Objective - Develop methodologies to determine the resilience increase or loss of infrastructure by the application of FR composites to structural components.
What is the technical idea? FR composites have been used in infrastructure applications to repair, retrofit (defined as strengthening and/or seismic retrofitting of existing structures), strengthen new support structures, build lightweight bridge decks, and provide corrosion-resistant internal reinforcement (i.e. as reinforcing bars) for concrete.1-4 In comparison to steel, FR composites offer the advantage of being corrosion-resistant, lightweight, easy to apply to a variety of support structures (especially in terms of reduced lifting, space use, and time restrictions), adaptable for a particular need (fiber orientation, fiber type, and resin type can be varied), and elastically responsive to seismic activity.1, 5-6 In infrastructure, carbon fibers are most commonly employed in the form of fabric, tape, or individual tow. These fibers have a high strength and stiffness-to-mass ratio.6 Fibers are typically combined with a matrix before (prepreg) or during (wet layup) application: thermoset polymer matrices (typically epoxy) are used in fiber reinforced polymer composites (FRPs) while mortar is used as the matrix in fiber reinforced fabric cementitious materials (FRCMs).3, 6-9 Several organization, such as American Concrete Institute (ACI) committee 440, the International Federation for Structural Concrete (fib), American Association of State Highway and Transportation Officials (AASHTO) and Transportation Research Board (TRB), have an active presence in FR composite research. It is well known that FR composites can strengthen structures, but durability is seldom addressed and performance based tests are lacking.
Infrastructure repair needs have been expressed by US Senator Sheldon Whitehouse from Rhode Island who said “While Rhode Island is directing millions of state funds to the repair and replacement of these (transportation) structures, we need federal financing to ensure this work gets done before a serious failure occurs, which could disrupt commerce along the entire Northeast Corridor or potentially cause injury.”17 Gangarao Hota, Professor at West Virginia University, conducting research on FR composites, said “I think we should not be thinking about ripping out or replacing infrastructure. What we should be thinking about is renewing infrastructure. This would save a tremendous amount of dollars, time, and user inconvenience.”18 FR composites are being widely used across the US as indicated by a recent US domestic field scan program, in which the National Cooperative Highway Research Program (NCHRP) assessed the current practice of incorporating FR composites into highway structures across the US. However, the domestic scan program recommended that “guidelines, commentary, and examples are needed for design, construction, and maintenance of FR composites before the material can fully mature and proliferate.” The importance of assessing appropriate and realistic environmental conditions that affect FR composites deployed in the field was also described.5 Dr. Antonio Nanni, Prof. at the Univ. of Miami, Fellow of the American Concrete Institute (ACI) and member of the ACI-440 Committee on Fiber-Reinforced Polymer Reinforcement provided some background on the FR composite materials most widely used in the field and some of the areas of research that are needed: “Carbon FRPs are used for almost all FR composite strengthening applications in the US. Carbon is used almost exclusively for fibers (10x more expensive than glass), but viewed as more durable. The areas of most interest for research are their fire resistance, the durability of the resin in general, and the use of anchor systems (nails, etc.) in bond critical applications.”
The durability of FR composite materials and the systems they are applied to are critical to both the functionality and safety of our nation’s building stock and infrastructure where FR composites are used. Resilience is intimately linked to durability of each component and the entire system, through either the long-term performance of a system or its response to a critical event; both situations will depend on the state of the materials within the system at any given time and the demands placed on the system. Factors that can affect durability may include age, UV radiation exposure and moisture-induced damage, indoor air quality, as well as thermal degradation and flammability during fire exposure.2, 6, 10-12 To date, some durability studies have been conducted by academia and acceptance criteria have recently been proposed for buildings by the International Code Council (ICC) Evaluation Service, but these studies and criteria are somewhat limited in their applicability to FR composites in the field.10, 13-14 Durability depends on the specific material constituents of the FR composite and the conditions present during matrix curing.15 Since strength retention (including modulus and ultimate strain at rupture) of the FR composites alone (micro-level) and when bonded to concrete (meso-level) are the key requirements to sustaining the structural performance of the FR composite system, mechanical measurements in these configurations are important. Furthermore, chemical measurements that provide mechanistic understanding of the mechanical changes that occur in FR composite systems can enable broader understanding of performance loss and failure modes.6 Methods to measure the structural improvement by FR composites applied to existing structural components such as masonry and reinforced concrete walls are also needed.
Currently, a general lack of data and test methods to assess the health and performance ability of FR composite materials in indoor and outdoor environments is a major barrier to their use.6 An evaluation of performance and health of FR composites in actual systems requires investigation to inform stakeholders and make progress on implementing the necessary method development and testing for this relatively new material. Once the performance and health of FR composite systems has been grounded in measurement science, the resilience of FR composite systems to earthquakes and other extreme events can be modeled and evaluated.16 Through field investigation and identification of important material property changes and failure modes, the performance of a large-scale structure can be assessed.
What is the research plan? This project will seek to develop a strategic plan that prioritizes the current research needs for FR composites used in structural systems. The research plan will 1) investigate FR use in the field 2) identify the degradation and/or failure modes in these FR composite systems to determine the most important material characteristics that impact structural response , 3) design durability experiments of FR composite systems from micro to macro level that focus on the critical modes of failure identified, and 4) assess performance of FR composites in structural systems using laboratory tests and numerical analysis.
