# Sustainable Development of Concrete through Aggregates and Innovative Materials: A Review
## 3 Synthetic Fibers  
### Steel Fibers  
Steel fiber is usually recovered from scrap tires and used to produce steel fiber reinforced concrete (SFRC). The mechanical properties of SFRC can be affected by factors such as water-cement ratio, steel fiber geometry, steel fiber volume fraction and ratio aspect and curing time [62]. Based on the investigation of [PERSON] et al. [63], the steel slag aggregate incorporated in high performance concrete (HPC) can improve its mechanical strength and durability. However, high expansion steel slag should not be used in concrete constructions without any treatment of free oxides to decrease its expansion capacity [63].  
[PERSON] [64] reported that the percentage of waste steel fibers added in concrete affects its workability. Although, compressive strength of this concrete is not significantly affected. [PERSON] et al. [65] researched concrete samples with different types of aggregates such as steel slag and waste glass. They demonstrated that these aggregates can improve the fire resistance of the concrete samples. [PERSON] and [PERSON] [66] studied the frost resistance of concrete using steel fibers as reinforcement and established that steel fibers can enhance the strength and durability to cracking even under freezing temperatures. Another type of steel fiber (steel slag and steel sludge) in concrete was used by [PERSON] et al. [67]. The steel slag and steel sludge were generated from the melting process of metal scrap and steel wire production, respectively. They identified the composition of the fibers using X-ray fluorescence and X-ray diffraction tests. The incorporation of these fibers in concrete improved its compressive strength, achieving the best strength with 10% replacement of steel slag. They recommended to use up to 15% steel sludge and 20% steel slag in concrete.  
Generally, reinforcing fibers have been used to reduce cracking in structural elements [68]. The inclusion of fibers in conventional reinforced concrete can improve the structural and functional performance of structures, for instance, pressurized structures such as pressure tanks, steel and polyamide fibers. The pressure capability can be greatly improved by adding steel and polyamide fibers to conventional reinforced concrete. Steel fibers are more effective when improve the containment performance of a prestressed concrete containment vessel (PCCV) in comparison to polyamide fibers. Fiber reinforcements have effectiveness at high loading pressure and low prestress level. These fibers can be used in structural elements such as beams and columns, which are extensively studied due to the exhibited tensile strength and postcracking deformation capacity. Concrete elements reinforced with steel fibers present correlations, according to the guidelines established by the ACI-318, which allow the estimation of basic mechanical properties of steel-reinforced concrete [69].  
Figure 8 shows the steel fibers incorporating in concrete to decrease its cracking. The mechanical behavior of concrete with steel fibers are evaluated to identify the optimal replacement ratio that improves its mechanical properties. The steel fibers can improve the maximum strain and shear behavior of recycled aggregate concrete beams [70].