Abstract
Concrete structures may suffer considerable restraint stresses during their hardening period. This is caused by several deformation impacts, especially temperature field changings due to hydration heat and volume changes due to autogenous shrinkage. Mainly affected are massive concrete members, but also the application of new concrete types or the erection of outstanding constructions requires further investigations in this context. 3D-FEM analyses of hydration heat induced temperature development in combination with the well known shrinkage give sufficient results for the deformation impact. The according elastic restraint stresses can be determined with consideration of the concrete’s rising elastic modulus and the restraint degree of the system. But due to duration of the heat flow process, the height of restraint stresses is strongly dependent from the viscoelasticity of the concrete. The viscoelastic effects consist of many components constituted by changing material properties influencing themselves. In practice, different simplified approaches are available for considering this in calculations. Their implementation in time step analyses is not generally admitted and requires expertise. In contrast, present research develops material models needing specific input parameters for every use case. This contribution focuses on a practicable approach considering the superposition of the viscoelastic behaviour of every stress increment in time step FEM analysis. The differentiation between the pure viscoelastic material behaviour (as it is given in the codes for idealistic conditions like creep or relaxation) and the according viscoelastic system response (addicted to the systems variable restraint degree) allows the transfer of this model into practice. As one application example of this approach, the compatibility check and the FEM-based recalculation of the monitoring program of a massive power plant slab will be presented.
Highlights
The behaviour of concrete structures could be considerably influenced by hardening caused stresses due to hydration heat and shrinkage—especially in case of massive concrete members, and at the application of new concrete types or at the realization of outstanding constructions
Viscoelasticity can be seen as an additional deformation impact, which occurs by time
The following issues should be pointed out: 1) Due to hydration heat, high temperature gradients occur in thick concrete members
Summary
The behaviour of concrete structures could be considerably influenced by hardening caused stresses due to hydration heat and shrinkage—especially in case of massive concrete members, and at the application of new concrete types or at the realization of outstanding constructions. In practice different simplified approaches are available for considering this in calculations Their implementation in time step analyses is not generally admitted and requires expertise. Present research develops material models needing specific input parameters for every use case This contribution focuses on a practicable approach considering the superposition of the viscoelastic behaviour of every stress increment in time step FEM analysis. Thermal restraint of hardening concrete is a superposetion of stress increments built-up by hydration induced deformation impacts (heat + shrinkage), which were restrained by different reasons. Because of the evolution of stiffness, the length of the time step, in which the temperature change occurs, is restricted to a period, in which nearly constant stiffness can be assumed This initially stress increment causes additional deformations in the concrete, what is called creep in general. Following present approaches were introduced and will be evaluated with regard to practicability and validity limits
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