Enhancing concrete’s ballistic impact resistance using Alfa fiber

Abstract

Natural fibers are gaining interest in recent years due to the increasing calls for eco-friendly materials production. Concrete reinforcement with Alfa fiber is attracting attention to take benefit of natural fiber advantages in the construction sector. The mechanical and physical characteristics of concrete reinforced with Alfa fibers have been primarily investigated. The current study seeks to analyse the ballistic protection performance of Alfa-reinforced concrete in response to the emerging need to withstand bullet impacts and protect occupants in light of the increasing armed hostilities. Eight formulations were selected for the concrete preparation by varying the Alfa fiber content from 0 to 100 kg/m³. After determining the mechanical characteristics of the concrete such as compressive, flexural and tensile strength, ballistic resistance to bullet impacts is determined by measuring the penetration depth, the crater area and the lost mass after impact in concrete slab 10 and 15 cm thick. The formulations of Alfa-reinforced concrete exhibited lower penetration depth, crater area, and lost mass compared to the reference concrete. Optimal values were achieved with 15 kg/m³ of fibers. A dependency between the ballistic response on the fibrous concrete and flexural strength of concrete is also remarked. In conclusion, the study demonstrates that incorporating Alfa fibers into concrete significantly enhances its ballistic resistance, with optimum performance observed at 15 kg/m³ of fiber inclusion. These findings underscore the potential of Alfa-reinforced concrete for use in military constructions and structures vulnerable to impact risks, paving the way for further research and practical applications in enhancing structural and occupant safety.

Keywords

Alfa fiber fiber reinforced concrete ballistic resistance bullet impacts penetration depth crater damage lost mass

Introduction

Security is of crucial importance for both civil and military structures. One of the commonly encountered risks for these constructions is ballistic impacts, which necessitates the implementation of adequate protective measures. Concrete remains the predominant construction material due to its versatility, strength, and ability to be moulded and prefabricated into various shapes and sizes. However, its inherent brittleness limits its energy absorption capacity during impacts. Indeed, when subjected to ballistic impacts, concrete exhibits several types of damage, including surface spalling and scabbing and radial cracking around the impact point, especially for high-velocity projectiles or thin target thicknesses (Arias et al., 2009). This is why it is imperative to enhance concrete’s resistance to ballistic impacts.

Various approaches have been developed to enhance the resistance of concrete to ballistic impacts. One method involves increasing the intrinsic strength of concrete by using high or very high-strength concretes, thereby improving the material’s ability to withstand the stresses generated by impacts. For instance, Zhang et al. (2005) presented the results of an experimental study investigating the impact resistance of projectiles on concrete with compressive strengths ranging from 45 to 235 MPa. The findings showed that an increase in the compressive strength of the concrete was associated with an overall reduction in penetration depth and crater diameter in the target specimens.

A second method involves the addition of reinforcements, such as wire mesh, as demonstrated in (Kim et al., 2018). The study examined the impact resistance of concrete panels reinforced with wire mesh and steel fibers when subjected to high-speed impacts. The results obtained indicate that the incorporation of crimped wire mesh enhances overall impact resistance, particularly in the spalling area on the front face. Concrete reinforcement can also be achieved by incorporating fibers into the fresh mix. Numerous studies have explored the impact of steel fibers on concrete’s resistance to impact. For example, Ong et al. (1999) investigated various types of fibers and their volume fractions, concluding that concrete slabs reinforced with steel fibers featuring hooked ends exhibited superior cracking and energy absorption characteristics compared to other fiber types. Mu et al. (2010) demonstrated that steel fiber-reinforced concrete reduced crater diameter and minimized cracking, albeit with no significant effect on penetration depth. Wang et al. (2013) found that the addition of ultrashort steel fibers significantly enhanced concrete’s impact resistance. In another study (Prakash et al., 2016), the effect of steel fiber volume fraction was examined, revealing that steel fiber-reinforced concrete panels performed exceptionally well against multiple impacts.

