A comprehensive review: The impact of recycling polypropylene fiber on lightweight concrete performance

2.1 Physical and chemical properties

PPF is effective in keeping cracks to a minimum, as shown in Figure 4, to strengthen the concrete’s ability to resist heat loss. The concrete’s fibers also distribute the tension stresses throughout the material.

Figure 4

PPF used in an investigation [111].

Table 2 displays the PPF’s physicochemical characteristics [110], and Table 3 presents the geometrical properties of PPF [111]. The physicochemical attributes and geometrical properties of PPF play pivotal roles in improving concrete to be green concrete by utilizing PPF. The findings not only have direct implications for the development of durable and energy-efficient structures but also contribute to the broader discourse on sustainable building materials.

For high-strength concrete, PPF was used as a reinforcing material [112]. These fibers are perfect for this application due to their high tensile strength, enhanced elongation, and low elastic modulus. Table 4 lists the physical attributes of the fiber in further detail.

Eidan et al. [113] developed PPFs with fibrillated shapes, as presented in Figure 5. Moreover, the characteristics of the fiber are presented in Table 5.

Figure 5

Shape of PPFs [113].

The term “straight or distorted fragments of extruded, oriented, and cut polymer material” refers to a particular kind of polymer fiber.” PPF is of two different types: microfiber and macrofiber [113], as presented in Figure 6. Their length is the most distinguishing feature but they also serve distinct purposes in the poured material. Fibers increase LWC’s flexural tensile strength, impact resistance, and postcracking ductility [113].

Figure 6

(a) Microfiber-PPF and (b) macro-PPF [114,115].

In this way, steel reinforcement can be made faster, reducing the initial outlay costs. Typically, they range in size from 30 to 50 mm. Microfibers are less than 30 mm in length, thus they cannot support weight [114,115].

Instead of using a mesh to prevent cracks, micro-PPF can be used. Either a single filament or a network of fibrils characterizes them. Melt spinning yields monofilaments, which are then used to create PPF, or one can use a sheet of polypropylene film to create fibrillated fiber [116]. According to calculations, micro-PPF has an elastic modulus of around 3.8–7.0 GPa and a tensile strength in the range of 300–450 MPa. At 400–760 MPa and 3,512.0 GPa, macro-PPF has a stronger tensile strength and elastic modulus [114].

Fiber hybridization, or incorporating fibers of different materials and/or lengths into the concrete to make HFRC, is described. It is a reliable method to regulate cracks of varying widths and depths over a range of cure times and loading conditions [117,118]. That is, when tiny cracks emerge in the concrete, short fibers can be used to patch them up. Given that they are greater in amount in the mixture, microfibers are more efficient than macrofibers. Thus, the concrete’s early tensile phase is characterized by an increase in strength. When the microscopic fissures join together and macrocracks form, the longer fibers become dynamic and work to slow down the crack expansion, thus increasing the ductility of the concrete and allowing them to withstand a greater load despite the ongoing deformations. For example, the efficacy of combining microfibers and macrofibers was confirmed by showing that after combining the toughness, compressive, tensile, and flexural strength of a material made from polypropylene microfibers and macrofibers increased, whereas drying shrank the material less [119]. A polyethylene/PPF blend improved the postcracking behavior of concrete, as shown in the study of Jongvivatsakul et al. [120].

To improve the mechanical properties of LWC, including its compressive and tensile strengths, without considerably increasing the related heat conductivity, Wang et al. [86] used PPF. Table 6 displays the mechanical properties of polyester fiber (PF), revealing that its tensile strength is pointedly greater than that of normal lightweight concrete. As a result, PF is a great option for enhancing the durability and strength of lightweight concrete.

Badogiannis et al. [93] used a synthetic (polypropylene) macrofiber and three steel fibers of varying total lengths (30, 36, and 60 mm) in the experiment. Steel fibers of 36 and 60 mm length were found to have the standard double-hooked end shape, but steel fibers of 30 mm length were found to have an opposite-hooked end shape. As shown in Figure 7, the fiber and pumice (8–16 mm) penetrate the concrete. Table 7 summarizes the geometrical and mechanical parameters of all fibers.

Figure 7

Materials used in the concrete mixtures examined [93].

In addition, Hussain et al. [121] reported that PPFs, steel fibers, and glass fibers, shown in Figure 8, provide a comparative perspective of fiber reinforcements. Table 8 details various characteristics of the fibers.

Figure 8

(a) Steel fibers, (b) glass fibers, and (c) PPF [121].

To sum up, the three kinds of fibers that are most frequently utilized in LWC are PPF, steel fibers, and glass fibers. The geometrical and physical characteristics of these fibers are compared in Tables 2, 7, and 8. PPF is resistant to mildew and moths and provides great insulation from electricity. Glass fibers have a larger dimension ratio and a smaller density compared to steel and PPF fibers, while steel fibers have a greater tensile strength and elastic modulus compared to PPF fibers. The mechanical characteristics of various types of LWCs, such as those with and without PPF, are compared in Tables 3–6. They demonstrate that the inclusion of PPF can dramatically enhance the tensile and compressive strengths of LWC without significantly raising the associated heat conductivity. Although to a lesser extent than PPF fibers, steel and glass fibers can also enhance the mechanical qualities of lightweight concrete.

