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Engineered Materials for Resilient Infrastructure Program

The resilience of the nation’s structures and infrastructure to aging and disaster ultimately rests on the quality of the materials used in its fabrication. The Infrastructure Materials Group contains 17 federal employees (13 permanent staff, 4 term staff) and 2 associates and has appropriately chosen to focus its attention on concrete and polymers. The average years of service of the staff in this program is 12 years. Polymers are essential because they protect the envelope of buildings against water intrusion (sealants, window claddings, thin films to block ultraviolet [UV] light and to resist windows from blowing out in hurricanes and tornadoes). With polymers, the focus has been on UV degradation, and this is certainly important. Concrete analyses and experimentation/modelling across size and length scales have been a traditional strength of NIST, with recent works continuing in the development of standard reference materials and analyses of degradation mechanisms/problems (alkali-silica reaction [ASR], Pyrrhotite) as well as potential repair strategies for aging infrastructure.

TECHNICAL MERIT OF THE PROGRAM

SPHERE Operations

The Simulated Photodegradation via High Energy Radiant Exposure (SPHERE) program in the Engineering Laboratory (EL) is comprised of 11 people and is directed at stakeholders in the polymers, coatings, sealants, and construction industry to aid in the selection and design of appropriate materials across a range of sectors, including new construction. The SPHERE program and its unique equipment enables assessment of accelerated weathering and service life prediction by providing a fundamental understanding of the deterioration and aging mechanisms of materials. The staffing appears to be appropriate for the ongoing mission of the program and combines a number of junior and more senior investigators.

The SPHERE provides an accelerated aging environment to gather data on how materials experience changes in their properties when exposed to outdoor weathering, such as water, UV, elevated temperatures, thermal cycling, and mechanical fatigue. These property changes result in lower performance of the materials than assumed when the materials were selected and initially installed. The SPHERE can expose these materials to UV, under controlled temperature, relative humidity and strain. The 2m SPHERE has been operating for more than 15 years and is heralded as a benchmark for the SPHERE technology such that its well-documented environmental conditions data (temperature, UV, and relative humidity) will be used to validate any new SPHERE devices. NIST is developing the capability of a smaller (0.25 m) more economical 6-port SPHERE. The exit apertures in the 6-port SPHERE can be

connected to specimen chambers, with the specific type of chamber utilized being dependent on the material being tested.¹

Accomplishments

This program has made significant advances in its 2m SPHERE testbed for understanding aging of polymer materials. The initial demonstration that links field and laboratory exposures was successful and although the initial model was for pure epoxy without fillers and other additives for mitigating UV degradation, more recent work has investigated polyethylene (PE), polyethylene terephthalate (PET), and polyethylene terephthalate glycol (PETG) with ongoing comparisons to field exposures. Of significance is the development and design of a smaller 6-port SPHERE that can be deployed by end users, such as manufacturers of polymer products for the construction industry, as well as other market sectors (e.g., sealants, coatings, polymer fiber-reinforced plastic [FRP] composites). The aging mechanisms are being comprehensively studied with 2m SPHERE testing, and robust models are being developed that demonstrate good correlation with field testing. External support from the Nuclear Regulatory Commission (NRC) on the NIST Cables Project was conducted to confirm the NRC condition-based qualification methodology. The design of a smaller commercial prototype SPHERE has been completed. One of these units has been purchased by an industry partner from a third-party vendor.

Challenges and Opportunities

There is some work to be accomplished to successfully deploy the smaller 6-port SPHERE for industrial applications. The initial models and experiments using the 2m SPHERE for pure epoxy without fillers and other additives for mitigating UV degradation, and the more recent work on PE, PET, and PETG has shown good correlation with field exposures. The next steps will be to include these factors to provide a more realistic test material to real-world polymer materials containing additives/UV absorbers and transition to a 6-port SPHERE Data/Validation Project, with comparisons to both field and 2m SPHERE data. The deployment of the smaller SPHERE may have a significant impact on industrial manufacturers who want to provide robust and long-lasting construction materials.

