Investigating the Impact of Stainless Steel Shavings Fibers 316L on Enhancing the Properties of High-Flow Sand Concrete in the Long-Term

In this study, the following materials were utilized to produce high-flow sand concrete reinforced with stainless steel fiber (316L):

• Sand (S): Class 0/2 sand, with a density of 2.6 g/cm³ and an equivalent sand content of 78.80%, sourced from a quarry located in Oued Z’hor, Skikda, Algeria.
• Cement (C): Sulfate-resistant cement (SRC), also known as NA 442. CEM I 42.5 N SR3, from the Moukaoum Plus brand, LAFARGE ALGERIE, Algeria. This cement, with a density of 3.1 g/cm³ and a Blaine fineness of 3450 cm²/g, underwent detailed chemical analysis as presented in Table 1.
• Fine limestone (FL): Type ALCAL F15, with a density of 2.7 g/cm³ and chemical analysis outlined in Table 1, was sourced from the El-Kheroub unit of the National Aggregates Company (ENG) in Constantine, Algeria.
• Silica fume (SF): Marketed under the name SILTEK and provided by the TEKNACHEM company in Sidi Bel Abbes, Algeria. This silica fume, with a density of 0.3 g/cm³ and chemical analysis as detailed in Table 1, was utilized in the study.
• Water reducer (SP): A super high water-reducing plasticizer, viscocrete 665, from Sika-El Djazair, with a density of 1.085 g/cm³ and a dry extract content of 32%.
• Mixing water (W): tap water.
• Fiber: Stainless steel shaving fiber (SSF-316L), with a density of 7.9 g/cm³, was procured from the Algerian Equipment and Machine Tools Company (ALEMO), situated in El Kheroub, Constantine.

Table 1. The chemical analysis of C, FL and SF

These fibers, which came from shavings of spare parts, were sorted and chopped into lengths of 25 to 40 mm, with 0.75 to 2 mm of breadth projecting from them, as shown in Figure 1. The morphological characteristics of the SSF-316L, including their length, width, percentage of content, and distribution, are also shown in Table 2. Table 3 elaborates on the chemical characteristics of stainless steel 316L.

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Figure 1. 316 L stainless steel

Table 2. SSF-316L morphology

Table 3. The chemical analysis of SSF-316L

Figure 2 is SEM-EDS cartography, which combines the spatial resolution of a scanning electron microscope with the power of X-ray spectroscopy. By scanning an electron beam across the specimen surface, the green color represents the calcium (Ca) element. It is the principal component of calcium carbonate (CaCO₃), commonly known as fine limestone, and the blue color represents the silicon (Si) element. It is the principal component of silicon dioxide (SiO₂), commonly known as silica.

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Figure 2. SEM-EDS cartography of Ca and Si

Figure 3 shows the SSF-316L fiber shavings via SEM-EDS cartography. The specimen surface is scanned by the electron beam, with light blue indicating the iron (Fe) element, pink representing the chromium (Cr) element, and light green standing for the nickel (Ni) element. These are the principal components of SSF 316L.

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Figure 3. SEM-EDS cartography of Fe, Cr and Ni

4.1 Physicals properties

Standard test methods outlined in Table 5 were utilized to conduct tests on physical properties. During the sag test, we proceeded by filling the Abrams cone with the mixture, then we lifted it constantly, and we measured each time the corresponding sag height. In the density test, both the void mold (M_v) and the filled mold (M_f) were weighed, with the volume of the mold (V) known (15×15×15) cm³_. Eq. (1) was then used to compute the results. The air content in the high-flow sand concrete was measured using a 1-liter air entrainment meter device.

$\rho=\frac{(M v-M f)}{V}$             (1)

Table 6 shows the impact of SSF-316L on the workability of fresh high-flow sand concrete. The slump value for the M0 mix was 215 mm. It increased by 13.95% for the M1 (0.25%) and M4 (1.00%) mixes. The reason for the result is that evenly dispersed 316L stainless steel fibers improve the mixture's homogeneity and consistency and strengthen the concrete's cohesiveness by strengthening the bond between the fibers, sand, and cement matrix. This configuration makes the concrete easier to deal with by making the pumping and placing operation easier.

It is evident that the inclusion of SSF-316L in mixes M2, M3, and M5 resulted in decreased workability. Furthermore, an increase in the SSF-316L content led to additional reduction in workability in mix M6, indicating a potential alteration in the properties of fresh concrete that could affect its strength and performance post-hardening [27, 28].