2.2 Microstructure properties

Jeyanthi and Revathi [110] stated that a comparison of the SEM images of PPF in Figure 9 reveals that the fibers are not sticky and have a shining surface, both of which indicate the presence of fibers within the concrete.

Figure 9

SEM image of PPF: (a) 200 lm, (b) 100 lm, and (c) 50 lm [110].

Additionally, Givi et al. [122] stated that the great adhesive strength of fiber to the composite binder is evidenced by the fact that when breaking a fiber-reinforced concrete sample, cement paste fragments closely fitting microfiber fiber are plainly evident at a crack (Figure 10). Therefore, under all conditions, the composite binder (containing PPF) concrete has superior qualities over the Portland cement concrete.

Figure 10

An example of the microstructure of the fiber’s interfacial transition zone in modified cement [122].

According to Ahmed et al. [112], given the high tensile strength, high elongation, and low modulus of elasticity of PPFs, they were utilized as reinforcing materials in high-strength concrete. Moreover, Yoosuk et al. [67] stated that the SEM images of the PPM samples shown in Figure 11 exhibited interfacial transition zones between fiber and matrices, which were produced by PPF [37]. Mixing became more challenging when the fiber content was higher than optimal. Consequently, the fiber began to ball up, resulting in the matrix developing large holes. In addition, the voids inside the matrix increased with the foam content (Fc). Thus, the fiber and geopolymer paste had less adhesion.

Figure 11

SEM images of PPF reinforcement. Fc = 0%, and PPF content: (a) 0%, (b) 1.5%, (c) 2.5%, and (d) 3% [67].

According to Bentegri et al. [123], given their high tensile strength, fibrillated twist PPFs of 19 and 54 mm are designed to be utilized in reinforcing and precast concrete (689 MPa). However, PPFs that are 30 mm in diameter are less tensile and have a wave-like appearance (486 MPa). Additionally, in contrast to 19 and 54 mm fibers, this type of fiber is more flexible due to its high elastic modulus (6.9 and 5.75 GPa, respectively). Scanning electron microscopy (SEM) analyses reveal distinct differences in the fiber form (Figures 12 and 13).

Figure 12

SEM images of the fibrillated twist PPF [123].

Figure 13

SEM images of the wave PPF [123].

Figure 14 displays the microscopic morphologies of PPF and cementitious materials, as reported by Chen et al. [124]. The fibers retain their morphology despite the attachment of a small quantity of cementitious elements to their surfaces. This result suggests that the fiber and cementitious components do not appear to react chemically. Alkali-activated composites demonstrated superior interfacial binding strength with various fiber types, as demonstrated by Bhutta et al. [125].

Figure 14

SEM images of the PPFs [124].

Hydrophilic fiber, such as basalt fiber (BF), and hydrophobic fiber, such as PF, are described in the study of Jiang et al. [126]. The bonding performance between BF and the shotcrete matrix is considerably superior to that between PF and the shotcrete matrix owing to the alteration in physical and mechanical properties of BF and PF. Therefore, as shown in Figure 15, the BF has a shorter pull-out length than the PF. Consequently, the ductility, toughness, and rigidity of LWC shotcrete can be improved by a two-fiber material and can prevent the improvement of various cracks in the course of the lightweight aggregate fiber shotcrete (LAFS) impact test. Smaller fibers tend to prevent cracks from occurring, whereas larger fibers prevent cracks from spreading and penetrating at a macroscopic scale [127]. As a result, PF is more flexible and has less stiffness than BF. As an added advantage, PF has a substantially longer draw-out length than BF. After peak stress, ductile deformation in concrete is enhanced further by PF.

Figure 15

Reflection of BF and PPF in the LAFS matrix: (a) BF and (b) PPF (PF) [126].

SEM images for ultralightweight high-strength concrete (ULHSC) reinforced with PP fibers are shown in Figure 16. Large gaps are observed among the PP fibers and the cement matrix, suggesting that the PP fibers are only tenuously bound to the matrix [128]. Loose bonding and poor mechanical properties can be traced back to the hydrophobic nature of synthetic PP fibers. Matrix components of ULHSC-PP include Ca, Si, and O, with a lower Ca/Si ratio in ULHSC-PP compared to ULHSC-0. Only a slight increase in static mechanical characteristics was observed, and numerous PP fibers ruptured and broke due to their low tensile strength. The dynamic mechanical capabilities of the ULHSC reinforced with PP fibers were high, showing that the bonding strength between the fiber and matrix is not the only determinant of the dynamic qualities [128].

Figure 16

SEM images of (a) and (b) ULHSC-0.5 PP- and (c) and (d) ULHSC-1.0 PP-reinforced ULHSCs [128].