In addition to steel fibers, various other types of fibers can be employed to reinforce concrete. For instance, Nia et al. (2012) compared the results of experimental and numerical impact tests conducted on ordinary concrete and fiber-reinforced concrete using hook-ended steel fibers and polypropylene fibers. The findings indicated that the impact resistance of the concrete specimens increased as the fiber volume fraction increased. Richardson et al. (2016) conducted a comparative study involving different types of synthetic fibers and steel. Their results demonstrated that synthetic fibers offered high toughness and enhanced protection against spalling in concrete slabs subjected to rifle fire. Nam et al. (2016) investigated the effects of short fibers, including polypropylene (PP), polyvinyl alcohol (PVA), and steel. They observed that the reduction in the diameter of the area damaged by projectile penetration was strongly influenced by the fiber blend’s quantity, and the inclusion of fibers significantly reduced damage to cementitious composites. Smaoui et al. (2023) conducted an investigation into the performance of concrete containing polyethylene terephthalate (PET) fractions of up to 20 kg/m³, incorporating thermally treated PET strip-shaped inclusions. This resulted in a significant enhancement of the concrete’s mechanical properties and ballistic impact resistance. Notably, it led to a penetration depth 2.5 times lower than that observed in normal concrete, a reduction in the crater area for all PET concrete specimens tested, and a remarkable decrease in the volume of impact debris generated, reaching up to 95% reduction.

Lastly, bio-based materials (Ibrahim et al., 2021), such as plant fibers also present an option for reinforcing concrete. Trabelsi and Kammoun (2020) reported that the use of prickly pear fibers increased both strength and high-velocity impact resistance. Alfa fiber (Stipa tenacissima L.), also known as esparto, is derived from plants that grow abundantly in the Mediterranean region and do not require irrigation. These fibers can be effectively used as reinforcement in cementitious materials, as suggested by several studies. Ajouguim et al. (2019) demonstrated that adding Alfa fibers improves compressive strength while reducing the composite weight. Sakami et al. (2020) observed that incorporating Alfa fibers in mortar reduces its weight and improves thermal insulation. Additionally, the mechanical strength of the composite increased by up to 10% when the fiber content reached 1%. These findings align with those of Bernat Masó et al. (2018) who also found a significant increase in flexural strength and toughness factor after adding Alfa fibers, although compressive strength slightly decreased. Miraoui et al. (2016) found that the use of unidirectional Alfa fibers at a concentration of 0.8% to 1.1% in mortar resulted in an increase in flexural strength of up to 27%. On the other hand, Helaili et al. (2023) showed that the incorporation of treated short-chopped Alfa fibers at a concentration of 1% had only a minor effect on the compressive and flexural strength of mortar. However, Ajouguim et al. (2021) found that the increase in flexural strength was accompanied by a decrease in compressive strength, depending on the length and concentration of Alfa fibers. Furthermore, Garrouri et al. (2022) investigated the use of Alfa fibers in a hydraulic lime matrix and observed a moderate reduction in compressive strength but an increase in ductility and flexural strength with the addition of 7% Alfa fibers. In the case of concrete, Khelifa et al. (2018) concluded that Alfa fibers improved tensile strength, exceeding that of both normal and polypropylene-reinforced concrete. Similarly, Messas et al. (2022) found a significant increase in tensile strength in concrete reinforced with Alfa fibers. However, it’s worth noting that excessive fiber quantity and length can lead to a reduction in compressive and tensile strength and the formation of voids in the concrete. Finally, Necib et al. (2022) concluded that the addition of Alfa to concrete significantly reduced thermal conductivity by up to 50.61%

In this context, the aim of this work is to examine the effect of Alfa fiber addition on concrete performance when subjected to the impact of a ballistic projectile. The purpose of this contribution is to enhance the tensile capacity of concrete by adding fibers, aiming to increase the concrete’s resistance to impact. Eight different fiber dosages, ranging from 0 to 100 kg/m³ of concrete, were considered. In addition to mechanical tests, projectile penetration tests were conducted on Alfa fiber concrete targets at impact speeds of approximately 700 m/s. The anti-impact capacity of fiber-reinforced concrete was assessed by examining and discussing the damage inflicted on the specimens, including the evaluation of the surface and depth of the crater created by the projectile impact and the measurement of the lost mass after impact. Furthermore, the experimentally measured penetration depth is compared with theoretical models for further analysis and discussion.

Materials and methods

Aggregate and binder

The study used gravel from the Jbel-Ressas quarry and sand from Khelidia quarry, both located in Tunisia. The gravel had a size range of 4 to 12 mm and served as the coarse aggregate, while the sand ranged from 0 to 2 mm in particle size.

