Mechanical Characterization of 3D Printed Ultra-High-Performance Concrete (2024-09)¶

As additive manufacturing of cementitious materials becomes more adopted, the need for accurate modeling and experimental validation grows. Currently, mechanical properties of 3d-printed concrete are measured in the materialscale with cast, cut, or cored specimen such as cylinders or cubes, or at the structural scale with full 1:1 replicas of the printed elements. This work addresses this gap by proposing a novel technique to characterize the mechanical properties of 3d-printed ultra high-performance concrete (UHPC) and ultra high-performance fiber-reinforced concrete (UHPFRC) by accounting for the unique geometric features of printed samples which are overlooked by cast, polished, or cored specimen while using less material than full-scale tests. A 3d-printable UHPC mix is developed using class-H cement, silica fume, silica sand, silica flour, and superplasticizer with a water-cement ratio of 0.2. This mix acquires extrudability and shape stability through the addition of NanoClay (purified magnesium aluminosilicate) in powdered form at 0.7% of the binder weight, which successfully modifies the fresh state properties to acquire early strength while not significantly reducing the flowability [1]. Brass-coated steel fibers with a length of 6mm are added as 2% of the total volume to make the UHPFRC. To ensure the desired rheological properties, the mix is extensively studied using a rheometer and each batch is validated using the manual flow table test. The concrete is printed using a piston extruder with a rectangular nozzle aligned horizontally with the print bed and controlled by a three-axis gantry robot (see Fig.1a). Hardened printed samples are tested under uniaxial compression and notched threepoint bending through their longitudinal (Fx), transverse (Fy), and normal (Fz) directions. Printed specimens have a cross section of 30mm width by 40mm height with a 150mm span for flexural tests. To allow representation of geometric features along the layers and boundaries of the printed samples while maintaining smooth, parallel surfaces for load application, a highly flowable variation of the UHPC mix is used as capping material. Properties including compressive strength, flexural strength, and fracture energy are obtained and compared to cast specimen from the same concrete batch (see Fig.1b, c). The effect of fiber addition in these properties as well as their mode of failure compared to cast specimen is also studied. Experimental tests show compressive strengths of over 170 MPa at 28 days, and anisotropic behavior of printed specimen is captured with the procedure. A compressive strength decrease of around 7, 16, and 30% is observed for the Fx, Fy, and Fz directions respectively when compared to cast specimen. Although fiber addition provides a small increase in compressive strength (around 2.5% for cast specimen), it provides significant increases in flexural strength and fracture energy. Results are validated computationally using the Lattice Discrete Particle Model (LDPM) implemented in ABAQUS to simulate the failure behavior at the heterogeneity scale [2], where simulations for unconfined compression test of cast and printed specimen are performed. To capture accurate geometry of printed samples, they are scanned using lidar and their geometry is imported in CAD and meshed accordingly. Results from simulations are compared with experimental data, focusing on both numerical predictions of material properties as well as crack initiation and propagation and modes of failure. The obtained results provide insight into the role of layer surface and shape in the strength of printed UHPC in comparison to cast specimen, while utilizing significantly less material than full-scale structural testing, in addition to providing important calibration data for computational models.