Objective - To develop the measurement science needed to enable greater industry use of innovative concretes, such as 3D printing, more durable and limestone replacement concretes.
What is the new technical idea? Manufacturing of cement and production/placement of concrete requires significant energy (12 % - 15 % of total industrial energy use). For the industry to develop innovative concretes, several technical barriers need to be overcome. The raw materials need to be properly characterized, innovative way to reduce CO2 emissions (producing 1 ton of cement generates one 1 ton of CO2), performance based concrete design and innovative constructability would allow the industry to better use raw materials available with a reduction of cost (material and labor) and increased quality.
The common approach to reduce CO2 emussion is to replace a portion (40 % or more) of the cement with a alternative materials such as fly ash, limestone powder, or furnace slag. At large replacement percentages, the chemistry of these alternative materials can dominate over that of the portland cement reactions, which can impact both the constructability and the durability of these blended systems. Furthermore, some of the standard test methods (ASTM or ACI) were developed with only portland cement-based materials in mind; they are not always applicable for these new innovative green concretes. To succeed in this replacement, the performance of new blended binders needs to be quantified. Thus, characterization of the raw materials and ASTM performance-based standards need to be developed/modified to support assessment of the blended binders.
In the area of concrete manufacturing, a new technology is being developed that would allow entire structures to be built by robots using 3-D concrete printing. This process could reduce the carbon-footprint of concrete and reduce construction costs by as much as 50 % (reduction in labor, less material than traditional construction). This is a revolutionary paradigm shift that needs new metrology to qualify the materials' requisite performance. Currently, this technology is widely used for polymers or metal (although using different principles of operation). To adapt this construction technology to large objects such as a house or at least components of it, such as a wall, many technological barriers will need to be overcome. The University of Southern California and other companies are starting to develop the technology for this type of construction, but measurement science and standard test methods to characterize the grout used in printing, usually including high volume replacement of cement with SCMs, are non-existent. NIST, and particularly EL, is in a great position to help the industry develop the metrology needed, such as characterization of the rheological properties of the grout, setting time, and durability. In support of this effort, the Intelligent Systems Division of EL will provide a robotic printer (NIST RoboCrane), expertise to set it up, and the software to control it.
This project would address these issues by conducting research in four thrust areas: 1) characterization of the chemistry and structure of the raw materials, specifically fly ash. This is built on NIST expertise using SEM and XRD to provide the industry with guidance on how to select fly ash by its composition rather than by the current simple arbitrary classification scheme; 2) Replacement of cement with limestone powder (portland-bonded limestone concrete) at a level over 40 %; 3) Determination of the changes needed in ASTM standards so that these new innovative green concretes can be accurately evaluated on a performance basis with the traditional portland cement-based materials; 4) Explore 3D printing of concrete by attempting to build a wall and examine the performance requirement of the material.
What is the research plan? he research plan is to address the four thrusts highlighted in the technical idea, providing the industry with needed guides and tools to adopt innovative green concrete.
Thrust 1 (Materials Characterization): Building on NIST expertise in materials characterization, critical performance-related components of fly ash will be identified, and the corresponding measurement science will be developed to quantify the proportions of those components instead of relying on the current broad classifications as Class C or Class F (sum of aluminum, iron, and silicon oxides). Such an effort will provide the correlation between the fly ash classification and its performance in concrete that is lacking in current classifications. The technical undertstanding gained from this activity would allow for the reliable use of fly ash that is otherwise discarded because its performance is uncertain.
Thrust 2 (Limestone-Cement): Limestone-cement concrete is based on an innovative concept that the cementitious component (water and cement powder) fills the gaps between the limestone powder particles and binds the limestone particles together. This innovative material uses limestone powder with particle sizes 1 µm to 5 µm, instead of the range of 1 mm to 5 cm usually present in concrete. Typical limestone concrete replaces only the cement powder. This approach, instead, focuses on replacing both cement and water with limestone powder, to minimize the paste content in these innovative concretes. Currently, replacement with limestone rarely exceeds 20 % and is limited to 15 % in existing ASTM standard specifications. These innovative materials could achieve 40% replacement or greater and the concept was proven in paste and mortar, but now it needs to be applied to concrete.
Thrust 3 (Durability): ASTM has numerous test methods to test durability, such as sulfate attack or alkali-silica reaction (ASR), but they were generally designed with portland cement in mind. This creates an undue burden on cements with high contents of SCMs (blended cements) as they are usually slower in gaining strength and exhibit delayed setting times, while actually achieving higher compressive strengths and superior durability at later ages. In anticipation of future innovative materials, NIST will build upon existing expertise with sulfate durability testing of materials by developing a test method for concretes made with fly ash. The NIST approach would reduce the testing time for sulfate resistance of a cement by 25 % (3 months instead of a year), NIST would collaborate with ASTM to ensure that the curing conditions of such tests (current and proposed) are adapted for blended cements as per sulfate resistance. Upon completion, NIST would identify other standard test methods that would also need to be modified and propose the corresponding changes.
Thrust 4 (3D-Printing): Partnering with the EL Intelligent Systems Division, NIST will build a 3-D printer for cementitious materials and then develop the necessary metrology to qualify the materials used for such applications. The grouts typically used for this application are composed of cement with SCMs replacements and chemical admixtures. The performance requirements, however, demand rapid rigidity (to withstand the deadload of subsequent layers), a mechanical bond between adjacent layers, and sufficient structural capacity. These are high-tech requirements of a material that will ultimately be placed outdoors, exposed to the environment, and for which the long-term durability is unknown.
1) Madlool N.A.,Saidur, R., Hossain, M.S., Rahim, N.A., "A critical review on energy use and savings in the cement industries", Renewable and Sustainable Energy Reviews, Vol 15 pp 2042-2060, 2015
2) K.A. Snyder et al., "Measurement Science Needs for the Expanded Use of Green Concrete," NIST Technical Note 1783, National Institute of Standards and Technology (2013) – http://dx.doi.org/10.6028/NIST.TN.1783