC are the high gain in mechanical strength, both in compression and in traction, and the increase in ductility. Disadvantages include the reduction in the workability of concrete, which can be solved using 3rd generation of superplasticizer reducers, and the higher cost of the material, especially with the use of synthetic fibers. These issues will be detailed in this section.

**Figure 5.** Stress x strain diagram of fibers used in HPC and UHPC [191,192].

**Table 5.** Properties of fibers used in HPC and UHPC. Source: [193–199].


*3.1. Steel Fibers*

Through the consulted database, it is observed that the majority of research carried out with HPC and UHPC, mainly use steel fibers, produced through essentially ferrous metallic alloys, containing between 0.008 and 2.11% of carbon. These fibers present positive properties, such as high ductility and tensile strength, in addition to compatibility with concrete, which is typically observed in the use of steel bars in reinforced concrete [73,200–202]. However, they are prone to corrosion, which is why several authors have studied the effects of this pathology on the behavior of HPC. According to Shin and Yoo (2020) [203], Yoo et al. (2020) [204], Lv et al. (2021) [190], and Ngo et al. (2021) [205], corrosion reduces the strength of the cementitious composite and decreases its ductility to levels even worse than its behavior without reinforcement. Therefore, corrosion must be severely avoided.

On the mechanical properties obtained by the application of steel fibers, Gou et al. (2021) [206], studied the mechanical properties of HPC containing different fractions of steel fibers in an orderly and disorderly manner in the cementitious matrix. The authors observed, as can be seen in Figure 6a, that the use of 1.0, 1.2, and 1.5% of disordered fibers presents compressive strength after 28 days of cure, superior to equivalent compositions with oriented fibers. However, this pattern of behavior is changed with the use of 1.8% of fibers because, as the fiber content is high, the workability of the cement mortar reduces, resulting in an agglomerate of steel fibers, which causes the formation of inter-defects in the concrete and, consequently, the loss of strength.

Regarding the flexural tensile strength at 28 days, Figure 6b, it was observed that the oriented fibers had better results, regardless of the amount of fiber evaluated. This is attributed by the authors to the formation of stress transfer bridges and crack propagation. This effect is obtained in greater intensity when the fibers are properly oriented. That is also why the results improve considerably with the use of higher amounts of fiber, where the composition with 1.8% oriented showed a result of 40 MPa, for example.

$$\mathbf{(a)}$$

**Figure 6.** (**a**) Compressive strength, (**b**) flexural tensile strength fibers [206].

Other important works using steel fibers in HPC are now cited. Ashkezari et al. (2020) [201] studied the experimental relationships between the volume fraction of steel fiber and the mechanical properties of HPC. Bao and Pyo (2020) [67] evaluated the mechanical behavior and electrical properties of high-performance concrete containing fiber incorporation for application in railway sleepers. Park et al. (2021) [207] evaluated and verified the orientation and distribution of steel fibers in high-performance concrete pillars using computerized microtomography. Kim et al. (2021) [63] and Kim et al. (2020) [208] studied the benefits of applying curved steel fibers with a radius of 10–50 mm on the properties of high-performance concretes. Dingqiang et al. (2021) [209] evaluated the influence of the use of straight and short (6 mm), straight and long (13 mm) and hook-end fibers on the mechanical properties of UHPC, performing experimental tests and computational analyses. The best results were obtained using 2% steel fibers with a hook, obtaining approximately 150 MPa compressive strength and 45 MPa flexural tensile strength at 28 days. In this way, it was concluded that the steel fiber is compatible with the cementitious matrix for UHPC applications.

#### *3.2. Other Synthetic Fibers*

Other synthetic fibers used for HPC and UHPC applications are carbon, glass, and polymeric materials such as polypropylene and polyethylene. Carbon fibers, composed of thousands of unified filaments, have the advantage of greater adhesion to the cement matrix due to their high specific area.

Relevant researches with the application of carbon fibers in HPC and UHPC may cite the following: Afzal and Khushnood (2021) [210] evaluated the influence of carbon fibers on the performance of UHPC exposed to high temperatures; Jung et al. (2020) [211] reported on the changes in the structural behavior of HPC containing carbon fibers; Liu et al. (2020) [197] measured the strength gain of HPC containing carbon fibers at early and intermediate ages; Zhou et al. (2020) [212] evaluated the behavior of HPC e UHPC containing several types of fiber, including carbon, exposed to high temperatures.

Glass fibers are produced using borosilicate glass (E-glass) or soda-lime-silica (Aglass), with the advantages of being able to apply thin panels and elements without the occurrence of corrosion. However, they can present durability problems, since the cementitious matrix is highly alkaline, which can degrade the fiber and promote the embrittlement of HPC and UHPC [12,64]. Some important works with this fiber are worth mentioning. Bilisk and Ozdemir (2021) [213] studied the three-dimensional configuration of glass fibers in cementitious composites composed of HPC and UHPC. Al-Khafaji et al. (2021) [214] evaluated the behavior of sustainable HPC with ecological materials and fiberglass application. Kumar et al. (2021) [215] studied the application of UHPC containing glass fiber, and other fibers, for structural reinforcement of concrete and reported corrosion problems. Ali et al. (2020) [216] investigated the behavior of beams subjected to bending of UHPC containing recycled aggregates and fiberglass.

Regarding the application of glass fibers, the work by Mohamed et al. (2021) [217] in which the authors applied glass fibers at various levels of UHPC with a w/c ratio of 0.12 and 0.14, varying the curing age in 7, 14, and 28 days, as shown in Figure 7, is worth mentioning. The authors observed that after using 1.5% fibers, the compressive strength results did not change. This happens because the composite has reached the fiber saturation point, which is the point at which the matrix is not wettable to absorb the fibers used. In concrete, this can cause a drop in strength or a loss of workability.

**Figure 7.** Compressive strength of HPC containing glass fibers [217].

The most used polymer fibers are polypropylene (PP) and polyethylene (PE). The main advantages are the low density of fibers when compared to others already mentioned in this review. As for disadvantages, the mechanical behavior is mentioned, which is inferior to steel, glass, and carbon fibers. Hussain et al. (2020) [218], for example, compared the mechanical behavior of CC and HPC containing 1% steel, glass, and PP fibers, as shown in Figure 8; as for HPC, steel fibers are the best performers, followed by glass. PP fibers have the worst results, even though they are superior to the reference concrete. However, as these fibers are lighter, their application should not be discarded, mainly because they help to reduce HPC and UHPC retraction.

**Figure 8.** Compressive strength of HPC containing fibers: S (steel), G (glass), P (po polypropylene) and R (reference) [218].

Other important research that used polymeric fibers are now cited. Behafarnia and Behravan (2014) [219] evaluated the application of PP fibers in HPC to be used in water tunnels; Zhu et al. (2017) [220] evaluated the effect of the degree of aggregate saturation on the freeze-thaw strength of HPC e UHPC with light PP fibers; Li et al. (2018) [221] and Li and Zhang (2021) evaluated the influence of aggregate size and inclusion of PP and steel fibers on the hot permeability of UHPC at high temperature; Yan et al. (2021) [12] carried out experimental research on the increase of HPC ductility using basalt fiber, PP fiber, and glass fiber; Shen at al. (2020) [222] analyzed the effect of the length of PP fibers on the crack strength of high-performance concrete at an early age; Yoo and Kim (2019) [223] evaluated the influence of PE fibers on the mechanical and impact strength properties of HPC; and finally, He et al. (2017) [224] evaluated PE fiber coating mechanisms to improve the adhesion and mechanical properties of UHPC.
