Assessing and Modelling the Structural Build-up of Concrete in the Context of Digital Fabrication

Digital fabrication with concrete, accomplished through layered extrusion, enables formwork-free production of structures via layer-by-layer concrete printing. This process leads to increased speed, customization, and efficiency in construction. The freedom from formwork has the potential to make the construction process more cost- and time-saving, but it poses multiple challenges for mix design and testing methods used to characterize the rheological behavior of fresh concrete. Special attention must be given to the static yield stress and structural build-up of concrete at rest, which define its buildability.
This research investigates the structural build-up of cementitious materials, which refers to the evolution of their strength and deformation properties over time at rest. The study aims to develop and refine a methodology to assess structural build-up, as well as its modeling and prediction. Regarding methodology, the research focuses on test methods that can be applied under field conditions and are capable of being automated to accurately predict the buildability of printed concrete structures. The study juxtaposes the results of constant rotational velocity (CRV) tests, unconfined uniaxial compression tests (UUCT), fast penetration tests, and newly proposed confined uniaxial compression tests (CUCT). These tests are compared to the results of buildability tests, which involved producing hollow cylindrical structures using a laboratory-scale 3D printer. Correlations are established between the data to better understand the structural build-up of cementitious materials.
The developed methodology was further utilized to explore the mechanisms behind the effects of aggregate properties on the structural build-up of fresh concrete. The study discovered that aggregate-induced inhomogeneity of constitutive paste in concrete allowed for the introduction of a three-component model of fresh concrete. Additionally, to find models capable of predicting the structural build-up of concrete, the study viewed concrete as a suspension of aggregate particles in a suspending medium. Three approaches were employed to define the suspending medium: plain cement paste, screened cement paste, and fine mortar, comprising particles below 0.5 mm. The study compared and discussed the applicability and limitations of the models based on all three approaches.