3.1 Compressive strength

Badogiannis et al. [93] explored how the addition of PPF affected the mechanical properties (i.e., compressive strength) of lightweight concrete. A total of nine mixes (six containing steel fiber, two containing PPF, and one serving as a reference without fiber) were tested as part of the experimental inquiry. The data collected during the experiments revealed that the totaling of either type of fiber remarkably increased the compressive strength compared with plain concrete between 16 and 76%. Strong dependence on the term Vf*(l/d) was found for the magnitude of the compressive strength for steel and PPF. Compressive strength was assessed and found to be stronger in all mixes than in the plain concrete by as much as 110%. The term Vf*(l/d) was found to be dependent on compressive strength. Instead, postcrack behavior was substantially linked with the fiber type [93].

El Ouni et al. [132] mentioned that natural aggregate concrete (NC) and recycled aggregate concrete (RAC) saw different effects when a single PPF was added to the mix (RC). Compressive strength in NC and RC is reduced by 4–6% when single synthetic fiber (i.e., PPF) are used because PPF has a lower density than the plain concrete matrix. The difficulty in dispersing microfiber, which leads to buildup of fiber filaments, is another reason [133] for the detrimental effect of PPF on compressive strength. As a result, the concrete’s total porosity improves. Compressive strength was increased when hybrid SF and PPF were used. In high-strength (54 MPa) applications, RC with hybrid fiber can perform up to 94%, as well as plain NC. The compressive strength of RC is increased by 17.2 and 19.6%, respectively, while using 0.75% SF/0.25% PPF and 0.85% SF/0.15% PPF instead of plain RC. PPF reduced the compressive strength when applied alone, but adding SF would have a synergistic effect. Thus, the combined effect of polypropylene and SFs is more beneficial than the sum of the effects induced by individual fiber. By utilizing fiber of varying lengths and diameters, the fracture bridging limit of concrete can be increased, resulting in synergistic effects. When polypropylene and SF are combined, a more corrosion-resistant RC can be produced than when using only SF. In addition, compared with single-SF RC, hybrid fiber RC is lighter and less expensive. PPF can provide an effective hybrid fiber RC because it has a smaller carbon footprint and is less expensive than SF per unit volume [134].

Figure 17 depicts the compression failure of several fiber-containing and fiber-free concrete compositions. PPF-reinforced concrete is more robust than regular concrete, even if it has a low compressive strength because the fiber fills in the gaps caused by the macrocracks, preventing the concrete from breaking apart too easily once the maximum stress has been applied. The single SF-reinforced concrete and the hybrid fiber-reinforced concrete samples maintained a high compressive strength when the maximum load was applied [132].

Figure 17

Patterns of compression failure in concrete with (a) no fiber (b) one SF, (c) a single PPF, and (d) an SF-PPF hybrid [132].

PF is a popular choice for reinforcing the strength of lightweight concrete [86] because it offers high-performance abrasion, corrosion, and fire resistance without considerably increasing the thermal conductivity of the concrete. While studies have shown that the addition of PF can enhance the compressive and tensile strengths of lightweight concrete, its effectiveness in increasing compressive strength is found to be less than that of its tensile strength. Therefore, PF can be an excellent option for enhancing the durability and strength of LWC, particularly when tensile strength is a priority. Furthermore, the selection of an appropriate fiber type and configuration for reinforcing lightweight concrete should be the result of the specific performance requirements of the concrete and the conditions in which it will be used.

Compressive strength of fiber with lengths of 3, 6, 9, 12, and 19 mm was reported to be raised by 57, 46, 57, 57, 71, and 6%, respectively, by Wang et al. [86] (Figure 18). The bridging action of the fiber at the crack face shows that its insertion can increase the compressive strength. An anisotropic sample was produced because disseminating the longer PPF uniformly was difficult. The improvement is less pronounced when the fiber length is more than 12 mm.

Figure 18

Effects of fiber length on the compressive strength [86].

According to Jeyanthi and Revathi [110], compressive strength is measured to determine the overall strength of composite concrete. Casted samples (0, 15, and 30% cenosphere with 0.5% PPF) were cured for 28 days before being put through their paces. The following is a summary of the findings based on the mean values of the strengths: The test may be conducted at day 28 because the minimum strength criteria will have been met by day 7. Levels of maximal strength might be from 0 to 30% (without any admixture) (18.503 and 17.96 MPa – 0 and 15% – on day 7, 25.09 and 23.17 MPa – 0 and 30% – on day 28).

Amran et al. [135] further reported that after 28 days, the composition’s compressive strength had reached 88.5 MPa, which was higher than the control nonreinforced composition (76%) and the reinforced composition (74%). This result proves that the fiber reduces the compressive strength, with the produced polymineral modifier providing majority of the effect. According to Ahmed et al. [112], several scientists have shown that adjusting the proportion of coarse recycled gravel to PPF considerably affects the concrete strength. However, Ahmed et al. [112] demonstrated the effect of PP fiber in concrete with silica fume and coarse recycled material (readily available due to the numerous demolished structures in the Mosul City research zone) on the basis of criteria, such as compressive strength. To achieve this goal, coarse RA concrete (at 0, 50, and 100% volume) is combined with varying percentages of PP fiber (at 0, 0.15, 0.3, 0.45, 0.6, 0.7, and 0.9% volume).

