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Adjusting Workability of Successful 3-D Concrete Printing

2/15/2021

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The ACI Virtual Concrete Convention – known as the world’s gathering place for advancing concrete – will be held virtually from March 28-April 1, 2021, and will feature 45 total sessions. All attendees will be provided the opportunity to advance their knowledge and earn Continuing Education Units (CEUs)/Professional Development Hours (PDHs). The 2-part featured session titled Adjusting Workability of Successful 3-D Concrete Printing is moderated by Nicos Martys and Scott Jones, and will highlight the importance of controlling fresh-state properties for the successful execution of 3-D concrete printing (3-DCP). 3-DCP is a new method of casting that can introduce a number of advantages but there are also significant challenges, especially in controlling the viscoelastic properties of the concrete in the fresh state. This featured session is sponsored by Advancing Organizational Excellence (AOE).

Presentations include:

1. Buildability, Rheological Properties, and Early-age Deformations of 3-D-Printed Cement-based Materials
   Speaker(s): Mohamadreza Moini, Jan Olek, Pablo Zavattieri, and Jeffrey Youngblood
   Ability to extrude and to achieve shape stability of layer-wise additively manufactured cement-based elements depends upon the early-age rheological properties (shear moduli, yield stress, viscosity) of the deposited materials. Upon successful extrusion, buildability challenges can originate from two common failure mechanisms: yielding of the material in lower layers and buckling of the element. However, it is yet unclear which among various rheological properties controls the early-age materials’ deformation during printing processes and thus contributes to the resulting buildability of the elements. This presentation focuses on how buildability is dependent upon rheological properties as well on predicting the buildability using a buckling theoretical framework. Specifically, the relationship between early age rheological properties of various cement pastes and the buildability of hollow cylinders dominated by buckling failure mechanism was investigated. It was found that certain shear moduli of the fresh pastes (G, G*, and G’) obtained from oscillatory shear stress sweep tests performed within the first 30 minutes after combining water and cement showed better correlation with buildability of hollow elements than some other rheological properties (i.e., loss modulus G”, yield stress yield, yield strain yield, and complex viscosity η*). Measured shear modulus (G) was used to calculate elastic modulus (E) of the pastes using the assumed value of the 0.5 for Poisson’s ratio () for fresh cement paste. Euler’s buckling theory was used to predict buildability (i.e., ultimate height) of hollow cylinders. It was found that Euler’s theory overestimates the buildability from 93% to 194%, mainly due to assumption of ideal geometry (i.e., no initial or printing imperfections) and linear elasticity. An isosceles triangle was also developed and used as a support structure for assessment of early-age deformation of individual cement paste filaments over varied.
   
   
2. Dispersed Nano- and Micro-sized Portlandite Particulates via Electrosteric Exclusion at Short Screening Lengths
   Speaker(s): Iman Mehdipour, Gaurav Sant, Jason Timmons, Shang Gao, Hakan Atahan, Narayanan Neithalath, Mathieu Bauchy, Edward Garboczi, and Samanvaya Srivastava
   In this presentation, we show that the high native ionic strength of portlandite suspensions strongly screens electrostatic forces, which simple Coulombic repulsion alone is insufficient to affect rheology. On the other hand, the longer-range geometrical particle-particle exclusion that arises from electrosteric hindrance is far more effective at affecting rheological properties. We reveal this behavior to be on account of a generalized scaling between the thickness of the adsorbed polymer layer and the observed yield stress reduction. As a result, electrosterically-based dispersants reduce the suspension’s yield stress by nearly 10x at similar dosage as compared to electrostatic action alone.
   
   
3. Enabling 3-D Concrete Printing Through the Use of Ternary-blended Binder and In-line Mixing Nozzle
   Speaker(s): Wilson Ricardo Leal da Silva and Thomas Andersen
   With the concepts for 3-D concrete printing established and first demonstrators constructed, robustness, flexibility and productivity become important benchmarks for the applicability of 3-D printed concrete in the construction sector. This means, ensuring that concrete can be extruded and stays in place after printing and that components can be constructed seamlessly without process interruptions. To achieve this for substantially high structures means controlling at the same time the yield stress at the time of extrusion and its evolution over time. This is achievable by tailoring rheology and hydration with admixture additions in the extruder, allowing to adjust rheology and structural build-up during printing operation, which besides robustness opens the door for more advanced processing approaches.
   
