Determining the stressed-strained state of concrete in the zone of exposure to local laser radiation

Abstract

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