Investigate FR use in the field and prioritize research needs
The initial goal of this project is to review the state of the science on FR composites in use for indoor and outdoor infrastructure applications. Through site visits and correspondence with stakeholders, the goal will be to identify research gaps with regards to FR composite application, performance, and health in the field.
In the process of identifying the current research gaps, experts from academia and industry will be invited to NIST to give seminars and discuss their perspectives on the topic. The collected data from site visits and communication with experts will be compiled into an information repository and presented to all stakeholders. A workshop can be organized during this process, with members from academia, industry (those that manufacture FR composites/components and those that apply FR composites to structures), structural engineers and departments of transportation invited to participate. Thoughtful discussions and thorough analysis of the facts compiled from investigating FR composite use in the field will be used to start a roadmap document for this project.
Identifying critical failure modes in FRP-concrete composites
The sensitivity of the loss of structural capacity with respect to the changes in the mechanical properties will be investigated through conducting data collection, literature review, and analyzing FEM models (depending on the capabilities of the available software packages). The mechanical properties that might be important include the tensile and shear moduli of the bond between the FR composite and concrete, the bond strength of the fiber to the matrix, and the ‘erosion’ of the FR composite (loss of thickness) as a result of weathering. The results of this analysis will determine whether the chemistry of the FR composite-concrete bond, the UV/temperature/ humidity degradation, or other factors are the highest priority research areas, and will inform initial work in developing methods to detect degradation in existing structures. After identification of the important research needs for FR composite systems based on the failure mode analysis and observations made in the field a roadmap of the findings will be completed from which experiments will be designed that are impactful to stakeholders with an emphasis on connecting measurements at the micro-and meso- level (FR composites and FR composites + concrete) to those on the macro level (FR composite + concrete in a structure). The roadmap will be reviewed externally to validate the direction of experimental design.
Design durability experiments of FR composite systems from micro to macro level
From the roadmap, the most relevant durability studies will be planned. Material selection and degradation factors will be carefully selected based on prioritized research needs. The PMG and IMG are uniquely equipped to measure polymeric materials and concrete as they age under different conditions which can involve using the NIST SPHERE (UV, temperature, humidity) and concrete environmental chambers (temperature, humidity) at multiple scales (i.e. micro to macro scales). Durability of stand-alone composites (fibers + resin) and FR composites bonded to concrete will be considered to look at changes to both the micro- and meso-levels, respectively. Changes in material properties such as strength retention, FR composite adhesion to concrete, and performance under different environmental loads have the potential to provide value, depending on the research needs identified.19 Chemical tests can be used, when needed, to determine mechanistic changes of FR composites during degradation. Protective coatings (e.g. UV-resistant paint, intumescent coatings, etc.) used in the field will also be carefully considered in the design of durability experiments.16 Durability studies can identify the properties of the micro and meso-level FR composite systems that are the best indicators of FR composite health and performance ability. Metrology that indicates FR composite performance level will be useful and applicable to performance loss measurements due to degradation and potentially predict the point at which FR composite failure occurs. Non-invasive metrology, such as ultrasonic techniques, may be investigated for measuring FR composite strength retention depending on identified performance metrics.20-22
Assess performance of FR composites in structural systems
A set of macro- (component) level tests will be developed using the findings of micro- and meso- level phases of the study to measure the performance of structural components including beams, columns, joints, and walls enhanced by FR composites. These tests will be conducted with two goals: 1) to quantify the improvement in the response of retrofitted (defined as strengthening and/or seismic retrofitting of existing structures) components in existing structures, 2) to measure the impact of FR composite degradation over time (using accelerated weathering data obtained with techniques outlined in the previous section) on the response of the structural components. The experiments will be designed and conducted to quantify the change in the force-displacement response of structural components retrofitted by FRP with and without consideration of deterioration over time. Three types of tests specimens will be assembled (FR composite + concrete structural component) for each test, where the first specimen is not retrofitted, the second specimen is retrofitted, and the third specimen is retrofitted and exposed to the accelerated degradation to simulate the filed observations. The force-displacement response of two tests will be measured under cyclic and monotonic loading to quantify 1) the improvement in the response of the structural components retrofitted by FR composites, 2) the degradation in the structural component level response due to the FR composite degradation over time.
Numerical analytical models will be explored and further developed to represent reinforced concrete structures retrofitted with FR composites. The outcome of the macro-level experiments will be used to model the improvement in the strength and ductility of the structural components retrofitted by FR composites (for components with the lack in the available data) as well as the degradation of the component response over time into the models. Similar to the component-level experiments, the nonlinear models will be developed for three cases: 1) non-retrofitted structures, 2) structures retrofitted with FR composites without consideration of FR degradation over time, 3) structures retrofitted with FR composites with consideration of FR degradation over time. The numerical models will be analyzed dynamically for a suite of ground motions. The performance of the structures will be quantified in a probabilistic framework in terms of exceeding a specific target performance at a specific excitation level. The performance of three cases will be quantified and compared against each other to measure the improvement of the system-level response of the structure retrofitted by FR composites as well as deteriorated response of the retrofitted structures over time.