Cement (1. P.O. 42.5) and Class I fly ash, substituting 10% of the cement's mass, served as the binder materials. Tables 1 and 2 detail the physical and chemical properties of these binders, respectively. The gravel, featuring a maximum particle size of 12 mm, was chosen as the coarse aggregate to optimize fiber dispersion and the requirement of slab test (15, 19). Additionally, the river sand with a fineness modulus of 2.4 was used in PVA-FRC as fine aggregates. The remaining physical properties of coarse aggregate included a moisture content of 0.17% and a crushing index of 8.2%. In addition, the coarse and fine aggregates were sourced from Inner Mongolia Huameng Aggregate Co., Ltd. (Inner Mongolia, China), with bulk densities of 2750 kg·m^-3 and 2510 kg·m^-3, respectively. Tap water was used for casting and curing all concrete mixtures, ensuring that the maximum chloride content did not exceed 500 ppm. To adjust fluidity performance, the dosage of naphthalene sulfonate superplasticizer (NSS) was 1.4% of the binders material mass, which is supplied by Baotou Anshun New Building Materials Co., Ltd (Inner Mongolia, China) (20). The NSS dosage remained consistent to explore the impact of PVA fiber length and dosage on concrete workability. The microscopic morphology and mechanical parameters of the PVA fiber are illustrated in Figure 2 and Table 3, respectively.

Nine PVA-FRC mixtures incorporating lengths of 8 mm, 12 mm and 18 mm at volume fractions of 0.05%, 0.15% and 0.25% were prepared. An ordinary concrete mix, referred to as NC, was utilized for comparison purposes. The proportions of a typical PVA-FRC and NC, as shown in Table 4, were determined and compared.

To enhance dispersibility, PVA fiber was premixed in water as part of the pretreatment process. Binders and aggregates were dry-mixed for three minutes. Then, water containing dispersed PVA fiber and NSS was added and mixed for an additional three minutes. Finally, the mixture was vigorously stirred for an additional five minutes to ensure optimal fiber dispersibility. Upon completion of the mixing process, the fresh mixtures of each type were cast into prismatic, cubic and steel plate molds. Only one specimen was cast in the steel plate mold, while the others were poured into three parallel specimens. The type and dimensions of the cast specimens are listed in Table 5. Concurrently, the specimens were covered with plastic wrap to minimize water evaporation. After being left outside for one day, the samples were demolded. All specimens were placed in a standard curing chamber at 20±2 °C with a relative humidity of 95% and continued to cure for 27 days.

The significantly different slump of each mixture is shown in Figure 6. It is apparent that workability inversely correlates with the incorporation of fibers. Specifically, maintaining constant fiber length, an increase in fiber dosage invariably leads to a reduction in slump. Practically speaking, greater fiber content correlates with a decreased slump, although employing an appropriate vibration technique can enhance the placement and uniformity of fiber-reinforced concrete, as referenced in (30). In this study, with a fiber content of 0.25%, the slump value does not exceed 10 mm. Comparisons of this series of mixture proportions with others have not been conducted. The relationship between the slump and the addition of two other fiber dosages is elucidated and depicted through a normalized folding line. Notably, the slump reduction is significant in the concrete series with 0.15% fiber content compared to others. Such as, comparing FX-0.05 with FX-0.15, the fiber dosage varying from 8 mm to 18 mm, the slump decreased by Such as, comparing FX-0.05 with FX-0.15, the fiber dosage varying from 8 mm to 18 mm, the slump decreased by 10.6%, 25.5%, and 47.8% respectively, all demonstrating poorer flowability compared to NC., 25.5%, and 47.8% respectively, all demonstrating poorer flowability compared to NC Evidently, PVA fiber detrimentally affects workability. This decrease in slump can be attributed to the reduced efficacy of cement in binding coarse aggregates and the obstruction caused by the three-directionally randomly distributed fiber grid, which impedes fluidity (31). In addition, the hydroxyl groups on PVA fiber surface tend to absorb water, thereby reducing the free flow of water between particles (32, 33).

The test results for flexural strength (Fs) are comprehensively detailed in Figure 7. To facilitate comparison, the Fs of each specimen has been normalized to that of normal concrete (NC), denoted as normalized f_f=1. All specimens exhibit a consistent increase in Fs across various fiber lengths and dosages. Nevertheless, the impact of 8 mm fibers on Fs is notably minor compared to fibers of 12 mm and 18 mm lengths, particularly at lower volumetric dosages. Specifically, the specimens designated F8-0.05 and F8-0.15 demonstrate equivalent flexural strength values. When the volume fraction is minimal, variations in parameters appear to exert a negligible influence on the flexural characteristics of fiber-reinforced concrete. This phenomenon may be attributed to a balance between the diminished air entrainment caused by shorter fibers and their limited capacity to halt crack propagation (15).

To effectively analyze the impact of fiber parameters, length and dosage, on flexural strength (Fs), a multiple linear regression analysis (MLRA) was performed, and the findings are presented in Table 6. The significance of the analysis of variance (ANOVA) is less than 0.05, indicating that the model developed with independent variables and dependent variable is scientific. The significant coefficient (0.033) indicates that fiber length more significantly affects Fs than fiber dosage. However, at higher dosages, the positive impact of increased fiber length decreases. For example, Fs in F12-0.25 and F18-0.25 is 1.324 times greater than in normal concrete (NC), mainly due to fiber length enhancing microcrack suppression and distribution uniformity in the cement matrix. Longer fibers effectively prevent microcrack development, improving flexural strength. Yet, their tendency to cluster can reduce their effective length and create weak points in the concrete, potentially undermining the benefits of longer fibers (19). Moreover, the concrete's tensile stress is transferred to the fibers, resulting in an increase in the stress borne by the fibers and ultimately enhancing flexural performance (34). These phenomena can be also obtained from SEM analysis.