   
4. A Physics-based DEM Model of Flow Through A Nozzle: Application to Cement-based Materials
   Speaker(s): Nicos Martys, Scott Jones, and William George
   Discrete Element Methods (DEM) are a minimalist approach to modeling granular materials, fluids, and suspensions in order to reduce computational demands and model large systems and systems with free surfaces. DEM generally relies on, but is not limited to, phenomenological interactions between objects, e.g., simple dissipative or spring like interactions to model flow like behavior. In DEM parameters are often chosen such that simulation results can match experiments and serves as a form a calibration. In this presentation, we discuss a DEM model, primarily for dense suspensions. Here the usual DEM interactions are replaced by lubrication forces. While not new, its robustness is evaluated and compared to fully detailed simulations which will give you an indication of its accuracy. Finally, this model is applied to modeling flow of a mortar or paste through two nozzle designs used in 3-D printing.
   
   
5. Scaling Relationships in 3-D Printing with Concrete
   Speaker(s): Timothy Wangler and Robert Flatt
   Digital fabrication with concrete, including 3-D printing with concrete, has emerged in the past few years as a topic of great interest in academia and industry, with numerous lab-scale and structural scale demonstrations being constructed to showcase the potential advantages of this new technology, especially improved productivity and more sustainable construction through shape efficiency of nonstandard elements. The material challenges inherent to these emerging technologies have been successfully addressed, at least as proofs of concept. Chief among these challenges is targeted rheological control, including set on demand, which generally requires the use of an inline mixing step just before deposition of the concrete. However, numerous challenges remain, primarily having to do with scaleup of these technologies, which generally have been demonstrated with fine aggregates and low flowrates. Additionally, these digitally fabricated “concretes” have generally had very high paste contents to maintain processability and rather high clinker contents to best control setting using current set on demand techniques, which negates any sustainability benefit from improved shape efficiency. Thus, new challenges for these technologies include 1) increasing sustainability by processing highly substituted cements, 2) increasing maximum aggregate size, and 3) increasing flowrates. This presentation will focus on how these challenges are being addressed in research at ETH Zurich.
   
   
6. Field Adjustments for Additive Construction
   Speaker(s): Peter Stynoski, Eric Kreiger, and Megan Kreiger
   Despite the rapid advancement of layer-by-layer additive construction technology that deposits mortar or concrete, the field lacks consensus guidance for mixture proportioning and performance-based specifications of fresh material properties. As a result, whether sourcing material from continuous mixing, onsite batch mixing, or redi-mix truck delivery, builders will generally require field adjustment capabilities in order to deliver consistent, printable material to the nozzle. In this presentation, we discuss the shortfalls of existing fresh property tests for additive construction applications and propose adaptation of standardized tests from other disciplines for rapid, cost-effective printability quality assurance. We examine the critical importance of water as both a mixture component and a corrective mechanism, plus other key proportioning variables which significantly impact viscosity and yield strength of mortars and concretes. We compare our experiences making field adjustments with those recommended in ACI documentation. Finally, we place field adjustments in context of the way forward: Standardization of adapted field tests, adaptation of existing proportioning guidance, and development of Unified Facilities Guide Specifications.
   
   
7. 3-D-Concrete-Printing by Layered Extrusion – Requirements for Fresh Concrete and Testing
   Speaker(s): Viktor Mechtcherine
   The interest in digital concrete construction is rapidly increasing, as well as the expectations of this new, promising technology. The methods of digital fabrication with concrete are becoming ever more mature, so that several pilot projects have already been realized worldwide. However, there are still some hurdles to overcome before 3-D-printing with concrete and other digital construction processes can be transferred to construction practice. One of the biggest challenges is the development of a practice-oriented approach for determining, adjusting, and testing the required material properties of fresh and hardening concrete. The difficulties in designing the rheological properties of fresh concrete result from different, sometimes conflicting requirements, which are determined by decisive technological steps – material conveying, shaping, depositing, and loading by subsequent concrete layers. To think through and to redefine are also relevant methods of material characterization. This article focuses on these issues in the context of what is by far the most commonly used digital manufacturing method, namely 3-D-printing by layered extrusion deposition. However, the conceptual reasoning, theoretical foundations, and recommendations with regard to material testing can be analogously applied to other methods.
   
   
8. Controlling Rheology of 3-D Printable Concrete Through Vibration
   Speaker(s): Karthik Pattaje, Chuanyue Shen, and David Lange
   3-D printing of concrete promises practical benefits such as customization of structural shapes, reduction of expensive formwork, and decreased construction time and costs. The 3-D print material must be practical to pump, place, and sustain adequate stiffness and surface quality. Vibration of a concrete mixture has been demonstrated to provide a near-instantaneous change in yield stress, allowing for laminar flow during pumping and then, when the material exits the nozzle, an instant recovery of yield strength so that placed material firmly holds its shape and supports subsequent layers of material. This study explores the use of vibration to control essential workability parameters. In the lab, a modified rheometer is used to characterize the rheology of paste, mortar, and concrete both under vibration and at rest. Accelerometers are used to measure the vibrations as they propagate through concrete. Numerical simulation, validated by experimental results, helps to understand the effect of vibration, and predict the behavior of concrete flow during full scale construction.
   