The sand equivalent (SE) value exceeds 75, indicating the high cleanliness of the fine aggregate, as per NF P18-597 standards. Table 1 provides details regarding the absolute and apparent densities of the aggregates used. Additionally, Figure 1 illustrates the grain size distribution of both the gravel and sand utilized in this research.

Table 1.

Densities of aggregates.

	Apparent densities (kg/ $m^{3}$ )	Absolute densities (kg/ $m^{3}$ )
Sand	1628	2505
Gravel	1535	2542

Figure 1.

Grain size curves of sand and gravel.

The binder utilized is Portland cement CEMI 42.5 from Tunisa, produced in compliance with the NT 47-01 and EN 197-1 standards. It has a density of 3210 g/cm³ and a compacity of 0.574.

Alfa fibers

The Alfa fiber used in this study is sourced from Tunisia. Figure 2 shows, from left to right, gathered yarns, a microscopic view of an individual yarn and its cross-section. The tensile strength of the Alfa fiber is assessed using a sample of three fibers through the MTS Criterion Series 60 testing machine, yielding an average ultimate strength of 425 MPa. The fibers utilized in this study possess an average density of 510 kg/m³. It’s worth noting that Hamza et al. (2013) reported differing density estimates of 454 kg/m³ and 890 kg/m³. This variability can be attributed to factors such as species diversity, climatic conditions, soil quality, and plant maturity.

Figure 2.

Alfa fibers.

The literature review reveals that subjecting plant fibers to hot water treatment offers significant advantages. This process effectively eliminates water-soluble sugars, alters the surface of the fibers to make them rougher, and improves their wettability. These transformations enhance the adhesion between the fibers and the cementitious binder, resulting in a substantial improvement in the compressive and flexural strength of the mixtures (Al-Kheetan, 2023; Kammoun and Trabelsi, 2018, 2019; Sellami et al., 2013). For this reason, in this study, the Alfa fibers underwent a process of water treatment before being utilized. This treatment involved exposing the fibers to water at a temperature of 60°C for a duration of 15 min.

Various concrete mixtures were prepared by adjusting the quantity of Alfa fiber. The fibers were cut to ensure a consistent length of 5 cm.

Elaboration of the concrete

The formulation of ordinary concrete (C₀), which serves as a reference, is used as the base formulation. The volume of Alfa fiber substitutes an equivalent volume of the aggregates, maintaining a constant volumetric ratio between gravel and sand. Eight formulations are studied, varying the fiber proportion from 0 kg/m³ to 100 kg/m³. Table 2 provides a comprehensive overview of the formulated mixtures and their compositions. The different mixtures are designated by the symbol C_x, where the subscript x indicates the Alfa fiber content in kg/m³.

Table 2.

Concrete compositions.

Designation	C₀	C₅	C₁₀	C₁₅	C₂₀	C₄₀	C₆₀	C₁₀₀
Gravel	1239	1211	1195	1179	1164	1101	1048	913
Sand	652	650	642	633	625	591	557	489
Cement	350	350	350	350	350	350	350	350
Water	175	175	175	175	175	175	175	175
Fibers	0	5	10	15	20	40	40	40

Concrete mixing (Figure 3) is performed at a consistent speed throughout the various stages of material preparation. Initially, the dry aggregates, including gravel, sand, and treated Alfa fibers are blended until a uniform and flawless mixture is achieved. Subsequently, water is gradually added to the blend. Mixing concludes once the concrete reaches a satisfactory level of uniformity. Three specimens are prepared for each composition of the fresh mixture as part of each test. After 24 h, the test samples are removed from the mold.

Figure 3.

Concrete mixing steps.

Experimental program

The compression test is carried out at 3, 7, 14, and 28 days on cubes measuring 15 cm on each side according to EN 196-1 (see Figure 4(a)). Two tests are performed to determine the tensile strength of the concrete: the three-point bending test (Figure 4(b)) and the splitting test (Figure 4(c)). The three-point bending test is conducted at 3, 7, 14, and 28 days on specimens measuring 7 cm by 7 cm by 28 cm according to EN 196-1. Flexural strength, measured by this test, assesses the concrete’s resistance to bending forces.

Figure 4.

Mechanical tests.

