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.

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).

Penetration testing provides a simple, depth-resolved measure of the strength of soft cohesive–frictional materials, but interpretation at finite penetration velocity is not established for rate-sensitive systems. This study extends a previously introduced depth-resolved bearing-capacity inversion to controlled penetration velocities and quantifies how rate effects enter the identified Mohr–Coulomb parameters. Penetration tests were performed on a mechani- cally inert montmorillonite–glycerin mixture at constant velocities from 1 to 600 mm min−1 using a circular punch (D = 30 mm). For each velocity, the internal friction angle φ(v) was obtained from the steady-state slope and the cohesion-like parameter cpen(v) from the intercept of the surcharge-corrected stress. Independent vane-in-cup measurements with matched surface velocities provided a geometry-distinct reference trend. Across the investigated range, φ remained approximately invariant (φ ≈ 7.8°), whereas cpen(v) increased mono- tonically with velocity (from 0.49 to 0.76 kPa), mirroring the trend in vane shear strength. The results indicate that finite-velocity effects are partitioned primarily into the cohesion-like intercept, while the slope-based friction angle remains a stable descriptor of granular resistance. This supports penetration testing at practical velocities when representative static parameters are required, and motivates application to time-dependent materials where short test durations are necessary.