Additive Manufacturing with Cement-Based Materials

Over the past 10 years, the construction industry has increased its use of additive manufacturing (AM) with cement-based materials, sometimes referred to as 3D printing of concrete (3DCP). Despite the fact that there has been relatively little increase in construction productivity due to 3DCP,² the technology provides an opportunity for game-changing productivity improvements in certain types of concrete construction, such as shelters, housing, wind turbines, and repair to existing elements in tight spaces. The NIST program is focused on the understanding the behavior of 3D-printed elements to loading, with the goal of providing performance standards, and is currently comprised of nine people.

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¹ National Institute of Standards and Technology, 2021, “Accelerated Weathering Laboratory: Metrology and Technology Transfer,” https://www.nist.gov/programs-projects/accelerated-weathering-laboratory-metrology-and-technology-transfer, updated January 7, 2021.

² Concrete Products, 2019, “ACI Foundation’s Strategic Development Council Brings the Future Faster,” http://concreteproducts.com/index.php/2019/04/23/aci-foundation-s-strategic-development-council-brings-thefuture-faster.

Accomplishments

The program is early in its development. The team has defined early-stage goals aimed at understanding the unique interplay between the material properties of the concrete “ink” (rheology, setting time, and layer-upon-layer adhesion) and the process of directionally dispensing concrete through nozzles to form structural geometries. A laboratory with robotically controlled AM equipment has been developed with a number of different nozzles for experimental examination of their effects on concrete printing. This group has shared information with other NIST groups that are developing 3D printing for polymers and metals and for small-angle scattering experiments. Cement pastes have been printed on a benchtop scale, constructing a single filament stacked structure.

Challenges and Opportunities

The early age of the program and the rapidly developing field of 3DCP represents both a challenge and an opportunity. It is important to develop a fundamental understanding of actual structural performance of elements that are 3D printed, which will depend on the types of robotic devices and technologies that are utilized for the manufacturing. This basic understanding will provide the industry with confidence that using 3DCP can in fact raise productivity and provide safe structural capacity to loading. The Infrastructure Materials Group at the EL has the advantage of bringing state-of-the art tools, such as small angle neutron scattering, to link material microstructure to bulk material properties, such as rheology and electrical conductivity. Of potential interest is the interfacial microstructure between subsequent layers of printed concrete and the resulting resistance to crack formation along those seams. The ability to collaborate with the NIST Performance-based Engineering Research for Multi-hazards (PERFORM) laboratory is likely to accelerate such efforts. The group could beneficially consider collaborating with ongoing international efforts and identifying U.S. universities performing research in this rapidly evolving area.

Direct Assessment of Concrete-Making Materials for Standards and Specifications

This program, currently staffed by five people and related to direct assessment of concrete-making material for standards and specifications, is a continuation of the long NIST tradition of investigating the fundamental nature, character, and performance of one of the world’s most used construction materials. The current specifications for cement are based on mid-century normative ones that are now outdated due to changes in the mineralogical, chemical, and physical characteristics of modern cement manufacture. Also, the basis for evaluating concrete in the field for acceptance is based on a slump cone test as a qualitative measure of concrete flowability and not on a physical and direct measure of rheological character. A detailed study has been conducted to evaluate various types of rheometer geometries and their stress-strain rate response as a direct measure of viscosity and yield stress and to better understand the performance of fresh concrete. This was combined with the development of well-defined, model pastes that will allow practitioners to properly calibrate their rheometers to produce reliable data on concretes formulated in the field.

Accomplishments

This work continues NIST’s fundamental investigations of cement-based materials. The hydration studies examining the reactions of cement and measuring and characterizing reaction rates are of high value and continue to be nationally and internationally recognized. The investigative methods using digital holographic microscopy, environmental powder reactions, and simulation are best in class.

This work has taken the fundamental understanding of cement hydration from macro- and meso-scale down to microscale. The research aimed at understanding and standards development of a device to measure the pertinent rheological character of fresh concrete has provided good advancement and understanding of its flow-processing behavior. The development of standard reference materials (SRMs) for concrete reference materials for Portland cement clinkers is recognized and utilized worldwide by cement manufacturers and research laboratories. In particular, the Portland Cement Fineness (114/46) has sold roughly 800 units annually, while the Portland Cement Clinkers (2686/2687/2688) has sold about 120 units annually. The Concrete Rheology SRMs are new products introduced in 2019: Concrete Paste (2492) has sold 4 units; Mortar (2493) has sold 1 unit; and Concrete (2497) has not sold any units yet. The combination of this materials development with computer simulations of particles suspended at high volume fraction and subject to particle-particle interactions have provided robust understanding of concrete microstructure and dynamic response. Four sold-out workshops on cement materials characterization have occurred over the past 6 years, encompassing more than 240 attendees from industry, academia, and government. These workshops produced a group of experts that have participated in round robin and proficiency testing using standardized test methods developed at NIST, such as ASTM C1365.