The tensile strength of concrete is assessed by the 28-days split tensile test to ASTM C496 (ASTM, 2004.) and BS 1881 117-83 (British Standards Institution, 1983) on cylindrical specimens of (16 × 32) cm². In this test, two compressive stresses are applied in two diametrically opposed lines across the diameter of the cylinder, resulting in the development of strong tensile stresses which cause the specimen to fracture. The concrete’s resistance to ballistic impacts is evaluated following the experimental setup depicted in Figure 5, which illustrates the bullet, the firearm, and the target slab. The test bullet is characterized by a 5.56 mm calibre, a mass of 3.61 g, and an impact velocity of 720 m/s. The impact velocity is measured using two light screens connected to a chronograph. The concrete slab measures (50 × 50) cm² in area and is available in various thicknesses (5 cm, 10 cm, and 15 cm). The shooting distance is carefully controlled to meet the necessary conditions for controlling the bullet’s impact angle.

Figure 5.

Ballistic testing equipment.

Three parameters are used to quantify the ballistic resistance of the tested slabs. The first parameter is the penetration depth, which represents the maximum depth within the damaged region in relation to the initial surface of the slab. The second parameter is the crater area, which measures the area of the crater created by the impact. The third parameter is the lost mass, which is the difference between the initial mass of the slab and the mass after the bullet impact.

Results of concrete proprieties

The obtained results of concrete properties are presented in the following sections. Firstly, the workability and density, of the concrete are examined and compared with those of plain concrete. Secondly, the mechanical properties of the fiber concrete are discussed.

Workability and densities

Figure 6 depicts the results obtained for concrete workability, characterized by both mean values and standard deviations. As the Alfa fiber content increases, the slump also increases until it reaches an asymptotic behaviour above an Alfa fiber content of 20 kg/m³. The values obtained fall within the range of [10–12] cm, classifying the Alfa concrete under the S3 consistency class.

Figure 6.

Workability and density of concrete.

Conversely, concrete densities decrease with increasing Alfa fiber content. This decrease can be attributed to the replacement of conventional concrete aggregates with the lighter Alfa fiber. Notably, the addition of 100 kg/m³ of Alfa fibers results in a 17.46% reduction in the mixture’s density, to reach a value of 1992 kg/m³. Consequently, according to the NF EN 206-1 standard, the obtained concrete with 100 kg/m³ of fibers can be considered as light fibrous concrete.

Compressive strength

Figure 7 displays the compressive strength of concrete mixes at different ages (3, 7, 14, and 28 days) concerning fiber content. Regardless of the concrete’s age, the same behaviour is observed when fibers are added. In fact, the addition of fibers results in a decreased compressive strength of the concrete mix, which becomes more pronounced as the volume of fibers increases. At 28 days, the reference concrete has an average compressive strength of 36 MPa. When 20 kg/m³ of Alfa fibers is added, a 24% reduction in the initial value is observed, and the addition of 100 kg/m³ causes a reduction of 46%. These findings align with the results of Khelifa et al. (2018), who reported a 14% reduction in concrete strength after 28 days when they added 20 kg/m³ of Alfa fibers.

Figure 7.

Compression test results.

In the current study, it was found that the compressive strength of the reference concrete at 28 days is reduced by 24% with 20 kg/m³ of fibers and by 46% with 100 kg/m³ of fibers. Previous research on reinforcing concrete with plant fibers supports this reduction caused by fiber inclusion. Several factors contribute to this decrease, including the comparatively lower inherent strength of plant fibers compared to traditional aggregates used in concrete, the increased void volume due to the higher porosity of plant fibers compared to traditional concrete aggregates, poor adhesion between the fibers and the cement matrix, and reduced compactness of the composite.

Flexural strength

The flexural strength results for Alfa concrete are shown in Figure 8. Regardless of the age of the concrete, the same behaviour is observed when fibers are added. In fact, when the amount of fibers varies between 0 and 15 kg/m³, the addition of fibers increases the flexural strength with a quasi-linear trend reaching its peak at about 15 kg/m³. Further increases in fiber content lead to a reduction in flexural strength. As the fiber content increases, the resistance decreases. Illustratively, the reference concrete has an average flexural strength of 3.22 MPa after 28 days. The addition of fibers increases the flexural strength by 134% and 28% compared to the reference concrete for 15 kg/m³ and 100 kg/m³ of Alfa fibers, respectively. Consequently, the inclusion of Alfa fibers reduces the flexural strength of the C100 formulation by 45% compared to the C15 formulation.

Figure 8.

Flexural test results.