PPF played an important role by enhancing the test findings; its height fineness and length variation in the staple made a bridge action to prevent further construction of microfractures. Compressive strength was maximized at 0.6% PP fiber, with increases of 24.8 and 24.4% for 50 and 100% RA replacement.

According to Meena and Ramana [136], fiber boosted the compressive strength of the concrete. Compressive strength was enhanced by over 17% in comparison with regular concrete after 28 days due to an increase in the fiber content of 0.0–0.5%. The effectiveness of the PF and the concrete mix in stopping cracks from forming is one possible explanation for the concrete’s enhanced compressive strength. Nonlinear regression analysis is used to convert PPF content into an expression of compressive strength. Meena and Ramana [136] noted that up to a PPF concentration of 0.5%, the concrete’s compressive strength increased. Compared with regular concrete, the compressive strength of mixtures containing 0.5% PPF increased by 17% (Figure 19).

Figure 19

Experimental compressive strength at various PPF contents [136].

Salehi et al. [79] reported that adding PPF to the control specimens had no appreciable effect on the size of their compressive strength. In contrast to the first group, the percentage of compressive strength increased when bacteria with 0.5% fiber were present. The 1% fiber group had a higher level of bacterial activity. The number of pores in the concrete increased with the fiber fraction.

For temperature effects, concrete with various PPF contents exhibited various compressive strengths both at room temperature and at higher temperatures [137]. After 28 days, the fiber dosage up to 0.5% polypropylene content increased the compressive strength at room temperature, achieving a peak increase of over 17% at 0.5% PPF, before starting to decline. At 200°C, concrete reinforced with PPF at doses of 0.20, 0.35, and 0.50% had compressive strengths that were, respectively, 2.87, 3.51, and 4.43% higher than those of concrete reinforced with PPF at ambient temperature. The compressive strength of concrete containing 0.75 and 1.00% PPF dropped by 3.8 and 4.61%, respectively, as compared to PPF concrete at 27°C [138]. All PPF mixes lost compressive strength at 400°C. In comparison to room temperature, the compressive strengths of PPF concrete mixtures with 0.20, 0.35, 0.50, 0.75, and 1.00% fiber dosage were 12.47, 13.50, 12.35, 15.51, and 17.38% lower, respectively [132,139].

To conclude, Wang et al. [86] showed that the maximal compressive strength of composite concrete can range from 0 to 30% (without any admixture), with mean values of 18.503 and 17.96 MPa for 0 and 15% fiber length, respectively, on day 7, and 25.09 and 23.17 MPa for 0 and 30% fiber length, respectively, on day 28. Wang et al. [86] also indicated that the compressive strength increases with increasing fiber length up to a certain point, beyond which the improvement is less pronounced. Fibers up to 12 mm enhanced strength significantly (up to 71%), but strength gains dropped for 19 mm fibers (only 6%). This can be attributed to dispersion challenges and agglomeration of longer fibers, leading to anisotropic behavior and poor bonding with the cement matrix. Therefore, the optimal fiber length for compressive strength improvement appears to lie between 6 and 12 mm, depending on the mix’s workability and compaction efficiency.

While Meena and Ramana [136] showed that the compressive strength of composite concrete increased with increasing mix content up to a certain point, beyond which the improvement was less pronounced. The optimal mix of content may depend on the specific application and desired strength requirements. It was also indicated that the compressive strength of mixtures containing 0.5% PPF increased by 17% compared with regular concrete. Similar patterns were observed in Surendranath and Ramana [138], where excessive fiber content (>0.6%) led to increased porosity and decreased strength, especially at elevated temperatures.

The hybrid use of steel and polypropylene fibers (SF + PPF) introduces a synergistic effect. According to El Ouni et al. [132], this combination improved compressive strength by enhancing both crack-bridging and toughness. For example, mixes with 0.75% SF + 0.25% PPF outperformed plain RAC by ∼17.2%, and the effect was attributed to better stress distribution and complementary mechanical performance of the fibers. While PPF alone may reduce compressive strength due to its lower modulus and dispersion issues, pairing it with SF can mitigate these drawbacks and yield a net improvement.

Table 9 provides a summary of previous studies on the compressive strength of PPF lightweight concrete. The table includes information on the field of use, mix proportion, PPF content, and compressive strength in MPa. By comparing the data in this table, we can see that the compressive strength of PPF lightweight concrete can vary widely depending on the specific application and mix proportion. For example, Hussein et al. [26] reported compressive strengths of 70 and 87 MPa for alkali-activated mortar with 0.5% PPF and 0.5% PPF + 1% NS, respectively. Qin et al. [3] reported compressive strengths ranging from 34.10 to 35.83 MPa for PPF fabric-reinforced concrete with different fiber contents and mix proportions. Yu et al. [140] reported compressive strengths of 28 and 90 MPa for PPF lightweight concrete used in different fields of application. These variations in compressive strength may be due to differences in the mix proportion, fiber content, curing time, and other factors. Overall, Table 9 highlights the importance of carefully selecting the mix proportion and fiber content to achieve the desired compressive strength for a given application.

