Reliability of Fiber Reinforced Polymer (FRP) Composite Systems for Resilient Infrastructure

In infrastructure, carbon and glass fibers are most commonly employed in the form of fabric, laminate, or individual tow. These fibers have a high strength and stiffness-to-mass ratio [Zaman et al., 2013]. Fibers are typically combined with a matrix before (prepreg) or during (wet layup) application [Ebead et al., 2016, Zaman et al., 2013; Bilotta et al., 2017; Pino, 2016; Awani et al., 2017]. Several organizations, such as the American Concrete Institute (ACI) committee 440, the International Federation for Structural Concrete (fib), the American Association of State Highway and Transportation Officials (AASHTO) and the Transportation Research Board (TRB), have an active presence in FR composite research. It is well known that FR composites can strengthen structures, but durability of retrofitted structures is not fully addressed.

The durability of FR composite retrofits and the systems they are applied to are critical to both the functionality and safety of our nation’s building stock and infrastructure. Factors that affect durability may include freeze/thaw exposure, moisture-induced damage, thermal damage, UV radiation exposure, indoor air quality, and flammability during fire exposure [Ma et al., 2015; Zaman et al., 2013; Barbosa et al., 2017; Kim and Alqurashi, 2017; Lau et al., 2016]. To date, some durability research studies have been conducted, and acceptance criteria have been proposed for buildings by the International Code Council (ICC) Evaluation Service but are somewhat limited in their applicability to FR composites in the field [Barbosa et al., 2017; AC125, 2007; AC343, 2016, ACI 440.9R-15]. Durability depends on the specific material constituents of the FR composite and conditions during matrix curing [Tumialan and De Luca, 2014]. Since strength retention (e.g., modulus and ultimate strain at rupture) of FR composites alone and when bonded to concrete are key requirements to sustaining the performance of the FR composite system, mechanical measurements in these configurations are important. Furthermore, chemical measurements that provide mechanistic understanding of the changes in FR composite systems can enable broader understanding of performance loss and failure modes [Zaman et al., 2013].

​​​In addition to the gaps in knowledge concerning durability of retrofitted components, the extent of short-term performance improvement of some structural components, such as reinforced concrete shear walls, is less understood. The ASCE 41, Seismic Evaluation and Retrofit of Existing Buildings [ASCE/SEI 41, 2023], which is the main U.S. standard for retrofit of existing buildings, currently provides no guidance on simulating the response of retrofitted components. 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, test methods, inspection methods, and standards to assess the health and performance of FR composite materials in indoor and outdoor environments is a major barrier to their use [Zaman et al., 2013, Tatar et al., ​​2021]. 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 material. Once the performance and health of FR composite systems have been grounded in measurement science, the resilience of FR composite systems to earthquakes and other extreme events can be modeled and evaluated [Wang et al., 2016].

The IMG is uniquely equipped to measure the composition and properties of polymeric materials and concrete as they weather under different environmental conditions. Durability studies of stand-alone composites (fibers + resin) and FR composites bonded to small-scale (102 x 102 x 356 mm) notched concrete beams are underway to evaluate degradation of FRP materials and their bond to concrete. Durability studies include outdoor weathering in two climatic zones, hot and humid and cold + freeze/thaw, with several types of glass and carbon FRP materials representative of externally bonded FRP products used nationally. Changes in material properties such as adhesion to concrete and strength retention of FRP composite/concrete assemblies under different environmental loads are being investigated to provide improvement to strength reduction factors for retrofit design [Dai et al. 2005]. Accelerated weathering protocols are also employed using at least two environmental conditions with high potential for causing FRP composite degradation. IMG conducts accelerated weathering experiments using environmental chambers (high temperature, humidity, low temperature, and freeze/thaw) and the NIST SPHERE (UV, temperature, humidity). Degradation modes from accelerated weathering tests will be compared to modes observed in outdoor exposure tests to determine when and if accelerated weathering can realistically determine FRP composite failure at a more accessible time scale than outdoor testing. Metrology that indicates FRP composite performance level will be incorporated into durability studies to measure performance loss due to degradation and potentially predict the point at which FRP composite replacement is necessary. Chemical tests can be used, when needed, to determine mechanistic changes of FRP composites during degradation. Non-invasive metrology, such as ultrasonic techniques and infrared thermography, may be investigated for measuring FRP composite bond adhesion and be related to strength retention depending on identified performance metrics. The applicability and practicality of FRP composite test methods for field inspections will be explored, as improved field inspection procedures are needed to monitor for long-term degradation or performance loss following a seismic event.

RT2-2: Bond quality testing

The quality of the bond between the FRP material and the substrate can be critical to the performance of the FRP retrofit. Oftentimes, the bond can be the limit state of the FRP retrofit. The quality of the bond can be affected by installation practices, environmental degradation, the substrate’s mechanical properties and durability, and hazard events such as earthquakes. Currently, the industry standard for assessing bond quality is the pull-off test, which is a pass/fail test that places direct tension on a disc adhered to the FRP surface and requires a failure in the concrete above a certain threshold strength. Based on input at the NIST stakeholder workshop, variability in the results of this test method leads to difficulty interpreting the actual quality of the bond, and further information is needed to understand the test’s limitations. An ongoing sensitivity analysis is underway to assess variables that may affect the test method’s results. Future work will include determining if there are available alternatives to the pull-off tests for bond quality assessment such as non-destructive testing, peel tests, and shear tests. It is important that new test methods are practical for field inspections of FRP retrofits.

RT2-3: Develop a database of FRP retrofitted shear walls and modeling parameters

All available data on the experimental testing of FRP-retrofitted shear walls will be collected for inclusion in a database. The database will include all relevant information on the tests, including specimen geometric and material data, design of FRP retrofit, test parameters, and experimental results. This database will be published for public use. One of the uses of the database in this project will be the development of modeling parameters of retrofitted shear walls. This is important because there are currently no modeling parameters in the building codes like ASCE/SEI 41 used for design that are directly applicable to retrofitted components. The modeling parameters will be developed using regression analysis of key design parameters of the walls in the database against control points of an idealized backbone curve. Once modeling parameters are determined, we will engage with stakeholders and code committees, such as the ACI 369 Seismic Repair and Rehabilitation committee (which provides concrete-specific content for ASCE 41), to discuss our findings and develop a change proposal to integrate into future versions of the design codes.

RT2-4: Develop a guideline for seismic assessment of FRP-retrofitted concrete structures

This RT is intended to produce design procedures for common high-use seismic retrofit techniques involving FRP materials. Of particular interest for this task is the seismic retrofit of deficient reinforced concrete buildings. The final product is intended to be a standardization-ready document targeted for adoption by ACI 369.1 (Guide for Seismic Rehabilitation of Existing Concrete Frame Buildings and Commentary) and ASCE/SEI 41 (Standards of Seismic Safety for Existing Federally Owned and Leased Buildings) standards, as well as the ACI 440 standard for seismic retrofit of concrete structures using externally applied FRP. The envisioned document would have standard language for ease of adoption in the standards, while also providing a robust background/commentary section. Under this RT, the project team will form a working group of subject matter experts (SMEs) from academia and industry to work with NIST researchers on developing the guideline document.

References
Ho, K.K.S., Leroi, E., Roberds, B. "Quantitative risk assessment: application, myths, future direction." ISRM International Symposium. International Society for Rock Mechanics and Rock Engineering (2000).