| چکیده انگلیسی مقاله |
Mass concretes including roller-compact concrete are materials with poor tensile behavior. When subjected to shrinkage or heat in their very early ages such concretes may easily crack. Thus for controlling and minimizing the risk of thermal cracks, it is crucial to study the effects of such parameters as the rate of concrete pouring in construction layers, seasons of start, pause of construction, and the extent of pre-cooling of concrete materials. Therefore, thermal stresses and probable cracks should be controlled based on a sound construction schedule. In practice, most cases are dealt with using a simple one-dimensional analysis pertaining only internal concrete evolution and thus disregarding the surface concrete story. At the same time, the induced surface stresses are not accounted for in such analyses. However, as a minimum requirement, a two dimensional model of the dam body across its vertical section is needed to account for the main effects mentioned above. Despite that many analyses have been carried out by others so far, in this research, concrete thermal conductivity coefficient is considered as a function of concrete temperature throughout a transient heat conduction analysis. The material is assumed as isotropic in both thermal and mechanical senses. The topology of model as well its top boundaries are continuously updated according to the construction schedule. Furthermore, accounting for the dam construction schedule, heat generation due to both ambient and cement hydration phenomena, as well as inclusion of convection and radiation boundary conditions due to solar effects are considered. In addition, when dealing with stress analyses and safety evaluation against cracks, the dependencies of concrete elasticity modulus on time and temperature, and concrete compressive as well as tensile strengths on time (i.e., the ages of layers) are all considered. Indeed, the thermal analyses are carried out after performing each single layer. Also after every 10 layers are performed, a full stress analysis is conducted under the current thermal and gravity loads. Safety factors are calculated considering the material properties and strength available at the same instance in each layer. To study the effect of these parameters on heat generation, and the subsequent thermal stresses in the body of RCC dam, "THA-DAN" dam in Thailand was chosen as a benchmark introduced by ICOLD. This dam has been built of 160 layers of 30 centimeter thickness. Program ANSYS-6.1 was employed in a special manner to allow such a coupled transient Abstracts 140 analysis for both thermal and stress parts. Initially, a basic verification of calculated temperatures versus the measured ones (as provided by ICOLD) was done for the layered construction at the dam site. The results of this study showed that maximum hydration temperature occurs at the layers poured in the hottest season. The efficiency of pre-cooling techniques is rather low, because by each 5˚C pre-cooling, only 1˚C drop in the temperature of internal concrete and 0.3 MPa drop in tensile stresses are gained. Tensile stresses are concentrated on the free surfaces of the concrete as well as on the ground interface due to constraints. This happens at a 3-4 meter thick layer there due to the high thermal gradients. Inclusion of gravity load in the stress analyses helps reducing the tensile stresses, particularly near the ground. Furthermore, if the frost of concrete materials could be avoided, winter would be the most efficient season for starting the construction. The most critical case is to start the construction in summer. Elongated construction pause in hot seasons is very effective for controlling the thermal cracks, although it has only negligible effect in cold season on the final distribution of heat and the maximum temperatures induced in the dam body. It is interesting to notice that the lower is the speed of concrete pouring, the cooler the core concrete becomes. At lowest speeds the warm core approaches to the ground surface. Although the above observations were found through a single dam being investigated, but they could also be mostly true for most of the gravity type RCC dams. |