Evaluating convergence between results of experimental studies on fire resistance of compressed reinforced concrete elements, having a higher percentage of reinforcement, and results of numerical calculations

Introduction. Evaluation of actual fire resistance of load-bearing building structures made of various materials and reinforced concrete, in particular, encompasses a rather complex and research-intensive case of analysis of integrated, mechanical and fire safety of buildings and structures. Current methods of analyzing the fire resistance of reinforced concrete structures do not take into account any thermal characteristics of reinforcement. Hence, values of the fire resistance limit of structures can only be consistent with experimental results only if the percentage of reinforcement in analyzed concrete structures and reinforced concrete structures is below 3.5 %.

The purpose of the work. Is (1) the pilot testing of a hybrid experiment method and (2) evaluation of convergence between (a) results of experimental laboratory studies on fire resistance of compressed reinforced concrete elements with a higher percentage of reinforcement, (b) theoretical data and (c) numerical calculations. The following tasks were solved:

• in accordance with the developed method, analytical, experimental laboratory and numerical studies of fire resistance of compressed concrete and reinforced concrete elements (including those that have a higher percentage of reinforcement) were carried out;
• convergence between the obtained results was evaluated;
• conclusions were made about the issues under consideration.

Research methods. Theoretical data, applied for comparison purposes, were obtained analytically (using formulas and nomograms of SP (Construction Regulations) 468.1325800.2019), experimental and calculated data were obtained in the course of concurrent experimental studies conducted at the fire testing laboratory of NRU MGSU, and numerical studies were obtained using the Abaqus PC software package. In the course of a laboratory experiment made using the loading equipment, the strength of reference concrete templates was identified, using the tensile machine to find the physical and mechanical characteristics of reinforcing steel. Next, reference concrete and reinforced concrete specimens with pre-set characteristics were made. Then temperature fields in concrete and cross sections of specimens, containing reinforced concrete, were identified using the thermocouple data. Centrally compressed concrete and reinforced concrete specimens were tested in a fire chamber at a standard fire temperature. The behaviour of concrete and reinforced concrete specimens was simulated using volumetric finite elements, having the size of up to 10 mm, and a built-in Heat Transfer Solver (Heat transfer). Temperature field distribution and fire resistance of reinforced concrete elements were analyzed at a standard fire temperature in concrete and reinforced concrete elements.

Results and their discussion. The following knowledge was gained by means of this research project:

• convergence between temperature values at control points in concrete specimens (according to thermocouples), acceptable for practical purposes. These values were obtained in the course of experimental laboratory studies and calculations made according to current standards, which confirms the validity of the experiment;
• convergence between temperature values at control points in concrete specimens (according to thermocouples), acceptable for practical purposes. These values were obtained in the course of experimental laboratory studies and calculations made according to current standards, which confirms the validity of the proposed hybrid experi­ment method and the trustworthiness of the research;
• a great difference reaching 10 to 30 % (19.3 % on average for a group of specimens) between experimental and analytical values of fire resistance of reinforced concrete elements, which proves the hypothesis about a reduction in the heating temperature of concrete in a reinforced concrete section if the thermal influence of reinforcement is taken into account;
• convergence between fire resistance values (the difference is 7 to 10 %) in reinforced concrete specimens, which is acceptable for practical purposes. These values were obtained in the course of experimental laboratory studies and calculations, and their convergence proves the applicability of the proposed hybrid experi­ment method with account taken of the hypothesis about less intensive heating of concrete, if the thermal impact of reinforcement is taken into account.

Conclusions. The hypothesis about a reduction in the heating temperature of concrete in the reinforced concrete section that has a high percentage of reinforcement (more than 3.5 %) was proven, taking into account the thermal influence of reinforcement. The consequence is the underestimated values of fire resistance of reinforced concrete elements, identified in accordance with a standardized approach. The applicability of the proposed hybrid experiment technique is also proven, taking into account the hypothesis about a reduction in concrete heating due to the thermal influence of reinforcement.

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