Interface Behaviour and Mechanical Anisotropy in Layered Concrete Applicable to 3D Concrete Printing: Experimental and Numerical Investigations

Abstract

Layered concrete systems are widely encountered in modern construction applications, including repair and strengthening of existing structures, precast-to-cast-in-place connections, and, more recently, 3-Dimensional Concrete Printing (3DCP). In such systems, structural performance is governed not only by the properties of the bulk material but also by the behaviour of interfaces formed between the layers. Among these applications, 3DCP has emerged as a rapidly developing construction technology in which cementitious material is deposited layer by layer to fabricate structural components without the need for conventional formwork. Although this technology offers several advantages, including reduced construction time, lower labor demand, material savings, and greater geometric freedom, several technical challenges still remain. One of the most important challenges is the formation of interfaces between adjacent filaments and successive deposited layers during the fabrication process. These interfaces act as critical weak links, degrading the hardened mechanical properties of the printed component and inducing pronounced mechanical anisotropy, in which the structural response depends on the loading direction relative to the layer orientation. Consequently, accurate characterization and modelling of interfacial behaviour are essential for the reliable structural assessment and design of layered concrete systems, especially 3D-printed components. This thesis uses destructive and non-destructive testing, along with Finite Element (FE) analysis, to characterize interfaces and investigate their influence on the mechanical response of layered concrete components fabricated with printable concrete materials. The first stage of this research investigates the flexural anisotropy of 3D-printed concrete beams using an interface-based finite element (FE) modeling approach. A Cohesive Zone Modeling (CZM) framework is used to explicitly simulate interfaces. The framework is calibrated and validated using experimental data from the literature and subsequently employed to evaluate the influence of loading direction, interface properties, and nozzle geometry on the flexural performance of 3D-printed concrete beams. The second stage of this research focuses on the experimental characterization and finite element modeling of concrete interfaces representative of those found in 3D-printed concrete. The primary objective was to determine the interface properties required for cohesive zone modeling through a comprehensive experimental program involving tensile, shear, and compression testing of monolithic and layered concrete specimens. Particular emphasis was placed on the use of Digital Image Correlation (DIC) to vi quantify interface deformation and obtain interface failure displacements, providing key input parameters for the finite element models that are difficult to measure using conventional testing techniques. The experimentally derived interface properties were subsequently incorporated into finite element models. Following validation, the modeling framework was employed to conduct a parametric study investigating the influence of the number and arrangement of interfaces on the compressive behaviour of layered concrete representative of 3D-printed concrete. The third stage of this research investigates the use of Ultrasonic Pulse Testing (UPT) combined with advanced signal-processing techniques for the non-destructive characterization of early-age interfacial bond development in layered concrete applicable to 3D-printed concrete. The primary objective was to evaluate the ability of ultrasonic parameters to identify weak interlayers and monitor changes in interface quality with curing age. To provide reference measurements and assess the influence of surface moisture on bond development, mechanical testing and ultrasonic measurements were performed on monolithic specimens and layered specimens with dry and wet interface conditions. The recorded ultrasonic signals were subsequently analysed using velocity-, attenuation-, frequency-, and time-frequency-based parameters to determine their sensitivity to weak interfaces and their potential for non-destructive assessment of interlayer bond quality. Overall, this research advances the understanding of interfacial behaviour in layered concrete systems applicable to 3DCP through the integration of experimental testing, finite element modelling, digital image correlation, and ultrasonic evaluation. The research provides validated interface-based numerical modelling approaches for investigating mechanical anisotropy, experimentally derived interface properties and failure parameters for cohesive zone modelling, and insights into the potential of ultrasonic techniques for the non-destructive assessment of early-age interlayer bond quality. Collectively, these contributions support the characterization, modelling, and quality assessment of interfaces in 3D-printed concrete and contribute toward the future development of reliable design methodologies, inspection strategies, and codification efforts for structural 3DCP.