Doctor Paper Abstract (English)

　The aim of this study was conducted to analyze accurately the heat conduction and thermal stress caused by thermal cracking of concrete box culvert walls on farm roads using the three-dimensional finite element method and to develop a curing method for preventing thermal cracking on the basis of the analytical results. The program used to analyze heat conduction and thermal stress by the three-dimensional finite element method was coded in FORTRAN language. The efficiency of this method was estimated by comparison between various analytical and measured values. Also, some consideration was given to input parameters. The curing method for preventing thermal cracking was devised. Thermal insulation was secured by tightly covering box culvert with tarpaulin, and then it was thermally cured by burning a briquete for increasing the interior atmosphere of a box culvert than the exterior atmosphere. The effectiveness of our newly devised curing method for preventing thermal cracking was estimated by applying the method to real concrete box culverts on farm roads. On the basis of these fundamental studies, simple systematization of an improved curing method was performed.

　The results obtained can be summarized as follows:

1. The calculated temperature by the three-dimensional finite element method was found to be lower than the measured one with increasing concrete structure size. This was attributed to the difference in the characteristic adiabatic temperature rise curve.
2. Concrete structure size and form influenced the time course of temperature. It was found that for a thin member such as a wall or slab, the three-dimensional method provided a better simulation than the one-dimensional method. This was due to the influence of atmospheric temperature.
3. Analysis of heat conduction by the three-dimensional finite element method, based on the Galerkin method, which can exactly approximate the inner temperature, was carried out in real concrete box culvert walls on farm roads.
4. The coefficient of thermal expansion was divisible into two sections, involving rising and falling of the inner temperature of the concrete. Also, for the case which involves changing point of the rising gradient during the period when the inner temperature was rising, the coefficient of thermal expansion was divisible into three sections, that between the initial temperature and the changing point of the rising gradient, between changing point of rising gradient and peak temperature, and falling section of the inner temperature of the concrete.
5. Analysis of thermal stress by the three-dimensional finite element method, which can exactly approximate the inner thermal stress, was carried out in real concrete box culvert walls on farm roads. The results of thermal stress obtained using the elastic modulus by a function of age was smaller than that using the elastic modulus by a function of maturity.
6. Our devised curing method for preventing thermal cracking is effective for cracks that occur in concrete box culverts. The validity of this method was assessed by analysis of heat conduction using the three-dimensional finite element method and its application to a real concrete box culvert.
7. Proposal of an empirical equation, R=V/84 (R: number of burst briquettes on the first day, V: volume of the interior box culvert) may make it possible to estimate the number of burst briquettes with this curing method for preventing thermal cracks.