# Impact of Fiber Type on Chloride Ingers in Concrete: A MacroXRF Imaging Analysis
## 4 Discussion  
In the analysis of the axial compressive strength, the concrete with the highest axial compressive strength was that produced without any percentage of fibers (RC), which reached an average of 35.40 MPa, while the second-best strength was that of the concretes with 0.25% fiber added (GF25 and PF25), which were statistically similar to each other with an average strength of 31.24 MPa. This value represents an axial compressive strength 11.75% lower than the reference concrete, followed by concrete with 0.50% polypropylene fiber (PF50), whose strength reduction compared to RC was 32.26%. On the other hand, the concrete with 0.50% glass fiber (GF50) alone had the lowest strength among the samples, with a value corresponding to 43.98% of the RC axial compressive strength. In light of the results obtained, it is important to note that concrete with a compressive strength of less than 20 MPa is not suitable for use in structural elements, in accordance with the requirements of NBR 6118 (the Brazilian standard that deals with the design of concrete structures) due to its insufficient load-bearing capacity to support the normal stresses of a structure [32]. This decrease in strength may be attributed to fiber packing and reduced workability, leading to increased porosity. The presence of excess fibers can hinder fluidity and densification, ultimately affecting mechanical properties. Previous studies support these findings, with [PERSON], [PERSON], and [PERSON] [40] reporting less than a 1% improvement in compressive strength with 0.5% glass fiber, and [PERSON] et al. [27] finding that 0.75% GF led to only a 6.6% strength increase. [PERSON] and [PERSON] [4] also confirmed that the compressive strength tends to decrease with increasing the fiber content, particularly for water-to-cement (\(w/c\)) ratios above 0.30. In their study, compressive strength reductions of over 67% were observed when 1.35% of polypropylene fibers were used, depending on the mix design. Similarly, [PERSON] et al. [41] reported a 40.3% drop in compressive strength when 0.5% polypropylene fibers were incorporated, reducing the strength from 21.45 MPa to 12.8 MPa and identifying 0.2% as the optimal dosage to minimize strength loss. These findings emphasize that an excessive fiber content can significantly compromise the mechanical performance of concrete. Confirming these results, the research by [PERSON] [29] showed that even self-compacting concretes (SCCs) without fibers (reference concrete) had a lower void index and lower water absorption by immersion. Samples containing steel fibers exhibited intermediate performance, while concretes reinforced with polypropylene fibers had the worst performance, indicating that the addition of fibers increases air incorporation in SCC. Similar results were observed by [PERSON] [13], who studied concretes reinforced with hybrid fibers (steel fiber and polypropylene fiber). The author found that an increase in the content of polypropylene fibers in hybrid fiber-reinforced concrete led to a reduction in compressive strength as well as an increase in the porosity of the hardened matrix. It can be seen that fiber reinforcement is not beneficial for this purpose. This is partly due to the way fibers interact with the cementitious matrix. Only the outermost glass filaments, because they are grouped together, come into contact with the cementitious matrix, unlike independent fibers, such as steel fibers, which are completely covered in material [6]. Therefore, due to the reduced space between the glass filaments, the cement grains cannot penetrate between the fibers, leaving spaces that increase the porosity of the matrix. In addition, as [PERSON] and [PERSON] [42] report, fibers increase the transition zone of concrete and, consequently, the porous area. Additionally, glass fibers, being stiffer than polypropylene fibers, further complicate compaction. Poor compaction leads to higher porosity, which negatively impacts both strength and durability.