   
9. Material Tailoring in The Extruder, Controlling Hydration, and Rheology
   Speaker(s): Lex Reiter
   With the concepts for 3-D concrete printing established and first demonstrators constructed, robustness, flexibility and productivity become important benchmarks for the applicability of 3-D printed concrete in the construction sector. This means, ensuring that concrete can be extruded and stays in place after printing and that components can be constructed seamlessly without process interruptions. To achieve this for substantially high structures means controlling at the same time the yield stress at the time of extrusion and its evolution over time. This is achievable by tailoring rheology and hydration with admixture additions in the extruder, allowing to adjust rheology and structural build-up during printing operation, which besides robustness opens the door for more advanced processing approaches.
   
   
10. Exploring the Influence of Material Characteristics and Extruder Geometry on 3-D Printing of Cementitious Binders
    Speaker(s): Sooraj Nair, Manu Santhanam, Gaurav Sant, and Narayanan Neithalath
    The steady state extrusion pressure (equivalent to extrusion yield stress) and the dead zone length (static zone of material buildup at die entry) are determined from the pressure-displacement response to characterize the extrudability and/or printability. A unique “geometric ratio” is used to account for the effects of barrel-die geometry on paste extrusion. The ratio of extrusion yield stress to the measured shear yield stress, the dead zone length, and the measured-to-designed filament volume at a given flow rate (print speed) are found to remain relatively invariant with geometric ratio for mixtures with high degree of microstructural packing, and thus better extrudability and shape stability.  This presentation discusses materials- and process-related issues and, could pave the way for test methods for 3-D printable mixture qualification.
    
    
11. Effect of Supplementary Materials and Fibers on Rheological Properties of 3-D Printing Concrete
    Speaker(s): Mohammed Sonebi, M. Dedenis, Sofiane Amziane, and Arnaud Perrot
    The aim of this presentation reports the effect of mix composition such as the percentages of metakaolin, fly ash and red mud on the fresh and rheological properties and also the percentage of fibers of 3-D printing concrete. Several tests were used to determine the rheological properties such as the flow table test, the cylindrical slump test and penetrometer test. Correlations between the results of rheological properties were established and the variation of the fresh properties on extrudability versus time was also investigated.
    
    
12. Printability Metrics for Additive Manufacturing of Cement-based Materials
    Speaker(s): Joseph Biernacki, Hajar Afarani, Edward Garboczi, Newell Moser, and Ebrahim Nasr-Esfahani
    Characterization of manufactured objects typically involves some form of physical measurement of dimension. This is true at all length-scales, from the smallest micro-machines to skyscrapers. Additively manufactured structures for infrastructure applications suffer from in-construction shape deformations, which are a strong function of process conditions and the rheology of the printing material (cement paste, mortar, or concrete). Thus, characterization of the shape fidelity of such manufactured objects is critical to establish and ensure shape-related performance. In this study, alternative metrics were used to compare the dimensional accuracy of printed cement paste objects. A simple visual index, a metric involving the comparison of external dimensions, and a metric that reflects deviations from the model (perfect, target or desired) volume were developed. X-ray computed tomography (XCT) of printed objects was used to evaluate the measurement-based quantitative metrics. Details of the logic and image processing requirements are given and typical sample irregularities that lead to quantification uncertainty are illustrated. The effect of sampling statistics on metric confidence was studied and guidelines are provided for good sampling protocols. The results show the extent to which different penalty logics provide sensitivity for the detection of specific types of flaws. Thus, multiple metrics may be required to characterize dimensional accuracy. Such quantitative printability metrics are necessary to establish a basis for evaluating the precision of 3-D-printed objects and for studying how rheology affects both the manufacturing process and the final built part.

Learning Objectives: Part 1
(1) Describe the current state-of-the-art of 3-DCP from a rheological point of view;
(2) Explain how rheology/workability directly impacts printing performance;
(3) Identify strategies to control and adjust workability for effective 3-DCP;
(4) Summarize the key challenges are in bringing 3-DCP to practice.

Learning Objectives: Part 2
(1) Discuss layered extrusion requirements for fresh concrete and testing;
(2) Review studies on how vibration controls workability parameters;
(3) Summarize influences of mix composition on rheological properties;
(4) Discusses material and process-related issues, for test methods for 3D printable mixture qualification.

Other Featured Sessions:

• Legal Issues in Concrete Construction – Lessons Learned
• The Concrete Industry in the Era of Artificial Intelligence
• Adjusting Workability of Successful 3-D Concrete Printing
• Productivity in the Concrete Industry – Why Has it Stagnated and How Can ACI Help?

View all 45 sessions

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