TITLE:
Consideration of Viscoelasticity in Time Step FEM-Based Restraint Analyses of Hardening Concrete
AUTHORS:
Dirk Schlicke, Nguyen Viet Tue
KEYWORDS:
Viscoelasticity; Hardening Mass Concrete; FEM Restraint Analysis; Time Dependant Material Properties; Hydration Heat; Evolution of Stiffness; Quantification of Restraint Stresses; In-Situ Measurements; Recalculation of Measurement Data
JOURNAL NAME:
Journal of Modern Physics,
Vol.4 No.10B,
October
30,
2013
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.