Challenges and Opportunities

There are significant opportunities to standardize the use of rheometry as a field test method for quality control. The slump test, which is in widespread use in the concrete industry, is a subjective and qualitative measure of flowability. The Engineered Materials Program recognized an important opportunity and has produced impressive work on the development and use of different rheometer geometries for reliable flow-curve measurement, the formulation of SRMs for proper calibration of instruments, and the use of large-scale simulations of particle suspensions to appreciate structure-property relationships. This team has also established and published a database of kinetic reactions of cements and their rates. There are ample opportunities to continue to educate and qualify the industry on the various characterization methods, especially the ASTM standard method for X-ray diffraction. Continuance of the program of concrete reference materials development is important.

Assessing Pyrrhotite in Concrete

There has been significant attention in the media³ concerning massive concrete cracking in home foundations in Connecticut. This project was initiated from congressional funding of $1.5 million and is expected to continue for another 2 years. The mechanism that has been identified is the use of a schist aggregate that contains pyrrhotite that is subject to expansion and subsequent concrete cracking. This has led to the need to replace concrete in tens of thousands of home foundations in Connecticut. This is a new research area for NIST. The goal is to develop a rapid test that can be standardized to assess the susceptibility of concrete damage to pyrrhotite and to develop reference materials for calibration of a test standard. The success of this program can have significant implications on the concrete materials industry. This program is currently staffed with six people.

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³ N. Post, 2016, “Connecticut Grapples with Failing Concrete Foundations,” Engineering News Record, June 21.

Accomplishments

Since this is a new research project, progress to date has been limited. Still, the program goals are well positioned to lead to a test method to assess potential pyrrhotite damage of concrete and a reference material for assessing pyrrhotite in concrete. As this is a new program, the materials submitted by NIST for this review do not include information about the accomplishments of this research to date, other than providing a grant to the University of Connecticut Civil Engineering Department to work on developing a test method for pyrrhotite in concrete as well as developing a risk assessment model.

Challenges and Opportunities

This research has great potential to minimize concrete damage due to deleterious aggregates that cause pyrrhotite. The challenge will be to develop a simple test that can be used by industry. This also represents the opportunity. This would mitigate the type of problems that occurred in Connecticut and other areas along the east coast.

PORTFOLIO OF EXPERTISE

SPHERE Operations

Accomplishments

The project team is staffed by very high-quality chemical and polymer engineers and material scientists with different levels of NIST service. There has been a strong continuity of the project team that has led to significant progress over the last 6 years. The expertise of the team members applied to develop the 6-port apparatus represents a good innovation that is likely to translate well to industry.

Challenges and Opportunities

The team as configured is well set to continue with its important industry-relevant work related to aging and deterioration of polymer materials. There are good opportunities to integrate the 6-port SPHERE testbed directly into industrial use. Continued engagement and outreach to other potentially relevant industry partners will be beneficial.

Additive Manufacturing with Cement-Based Materials

Accomplishments

The technical expertise of the program’s participants is strong and builds on the long history in cementitious material research, materials characterization, robotics, and structural performance testing.

Challenges and Opportunities

This nascent program affords the project team the opportunity to emerge as leaders in the field. However, toward this end it will be important that the team communicate with non-NIST personnel, including other national and international researchers as well as construction industry and standardization

organizations. Technology transfer through this program could be significant because the capabilities of the NIST 3DCP researchers are strong and unique.

Successful development of this technology will require a commitment of time by rheologists, microscopists, and theorists. This expertise does exist within the Engineered Materials Program, but the personnel are simultaneously engaged in other projects, and the scheduling of their time may become highly constrained. Additional staffing may be required to continue to elevate this area to a leading national effort.

Assessing Pyrrhotite in Concrete

Accomplishments

The technical expertise in the concrete materials team at NIST is well qualified for the goals of the project.

