Experimental and computational studies on the effectiveness of strengthening the concrete floor slabs of a hydroelectric power plant's powerhouse through the use of an external reinforcement system involving carbon tape application. Part I

Introduction. In accordance with the Federal Law of the Russian Federation ”On the Safety of Hydraulic Structures“, it is necessary to ensure the long-term safety and reliability of operating structures at hydroelectric power plants (HPPs). In a run-of-river HPP, the "headwall-turbine hall floor slab" system serves as the main load-bearing structure, absorbing a variety of external forces and loads during the operation of the turbine hall within the HPP. Over time, surveys have revealed the intensive cracking in reinforced concrete floor slabs, reducing their bearing capacity. To ensure safe operation and prolong the life cycle of the HPP, it is necessary to strengthen the reinforced concrete floor slabs of the turbine hall with external reinforcement, preferably with composite carbon tapes having high strength, low weight, dielectric qualities and other positive characteristics. At the same time, the method of installing carbon tapes should ensure that finely dispersed conductive dust does not form, since the reinforced concrete structures of the HPP turbine hall, saturated with electrical equipment that powers the HPP, are reinforced. It is also necessary to ensure that the operation of the strengthened reinforced concrete floor slab with an external reinforcement system with carbon tapes is monitored over time by means of embedded control and measuring equipment. The solution of the tasks set in the present experimental and computational studies made it possible to substantiate the effectiveness of strengthening the floor slab with external reinforcement based on the use of carbon tapes and to implement it in the HPP turbine hall. Materials and methods. The reinforced concrete floor slab in the HPP turbine hall, which has been subjected to both external and non-design loading during operation, is the object of this study. The cracking and deflections recorded by the surveys led to the need for experimental studies in field conditions, including using a load weighing 150 kN. The stresses in the working metal stretched reinforcement were determined before and after the external reinforcement device with FibArmTape-530/300 carbon tapes. The stress in the carbon tapes was also determined. The experiments of reinforced concrete floor slab models’ fragments of the stretched zone were carried out in laboratory conditions in order to determine the effectiveness of the reinforcement using a new method of installing carbon tapes on a concrete surface. These experiments were carried out using a tensile breaking machine and confirmed the positive effect of using a new method of carbon tape device, preventing the formation of finely dispersed conductive dust in the operating electrical installations of the HPP turbine halls. At the same time, the necessary strength of reinforced structures is obtained. In order to determine the formed stress-strain state of the reinforced concrete floor slab of the turbine hall, computational studies were performed taking into account the revealed nature of cracking. Stresses in stretched working reinforcement under various loads before and after strengthening, as well as stresses in carbon tapes, were determined. Using the calculations performed, a schematic diagram of reinforced concrete floor slab strengthening is proposed – the pitch of the carbon tapes and the number of layers of the composite material. Results. From the results of the experimental studies of models fragments of the reinforced concrete floor slab stretched zone that have characteristics of hydraulic structures (concrete of class B15 and B25, low coefficient of reinforcement μs = 0.00445÷0.00692, it follows that an increase in strength of 1.62–1.96 times was obtained for composite carbon tapes on a concrete surface strengthened by a new method (applied to HPP). As a result of the strengthening of the reinforced concrete floor slab with external reinforcement using a new method (applied to HPP) for the installation of carbon tapes, as a result of experimental studies in the turbine hall of the HPP, a stress redistribution was established between metal working reinforcement and carbon tapes, which were included in joint work with the floor slab with the formation of tensile stresses in the tapes. Under loading on a reinforced floor slab with 150 kN, the tensile stresses in the metal reinforcement decreased by 1.8–2.9 times (directions – along the flow and across the flow) with the formation of almost equivalent stresses in carbon tapes of the external reinforcement. In order to calculate the changes in the stress-strain state of the floor slab of the turbine hall of the HPP building (with cracking, with support that is not designed in some places under the influence of non-designed loads) strengthening with external reinforcement with carbon tapes, a spatial finite element model was developed and applied.