Modeling of Temperatures in Cementitious Monoliths
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MODELING OF TEMPERATURES IN CEMENTITIOUS MONOLITHS S. KAUSHAL, D.M. ROY* and P.H. LICASTRO Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802 *Also affiliated with the Department of Materials Science and Engineering Received 2 February,
1987; Communicated by G.J. McCarthy
ABSTRACT Temperatures in large cementitious monoliths (works) can rise very high due to unfavorable thermal properties such as low conductivity and high diffusivity of the monolith and the surrounding media. Heat moderation becomes necessary in such situations to avoid excessive thermal stresses. Moderation due to the addition of inert additives such as sand in mortars is compared to that obtained by the addition of reactive but low heat evolution substituents such as Class C and Class F fly ashes. Substitution of cement by slag has also been considered. The hydration temperatures for the extreme conditions (adiabatic) have been experimentally measured and compared to those predicted under real conditions. Such a simulation has been made by measuring the thermal properties and analyzing the temperature distribution due to exothermic reactions as predicted by a finite differences computer model. In general, lower temperatures can be maintained by increasing the thermal conductivity and heat capacity of the hydrating material. This material can be tailored for both heat evolution and setting times by incorporating inert additives such a sand (quartz) and/or reactive additives such as slag and fly ash. INTRODUCTION The hydration of cementitious materials is the result of a series of complex exothermic reactions. As a result of these hydraulic reactions, the temperature of a cementitious monolith increases significantly within three days [1]. Similarly high temperature increases have been predicted as well as reported in complex blended cements [2,3]. The increase in temperatures in large works is known to cause earlier strength development. This has led to fears about thermal cracking due to the large temperature differentials between the interior and exterior of concrete bodies [4]. At the same time, the appearance of fly ash as a cheap and abundant mineral source has made the equivalent of 10-20 million tons of cement available in the U.S. This has resulted in a lowering of the energy requirement in the production of concrete and directed more research toward incorporating fly ash in cement. However, very little effort has been directed toward recognizing blends of portland cement and fly ash as integral reacting systems and to study such a system at the more realistic elevated temperatures attained [5]. While it is accepted that the heat and the alkali hydroxides released during cement hydration are the causes for fly ash activation, it is not known whether fly ash causes retardation or acceleration of the kinetics of cement hydration [5,6]. The study of temperatures in a large cementitious work can be broken up into a prediction of the actual temperatures forecast and the simulation of such temperatures
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