## 1 Introduction and Scope
### Polymer Microfibers and Macrofibers Used in Concrete and Other Cementitious Materials  
Generally, there are two basic types of polymer fibers used in ordinary concretes and other concrete-like materials with cementitious matrices: micro- and macrofibers. Microfibers are fibers with very small cross-sections (diameters of several to a dozen micrometers) and lengths usually not exceeding 12 mm (see Figure 1). The dosing of such fibers is 0.2-2.0 kg per 1 m\({}^{3}\) of the composite, depending on the intended use. Microfibers are intended mainly for non-structural applications and can be considered for microrefinement, which reduces plastic shrinkage and prevents the appearance of microcracks in cement matrices in the early stage. It is advisable to use them in various composites (concretes; thin-layer screeds, including ones for underfloor heating and plastering; masonry mortars; repair mortars; adhesives; insulating coatings; etc. ), especially if they contain high-class Portland cements with high early strength, i.e., with an increased tendency to shrinkage and crack development, or, in general, in advanced cementitious matrices that are characterized by a refined microstructure and low porosity (e.g., a water/binder ratio lower than about 0.3). Due to their small size, microfibers can easily be evenly distributed in cement paste and create a spatial mesh that can additionally reduce water absorption and increase the water tightness and frost resistance of modified composites. The authors confirmed in previous research investigations [5] that it was possible to obtain a seamless polymer-cement insulating coating with polyacrylonitrile microfibers that showed excellent elasticity and water resistance and at the same time remained water-vapor permeable. Also, when mortars with PP microfibers (presented in Figure 1b) were tested, no negative impact on the mixes' consistency was observed, while hardened composites were characterized by assumed compressive strength and flexural strength. Therefore, it was not necessary to analyze or to improve the adhesion between the used microfibers and the composite matrices. The second type of polymer fibers used for concrete, i.e., macrofibers or structural fibers (sometimes referred to as \"traditional fibers\"), have much larger cross sections (diameters of 0.1 mm and more) and greater lengths. [PERSON] and [PERSON] [1] distinguished a third type of fiber, i.e., mesofibers, which have cross-sectional dimensions in the range of 0.1 to 0.3 mm; however, in most studies in the scientific and industrial literature and most technical data reports (including most of the mentioned references), fibers with such cross sections are classified as fine macrofibers. Unlike the situation with polymer micro-fibers, in the case of macrofibers made of polymers, the issue of adhesion is much more problematic. Polymer macrofibers are added to cementitious mortars and concretes (structural and non-structural) for several reasons:  
* To reduce plastic shrinkage and the formation of microcracks in cement matrices;
* To increase the mechanical strength of composites (including flexural and tensile strength, as well as the level of fracture energy absorption and impact strength);
* To reduce the possibility of crushing and spalling at the edges of composite elements. The positive effects of the presence of polymer macrofibers mentioned above are basically the same as the effects of steel fibers, but they cannot be treated as a direct replacement for steel fibers of the same geometry, mainly due to their much lower adhesion to cement binders. Figure 1: Commercial polypropylene microfibers of similar geometry (both with a length of 12 mm and a diameter of 18 μm) from different manufacturers: (**a**) fibers without an additional coating; (**b**) fibers additionally covered with a water-soluble coating. The authors have reviewed the range of polymer macrofibers currently available on the Polish market and used in concrete production.