ADEQUACY OF RESOURCES

SPHERE Operations

Accomplishments

The facilities continue to be some of the best in the world. The facilities do not appear to be an impediment to the continuation of the research program.

Challenges and Opportunities

There would be a great interest by industry in the availability of a portable test method for determining life of polymer materials. The challenge will be to gain wide acceptance of the 6-port device and to get it standardized.

Additive Manufacturing with Cement-Based Materials

Accomplishments

NIST has rapidly put together a team of experts and facilities, including an additive manufacturing testbed for 3DCP. The program will have a good blend of experts from various fields and will include structural testing of elements for bond characterization and shear in the PERFORM program.

Challenges and Opportunities

One challenge will be for the team to be agile and facile in the rapidly changing 3DCP field. However, it will remain important to conduct good and reliable experiments that demonstrate the performance of 3D-printed elements and provide basic knowledge of the processes and materials.

The scientific team that is assigned to this project brings together expertise in cementitious materials, rheology, microstructural characterization, and the strength of materials. The PERFORM laboratory affords this program state-of-the-art measurement methods in rheology, the strength of

materials, X-ray diffraction, UV and Raman spectroscopies, nano-indentors, atomic force microscopy (AFM), and scanning electron microscopy (SEM). The diversity of scientific backgrounds gives this team a great advantage. However, this same team is tasked with advancing an equally diverse set of research programs within the purview of the Engineered Materials for Resilient Infrastructure Program. Additional staffing may be required to continue to elevate this area to a leading national effort, while the level of support staff (technicians and postdoctoral fellows) that augment the program activities was not made clear.

With regard to infrastructure, Building 206, where the 3D Concrete Printing project resides, lacks some environmental controls (no air conditioning in the summer), modern information technology (IT) infrastructure, and it is largely without utilities due to the building’s origins as an aggregate storage facility. These issues should be addressed for the safe and efficient use of this high-profile area.

Assessing Pyrrhotite in Concrete

Accomplishments

The facilities, equipment, and technical expertise are well positioned to handle the goals of the program.

EFFECTIVENESS OF DISSEMINATION OF OUTPUTS

SPHERE Operations

Accomplishments

The project team has prepared 85 technical publications since 2014 that span internal technical reports, 30 peer-reviewed scientific and symposia papers, along with 7 peer-reviewed book chapters and 3 books on service life prediction. Eight standards have been developed with the NIST SPHERE technology. In addition, there has been outreach to industrial partners in various consortia (e.g., photovoltaic, polymer surface interface, sealants) and workshops (e.g., Service Life Prediction of Polymeric Materials). As this continues, it will need to gain acceptance of the models and the SPHERE system as a means of assessing polymer materials across a wide spectrum of sectors.

Challenges and Opportunities

There are great opportunities to engage with end users of the SPHERE system, including manufacturers of new construction materials, the polymers AM and structural polymers community, and design professionals who specify those materials. The challenge is the same as the opportunity, which is to get buy-in on the experimental methodology and the modeling that have been advanced in the SPHERE program by the end users. Successful engagement of end users will lead to development of standards and acceptance by the industry.

Additive Manufacturing with Cement-Based Materials

Accomplishments

The 3DCP research group has produced a number of publications in a short period of time. These include a review of 3DCP for infrastructure construction, an article on the rheological control of 3DCP pastes, a paper describing a roadmap for extrusion of 3DCP pastes, a paper on formulating sustainable cementitious binders for 3DCP, and contributions describing two nonintrusive testing methods of extruded pastes—ultrasonic wave propagation and electrical conductivity. In addition, one of the members is chair of ACI 564: 3-D Printing with Cementitious Materials, which will continue to provide good visibility to this evolving program The MACE consortium was developed to determine the measurement science needs in this area. With regards to infrastructure, Building 206, where the 3D Concrete Printing project resides, lacks some environmental controls (no air conditioning in the summer), modern IT infrastructure, and it is largely without utilities due to the building’s origins as an aggregate storage facility. These issues should be addressed for the safe and efficient use of this high-profile area.

