# Impact of Fiber Type on Chloride Ingers in Concrete: A MacroXRF Imaging Analysis
## 4 Discussion
The impact of the fiber content on durability was evident in the chloride migration test. The samples with 0.25% polypropylene fiber (PF25) exhibited a 19.74% increase in chloride ion migration compared to RC. Between PF25 and PF50, there was an 18.81% reduction in the diffusion coefficient (Dns), indicating that an excessive fiber content can compromise performance. These findings are consistent with [PERSON] et al. [43], who reported that fiber \"skeletons\" can impede compaction, increasing voids, and [PERSON] and [PERSON] [44], who observed similar effects with glass fibers in concrete and mortar. Given the small statistical difference in the Dns means of the samples analyzed in the accelerated chloride migration test, it is worthwhile to examine the chloride distribution using MacroXRF. This quantitative and qualitative technique provides bidimensional mapping, allowing for the visualization of chloride pathways within the samples. With the addition of polypropylene fibers, as observed in Figure 10, there was a noticeable reduction in the intensity of chlorine, indicated by a decrease in the number of excited photons from a maximum count of 1200 (reference concrete) to a count of 375 (PF25 sample), as shown in the graphs on the right of the image. This suggests that polypropylene fibers were effective in reducing chloride penetration. The samples with a 0.25% fiber content demonstrated lower chloride presence from the initial scanning depths. In the PF50 sample, the presence of chlorine was more concentrated at the edge of the slice but with an intensity greater than the PF25 sample, reaching a count of approximately 800 photons. This behavior indicates some resistance to chloride penetration compared to the reference sample but with a lower performance than PF25. One possible explanation for this result is that the higher fiber content may have led to the formation of fiber clusters, hindering proper dispersion within the matrix. This phenomenon has been previously reported by [PERSON] et al. [8], who observed a decrease of about 4% to 6% in compressive strength in concretes with 1% polypropylene fibers due to poor fiber distribution. Furthermore, [PERSON] et al. [9] noted that an inadequate dispersion of fibers can increase porosity and cause surface finishing issues in concrete elements. The fiber-matrix transition zone exerts a significant influence on the behavior of the reinforced element, affecting both mechanical performance and durability. When fibers form clusters instead of dispersing uniformly, only their outermost filaments interact with the cement matrix. This contrasts with independent fibers, such as steel fibers, which distribute more evenly throughout the matrix [6]. As a result, the reduced space between clustered filaments prevents cement grains from fully penetrating, creating voids that increase the porosity of the composite. [PERSON] [45] describes this effect as the formation of a \"hedgehog\" structure, where fiber agglomerates generate highly porous weak spotswithin the matrix. This could explain the higher chloride penetration observed in PF50 compared to PF25. Several studies have explored this phenomenon. The research by [PERSON] et al. [46] investigated the effect of carbon nanofiber agglomeration on the micromechanical properties of a cement paste. The authors found that these agglomerations formed interfacial zones with higher porosity, affecting the macroscopic mechanical response of the material. The type of material composing the fiber and its surface roughness also have a direct influence on adhesion and the fiber-matrix transition zone. [PERSON] et al. [47] analyzed the interfacial properties between polyethylene (PE) and polyvinyl alcohol fibers (PVA) in cementitious composites. The study found that PVA fibers formed a stronger adhesion with the matrix, while the interfaces of polyethylene fibers with the matrix exhibited cracks and pores, compromising the compressive performance of the material.