Stresses in high performance concrete due to temperature and moisture variations at early ages

Sammanfattning: This thesis presents results from laboratory tests and numerical analyses of stress states in concrete structures arising from volume changes due to temperature and shrinkage during hydration. The laboratory tests presented here deal with: the determination of heat transfer coefficients for cooling pipes measurements of self-desiccation and shrinkage under sealed conditions deformation measurements under temperature and moisture changes and measurements of stress development during hardening for a fully restrained structural member. The thesis comprises three papers dealing with the following topics:
Paper A - Air cooling of concrete by means of embedded cooling pipes. Laboratory tests of heat transfer coefficients
This paper presents a laboratory method for determining heat transfer coefficients for cooling pipes. The embedded cooling pipes are used in order to reduce the temperature rise in massive structures as a measure against thermal cracking. When air is used as cooling medium, relative large diameters with surface profiles (roughness) causing friction losses along the pipe are preferable. Heat transfer coefficients for two different types of cooling pipes have been determined for varying pipe flows in combination with different temperature levels. Paper (A) constitutes the first part of the thesis dealing with the laboratory tests on heat transfer coefficients.
Paper B - Air cooling of concrete by means of embedded cooling pipes. Application in design
Paper B constitutes the second report on the subject air cooling of concrete. Embedded cooling pipes are used in order to reduce the risk of thermal cracking in early age concrete. Traditionally, water has been used as the cooling medium, but air cooling has shown to be advantageous in many applications. The experimentally determined heat transfer coefficients of cooling pipes, investigated in Paper A, have been used and verified in comparisons with in situ measurements at the Igelsta Bridge in Södertälje, Sweden. The close agreement between measured and calculated temperatures of air-cooled sections seems to justify the use of average heat transfer coefficients. A few exemplifying calculations are also given, and the general behaviour of cooled structures is discussed. The principles of designing cooling systems for the general case are proposed. It is concluded that it is possible to design prismatic structures, such as a columns, by the use of existing models and the measured heat transfer coefficients evaluated according to the method presented in Paper A.
Paper C - Deformation and Stresses in Hardening Concrete due to Simultaneous Changes in Humidity and Temperature. Laboratory tests and evaluation
This paper describes test methods and equipments for determination of strength and maturity growth, heat of reaction, and free thermal deformation. Measurements of shrinkage, relative humidity and stress development are carried out under sealed conditions during hardening of the young concrete.
Results for both high performance and normal strength concretes are presented and discussed here. Also, methods for describing shrinkage at sealed conditions as a function of maturity and as a decrease in relative humidity are presented. An evaluation is performed of the free thermal deformation during the hardening of concrete where the deformation is separated into pure thermal deformation and shrinkage. In this context some pilot calculations of non-linear modelling of stress development have been performed taking stress-induced deformations and simultaneous changes in humidity and temperature into account. Calculations are made for a structural member which is fully restrained, i.e. to a degree of 100%. A sensibility analysis of the non-linear stress model is performed and presented.