Overall, the inclusion of PPF in lightweight concrete demonstrates a variable yet generally positive impact on the compressive strength, with most studies identifying an optimal fiber content in the range of 0.5–1.0% (Table 9). Improvements up to 76% have been reported, particularly when PPF is used in hybrid systems or with optimized mix designs. The most significant gains occur around 0.5% PPF, where adequate fiber dispersion and bridging action at crack interfaces contribute to enhanced strength and toughness. However, discrepancies arise among studies due to differences in fiber dispersion, mix composition, and aggregate types. For instance, some studies report reductions in strength when PPF is used alone, citing issues such as fiber agglomeration, increased porosity, or poor compatibility with recycled aggregates. Others highlight the synergistic effect of combining PPF with steel fibers, resulting in improved mechanical performance and cost-effectiveness. Moreover, fiber length and aspect ratio (l/d) influence performance; overly long fibers may entangle, leading to anisotropy and reduced workability, while shorter fibers may inadequately bridge cracks. Environmental conditions (e.g., elevated temperatures) and additives (e.g., silica fume) also affect outcomes, further contributing to variability. Despite these inconsistencies, the weight of evidence suggests that carefully controlled PPF dosages between 0.5 and 1%, combined with appropriate mix designs and placement techniques, can reliably enhance the compressive strength of lightweight concrete in both conventional and recycled aggregate systems.

3.2 Flexural strength

The flexural strength of fiber cementitious composites is a crucial criterion in determining their tensile strength. Ductility increases with flexural strength [75,131]. The flexural strength of alkali-resistant PPF was found to decrease, whereas tensile strength increased [26,144]. The effectiveness of PPF and nanosilica in enhancing the flexural strength of concrete has been investigated in various studies. Comparisons have been made between mixes containing PPF and nanosilica, mixes with either PPF or nanosilica alone, and control mixes without either additive. PPF outperforms control mixes without fiber and nanosilica in enhancing the flexural strength. Furthermore, the combination of PPF and nanosilica has been found to offer even greater improvements in flexural strength. These findings highlight the potential benefits of using PPF and nanosilica as additives for improving the flexural strength of concrete, which can have important implications for the performance and durability of concrete structures. Furthermore, the specific mix design and conditions of use should be considered when selecting and optimizing the use of additives for concrete reinforcement. Fiber reinforcement is commonly used to boost the flexural characteristics and postpeak appearances of comparable composites by reducing the propagation of fractures under various mechanical loads [145].

When matched to other mixes, the ones containing 0.5% PPF demonstrate suitable and higher performance in terms of flexural tensile strength. Furthermore, when comparing the strength performance of test specimens with and without PPF, a difference of approximately 6.5 and 4.5 MPa can be observed when using alkali ratios of 2.5 and 1.5, respectively. Furthermore, when comparing 1% PPF specimens to 0.5% PPF specimens without nanosilica, the flexural tensile strength of the latter group is lower [26].

Amran et al. [135] observed that after 28 days, the flexural strength reaches 15.2 MPa, which is 217 percentage points more than the control nonreinforced composition and 111 percentage points higher than the control reinforced composition. This result proves that the improved flexural strength is due to a combination of the fiber itself and the created polymineral modifier. In addition, fibers have been shown to increase the flexural strength of concrete, as reported by Meena and Ramana [136]. Compared with regular concrete, the flexural strength of fiber-reinforced concrete is greater by over 7.53% after 28 days. Flexural strength is represented as PPF content using a nonlinear regression technique [146].

For flexural strength, the optimal percentage of PPF in the sample was found to be 2.5%. The consequence of fiber bridging across the cracks accounted for the increase in flexural strength shown with increasing PPF concentration up to 2.5%. Overshooting the optimal PPF level resulted in a steady decline in the flexural strength due to problems in mixing and a lack of uniform fiber distribution, which in turn reduced workability and increased porosity. For instance, the flexural strength of a sample with 8 M NaOH and 2% Fc was 1.27 MPa with 0% PPF, 1.47 MPa with 1% PPF, 1.88 MPa with 2% PPF, 2.19 MPa with 2.45% PPF, and 2.22 MPa with 3.0% PPF [67].

Transverse loads applied to a plain beam are used to measure the flexural strength of concrete. Flexural strength is determined using a three-point loading approach. Srinivas et al. [80] observed that the maximum flexure of lightweight concrete is obtained at 1% PP fiber, and with increasing fiber%, the strength decreased (Figure 20).

Figure 20

Flexural strength of PF [80].

According to Salehi et al. [79], the flexural strength and the postcracking behavior of LWC are enhanced when fiber is added. As a result, the bearing capacity is improved because the fiber stops the cracks from spreading and prevents them from breaking suddenly. The addition of 0.5% fiber improved the flexural strength. Given the balling phenomena that enhance porosity and heterogeneity in the concrete matrix, flexural strength might be expected to decrease if the percentage of fiber used is greater than the optimal amount.

