At a stage following the crack development, a reinforced concrete beam has a complex structure that does not correspond to such simple systems as the truss, arch or strutted frame. Therefore, to establish a possible design model for calculating the shear strength, it is necessary to know not only the failure pattern, but also the order of crack nucleation and propagation in the loaded structures. The pattern of nucleation and propagation of diagonal cracks differs significantly from that of vertical cracks, which are the reason for the stretching stress or bending moment. Diagonal cracks can be far more dangerous, as the time interval between the crack nucleation and failure is considerably shorter than for pure bending. The paper discusses the experimental study of diagonal cross-sections of single-span reinforced concrete beams under a concentrated load for a/d = 1. The tested beams had identical characteristics and differed only in the type of transverse reinforcement. The analysis of the experimental data revealed the specifics of nucleation and propagation of diagonal cracks in the zone of impact of the shear force and bending moment. The dynamics of propagation and width of diagonal cracks opening was recorded depending on the load. The effect of different types of transverse reinforcement on the bearing capacity of the beams was established.
The object of this research was the crack resistance of inclined sections of concrete and reinforced concrete fragments of protective structures under the action of emergency dynamic loads. The characteristics of dangerous emergency dynamic loads on protective structures (seismic, aircraft attack), the experience of increasing the crack resistance of inclined sections with various materials and design measures under static effects have been described. Areas of influence of dynamic loads on reinforced concrete structures reinforced with horizontal grids near the upper and lower faces need to increase crack resistance and eliminate the risk of splitting in the mesh plane. Comparison of the results of experimental studies of inclined sections of protective structures in the area of influence of local emergency load showed the feasibility of such structural measures. Additional horizontal reinforcement near the pushing face increases crack resistance by 55–65 %. When using the developed theoretical dependences, the error in determining the cracking forces and pushing strength does not exceed 20.7 %. Increased crack resistance is ensured by limiting the maximum diameters of the rods of horizontal grids and their pitch. Especially important is the arrangement of additional reinforcement in the middle zone, taking into account the actual tensile strength of concrete in the calculated dependences. Complete elimination of the danger of splitting in areas of probable action of emergency dynamic load in protective structures in the planes of the grids is recommended through the use of concrete of class not lower than C16/20, the use of reinforcement Ø12–14 mm. The optimal pitch of the rods is 50–125 mm. This makes it possible to increase the reliability of the design and operation of protective structures in case of emergency impacts, to reduce the cost of their repair after such impacts.
The experience of inclined cross-sections in the zones of influence of transverse forces and punching loads has been studied. The results of experimental studies of inclined cross-sections of protective structures in the area of influence of local emergency load on punching are presented. The article presents the reinforcement and strength of inclined cross-sections at the angle of destruction γ=40°. The analysis of the results was carried out and recommendations were developed for the design of inclined cross-sections of shells in the punching zone. The experimentally obtained values of the bearing capacity of concrete and reinforced concrete samples during punching correlate well with the results of theoretically determined dependencies that take into account the pin effect of reinforcement and the actual strength of concrete.
This paper reports the results of the physical and numerical experiments on determining the stressed-strained state of concrete in protective structures in the region of the effect of local point laser radiation. The software package LIRA10.8 (release 3.4) was used to build a computer model in the statement of a stationary thermal conductivity problem. To this end, the findings from the experimental studies were applied – the resulting temperature distribution and changes in the structure of concrete on the surface and deep into concrete cubes for more than 120 samples of concrete with three levels of moisture content: dried, natural humidity, and water-saturated. This paper gives the parameters of the simulation, the results of a numerical experiment, their analysis, and comparison with the results of a physical experiment. The temperature fields when establishing the dynamic temperature equilibrium, the level of stresses in concrete, derived from the physical experiments, correlate well with the results of the numerical experiment. The maximum temperature determined by the optical method at the surface of concrete was 1,350+50 °С. Deviations at control points do not exceed 12–70 °С in the temperature effect zone and 18–176 °С (1–11 %). At the rated radiation power of 30 W, the second stage of interaction was achieved; at 100 W – the fourth stage for concrete with a moisture content of 0–2.5 %; and, for water-saturated concrete, the fifth stage of interaction with the laser beam. A significant decrease in the thresholds between the stages of interaction between laser radiation and concrete was revealed, especially water-saturated concrete, compared to the thresholds for metals (the thresholds between the third and fifth stages were reduced by 103–104 times). The destruction of the walls of water-saturated pores in concrete occurred under the pressure of water vapor. The tangent stresses, in this case, were 1.7 MPa, and the values for the coefficient Kр, determined by the method of acoustic emission, were in the range of 4‒6. Such results explain the absence of normal microcracks due to the hoop effect. It was established that in the contact zone between a laser beam and concrete, about 90 % of the radiation energy dissipates, and in the adjacent heating zone ‒ up to 77 %. The optimal speed of beam movement when cleaning the concrete surface from organic, paint, and other types of contamination of 0.5–2 mm/s (surface temperature, 100–300 °С) has been proposed
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