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This paper discusses the flexural and tensile strength properties of 3D printed concrete, based on the results of a RILEM TC 304-ADC interlaboratory study on mechanical properties. These properties are determined using different testing techniques, including 3- and 4-point flexural tests, splitting tests, and uniaxial tension tests, on specimens extracted from large 3D printed elements in accordance with a prescribed study plan. The relationship between compressive and flexural or tensile strengths, cast or printed samples, different types of tests, and different loading orientations, are analysed to understand the influence of 3D printing. As expected, the strength can reduce significantly when the main tensile stress is acting perpendicular to the interface between layers. The role of deviations from the standard study procedure, in terms of the time interval between the placing of subsequent layers, or the adoption of a different curing strategy, are also assessed. While the increased time interval significantly impacts the strength in the critical direction, the use of variable curing conditions does not seem to have a clear-cut effect on the strength ratios of the printed to cast specimens. Additionally, the paper looks at the variability in the results for the printed specimens, in order to emphasize the need for multiple replicates for obtaining a proper result. An extensive insight into the aspects affecting the variability is presented in the paper. Finally, with the limited dataset available for specimens tested at a larger scale, it is difficult to arrive at a clear understanding of the role of specimen size (i.e., greater number of layers).
Traditional construction techniques, such as in-situ casting and pre-cast concrete methods, have well-established testing protocols for assessing compressive strength and modulus of elasticity, including specific procedures for sample preparation and curing. In contrast, 3D concrete printing currently lacks standardized testing protocols, potentially contributing to the inconsistent results reported in previous studies. To address this issue, RILEM TC 304-ADC initiated a comprehensive interlaboratory study on the mechanical properties of 3D printed concrete. This study involves 30 laboratories worldwide, contributing 34 sets of data, with some laboratories testing more than one mix design. The compressive strength and modulus of elasticity were determined under three distinct conditions: Default, where each laboratory printed according to their standard procedure followed by water bath curing; Deviation 1, which involved creating a cold joint by increasing the time interval between printing layers; and Deviation 2, where the standard printing process was used, but the specimens were cured under conditions different from water bath. Some tests were conducted at two different scales based on specimen size—“mortar-scale” and “concrete-scale”—to investigate the size effect on compressive strength. Since the mix design remained identical for both scales, the only variable was the specimen size. This paper reports on the findings from the interlaboratory study, followed by a detailed investigation into the influencing parameters such as extraction location, cold joints, number of interlayers, and curing conditions on the mechanical properties of the printed concrete. As this study includes results from laboratories worldwide, its contribution to the development of relevant standardized testing protocols is critical.
Accurate assessment of damage in concrete structures requires monitoring techniques that can capture both global stiffness degradation and local cracking processes. Existing structural health monitoring approaches typically rely on separate sensors for vibration measurements and acoustic emission (AE) monitoring, while conventional surface-mounted devices often suffer from poor and variable coupling. This study presents an embedded piezoelectric (PZT) sensor developed for dual mode vibroacoustic monitoring in concrete structures.
The sensor is cast within the concrete matrix to improve mechanical coupling and enable robust measurement of structural response during damage evolution. Dual-mode monitoring is achieved through sequential operation of
the same embedded sensor in two distinct modes passive acoustic emission (AE) monitoring during fracture loading and impulse-excited vibration testing conducted before and after fracture test. Benchmarking experiments include comparison with commercial accelerometers and AE sensors, confirming that the embedded configuration enhances high-frequency sensitivity and coupling performance. The fracture process is interpreted by correlating AE activity with Digital Image Correlation (DIC)-based crack kinematics, enabling zone-wise understanding of crack development. The vibration response is interpreted using a stiffness-reduction framework consistent with hinge-type crack formation, explaining the observed modal-frequency reduction and in crease in damping. Electromechanical impedance measurements quantify sensor–matrix interaction, highlighting the role of epoxy-mediated impedance matching. Overall, the results demonstrate that the proposed embedded sensor provides a unified platform for validated AE-vibration sensing, offering a promising approach for integrated structural health monitoring of concrete infrastructure
