This technical session is co-organized by ACI 564, ACI 236, and RILEM to disseminate information on scaling up of concrete 3D printing, with an emphasis on knowledge gained by the international community (in Europe, Asia etc. where concrete 3D printing is becoming more ubiquitous). These sessions are intended to: (i) disseminate the most recent information on scaling up of concrete 3D printing for different structural applications, and (ii) outline the challenges faced in printing real-world elements related to materials, printing system, architectural and design, structural testing, code compliance etc., and methodologies adopted to overcome those challenges. These sessions will incorporate international speakers who are at the cutting edge of research and development in concrete 3D printing, as well as practitioners who have implemented such structures in real world. The session is intended for professionals, students, and practicing engineers/architects.
Learning Objectives:
(1) Understand fresh properties relevant to upscaling 3D printing;
(2) Obtain an overview of optimization process with respect to 3D printing;
(3) Describe the mechanical characteristics of conventional and high performance 3D printed elements;
(4) Explore the applications of AI in 3D printing.
Twin-Pipe Pumping Method for Upscaled 3D Concrete Printing with Chemical Stiffening Control
Presented By: Geert De Schutter
Affiliation: Ghent University
Description: It is challenging to unify the contradictory rheology requirements for cementitious materials for 3D printing. In the first phase, the material needs to be sufficiently flowable to facilitate pumping towards the nozzle. In the second phase, the material has to be extrudable. In the third phase, once the layers are formed, the cementitious material needs to show a high buildability to support the weight of the next layers. Consequently, within a short time, the material needs to show a very quick change from more fluid-like to more solid-like. Different approaches could be followed to reach this goal, among which the addition of an accelerator in the nozzle in combination with the use of a dynamic mixer to homogenize. A new and alternative approach is now presented with the Twin-Pipe Pumping (TPP) method, basically consisting of two pumps combined with a motionless static mixer. During the printing process, two distinct mixtures are premixed. The first stream consists of a Portland cement-based mixture without accelerator, while the second stream is based on limestone powder and contains a relatively high dosage of accelerator. Both mixtures are easily pumpable, with a long open time. Two separate pipes convey the different streams towards an in-line static mixer, located immediately before the extrusion nozzle. By means of the chemical action induced by the accelerator, the stiffening rate of the combined mixture is then drastically increased. TPP enables a high construction rate with good shape stability. As an example, a 3 m high column could be printed within a record-low time window of only 9 minutes. As a further advantage, TPP can also be extended to potentially more sustainable alkali-activated materials, following the same principle of two separate streams and chemical activation within the static mixer. Some examples of remarkable 3D printed elements will be shown, including large overhang and bridging window and door openings.
Application of 3D Concrete Printing with Form-Optimization for Structural Application
Presented By: Kolluru Subramaniam
Affiliation: Indian Institute of Technology Hyderabad
Description: In construction, 3D concrete printing (3DCP) is emerging as a technology that has the potential of creating significant impact on the state of practice and overcoming several limitations of conventional construction processes. 3DCP is currently focussed on delivery systems which build a structure using layer deposition. 3DCP technology is however still in a nascent stage in construction related applications. Techniques for printing free-standing structures are not sufficiently developed for large-scale applications. In this paper, the concept of form specific structural system optimization that broadly follows ‘material-follows-force’ will be used to arrive at a shape that reduces weight while minimizing the requirement of conventional reinforcement. This concept of form optimization is demonstrated with the design of a bridge following the concept of a tied arch. The tied arch, which combines the tension ties with compression struts is used to develop a force transfer system that minimizes the requirement of reinforcement. The printed structural system of the tied arch is used to design a pedestrian bridge of 8 m span. The evolution of the structural system of the bridge that was optimized for the use of material, while ensuring stiffness is presented. The material testing, design, off-site printing and installation of the structure are detailed.