Furthermore, Salehi et al. [79] reported that 0.5 and 1% fiber concentrations in bacterial specimens resulted in increased flexural strength. Increased bacterial activity in the fiber-containing group led to greater flexural strength.

When compared to ordinary concrete, all of the combinations tested showed considerably higher flexural strength values, ranging from 47 to 110%. Furthermore, the flexural strength was dependent on the expression Vf*(l/d). However, post-crack behavior was substantially linked with the fiber type. For comparable values of Vf*(l/d), the postcrack behavior appeared to be governed by the fiber form, even among steel fiber blends. Polypropylene blends with oppositely hooked fibers demonstrated similar post-crack behavior, as noted by Badogiannis et al. [93]. However, considerable postcrack strength and toughness values appeared to result from the use of all fiber kinds and shapes investigated.

As shown in Figure 21, flexural strength increases of 10.2 and 43.4% were reported for 7-day and 28-day bimaterial concrete, respectively, as compared with the standard mix with fibers [147]; the strength of the interface concrete increased by 13.1–45.9%. The primary function of the fiber is to strengthen the concrete’s resistance to cracks, and their “bridge action” in the presence of voids makes the material stronger than regular or bilayer mixes. Strength is reduced by 22.60% when the fiber mix is added to concrete, which is already rather strong. The creation of a void between conventional and bilayer concrete may account for some of the strength loss.

Figure 21

Flexural strength of bimaterial concrete [147].

Using bilayer fiber concrete led to remarkable results and increased flexural deflection strength. In plain concrete, a fracture appears immediately after a load increase, and it widens and deepens as the load decreases. Afterward, the load is increased again because of the fiber’s bridging effect [147]. The load-bearing capacity of the fiber is increased over a standard mix, preventing the matrix from splitting under the strain [148]. Shilkrot and Srolovitz [149] found that as the fiber level increases, the performance of the fiber deteriorates and becomes more challenging because they are no longer interacting with the cement matrix.

According to the findings of a flexural strength test conducted by Ramana et al. [147], the specimen’s strength increased in comparison with plain concrete but decreased in comparison with fiber filling the entire volume of the specimen. The creation of cracks could be the result of a weak connection between various materials.

Overall, the flexural strength performance of PPF-reinforced concrete shows considerable enhancement across various mix designs and testing conditions, though the optimal dosage varies between 0.5 and 1%, as presented in Table 10. Most studies reported a significant increase in flexural strength with PPF addition compared to control mixes, with maximum strength typically achieved at 0.5–1% PPF. For example, Meena and Ramana [137], Salehi et al. [79], and Xie et al. [37] observed the highest strength at 0.5% or 1% PPF, attributing the improvements to enhanced crack-bridging mechanisms and improved ductility. However, some studies reported peak flexural strength at higher fiber dosages, up to 2.5% [67], suggesting variability due to factors such as fiber dispersion, mix proportions, alkali content, and use of supplementary materials like nanosilica. Discrepancies, such as reduced strength at 0.9% or 1% PPF in certain mixes [112,137], were often attributed to poor workability, fiber balling, and increased porosity at higher dosages. The synergistic effect of combining PPF with nanosilica has also been found to enhance flexural strength beyond what is achievable with either additive alone [26]. Therefore, while 0.5–1% PPF generally offers optimal balance between strength gain and mix integrity, the variability across studies highlights the importance of considering mix design parameters, fiber geometry, and supplementary cementitious additives when optimizing for flexural performance.

3.3 Failure mode of PF lightweight concrete

The toughness and failure morphology of a fracture specimen can be considerably altered by the inclusion of fiber, although these effects depend on the elastic modulus of the fiber and the method of incorporation. Yang et al. [96] found that low-modulus fiber aids in the management of microcracks, whereas high-modulus fiber aids in the management of macrocracks. In addition, Yang et al. [96] reported that when PPF is incorporated into the specimens for the type-I shear fracture test, the resulting crack is fine. All fiber-doped samples also retain their structural integrity after being fractured. By contrast, the initial fractures of most specimens for a type-II shear fracture test grow steadily along the reserved crack tips. The expansion path shifts, and the fracture type is a mixture of types I and II due to the varied fiber involved and the effect of type-I strain.

The addition of fiber to concrete has always been driven by the need to improve the material’s performance in finished constructions. Various types of fibers have been used in construction, but steel and polypropylene dominate. Steel fiber, for instance, is known to increase a material’s fracture energy, creep and shrinkage resistance [150,151], impact resistance, freeze–thaw resistance, abrasion resistance, and fatigue resistance, among other desirable properties. Steel fibers have limited application in concrete because they cannot prevent spalling even at high temperatures [152].

PPF has a low melting point; thus, when the temperature is beyond 170°C, the fiber melts and generates more pores in the concrete matrix, which allows the accumulated vapor to be evacuated more efficiently, resulting in lower pore pressure [153].