Figure 9 depicts the specimens after the flexural test, confirming the brittle behaviour of the reference concrete. However, the incorporation of fibers enhances the material’s ductility. Figures 9(c) and (d) clearly demonstrate the bridging effect resulting from fiber inclusion, which hinders crack propagation. This behaviour is also observed when other types of cellulosic fibers are added to concrete (Kammoun and Trabelsi, 2018, 2019)

Figure 9.

Experimental observation of the specimen response for various Alfa fiber amount.

After conducting the flexural test and examining the broken specimens (Figure 10), it is observed that the Alfa fibers remain intact at the point of fracture in the specimen. Some holes can be seen as a result of the fibers being pulled out of the matrix (Figure 10(a) and (b)). This indicates that the tensile bond strength between the Alfa fibers and the cement matrix is significantly weaker than the tensile strength of the Alfa fibers themselves. Figure 10(c) and (d) provides a microscopic view of the Alfa fibers anchored in the concrete. The image of the Alfa fiber, separated from the matrix, shows residual cement (Figure 9(d)). Therefore, the observed failure mode, which results in the preservation of the integrity of the Alfa fiber, can be attributed to the low relative adhesion of the cement matrix to the plant element.

Figure 10.

Aspect of the broken fibrous specimen at the surface of the crack.

Splitting tensile strength

The split tensile test provides tensile strength along the cylinder diameter under compressive load. Figure 11 indicates a similar shape of the tensile resistance evolution regarding the amount of fiber reinforcement as for flexural strength (Figure 8). Similarly, when the amount of fibers varies between 0 and 15 kg/m³, the addition of fibers increases the tensile strength with a linear trend, reaching its peak at about 15 kg/m³. Further increases in fiber content lead to a linear reduction in tensile strength. Compared to the reference concrete, the optimal formulation of fibrous concrete (C₁₅) exhibits an enhancement of 80%. While the increase in fiber inclusion amount in the C₁₀₀ Alfa-concrete leads to only a 10% enhancement in tensile strength.

Figure 11.

Splitting tensile test results.

The correlation between splitting tensile strength and flexural strength is illustrated in Figure 12. The trend curve obtained displays a linear relationship with a regression equation of y = 0.814x. An excellent goodness-of-fit is indicated by the coefficient of determination R², which is 0.9924.

Figure 12.

Correlation between flexural strength and tensile strength.

Flexural/tensile and compressive strength relationship

Flexural and compressive strengths in concrete are typically related, and various equations can be found in the literature. For instance, standards such as AS 3600 and CSA-A23.3 (Russell et al., 1997) specify this relationship as $f_{R} = 0.6 \sqrt{f_{c}}$ , while the ACI 363-92 standard proposes $f_{R} = 0.94 \sqrt{f_{c}}$ .

Several researchers have established relationships between flexural or tensile strength and compressive strength, as reviewed in (Chhorn et al., 2018). These equations typically follow the format $f_{R} = a f_{c}^{b}$ , where a and b are variable coefficients.

The present study demonstrates a clear bi-linear relationship between compressive strength and both tensile and flexural strength (Figure 13). The curves depicting flexural and tensile behaviour as a function of compression show an initial linearly increasing trend followed by a linearly decreasing trend, with peaks observed at a compression strength of 29.44 MPa. These peak values are associated with the addition of 15 kg/m³ of fibers into the concrete. This trend is attributed to the inherent characteristics of the added plant fibers, which reduce compressive strength while simultaneously increasing tensile strength until a specific threshold is reached.

Figure 13.

Flexural/Tensile and compressive strength relationship.

Ballistic performance

Penetration depth

Ballistic resistance is determined by measuring the penetration depth when the concrete slab remains unperforated. Effective resistance to bullet penetration is observed in both the 10 cm and 15 cm thick slabs. The results, shown in Figure 14, depict the correlation between the Alfa content in the concrete mixture and the degree of penetration. The addition of fibers, up to 15 kg/m³, resulted in a linear reduction in penetration. However, as the fiber content increased beyond 15 kg/m³, projectile penetration also increased. Nevertheless, the curve exhibited asymptotic behaviour, with values remaining below 3 cm.

Figure 14.

Penetration depth for various amounts of fibers.

All tested Alfa concrete compositions exhibited shallower penetration depths compared to regular concrete. Consequently, it can be concluded that Alfa fibers enhance the concrete’s resistance to ballistic impacts.

