Additive Manufacturing (AM) with cement-based materials is an emerging technology that opens opportunities in the construction industry by reducing labor costs and increasing reliability of concrete infrastructure. Several institutions have demonstrated using concrete in AM by automating portions of the construction process. The construction of concrete columns has been automated with the use of a robotic arm to control the assent of a form to create geometries that cannot be achieved with conventional static forms. Computer controlled gantry cranes have been adapted to dispense material in a layer-by-layer method to create structures resembling the walls of buildings. AM in the construction environment has the potential to im- prove construction efficiency by eliminating the time and resources, e.g., labor and material cost, required to erect temporary formwork required for conventional concrete construction. Rapid construction enabled by AM techniques can provide shelter to communities affected by natural disasters, build with local materials in hostile environments (e.g., military and mining applications), build taller wind turbine towers to access higher energy winds, enable cost-effective design strategies by precise control of materials, facilitate intricate architectural designs, and provide tools to repair concrete in hard to reach areas.
The complex flow properties of the materials and the layering nature of the AM process, however, will require new standard practices for specifying/accepting mixtures and for assuring the performance of as-built structures. This represents a paradigm shift for the concrete construction industry. New metrologies are needed to measure the complex rheological properties of these materials, and then relate these properties to the construction process.. In addition, existing structural testing methods (e.g., compressive strength) are not applicable to this type of construction.
Objective - Develop measurement science tools (metrologies, standards, and guidance documents) for accurately evaluating the critical material properties that ensure the desired field performance of cement-based materials for additive manufacturing.
What is the new technical idea? Additive manufacturing is a dynamic process whereby the time-dependent chemical processes (hydration) influence the rheology (flow) and the bonding of one layer to the previous layer. Therefore, not only must one characterize the time-dependent rheological properties, one must also understand the relationship between the hydration and rheology. NIST will leverage its world-class expertise in cement hydration modeling and concrete rheology to develop a standard practice that will help to ensure defect-free fabrication. Rheology modeling will be used to identify critical material parameters influencing dense suspension flow through a nozzle (viscosity vs. complex modulus). Hydration modeling and experimentation will be used to relate microstructural changes to rheological changes; field measurements (e.g., electrical tests) will be developed to identify these microstructural changes in the field. NIST expertise in concrete testing will be used to develop new standardized tests for the built structure, consistent with the AM layering process.
What is the research plan?
Four prong approach
A combination of experimental measurements, numerical simulations, and machine learning data processing will be used to assess the potential of a material system for AM. Experimental measurements of AM materials will be made prior to the AM process. The material will be used to fabricate an artifact using one of two AM robots, one bench scale robot for paste and one construction, pilot-size robot for grout. These measurements will be correlated with an assessment of the quality of the artifact after fabrication. Properties of setting time, viscosity, yield stress, isothermal calorimetry, and compressive strength will be explored to select the best combination for qualifying material composition for AM.
Numerical simulations of paste and mortar flow will provide insight into the stresses experienced by the material during the AM and will inform rheology experimental protocols. In-line measurements of the AM material?s viscosity may be critical to ensuring successful construction. A well-developed model can aid in interpreting data collected from instrumentation on the AM robot. Linking resistivity and rheology to changes in cement microstructure during hydration will be accomplished by parallel measurements of small angle oscillatory shear (SAOS), electrical resistance, small angle neutron scattering (SANS), and ultrasonics. Characterization of the structure fabricated by AM will be essential to ensure quality and integrity.
A critical unknown is the influence that the interface between layers on the strength of a structure. Traditional concrete compressive strength testing by ASTM C39 will not assess the impact of layer interface strength on compressive strength. Studying the effect of layer strength and layer orientation within a AM concrete cylinder or AM mortar cube will be conducted to assess the proper method to measure the compressive strength of AM structures. To address material measurement needs for AM, specific flow problems to be addressed include (a) flow through nozzle with aggregates, (b) creeping flow, (c) sedimentation, (d) capillary rheometry/viscometry, and (e) role of aggregate shape and distribution for nozzle flow and creeping flow. The quality of an AM structure is dependent on material and process control. To aid in material and process parameter selection, data collected during this project will be used to train machine learning (ML) algorithms to predict combinations that produce quality AM structures. Initial ML studies will be limited to published data sets and results to select the most appropriate inputs and ML algorithm. Collaboration with the industry will be sought to ensure that solutions would be applicable to the construction field. NIST will take the lead by creating a task group in ACI that would coalesce industrial efforts. Interfacing with ASTM and RILEM will be a focus of this project. Potential standards and test methods will be discussed with in ASTM C09 by establishing a task group focused on identifying urgent measurement science needs from the industry. A consortium of industry and academia will be created to discuss the measurement science needs to advance adoption of this technology.