Conference Agenda

3D-printing with cement-based materials has gained attractiveness in recent years thanks to freedom of form allowing for mass customization, as well as reduced material consumption through shape optimization. However, reaching repeatable quality standards and predictable mechanical properties for print pieces remains a challenge which modelling could help solving. We thus propose a simulation framework for 3D-printing of cement-based materials with two main components. Firstly, we present a fully coupled thermo-hydro-mechanical constitutive model, extended from classical poromechanics to chemically solidifying media, wherein material properties evolve with the extent of hydration reactions, allowing to cover behaviours from the very-early age to the hardened state, along with some experimental procedures to determine model parameters. Secondly, a finite element-based modelling strategy is introduced, aimed at creating a digital twin of the printing process, namely the sequential deposition of the material throughout the print path. Such a framework allows the investigation of common issues in 3D-printing of cement-based materials along with their multi-physics origins, ranging from printing failures to longer-term durability concerns. Influence of process-related parameters -such as layer-pressing- and environmental conditions on printing stability and accuracy are displayed, along with indication as to their mitigation. Durability issues related to drying, be it at the interlayer or at the exterior surface of a print piece, are also presented in light of a detailed modelling of unsaturated behaviour from the constitutive model.

Flexural tests were conducted on RC beams by using material extrusion 3D printing technology. The used mortar does not include any fibers, and the beams have horizontal and vertical interlayers. It was clarified that the compensation plane was maintained within the cross section, the neutral axis rose with the propagation of flexural cracks. The strain of the mortar at the extreme fiber in compression was about 3000 micron, and flexural failure with interlayer delamination in compression zone was observed finally. In addition, delamination of multiple filament interlayer was observed at the rebar position. The interlayer bond strength was measured and found to be about 30% of the splitting tensile strength of mortar itself.

This paper investigates the fracture process zone (FPZ) of mixed mode I-II fracture in concrete after sustained loading, employing the digital image correlation (DIC) technique. Concrete specimens were subjected to sustained three-point bending (TPB) and four-point shearing (FPS) loading for 90 days, with the sustained load level at 80% of the initial cracking load. Afterward, the specimens were unloaded and tested under static loading until failure. The DIC technique was employed to monitor the fracture process of concrete beams after sustained loading. The crack mouth opening displacements obtained from the DIC technique were consistent with those measured using clip gauges, validating the accuracy of the DIC technique. The crack opening displacement field derived from DIC technique was used to determine the size and shape of the FPZ. The lengths of FPZ (lFPZ) for all specimens were measured at peak load. The results indicated that as the ratio of mode II to mode I stress intensity factor (KII/KI) increased, the lFPZ increased. The lFPZ increase in became more pronounced with higher KII/KI ratios at peak load. The lFPZ of creep specimens were shorter than those of aging specimens. The increase in lFPZ for creep specimens was less than that for aging specimens. The findings indicated that that the crack propagation resistance improved with a higher mode II component in mixed mode I-II fracture. Additionally, the brittleness of the concrete increased after sustained loading.

In this study, an experimental work was carried to focus on damage evolution and fracture behavior of under-reinforced concrete beams by continuously monitoring using acoustic emission technique. Beams were of three different sizes with geometric similarity having single longitudinal reinforcement without stirrups. The specimens was tested in three point loading under CMOD control in the closed loop servo controlled hydraulic testing machine. The results of load, displacement, CMOD and strain in the steel are acquired in the data acquisition system. The results of acoustic emission such as location, hits, events, amplitude, absolute energy and time were also simultaneously stored in a computer during the testing. The results of acoustic emission such as spread of the events and its distribution within the beams help in understanding the fracture processes. They provide information regarding the sequence of mechanisms taking place such as micro-cracking, coalescing of microcracks to macrocracks, increased width of macro-cracking and final fracture in under-reinforced concrete beams.