Let P_d (in meters) represent the penetration depth, and d (in meters) the projectile diameter. Figure 15 presents P_d/d values obtained from experimental tests and calculated using the formulas from the US Army Corps of Engineers (ACE) and NDRC (Abdel-Kader and Fouda, 2019; Zhang et al., 2005), based on compressive strength.

Figure 15.

Penetration depth function of concrete compressive strength.

The ACE formula is defined as follows:

\frac{P_{d}}{d} = \frac{3.5 \times 10^{- 4} M V_{1}^{1.5}}{f_{c}^{0.5} d^{2.785}} + 0.5

(1)

The NDRC formula is defined as follows:

\begin{array}{l} G = {(\frac{P_{d}}{2 d})}^{2} for \frac{P_{d}}{d} < 2.0 \\ G = \frac{P_{d}}{d} - 1 for \frac{P_{d}}{d} > 2.0 \end{array}

(2)

The equation for G is derived as follows: $G = \frac{3.8 \times 10^{- 5} N M V_{1}^{1.8}}{f_{c}^{0.5} d^{2.8}}$ , where M(kg) represents the projectile mass, N = 1.14 denotes the shape factor for a sharp projectile, V₁ (m/s) represents the impact velocity, and f_c (Pa) is the compressive strength of concrete.

From Figure 15, it is evident that the ACE and NDRC formulas do not align well with the data related to fibrous concrete, as they yield penetration estimates significantly higher than the experimental values. Moreover, the evolution of penetration as a function of compressive strength appears to be piecewise polynomial, whereas the ACE and NDRC formulas suggest that penetration is inversely proportional to $\sqrt{f_{c}}$ . Hughes (1984)explains that the NDRC and ACE formulas incorporate the impact parameter $I = \frac{m v^{2}}{f_{r} d^{3}}$ , which is inversely proportional to the tensile strength fr. The latter is derived from the compressive strength f′c using the equation $f_{r} = 0.63 \sqrt{f_{c}}$ , which appears to be inappropriate for the data depicted in Figure 13.

The penetration depth curve exhibits a shape similar to the symmetry of that in Figure 11, illustrating the relationship between flexural strength and compressive strength. This confirm that the bullet’s penetration depth in Alfa concrete depends on the flexural strength rather than the compressive strength. Therefore, Figure 16 demonstrates the significant influence of the concrete’s flexural strength in reducing penetration depth. In essence, penetration depth decreases as the material’s flexural strength increases, and a linear trend is clearly observable. Hence, the ballistic resistance of fibrous concrete is primarily determined by its flexural strength. These findings align with the conclusions of Kristoffersen et al. (2021) which emphasize that the ballistic resistance of thin concrete slabs depends largely on tensile strength rather than compressive strength.

Figure 16.

Penetration depth function of concrete flexural strength.

This finding is also consistent with the result of the impact resistance of concrete slabs reinforced with prickly pear fiber (PP) (Trabelsi and Kammoun, 2020) and PET fiber (Smaoui et al., 2023), as presented in Figure 16 for the purpose of comparison. Within the same figure, a linear dependency between penetration depth and flexural strength increases is evident. An examination of the data reveals that the Alfa-reinforced slabs exhibit lower flexural strength, especially when compared to concrete with prickly pear fibers, which is based on a high-performance concrete formulation. Nevertheless, the penetration depth achieved with Alfa-reinforced concrete is comparable to or even less than the penetration depth obtained with concrete reinforced with PET and PP.

Impact area and lost mass

Concrete is the material used for construction in industrial applications and military fortifications. It is a brittle material which causes fragmentation ejection under ballistic impact. This spalling can be responsible for injuries to people near the incident area. One measure to quantify the risk associated with these ejections is the lost mass of the concrete slabs. Moreover, the crater area is indicative of the ejection extent and the ballistic resistance of the concrete material.

Figure 17 presents the progression of the crater area as a function of the Alfa content. It is apparent that the Alfa inclusion significantly reduces the damaged area of the concrete slabs compared to ordinary concrete. Additionally, the crater area decreases as the fiber content rises. The concrete reinforced with Alfa fiber displays a reduction of the crater surface from 60% to 90% compared to the plain concrete. Thus, the inclusion of fibers restricts the propagation of cracks, resulting in a decrease in the damaged area.

Figure 17.

Impact area.

