## 3 Results and Discussion  
It is well known that the brittle behavior of concrete fracture increases with the growth of its compressive strength [27; 28]. The properties of concrete with an increased compressive strength, obtained by optimizing technological factors on the different structural levels, were studied. The compressive strength of the designed concrete is shown in Figure 4.  
Figure 4: The effect of different technological factors on compressive strength of concrete. Figure 3: (**a**) General view of test apparatus and (**b**) scheme of measuring device: 1—strain gauge, 2—deflection gauge, 3—signal amplifier, 4—resistor bridge, 5—interface board, 6—power supply, 7—PC, 8—power distribution ring. As can be seen from the obtained results, the optimization of the concrete grain size results in the reduction of the compressive strength of reference concrete R by 13.3% after 1 day of hardening in comparison with concrete denoted R0. The more intensive kinetics of the strength development of concrete R are observed with the further hardening of such concrete. Thus, the strength of concrete R increases by 23.6% after 28 days. Following the optimization of various types of fibers [33], the appropriate dosages were incorporated into the reference concrete. The results show that concrete's compressive strength increases slightly compared to concrete R. [PERSON] [42] states that polypropylene fibers do not have a significant effect on the compressive strength of high-performance self-compacting concrete after 28 days of hardening because added fibers adversely affect the workability of fresh concrete and, as a result, voids can be created in the concrete, reducing a possible strength increase. Some growth of compressive strength is observed due to the retardation of matrix fracturing because PP fibers limit crack propagation. [PERSON] [43] also showed that a slight influence on the compressive strength was observed when fibers with a length of 5 mm, 10 mm and 15 mm in quantities of 0.25%, 0.75%, and 1% were used in the mixture. [PERSON] et al. [44] studied the influence of different dosages (2 and 4%) of polypropylene fibers with a length of 48 mm on the compressive strength of concrete and revealed a positive effect. The concrete incorporating air-entraining agent R+F+A is characterized by a 6.6% lower early strength and a 3.7 and 3.5% higher strength after 3 and 28 days, respectively. The use of an optimal amount of zeolitic tuff [13; 45] causes a slight decrease of 12.3 and 12.8% of the early strength of concrete R+Z+F+A compared to the reference concrete R after 1 and 3 days of hardening, respectively, and a 3.7% higher compressive strength after 28 days of hardening. It should be noted that concrete with such an increased strength is characterized by more brittle behavior compared to conventional concrete. As shown in Figure 5, the behavior of concretes, except concrete R+Z+F+A, do not differ significantly in the elastic stage, which the linear ascending branches of the load-deflection curves indicate. A more inclined ascending branch of the load-deflection curve in the pre-peak stage demonstrates the increase of the fracture toughness of R+Z+F+A concrete. A similar behavior is observed in the post-cracking stage for concretes R0 and R. The results show that this concrete exhibits a consistent behavior throughout all stages of deformation under load. A significant difference is visible in the post-cracking stage for other concretes. The descending part of the load-deflection curve is rather steep for all concrete series. The tail of the descending branch is short for concretes R and R0. The use of an optimal amount of both increases the volume under the descending branch tail significantly. This indicates that the toughness of the concrete increases in comparison with R0 despite the higher compressive strength class. [PERSON] and [PERSON] [28] reported that the tail of the descending branch would be longer in the case of conventional concretes and shorter in HSC.