Mechanical Characterization of 3D Printed Ultra High-Performance Concrete
Presented By: Shady Gomaa
Affiliation: Northwestern University
Description: As additive manufacturing of cementitious materials becomes more adopted, the need for accurate modelling and experimental validation grows. Currently there is a wide gap between material characterization for 3d-printable concrete and full-scale tests of printed structural 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 geometric irregularities between printed layers which is overlooked by cast, polished, or cored specimen. A nano-modified 3d-printable UHPC mix is developed for this study, and early-age printability properties are validated using the manual flow table test. Hard-ened printed samples are tested under uniaxial compression, notched three-point bending, and split-tensile loading along their longitudinal, transverse, and normal directions. To allow representation of geometric irregularities 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 was used as capping material. Properties including compressive strength, tensile strength, and fracture energy are obtained and compared to cast specimen from the same concrete batch. The effect of fiber addition into these proper-ties is also studied. Additionally, results are validated computationally using the Lattice Discrete Particle Model (LDPM) to simulate the failure behavior at the heterogeneity scale. The printed samples are measured and scanned, and their exact form is used for the simulations to accurately match the model to the physical samples. 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 ca
Ultra High Performance Concrete 3D Printed in Conjunction with Conventional Concrete - Composite Behavior
Presented By: Avinaya Tripathi
Affiliation: Arizona State University
Description: This presentation explores the composite behavior of ultra high performance concrete (UHPC) printed in conjunction with conventional concrete, and evaluates the flexural response and cracking behavior of the composite system. Digital image correlation to determine crack propagations and a composite model for stress-strain relations are proposed. The talk demonstrates the synergy that can be obtained in developing cost-effective, high-performance cementitious composites using 3D printing
A Numerical Framework for Fracture Mechanics of Additively Manufactured Materials and Interfaces
Presented By: Aimane Najmeddine
Affiliation: Princeton University
Description: Additively manufactured cement-based materials can exhibit complex fracture behaviors such as crack deflection, penetration, and bridging, under various loading conditions, due to the presence of heterogeneities such as interfaces. This can lead to significantly different mechanical properties such as strength and toughness and overall performance at a large scale. A major challenge encountered in designing additively manufactured concrete materials is understanding the contribution of interfacial cohesion between a material's constituents to the overall mechanical properties and performance of the structure. In this presentation, we propose a constitutive framework based on fracture mechanics to better elucidate the role of the interfaces in capturing and engineering the fracture response of additively manufactured cement-based materials. This framework integrates the phase-field approach to fracture with a Cohesive Zone Model (CZM) to capture various fracture propagation mechanisms within the bulk and across layered material’s interfaces. It has been demonstrated that the framework can accurately predict a range of fracture dissipation phenomena in brittle layered hard-hard cement-based assemblies as well as hard-soft composites with interfaces. These mechanisms include crack deflection and penetration in hard-hard assemblies and crack bridging in hard-soft assemblies along with other toughening mechanisms (e.g., progressive layer failure), all of which are crucial to properly design resilient additively manufactured materials and structures at large scale.
Towards a Rational Performance Based Mix-Design Approach for 3D Printable Concrete Mixes through AI Algorithms
Presented By: Liberato Ferrara
Affiliation: Polytechnic University of Milan
Description: 3D Concrete Printing (3DCP) has been gaining momentum in the last few years, because of its great potentials from the social, environmental and economic point of view. Although nowadays the research carried out is extensive, also complemented by an increasing number of practical applications, ranging from decorative to structural, it is still difficult to steer 3DCP towards a standardization which would permit a wider use of the technology in large scale civil engineering applications. This deviation from going straightforward to standards can be due, among the other reasons, also to the variety of parameters involved in the design of “printable” mixes, which often opens to new questions on the study of the optimal mix composition and on the influence of the latter of the “printability” performance. In this sense, it results extremely powerful to perform a systematic survey of the literature on 3DCP in order to investigate the parameters of interest and establish correlations among them which may be helpful in the design of a mix from scratch. This is exactly the aim of the present work, which describes the steps followed during a systematic survey of the literature with the aim of developing a database on 3DCP, and using this last for practical application as predictive tool for the printability of selected mix designs through Artificial Intelligence (AI) approaches, as Artificial Neural Networks (ANN). Due to the complexity of the problem, a method to collect data in an orderly manner was considered necessary. A relational database based on Structured Query Language (SQL) was selected as an efficient way to visualize and select stored data than a single spreadsheet. Therefore, an entity-relationship model suitable for the case study was developed with the aim of providing a database structure that can be easily handled and modified in the next future by any researcher willing to integrate own results to broaden the size of the same database. The developed database