Monitoring of the Thermal and Autogenous Strain

Abstract

During early age, the autogenous and thermal deformations are two of the most important concrete properties that are involved in the cracking risk in cement-based materials and concrete structures (especially for massive structures). For that reason, several test rigs were developed in the past for the monitoring since casting time of these deformations. During early age, the thermal strain depends strongly on the evolution of the coefficient of thermal expansion (CTE), the thermal conductivity, the heat capacity, the heat release of the cement paste and the boundary conditions. During hydration process, both autogenous and thermal strains occur simultaneously and cannot be directly distinguished. In previous work, two kinds of methodology were used to monitor the CTE. The first method uses two distinct tests with two different histories of temperature in order to define autogenous and thermal strain. The second method uses only one test during which repeated thermal variations are applied to a sample. This chapter reports on the most recent advances on the physical mechanisms associated to the development of the autogenous strain and the CTE. The recent devices and test protocols developed for the simultaneous monitoring of the coefficient of thermal expansion and the autogenous strain at three different scales (cement paste, mortar and concrete) are presented. The global testing methodologies bear some similarities but major differences remain in the test set up designs, in the testing processes and also in the data processing. Based on the physical mechanisms, existing test facilities and test protocols, a new test protocol is established and is compared to existing test protocols. Finally, recommendations are proposed for the determination of the autogenous strain and the CTE.