The paper presents the results of experimental and theoretical investigations of the mechanisms of crack formation in reinforced concrete beams subjected to quasi-static bending. The boundary-value problem has been formulated in the framework of brittle fracture mechanics and solved using the finite-element method. Numerical simulation of the vibrations of an uncracked beam and a beam with cracks of different size serves to determine the pattern of changes in the spectrum of eigenfrequencies observed during crack evolution. A series of sequential quasi-static 4-point bend tests leading to the formation of cracks in a reinforced concrete beam were performed. At each loading step, the beam was subjected to an impulse load to induce vibrations. Two stages of cracking were detected. During the first stage the nonconservative process of deformation begins to develope, but has not visible signs. The second stage is an active cracking, which is marked by a sharp change in eingenfrequencies. The boundary of a transition from one stage to another is well registered. The vibration behavior was examined for the ordinary concrete beams and the beams strengthened with a carbon-fiber polymer. The obtained results show that the vibrodiagnostic approach is an effective tool for monitoring crack formation and assessing the quality of measures aimed at strengthening concrete structures.
The paper presents a block diagram of an automated inclinometer-based system for monitoring the inclination of a high-rise metal structure of the shaft headframe. The stress-strain state of this structure is defined by the loads associated with the operation of shaft skip and the impact of the environmental factors on the structure. The data obtained by 6 inclinometers located at different height were used to determine the inclination of the headframe and distortion of its shape. Analysis of long-term observation data revealed the existence of deformation processes having different time scales (long-term, seasonal and daily). A correlation between changes in the tilt angle of the structure and ambient temperature was established. Long-term monitoring of tilt angle showed that the displacements of the structure were within the permissible limiting values. The developed monitoring system is of great theoretical importance for understanding the deformation processes arising during the operation of such structures.
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