Challenges and Opportunities

There are significant opportunities to advance the field and provide free flow of information and findings to other researchers, industry, and other stakeholders. The 3DCP effort at NIST brings together an outstanding cast of experts in the key areas of importance to this emerging construction technology (cementitious science, suspension rheology, simulation of non-Newtonian materials, strength of materials measurements, and microstructure determination). The major pieces of scientific and robotic-controlled 3DCP equipment have been established, and the team is positioned to develop printing strategies, materials, and standards that can be translated into practice. Linkage with ongoing efforts at other international locations and standards developing organizations is needed, in addition to linkage with other (polymer) AM efforts at NIST, where a Measurement Science Roadmap for Polymer-based AM (NIST.AMS.100-5) has also been prepared.

Direct Assessment of Concrete-Making Materials for Standards and Specifications

Accomplishments

The outreach of the team members in the form of technical reports (7 NIST internal), 31 peer-reviewed publications, at least 9 standards and test methods development, workshops (e.g., Cement Workability, Green Concrete, Cement Microscopy) with academia, government, and industry and consortia (e.g. COMEX, CREME) is notable. Three separate SRMs for concrete rheological measurements have been prepared along with international round-robin testing. Two separate ASTM standards on cement characterization were also revised with NIST input, while the work in cement characterization has also garnered a prestigious paper award.

Challenges and Opportunities

The team has developed important insights into the characterization of cement over a wide range of length scales and has made important contributions to the rheological properties of fresh concrete. This has included the development of measurement tools, standardized materials, and theoretical modeling. This will assist in the development of alternative concrete mixtures using less cement and for the use of alternative cements and pozzolanic materials for sustainability. There are opportunities to impact the

testing and quality of fresh concrete with further development and standardization of the helical rheology device.

This research group is composed of accomplished scientists and engineers. However, there has been a notable reduction in the size of the team but not in the scope of the important problems where materials impact infrastructure resilience. Cementitious materials are central to infrastructure, and these are flowable, non-Newtonian pastes while structures are being created. One concern is that this group does not currently have a dedicated rheologist following the departure of its expert in this area.

Assessing Pyrrhotite in Concrete

Accomplishments

Accomplishments in the area of dissemination of outputs were not presented because this is a new program initiated in response to congressional funding. The legislation calls for NIST to partner with academic institutions to establish standards, guides, and specifications for acceptable levels of pyrrhotite in concrete aggregate. A grant was awarded to the University of Connecticut’s Department of Civil Engineering to work on developing test methods and risk assessment models.

Challenges and Opportunities

There will be ample opportunities to disseminate the research and to develop a standard test method for ensuring aggregates are not susceptible to pyrrhotite expansion.

CONCLUSIONS AND RECOMMENDATIONS

Technical Merit of the Program

SPHERE Operations

This has been a strong research area, and expanded focus of its attention would maximize the development of the 6-port testing device for industry. The testing device could be standardized with the potential of eventual round-robin testing. Expanded focus on the polymer additives would assess more realistic industry products. Opportunities for collaboration with the AM polymers and structural polymers/composites communities would be beneficial in addition to FRP composites for infrastructure repair.

Additive Manufacturing with Cement-Based Materials

This project, which will combine the fundamental science of cementitious paste flow and curing with structural and mechanical measurements over a wide range of length scales, promises to advance 3DCP and increase its utility and capability.

Direct Assessment of Concrete-Making Materials for Standards and Specifications

There is great value in continuance of the research of a rheological test apparatus for viscosity measurement of fresh concrete, with the goal of developing a standard for industry, and great value also in the work on the development of concrete reference materials.

A better understanding of the atomic-scale mechanisms responsible for ASR and insights into methods to mitigate its effects justifies the exploration of atomistically based modeling tools that are proving generally useful across the disciplines of materials design. Although ASR and other aspects of cement chemistry are among the more complex phenomena of engineering science, tools for atomistic and quantum mechanical modelling have advanced to a point justifying the exploration of these as part of a more expansive effort to understand and model the properties of cementitious materials. In particular, as a starting point, these tools may be particularly useful in investigations of siloxanes to mitigate ASR effects.

Assessing Pyrrhotite in Concrete

Development of a standardized test will represent a major improvement in production of concrete and concrete durability.

Portfolio of Expertise

Additive Manufacturing with Cement-Based Materials

The assembled team and the instrumentation at its disposal is highly competent and appears to be unique. This group of scientists and engineers, however, consists of individuals with responsibilities in several other projects of equal interest and importance. These competing interests will need to be managed, and this may be difficult at the current staffing levels.