As the SSF 316L shavings fibers content rises in HFSC, the density gradually increases after 0.50%. At the peak percentage of 2.00%. Indeed, the shape, size, dispersion, and bulk density of SSF-316L shaving fibers can affect the concrete's apparent density.

Table 6 demonstrates how the air concentration in both mixtures M1 and M6 decreases when fibers are present. This suggests that fibers can be incorporated into the concrete matrix at rates of 0.25% and 2% to effectively disperse the fibers, improving the concrete's interaction with its constituent parts and minimizing air gaps. Compared to carbon fibers, stainless steel fibers (SSF) are recognized for their strong interfacial connection, durability, and capacity for dispersal within the concrete matrix. On the other hand, the air content in M5 has stabilized, and the compositions of M2, M3, and M4 concrete have increased.

Table 6. Physicals properties of the mixtures

As evidenced by the results reported in the literature [29], the authors reinforced concrete using two distinct types of fibers: waste metal fibers (WMF) obtained from a local lathe workshop byproduct, which had an irregular shape; and waste polypropylene fibers (WPF) obtained from polypropylene storage bags, which had a straight shape and behaved differently from the fiber-reinforced sand concretes in this study.

Researchers found that adding waste fibers to a cement matrix can be difficult when mixing and consolidating the fibers, particularly when there is a large volume proportion of irregularly shaped and complicated fibers, such as the fibers (WMF) used. In addition, compared to concrete reinforced with WMF fibers, the addition of WPF fibers had no discernible effect on workability and was easy to consolidate. Additionally, they came to the conclusion that adding fibers to the cement matrix upsets the granular skeleton and that, in order to achieve the proper workability and compactness of the fiber-reinforced concrete (FRC), the composite material's proportions must be re-optimized [30].

4.2 Mechanical properties

Mechanical property tests were performed by standard test methods specified in Table 5. The samples were submerged in water at 20℃ until the day of the test.

4.2.1 Compressive strength

Table 7 illustrates how the addition of SSF-316L to high-flow sand concrete first increased and subsequently lowered its compressive strength. It is determined that 0.25% of SSF-316L is its ideal content. The compressive strength (fc) of mix M1 was 23.21% greater than that of reference mix M0. There are numerous causes for this growth, including: First, the high-flow sand concrete's micropores were filled with fine limestone and silica gasses; Secondly, the addition of SSF-316L enhanced its density while reducing air content (as indicated in Figure 4 and Figure 5, respectively, limiting the formation of micro-cracks [31]. Furthermore, by dispersing stresses throughout the concrete matrix, 316L stainless steel fibers in concrete act as tiny reinforcement bars and help avoid cracks.

Table 7. Mechanical properties of the mixtures in 90 days

On the one hand, this can reduce the width and propagation of cracks. The compressive strength of this concrete was improved by the uniform distribution and the strong adhesion to the aggregates and the cement matrix. On the other hand, increasing the SSF-316L content does not cause a systematic improvement in the compressive strength of high-rate sand concrete, since excess SSF-316L hinders the mixing process and d homogenization of fibers, which causes a negative effect on workability [27].

The best results were obtained at 1.2% by volume, using the substitution method, according to Ndayambaje's research [32], which examined the evolution of compressive strength in concrete mixes incorporating various percentages of steel fibers extracted from abandoned tires. On the other hand, when waste polypropylene fibers were added, the samples' compressive strength increased in comparison to the control mix that lacked steel fibers [33].

4.2.2 Flexural and split tensile strength

As detailed in Table 7, Raising the content of SSF-316L led to enhancement in both flexural (ft) and split tensile (fs) strength [34]. The best mix for both strengths, M6, was determined to be 2.0%. Bending strength saw an increase of 52.18%, and split tensile strength improved by 42.36% compared to the reference mix M0. The improvement of fibres in split tensile strength can be attributed to their ability to serve as stitches between two cracked parts, to transfer the stresses and improve tensile capacity [27]. Additionally, this improvement attributed to the excellent properties of SSF-316L, with its corrugated shape playing an important role, as supported by the Fox study [21, 22].

Xu et al. [35] found has demonstrated that increasing fibre content reduces the average distance between fibres, and the increased fibres shared the load, consequently reducing the average bonding stress between the fibres and the matrix.

The decrease in bonding stress effectively restricts crack expansion, thereby enhancing both flexural and splitting tensile strength.

Furthermore, Kalpana and Tayu [4] found that the addition of steel waste substantially improves the mechanical properties of concrete, particularly impacting on tensile and flexural properties compare to compressive property. This finding corroborates our study's results [4, 36, 37].

