Objective - Develop measurement science tools (metrologies, standards, and guidance documents) for quantitatively evaluating the critical material properties and ensuring the desired field performance of cement-based additive manufacturing.
What is the new technical idea? Additive manufacturing in the construction industry is garnering much attention for its potential to reduce cost by eliminating formwork, reducing labor cost, and improving quality by maintaining precise control of concrete placement and curing. Successful adoption of this technology by industry will require new measurement techniques (1) for assessing the state of a material during the AM process to ensure defect-free fabrication, and (2) for assessing the final structure for manufacturing defects. This project will address these measurement challenges by developing measurement science tools to assess the rheology of a material during AM and to assess final structure strength by means of non-destructive testing (NDT) or destructive material testing. The approach will use numerical simulations to inform rheology experiments and machine learning algorithms to identify material properties that influence and predict the quality of a final AM structure.
What is the research plan? A combination of experimental measurements, numerical simulations, and machine learning data processing will be used to assess the potential of a material system forAM. 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. NDT techniques will be used to assess the structures’ build quality by detecting defects such as cold joints. 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.