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

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.

The impacts of fabrication systems on 3D concrete printing building forms

Recently,3D concrete printing(3DCP)technology starts entering the market from factories and laboratories,contributing to the creation of new construction methods and architectural forms.However,since the technologies of most 3DCP institutions are independently developed,there is a lack of consensus in terms of construction methods and development approaches in the industry.In this paper,based on 423DCP architectural works completed in last five years,a quantitative analysis was made to evaluate the impacts of the fabrication system on 3DCP building forms.The paper introduced three criteria,including Workspace Index,Geometric Complexity Index,and Tectonic Prospect Index,analyzing and answering the discussions about"adopting in-situ printing or prefabrication","using gantry printers or robotic arms"from the perspective of architectural form.By analyzing specific construction methods and design strategies in these projects,the research summarized three development trends,"mobile equipment,algorithmic structure,and intelligent construction",which will affect the future development of 3DCP building forms.Finally,the paper discussed the advantages,limitations,and potential of four different 3DCP fabrication systems,expecting to point out the directions to further optimize each system and realize more diverse 3DCP buildings.