Additive Manufacturing (AM) with concrete, also known as 3-D Concrete Printing (3DCP), is an emerging technology in the construction industry. This approach to concrete construction has the potential to change the way cementitious materials are used to create infrastructure components. Automating the placement of concrete materials may improve construction efficiency by eliminating the need to erect formwork, improve infrastructure durability by providing precise control of concrete formulations, and improve construction safety by removing humans from hazardous working environments. Rapid construction enabled by 3DCP 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, and repair concrete in areas which are hard to access with conventional construction equipment. The concrete design and engineering community lacks sufficient knowledge about the performance of 3DCP structures subjected to designed loading scenarios to properly design 3DCP structures for a given application. The role of the printing process in determining the failure mode and a detailed understanding of the relationship between the 3DCP structure’s constituent material properties and structural response are critical to developing performance-based standards and guidelines for 3-D printing construction techniques. Developing this knowledge base and coupling it to measurements of concrete materials in the 3DCP process would mark a significant step toward revolutionizing concrete construction.
Objective - Develop the measurement science tools and scientific knowledge base for performance-based standards for reinforced 3-D printed concrete structures.
What is the new technical idea? To develop an understanding of the connection between material properties and structural performance, NIST will leverage existing expertise in cement hydration and rheology modeling as well as new experimental capability in large-scale structural testing and concrete 3-D printing. Continuing on prior work, the relationship between the C-S-H microstructure and rheological properties of ordinary and alternative cements will be studied using advanced material characterization techniques such as dielectric RheoSANS. New approaches to coupling existing hydration and rheology models will be explored. Developing this modeling capability could provide a critical design tool to the concrete community. Expanding on this knowledge, new experimental techniques will be developed to assess the hydration state of cements. For example, using Raman spectroscopy to monitor the water content of cement slurries and assess the potential to track changes during cement that indicate changes to the mix rheology. 3DCP takes place primarily during a period of relatively slow hydration kinetics. New analytical techniques to assess cement chemistry during this time may be required to develop important quality control mechanism. Newly developed experimental assets in Materials and Structural Systems Division will be utilized to connect material-scale measurements to the large structural element scale. The seven-axis robot mortar printer (RMP), located in the Additive Construction Laboratory (ACL), will be used to construct medium and large-scale specimens which will be tested in the newly equipped Structures Laboratory.
What is the research plan?
Connecting the structural performance and failure modes to material properties requires a multi-scale research approach, summarized in Figure 2. The approach is organized into three tracks: fundamental studies of the relationship between hydration product formation and the setting cementitious slurries, understanding the relationship between material properties and printing performance, and the testing of the response of 3-D printed concrete structures to engineering design loads.
3DCP by extrusion of material requires a phase transition from one exhibiting fluid and flow properties to a solid exhibiting strength and rigidity. Conventional cementitious materials, such as concrete, mortars and grouts, exhibit a fluid-to-solid transition as a result of the formation of hydration products which act to link particles together creating a porous microstructure with strength and rigidity. In conventional concrete construction, the cementitious material must remain in the fluid state for a period of time required to cast the material in a mold. AM techniques require precise control of the fluid-to-solid transition to ensure defect-free fabrication. Fundamental studies on morphology of hydration product microstructure will be related to macroscopic rheology measurements using NIST NCNR dielectric RheoSANS instruments. Developing an understanding of how the morphology of hydration products influences rheology and the fluid-to-solid transition will provide the basis of understanding for tailoring the properties of cementitious material for a particular construction application. These measurements will also provide experimental validation to simulations of concrete setting and rheology. Robust, experimentally validated models of the setting and rheology can be used to develop models for cementitious materials in 3DCP applications. Linking microstructure and flow models will create a unique tool for conducting virtual tests of formulation for 3-D printing. The models can further be used to aid in the development of in-situ measurements material in the 3DCP process. New experimental techniques such as Raman spectroscopy, will be explored for suitability as an in-situ measurement of cement hydration. Developing these measurements become critical quality control measurements as they may indicate the presence of a defect in the 3DCP structure or may be used to evaluate hydration products in real time.
Much of the 3DCP process occurs during a period of cement hydration where the hydration reaction kinetics are slow prior to the initial hardening. New analytical techniques will be explored to assess the chemistry of cement during this dormant period. These techniques could provide the basis for new measurements to assess the state of cement hydration during the 3DCP process
With an improved understanding of the relationship between the forming microstructure and flow properties, the influence of the printing process on the resulting performance of AM structures can be assessed. In reinforced 3DCP, shell structures are created with an open cell pattern in the interior. Reinforcing bars are added through the openings in the infill and conventional grout is used to bond the rebar to the 3-D printed shell.
The performance of this type of structure subjected to forces expected in the built environment has not been evaluated. The bond of the rebar to the concrete is critically important to the performance of any reinforced concrete structure. Figure 3 shows a schematic representation of a specimen designed to test rebar bond strengths.
Grouting rebar into 3DCP structures creates core and shell geometry, where the shell is additively manufactured while the core is composed of conventionally cast concrete materials. Potential failure mechanisms with this type of construction are de-bonding of the core from the shell, crack propagation along shell layers and failure of the shell prior to rebar yielding. The likelihood of these failure mechanism can be studied by measuring the force and displacement during a rebar pullout test.
The specimen consists of a series of 3-D printed shell layers with a conventional concrete core. Printing parameters such as the number of shell layers, np, the layer height, hl, core diameter, dc and specimen diameter, D, can be evaluated. Additionally, the compressive strength of the concrete core and shell layers can be evaluated. Combining these parameters in a design of experiment (DoE) will provide insight into the pullout strength of the rebar and the failure mechanism of the specimen. Following the DoE approach, important factors governing the structural response of the specimen can be identified and mapped to the failure of the specimen.
Understanding the relationship between the failure mechanisms in the rebar pullout test and printing parameters is a critical first step toward developing design guidelines for reinforced 3DCP construction. This information will directly inform large-scale testing of reinforced 3DCP structures such as the shear wall test depicted in Figure 4. The shear wall test will study the structural response of a wall structure subjected to combined compression and shear loading. The number of 3-D printed shell layers, reinforcing bars, and grouting material will be selected, guided by the rebar pull out results. This test will further explore the effects if printing parameters by studying the effects of the infill pattern angle, α, the density of the infill, and the cycle time between deposited layers (print speed). Studying the effects of these parameters on the failure mechanism of this structure will help identify the suitable design guidelines and printing parameters.