Figure 18 presents a comparison of crack propagation on the front surface of plain concrete (a) and Alfa-concrete (b). Severe radial cracks are visible beyond the crater surface in the former, while the latter shows no such cracks. The findings are consistent with those of Zhang et al. (2005), which concludes that adding steel fibers to plain concrete permits arresting crack propagation.

Figure 18.

Crack propagation on the front face.

Furthermore, it is apparent from Figure 17 that there is a decrease in the impact area as the thickness of the plain concrete slab increases, although this difference is diminished with an increase in the amount of added fiber. As a result, the effect of slab thickness becomes negligible beyond the 15 kg/m³ composition of slabs.

The impact caused by a ballistic event on people nearby is directly related to the amount of concrete that is displaced. Figure 19 displays the relationship between the lost mass of the target, fiber content, and slab thickness. The inclusion of fiber reduces the lost mass, which represents the difference between the target’s initial mass and post-impact mass.

Figure 19.

The lost mass of concrete slabs.

Figure 20 presents a series of frontal impacts on the reinforced concrete. The red dots mark the fragments that remained attached to the impact locations on the concrete blocks. This behaviour is another advantage of fiber inclusion, contributing to the reduction in mass loss. Similar to the depth of penetration results, concrete reinforced with 15 kg/m³ of Alfa fiber demonstrates the best performance in ballistic impact.

Figure 20.

Face damage of concrete with Alfa fiber.

In conclusion, it can be inferred that when subjected to an impact test, fibers act as reinforcing components for the concrete matrix, generating a bridging effect at the crack level. This disperses the energy generated by the impact. Additionally, the inclusion of fibers enhances the concrete’s ability to withstand tensile forces and prevents cracking, thereby enhancing the material’s impact resistance.

Conclusions

Concrete reinforcement with Alfa fiber has already been examined to validate the material’s ability to fulfill the main requirements of the civilian construction industry. This research investigates the ballistic resistance of Alfa-reinforced concrete subjected to bullet impact. The added amount of Alfa fiber was varied from 0 kg/m³ to 100 kg/m³ to examine the performance dependence on the inclusion percentage. First, slump, compression, flexural, and split tensile tests were carried out to investigate the effect of the Alfa fiber inclusion on the mechanical and physical properties of the reinforced concrete and to compare the obtained results with existing findings. Next, the impact response of the fibrous concrete is analysed and discussed based on the mechanical properties of the proposed material. The gathered results draw the following conclusions:

• The compressive strength results are in line with existing findings and show a decrease in concrete strength as the fiber proportion increases in the mixture. This behaviour is mainly attributed to the higher compressibility and porosity of natural fiber compared to ordinary concrete aggregates.

• The flexural and tensile strengths of the prepared formulations of the fibrous concrete are higher than the ordinary concrete results, with a maximum achieved for the 15 kg/m³ amount of Alfa fiber inclusion. The inspection of failure planes demonstrated a pull-out of intact fiber before the damage propagation. This finding suggests the seeking of methods to improve fiber-matrix bonding.

• All the tested formulations of the Alfa fiber-reinforced concrete manifested a lower bullet penetration depth than ordinary concrete. The best-achieved performance is obtained by inclusion of Alfa fiber equal to 15 kg/m³. The analyses of the ballistic resistance dependence on the material mechanical properties showed that the bullet penetration depth is primarily controlled by the flexural strength of the fibrous concrete. Moreover, ballistic performance comparison of reinforced concrete with Alfa, prickly pear, and PET fibers reveals that the Alfa fiber is the very beneficial in terms of the penetration depth reduction of the impacting bullet.

• Concrete materials are brittle materials, which cause multiple cracking and fragmentation upon bullet impact. The present study demonstrated that Alfa fiber inclusion in concrete mixture imitates synthetic fiber inclusions in arresting crack propagation and limiting fragment ejection. Indeed, the Alfa-reinforced concrete registered a drop of approximately 60% and 80% in overall lost mass and impact area, respectively.

The present research explores the extent of reinforcing concrete material with Alfa fiber to improve the ballistic resistance of the material compared to the ordinary one. Overall, the research findings suggest that incorporating Alfa fiber into concrete formulations has promising implications for both civilian and military construction. This could lead to the development of more resilient, durable, and safer structures, addressing the needs of diverse sectors ranging from infrastructure development to national defence.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Zied Kammoun

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