4.2.3 Relationship between properties in the fresh and hardened state

There is a link between the mechanical characteristics, air content, and density of concrete following the addition of SSF-316L shaving fibers, as illustrated in Figure 4 and Figure 5. It is seen that SSF-316L was added to mix M1 at a rate of 0.25%, seems to an increase in density and a reduction in air content, leading to enhancement in compressive, flexural and split tensile strengths.

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Figure 4. Mechanical proprieties and density of the mixtures

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Figure 5. Mechanical proprieties and air content of the mixtures

4.3 Chemical properties

4.3.1 Sulfuric acid

A study was conducted to assess the durability of samples exposed to 5% water concentration sulfuric acid for 90 days by examining the weight loss caused by attack [25].

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Figure 6. Chemical properties (H₂SO₄) of the mixtures

Based on the results obtained in Figure 6, the best combination to withstand sulfuric acid is M0, without SSF 316L, this is attributed to the quality of the sulfate-resistant cement (SRC). It was found that the mix closest to it is M4, with a 1% SSF 316L content. In mix M4, the weight loss percentage for the high-flow sand concrete was 3.50%, matching what was observed in mix M0. This is because the fibers made of stainless steel help to reinforce the concrete structure and prevent cracking brought on by sulfate assault. Concrete constructions that exhibit porosity are vulnerable to material degradation as a result of external substance penetration [38].

The mechanical qualities of both steel- and polypropylene-reinforced concretes improve as the fiber content of the concrete rises, providing an adequate solution. This is because fibers have the capacity to limit the spread of cracks, lessen the amount of stress concentrated at the tips of cracks, and slow down the rate at which cracks expand [39].

4.3.2 Hydrochloric acid

Research was conducted to assess the durability of samples exposed to a 5% water concentration of hydrochloric acid for 90 days. The objective of the research was to study the decrease in weight due to this attack.

According to the results shown in Figure 7, it is evident that the high-flow sand concrete reinforced with SSF-316L exhibits resistance to acid attack compared to mix M0, resulting in less weight loss. Among the different ratios, mix M5 reinforced with 1.5% of SSF-316L by weight demonstrated the highest resistance against acid attack. These results agree with those found by Dsouza Nithin et al. [25]. Stainless steel fibers are more resistant to corrosion in an atmosphere containing chlorides, which can corrode conventional steels. By doing this, concrete's mechanical qualities can be preserved and its longevity increased. Moreover, adding 316L stainless steel fibers to concrete can lessen its permeability, which will stop chloride from penetrating the concrete structure.

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Figure 7. Chemical properties (HCl) of the mixtures

4.4 SEM testing

After the mechanical tests mentioned above, the specimens were cut into small discs measuring 10 mm in diameter and 2mm, in thickness. Following they were cleansed with water and drying in an oven. The microstructure of the specimens was observed and imaged using a FEI Quanta 650 FEG scanning electron microscope (SEM).

A scanning electron microscope (SEM) be used to examine specimens analyzing their structure and physical proprieties within a scale of 50 µm to 1 mm. By comparing Figure 8 representing the reference mix M0, to Figure 9 illustrating the enhanced mix M1 containing high flow sand concrete reinforced with 0.25% SSF 316L by mass, differences in diameter and number of pores are apparent.

The incorporation of SSF 316L into mix M1 led to a reduction in pore diameter and quantity. This led to a 9.67% reduction in air content and a 17 kg/cm³ increase in the density of the mixture. As a result, there was an improvement in the mechanical properties of the high-flow sand concrete.

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Figure 8. SEM of the mix M0

Figure 9 shows a cross section of a specimens from mix M1. The SSF-316L fibers have irregular edges blending well with C-S-H (calcium silicate hydrate). This enhanced bonding and cohesion between the sand concrete and the fibers explain the progressively increasing flexural and split tensile strength results.

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Figure 9. SEM of the mix M1

Additionally, Figure 10 illustrates a longitudinal section of a sample from the same mix M1. It is apparent that the SSF-316L fibers do not have smooth surfaces like standard steel fibers, but they have graduated protrusions which enhance their adhesion with the cementitious matrix. As a result, this enhances the cohesion between the high flow sand concrete and the SSF-316L.

Sun et al. [40] found that SSF fibers mainly play a bridging role through the bonding force and friction force formed by the hydration reaction between the fiber and the cementitious matrix.

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Figure 10. SEM of the mix M6