When polypropylene melts, the pore pressure of plain concrete always becomes larger than the pore pressure of similar concrete with the addition of PPF [152]. This benefit is crucial for high-risk buildings that could be destroyed by fire. Many experiments have shown that the presence of PPF degrades the residual mechanical qualities of concrete subjected to high temperatures [154]. Thus, most ongoing studies focus on hybrid fiber-reinforced concrete, which combines steel and PPF.

Fibers enhance flexural strength and substantially improve toughness. In trials with polypropylene–BF-reinforced concrete, the incorporation of fibers enhanced the material’s flexural strength [155]. Under bending stress, fibers significantly decreased fracture propagation, exhibited increased ductility, and hence enhanced the material’s toughness (Table 11). They also provide a convoluted fracture propagation pathway. This augments resistance to fracture propagation and improves the overall toughness of concrete. In contrast to traditional concrete, hybrid fiber concrete exhibits more uniform cracking, hence diminishing localized crack development and enhancing energy dissipation. Test findings indicate that hybrid fibers can markedly improve the fracture toughness of concrete.

Table 11

Crack patterns of fiber-reinforced LWC [155–158]

Fiber concentration	Crack pattern
Without fiber and PPF-based specimens (compressive strength):
0% fiber 0.5% fiber 1% fiber
Hybrid fiber (PPF + steel):
Steel fiber PPF Steel fiber + PPF
Hybrid fiber (PPF + steel after flexural strength test)

The flexural schematic diagram in Table 11 illustrates the primary fiber bridging process of hybrid fiber-reinforced concrete under flexural loading: Initial loading causes interior micro or macro fractures due to inherent flaws and shrinking, but no outward major cracks form [156]. Randomly dispersed fibers can bridge cracks and relieve matrix tension. Due to its poorer matrix contact and lower tensile characteristics, PPF can only withstand the formation of tiny microcracks, resulting in lower pull-out loads. Loading causes the primary fracture to propagate, while fiber bridging enhances flexural behavior. Fiber sliding and de-bonding may occur at the PPF–matrix interface due to minimal contact between fiber and the matrix. Fiber rupture occurs when stress exceeds the reinforcing limit and the fiber is strongly attached to the matrix (e.g., at the steel fiber–matrix contact). Both approaches can improve post-cracking performance, particularly energy absorption capacity. However, energy absorption depends on parameters like fiber quantity and spacing surrounding the fracture. Overall, fiber behavior significantly improves FRC flexural behavior, with greater enhancement when more fibers align perpendicular to the loading direction.

3.4 Stress–strain behavior

Wu et al. [159] suggested that the ductility of core concrete can be increased through a combination of transverse reinforcement and fiber-reinforced lightweight concrete. Ductility refers to the ability of a material to deform under stress without breaking. In the case of concrete columns, ductility is important because it allows the column to absorb energy during an earthquake or other severe loading event without collapsing. Wu et al. [159] also noted that columns reinforced with fibers saw a 50% increase in ductility when the spiral spacing was reduced from 45 mm to 30 mm. This decrease suggests that the use of fiber-reinforced concrete can be an effective method for increasing the ductility of concrete columns. The reduction in spiral spacing likely contributed to the increased ductility by increasing the amount of transverse reinforcement in the column. However, Wu et al. [159] also noted that unreinforced columns saw almost no benefit from the change of spiral spacing. This finding highlights the importance of reinforcement in increasing the ductility of concrete columns. Without reinforcement, the concrete may not be able to withstand the forces acting on it during a severe loading event. Finally, no remarkable increase in peak stress in the column was observed due to the reduction in spiral spacing. This consideration is crucial because excessive stress can lead to the failure of the column. The fact that the difference between the two tie spaces was negligible suggests that the reduction in spiral spacing did not considerably increase the stress on the column. Overall, Wu et al. [159] suggested that a combination of transverse reinforcement and fiber-reinforced lightweight concrete can be an effective method for increasing the ductility of concrete columns. However, it also highlights the importance of proper reinforcement and the need to avoid excessive stress on the column.

Libre et al. [44] concluded that PP fiber did not affect concrete compressive strength. PP fiber slightly improved the concrete’s energy absorption and durability under compression. Moreover, Libre et al. [44] highlighted that the primary objective of adding fiber to LWC is to enhance its ductility and postpeak behavior. In this regard, PP and steel fiber can expand the ductility and postpeak behavior of the concrete. The descending part of stress–strain curves in concrete, which represents the postpeak behavior, is mainly affected by the addition of steel fiber. However, the ascending part of the curve, which represents the elastic behavior of the concrete, is less influenced by the addition of steel fiber. In summary, Libre et al. [44] suggested that the addition of fiber can remarkably develop the compressive strength, ductility, and postpeak behavior of LWC. Although PP fiber have a relatively minor effect on the compressive strength of the concrete, they can enhance its energy absorption and toughness characteristics.