The cracking parameters (max width, max length, total area and index of anti-cracking) of all type mixtures are depicted in Figures 8 and 9. It is clear that PVA fiber significantly enhances cracking resistance compared to normal concrete (NC). As detailed in the partial enlarged view of Figure 8, the 12 mm fibers outperform both the 8 mm and 18 mm fibers in terms of crack parameters. Specifically, the maximum width, maximum length, and total area of cracks for the F12-0.25 series are reduced to 96.1%, 93.1%, and 92%, respectively, compared to NC. For the 8 mm series at a fiber volume fraction of 0.25%, the improvements are noted as 83.3%, 82.3%, and 86.4% in these parameters, respectively. Meanwhile, the 18 mm series shows increases of 92.3%, 91.9%, and 88.9% in these parameters under identical conditions. There is a positive correlation between the maximum width of a crack and the total area of cracking as depicted in Figures 8(a) and (c). Furthermore, Figure 9 shows that the crack resistance index varies with fiber length, with indices of 0.65, 0.99, and 0.95 for 8 mm, 12 mm, and 18 mm fibers at a 0.25% volume fraction, respectively.

The improvements in cracking resistance can be attributed to the interactions between PVA fibers and the cement matrix. When subjected to tensile stress from shrinkage, the fibers inhibit crack formation by providing shear resistance at the fiber-matrix interface (35). The length of the fibers plays a crucial role: shorter fibers might not bond effectively with the matrix, resulting in slippage and reduced crack resistance, whereas longer fibers enhance the contact area and strengthen the bond (36). Moreover, the high polarity of PVA fiber serves as nucleation sites for calcium ions, improving adhesion at the interface and consequently increasing crack resistance (37). This study finds that 12 mm PVA fiber is optimal, offering the best balance between adhesive forces and mechanical interlocking, essential for controlling crack propagation.

To evaluate the overall properties of fiber concrete, five dimensions were selected for evaluation: crack width, crack area, index of anti-cracking, flexural strength, and slump. It is evident from Figures 7 and 8 that Fs and anti-cracking are notably affected by the 0.25% fiber content. To further analyze performance variations with different fiber lengths, FX-0.25 was examined in Figure 10. It becomes apparent that concrete samples containing 12 mm and 18 mm PVA fibers demonstrate similar levels of performance in terms of flexural strength and index of anti-cracking. Specimens with 8 mm and 12 mm PVA fiber show comparable workability. By evaluating the graphical representations of these five dimensions, it is evident that the performance of the 12 mm specimen surpasses that of the 8 mm and 18 mm specimens.

After the overall performance analysis, combining with the improved mechanical performance and resistance of cracking, only the performance of 12 mm series specimens was tested to characterize chloride penetration profiles.

NMR techniques are sensitive enough to distinguish between different proton populations in concrete (44). Free water in the inner concrete typically resides in air voids and cracks, while bound water is found in the capillary pores within the concrete. Consequently, NMR techniques can be employed to analyze the porosity of selected samples using T₂ spectrum curves.

The curves with various PVA fiber dosages and length are shown in Figure 14. For each PVA-FRC and NC, three signal peaks are basically displayed by T₂, except F18-0.05 with two peaks. The first signal peak occupies the mainly dominant position. Specifically, the first peak signal changes from 0.01 to 0.1, namely, two orders of magnitude of distribution. Particularly, at 0.25% dosage of PVA fiber, the emergence time of T₂ spectrum curve is earlier and the distributions of main signal peak are narrower with respect to the other group specimens. The above all indicates that the pores in the interior region of the concrete are mainly micro-pores (45), in this investigation.

The existence of porosity can be characterized by integrating the T₂ curve. To facilitate the comparation, the total area of peak of various specimens are normalized with the NC in Figure 15. Adding 0.05% and 0.15% to mixture, the content of small pores (i.e. the area of the main peak) is positively correlated with fiber dosage. Based on low dosage fiber, 18 mm PVA fiber plays a marginally negative role in the first signal peak area. The similar phenomenon also can be seen at specimens incorporating 12 mm PVA fiber. The negligible alteration in the peak area of F18-0.05 and F12-0.05 can also serve as an additional explanation for the absence of notable changes in 3.2. flexural strength. It is differential in relatively higher dosage groups. Adding 0.25% to mixture, the better performance of PVA-FRC is explained by T₂ spectrum in consideration of the total areas and signal peak ratio. The proportion of first signal peak and the total area of peak, regardless of PVA fiber length both undergo optimization. Compared to NC, F12-0.25 and F18-0.25 show improvements of 9.31% and 4.65%, as well as 54.61% and 35.25% in two parameters, respectively. This is a plausible explanation as to why the erosive properties of chlorine salts have increased with the addition of fibers. The refinement of the porosity, a declined total peak area and a ratio of first signal peak, can be observed.

As the Figure 16 shown, PVA fiber tends to locally act as reinforcement through anchorage and confinement, and it possesses a rough surface. Moreover, the fiber plays a crucial role in the bridging mechanism, which helps impede the propagation of cracks. With a moderate dosage of PVA fiber, enhanced strength properties can be achieved due to the reduction in internal defects, as confirmed by the SEM image (Figure 16(b)). The process of fiber bridging and deflection contributes positively to improved mechanical properties and energy dissipation (46). Additionally, due to the hydroxyl group on the molecular chain, the PVA fiber acts as nucleus on which the hydrated production could grow (47). This interaction can be clearly seen in the way fibers are embedded within the matrix, enhancing the bonding between the surrounding mortar and the fibers, as visualized in (48). These characteristics yield significant effect to present better mechanical properties and durability performances.