The article presents the results of computational studies that are part of the authors' comprehensive research on the development of a methodology for predicting the resistance of steel fiber concrete to thermal effects at NPP facilities with various parameters of dispersed reinforcement. The studies were carried out using selected models to describe the processes of thermal deformations and changes in the most important properties of steel fiber based on previously obtained experimental and calculated data on thermal changes of steel fiber on an optimized cement-sand matrix with five types of steel fiber with a volume content of 0.5% to 6% after heating at 110-1100 oC. Based on calculations in accordance with the accepted hypothesis and the selected models, the dependences of thermal changes in the properties of steel fiber concrete relative to matrix changes from the calculated changes in linear dimensions due to the formation of cracks in the fiber concrete are established. The dependences of thermal changes in the linear dimensions of steel fiber concrete due to the formation of cracks on the ratio of the calculated unreliaxed tensile structural stresses in the matrix to the tensile strength of the matrix are established. Based on the performed studies, a method for predicting thermal deformations, changes in mass, linear dimensions due to the formation of cracks, strength, water resistance of steel fiber concrete, based on thermal deformations and changes in the properties of the matrix, parameters of dispersed reinforcement is formulated.

Keywords: steel fiber concrete, prediction of resistance to thermal effects, cracking, thermal changes in linear dimensions and properties

The work is devoted to the study of thermal deformations and changes in the properties of steel-reinforced concrete after heating them to high temperatures in connection with the use of steel-reinforced concrete panels as permanent formwork of prefabricated monolithic structures of nuclear power plants. Thermal changes of steel-fiber reinforced concrete have been studied in other works. However, the results obtained in these studies do not correspond to the features of the work of steel-fiber concrete in the panels of fixed formwork. Not all major thermal changes have been investigated. In this regard, in this work, tests are carried out on sample plates. Thermal changes after heating at temperatures from 110 to 1100 oC of mass, density, size, flexural strength, ultrasound transmission rate, and water resistance are studied. Samples of 23 compositions made on the basis of a cement-sand matrix using five types of steel fiber with a volume content from 0.5% to 6%, as well as samples of the matrix, were studied. It was found that after heating, there is a decrease in mass and density, a change in size (mainly a decrease after 110-600 oC and mainly an increase after 800-1100 oC), a change in bending strength (mainly an increase after 110 – 300 oC and a decrease after 400 – 1100 oC), a decrease in the speed of ultrasound transmission and the water resistance of steel fiber concrete. It is found that the dependence of thermal deformations and changes in the properties of steel-fiber concrete on the heating temperature, volume content and type of fiber is not monotonous and variable at different values of these factors. Moreover, the thermal changes of fiber-reinforced concrete do not always correspond to the expected ones, based on theoretical concepts. The reasons for this phenomenon are shown. It is noted that it is necessary to conduct further numerical analysis of the obtained results in order to identify the true, not masked by other processes, influence of temperature, type and volume content of fiber on the thermal changes in the properties of steel fiber concrete after heating.

Keywords: steel fiber reinforced concrete, temperature impact, change in mass, change in ultrasound speed, change in water resistance, change in strength