Consideration of Viscoelasticity in Time Step FEM-Based Restraint Analyses of Hardening Concrete

Dirk Schlicke, Nguyen Viet Tue

Institute of Structural Concrete, Graz University of Technology, Graz, Austria.

DOI: 10.4236/jmp.2013.410A2002                

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.

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

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	EN 1992-1-1: Eurocode 2: Design of Concrete Structures, Part 1-1: General Rules and Rules for Buildings, 2011.
[2]	L. Nietner, D. Schlicke and N. V. Tue, Betonund Stahlbetonbau, Vol. 106, 2011, pp. 169-177. http://dx.doi.org/10.1002/best.201000051
[3]	N. V. Tue, D. Schlicke and H. Schneider, Bautechnik, Vol. 86, 2009, pp. 142-149. http://dx.doi.org/10.1002/bate.200910013
[4]	D. Schlicke and N. V. Tue, Measurement of Thermal Re-Straint in Hardening Mass Concrete—A New Approach Exemplified by a Thick Foundation Slab. In Concrete Structures for Challenging Times, pp. 179-184. Marianske Lazne 2010, 6th Central European Congress on Concrete Engineering.