Abstract
In recent development of mass concrete structures, the problem of cracking associated with thermal expansion has been a matter of great concern, especially in those structures requiring high watertightness. This paper focuses on the effects of placement conditions of mass concrete structures, such as the proper season for placement, height of lift, interval of placement and curing time, on the development of cracks due to thermal expansion. Numerical and parametric studies were carried out using FEM, on the basis of an experimental planning method, to examine quantitative relationships between them. Concluding remarks drawn from the present study are summarized as follows.
(1) The height of lift is to be the most influential factor on the temperature increase in concrete. The variance analysis on the temperature increase showed that the height of lift and the interval of placement were of 1% significance and the curing time of 5% significance in the variation. The contribution ratio to the temperature increase was extremely large for the height of lift as compared to other factors, showing 94-95%.
(2) The height of lift also showed a large influence on the development of thermal stresses in concrete. According to the variance analysis on the maximum tensile stress, all factors exhibited 1% significance in variance. The contribution ratio to the maximum tensile stress were 84-87% for the height of lift, 7-10% for the interval of placement, and 3% for the curing time, indicating a predominantly large contribution of the lift height.
(3) The maximum tensile stress in the surface part of concrete showed a considerable decrease due to the decrease in the lift height, its decreasing rate being almost exponential.
(4) Evaluation of thermal-crack index also revealed a large influence of the lift height on craking potential. This again confirmed the need for special consideration regarding the height of lift in the design and construction of mass concrete structures.
Practical measures to control thermal cracking have already been discussed and taken into account in the actual construction procedures, though they are from qualitative points of view as a result on experience. Numerical experiments performed here, on the other hand, made it feasible to conduct quantitative evaluation of the influence of placement conditions on thermal stresses and cracking potential.
