Temperature Changes in Mass Concrete of the Hydro Power Plant Lešće in Croatia

Following the needs of the hydro power plant (HPP) Lesce construction on the river Dobra in Croatia, the development of maximum temperatures in mass concrete blocks of the power-plant’ s gravity dam is analysed. Prediction of the temperature and stress distribution in concrete is of great importance to designers, contractors and investors. The appearance of cracks in concrete can represent a serious problem for the stability of an entire structure or its durability by allowing penetration of aggressive substances from the environment into concrete which results in the corrosion and deterioration. For this reason, it is necessary to make a temperature calculation. Hardening of early age concrete is connected to chemical processes which go along with a severe temperature and dimensional changes depending on the quantity of cement, heat of hydration, initial temperature of concrete and temperature changes of the environment. For the purpose of preventing cracks in concrete two basic conditions are set. One is the maximum temperature that concrete is allowed to achieve because of hydration process. The other condition is much more complex and it refers to the temperature gradient which is forming from the inside out to the surface and it is a consequence of the heat transfer from concrete block to the environment and of the small thermal conductivity of concrete. This results in the tensile stresses and tensile creep of concrete. The tensile stress increases because of the concrete shrinkage and if it becomes greater than tensile strength, cracks appear. In the first few days of concrete hardening the relationship between tensile strength, modulus of elasticity, tensile creep and stresses in concrete change rapidly. Thus, it is very difficult to prescribe limiting conditions for temperature gradient as a basic criterion for the mechanism of cracking in mass concrete. The limiting temperature gradient according to literature is 25 C /25cm in the surface layer of concrete. The process flow of temperature changes in concrete is very difficult to predict in calculations because it combines simultaneous increase in temperature in concrete during the release of cement hydration heat and the heat transfer from concrete block, taking the environmental temperature into consideration. The problem is mathematically solved using two different methods for the heat transfer problems. First method is Schmid’ s numerical method. With this method we solved one dimensional transient heat conduction. Second method of solving was a finite element method with the software package Cosmos/m. Problem was with this method solved on the two dimensional model which is subjected to a simetrical load so that results from both methods can be compared. With this method transient analysis was performed taking into account heat transfer through conduction and convection. Computer calculations of different extreme initial conditions and environmental conditions enable detailed analysis of different parameters of construction technology such as: height of blocks, quantity and type of cement, time-table of concreting and so on. During the execution of temporary structures for the HPP Lesce, the temperature measurements in mass concrete have been performed. Obtained data have been compared with the computer calculations. The analysis of data will be used in the process of the gravity dam execution in order to avoid the thermal cracking in concrete placed in the local climate conditions.