Hydration heat of concrete mix has an important role in the process of temperature increase in mass concrete at an early age. Thermal stress caused by the temperature difference between the core and the surface of mass concrete is the main reason in making thermal cracks if the stress is larger than the tensile strength of concrete. The aim of this study is to investigate the effect of placing temperature on temperature distribution and thermal stresses of mass concrete. A proportion of concrete mix of interest is used for the thermal analysis of a mass concrete sample sized 8×6×3 m. The finite element Midas Civil program is used to conduct the three-dimensional thermal simulation. Four typical scenarios of placing temperature in the range of (15-30)°C of the concrete mix, which is commonly used in actual construction condition, are presented. The analysis results of temperature distribution and thermal stress indicate that the value of 30°C of placing temperature according to the selected proportion of concrete mix may cause thermal crack in the mass concrete. The study gives a useful way for practical construction application to avoid the risk of thermal crack in mass concrete at an early age.

Synopsis:To construct a calculation model of flow loss in a helical-type seawater magnetohydrodynamics (MHD) power generator, it is important to elucidate the effects of the rotation number, the pitch length and the inner diameter of the helical wall on the pressure distribution and flow loss. The pressure distribution and flow loss in experimental devices constructed to model generators with a helical wall made of polyvinyl chloride with an outer diameter of 100 mm, inner diameters of 10, 20 and 30 mm, pitch lengths of 30, 37.5 and 45 mm and rotation numbers of 3, 5 and 7 were measured systematically. The effect of the rotation number on flow loss was small at flow rates of 20 m 3 /h or less and relatively large at flow rates of 30 and 40 m 3 /h. The effect of the pitch length was small at flow rates of 20 m 3 /h or less and relatively large at flow rate of 28 m 3 /h. The effect of the inner diameter was small at flow rates of 20 m 3 /h or less and relatively large at flow rates of 30 and 34 m 3 /h. The pressure drops in the vicinity of the entrance and / or the exit were the main causes of the disagreement in the flow loss between the experimental and computed values. The equations for the coefficient of entrance flow loss and exit flow loss were modified on the basis of the experimental data.

This article proposes a formula to determine the required amount of ice to partially replace the water in the concrete mix to control the initial temperature of the concrete mix and reduce possible cracking. The formula was created based on the principle of energy balance in the heat transfer process. At the same time, the obtained results were compared with the other methods. Besides, an example of the calculation for a concrete block during the construction was performed. The maximum temperature and temperature difference in mass concrete obtained depend significantly on the initial temperature of the concrete mixture. The research results and the proposed techniques can be used in the practical design of mass concrete structures.

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