## 3 Results and Discussion
### Visual Inspection of Composite Fractures and Fibers  
The polymer fiber modification was not expected to drastically improve the mechanical properties of the mortars, as such an effect would require changes in the composition of the  
Figure 5: Mechanical properties of mortars with fibers after 7 days and 28 days of curing (SD—standard deviation, CV—coefficient of variation). target composite. The main aim of the performed investigation was to determine whether the modified fibers were better anchored in the cement matrix, as that would be a sign that the tannic acid modification was effective in roughening the surface of the polypropylene fibers and improving their adhesion to the mineral matrix. Here, the visual inspection of the damaged specimens was helpful--visual inspection of the surfaces of scratched specimens (after bending), as well as the fractures of the broken specimens and the conditions of fibers themselves. In Figure 6, one can see the comparison of the specimen's outer surfaces with both fiber variants after the flexural strength test. The test procedure did not assume loading the bending elements until the specimen wholly disintegrated, but until the stress dropped, which meant damage to the brittle material structure. In the case of standard mortar testing, as a result of exceeding the maximum stresses, the brittle cracking occurred in the area of the cementitious matrix and the aggregate-matrix interface, and the visible crack appeared, leading to the separation of the halves of the bent prism. However, in the testing of mortars with polymer fibers, the polymer fibers ensured the stability of the specimen even after the failure of the brittle matrix. Despite the destruction and significant decrease in the load capacity, the specimens retained their integrity. However, the nature of the destruction differed in the case of mortars with unmodified and modified fibers. The specimens with unmodified fibers were cracked across the entire thickness of the bent element (Figure 5, upper specimen). However, the specimens with modified fibers were only scratched (up to a maximum of 2/3 of the element thickness), and the crack width did not exceed 0.3 mm (thus fulfilling requirements of Eurocode EN 1992-1-1 [25] in terms of carbonation, chlorides other than from seawater and most of the chlorides from seawater exposure classes). Specimens with modified fibers clearly showed significantly improved integrity after failure. Figure 6: The outer surface and the mechanical damage (the length of the crack corresponds to the entire thickness of the specimen, i.e., 40 mm—see upper arrow) of the mortars with non-modified PP fibers (upper specimen) and PP fibers modified with tannic acid (lower specimen; crack range is 24.9 mm—see lower arrow) after flexural strength test (three-point bending test). In order to measure compressive strength, which was the next phase of the experiment, the prisms had to be fully broken, which required additional loading of the specimens. After separating the prisms' halves, the fibers' fractures and condition were inspected. It was observed that the unmodified fibers were not broken but pulled out, often almost entirely, from the cementitious matrix (Figure 7a). Meanwhile, the tannic acid- modified fibers remained anchored in the matrix, and their breaks occurred close to the specimen fracture surface. Also, their ends were frayed, confirming breaking when their tensile strength was exceeded (Figure 7b). The above observation proves they better adhered to the matrix, which the authors attribute to the roughened surface of the fibers. The last conclusion was also supported by microscopic observations that showed that the surface of unmodified polypropylene fibers was smooth before their implementation into the mortar mix and after their pull-out from the broken specimen.