An increase in the foam agent content reduces mass loss and increases compressive and flexural strengths of foam concrete when the material is subjected to high temperatures, cooled by air and water [160]. High temperature has less of an effect on foam concrete because of the enhanced porosity made possible by the improved foam agent content. Cement content has a considerable effect on the compressive and flexural strengths of foam concrete, with more cement content resulting in lower mass loss and higher strengths across the board. However, Gencel et al. [160] stated that a higher proportion of PPF increases the mass loss of foam concrete. Cracks and interconnected pores can be seen in SEM images of samples after they have been heated to high temperatures and subjected to thermal stresses caused by the breakdown of calcium silicate phases.

PPF fabric-reinforced concrete had a uniaxial compressive constitutive model, which was established by Qin et al. [3]. The correlation coefficients b, αa, λ, β, and αd for PPF-reinforced concrete at varying ages and fabric contents were calculated using the constitutive equation for uniaxial compression (Figure 22). The stress–strain curve of PPF fabric-reinforced concrete was also investigated, as reported by Qin et al. [3]. The falling part of the stress–strain curve was more affected by the fabric dosage and curing age than the growing half. This result is a direct result of the fact that as fabric percentage increases, the material’s toughness and the value of d drops increase, and the steepness of the curve’s decline is mitigated. As the curing age of a material increases, its internal water content drops, its brittleness grows, its αd value increases, and the slope of the corresponding descending section of the curve steepens.

Figure 22

Diagram illustrating the constitutive relationship that is impacted by the age and amount of fabric and fiber [3].

3.5 Ductility

Orouji and Najaf [142] found that the glass fiber-reinforced plastic (GFRP) rebars had lower flexural strength than steel rebars, which resulted in lower ductility and overall strength in specimens reinforced with GFRP rebars. However, the addition of fiber proved to be beneficial in compensating for this weakness. By increasing the amount of fibers used, the ductility of hybrid beams was enhanced. For example, using 1.5% fiber in combination with GFRP rebars resulted in a fully ductile beam that exhibited flexural strength similar to that of specimens reinforced solely with fiber but without any rebars.

Orouji and Najaf [142] highlighted the importance of carefully considering the properties of different reinforcement materials and how they interact with one another. While GFRP rebars may have certain advantages over traditional steel rebars, their lower flexural strength can affect the overall performance of a reinforced structure. However, by using a combination of materials and adjusting the amount of fibers used, a balanced and effective reinforcement system can be achieved.

Fiber reinforcement is a commonly used method to enhance the properties of lightweight concrete, such as its flexural capacity, toughness, post-failure ductility, and crack control. Among the different types of fiber available, PPF is often preferred for its effectiveness in providing thermal insulation and improving ductility. PPF is also popular due to its affordability and excellent resistance to environmental factors [86,161].

Wang et al. [86] mentioned that the addition of fiber to lightweight concrete can help mitigate the inherent weaknesses of this material, such as its low tensile strength and brittle behavior. IN particular, PPF can improve the toughness of the concrete by absorbing energy during deformation and preventing the propagation of cracks. This improvement can lead to a more ductile behavior, which is desirable in many applications, including earthquake-resistant construction. In summary, fiber reinforcement, especially using the PP fiber, is a widely recognized and effective method for improving the properties of lightweight concrete. Its cost effectiveness, resistance to environmental factors, and ability to enhance ductility and crack control make it a popular choice in the construction industry.

Numerous parameters, including high-performance PPF content, stirrup ratio, and shear span to depth ratio, can affect the failure mechanism of LWC beams, according to Xiang et al. [162]. It has been proven that adding more high-performance PPF to the concrete mix will effectively halt the spread of diagonal fractures. By altering the fiber content and stirrup ratio, the specimen’s failure mode can also be changed. When specimens break under diagonal tension with evident brittleness, they can change from shear compression failure to shear bending failure with enhanced ductility. This change demonstrates how fiber and stirrups may help a design achieve a less brittle failure mode, which is frequently desired in structural engineering.

Ekanayake et al. [163] reported that fiber added to concrete can boost the material’s strength and flexibility. To this end, this research looked into the outcomes of reinforcing concrete beams with either macrosynthetic polypropylene (MSPP) fiber, PPF, or both. Findings revealed that the addition of MSPP fiber enhanced beam load capacity by 13%, the addition of MSPP and PPF raised it by 6%, and the addition of PPF alone increased it by 5%. In addition to enhancing the initial stiffness by 3% and the ductility by 12%, MSPP fiber was added to the beam to make it stronger than the fiber-reinforced concrete alternative. However, PP fiber had only a moderate effect on the beam’s ductility and initial stiffness.

The beams’ failure mode varied depending on the length of the lap splices. Failure occurred before the expected performance was reached when the lap splice length was insufficient (20ϕ or 0.5 lb). However, when a sufficient amount of lap splice (40ϕ or 1.0 lb) was used, the beams did not fail in the middle even when subjected to a displacement of 130 mm. The beams in this instance behaved ductilely. Overall, Ekanayake et al. [163] demonstrated that the addition of MSPP and PP fiber can improve the load capacity and ductility of concrete beams. However, the effect of PP fiber on the ductility and initial stiffness was found to be limited. Additionally, providing sufficient lap splice length is crucial to ensuring the desired performance of the beams.