Validation of Frp Composite Technology Trhough Field Testing

This paper identifies assessment techniques for critical parameters relative to installation and performance of fiber reinforced polymer (FRP) materials as a way to strengthen structurally deficient concrete bridges. Various Nondestructive Testing (NDT) techniques, representing viable solutions for identification and monitoring of such critical parameters on structural members strengthened with FRP, were implemented in a research program consisting of five upgraded concrete bridges in the State of Missouri. The research program was designed to provide information on installation aspects such as optimal roughness of the concrete substrate surface, fiber alignment, FRP debonding, FRP-concrete bond quality, concrete cracking, and FRP strain level. These parameters are related to both initial and long-term performance of a bridge structure. The five upgraded concrete bridges strengthened with composite materials are to be monitored semiannually for five years, including load tests. The NDT evaluation mainly focused on one of the bridges, where three different FRP strengthening systems were used. Introduction: Fiber-reinforced polymer (FRP) as an emerging strengthening technology for concrete structures has been tested and proven successful because of its inherent characteristics [1, 2, 3] such as corrosion resistance, high strength, light weight and anticipated long-term durability. However, some important installation and performance issues remain to be fully understood. In order to make FRP composites as popular as conventional strengthening materials, it is necessary to develop suitable methods for quality control (QC) and longterm monitoring. Even though FRP systems are relatively easy to install, they are also highly susceptible to handling and construction errors. The purpose of this paper is to explain what issues have been recognized as critical and need to be monitored with NDT technologies. Methods for QC must meet basic requirements to be suitable for effective field use such as: simplicity, ease of handling, and lack of complex and/or heavy equipment. A five-bridge strengthening program, using five variations of composite strengthening techniques, was undertaken to offer a comprehensive case study, addressing several aspects of installation and long-term monitoring. The project is intended to validate the use of FRP materials, and carbon FRP (CFRP) in particular, to strengthen structurally-deficient concrete bridges. As part of a five-year long monitoring program, an NDT research will be concentrated on one structure, namely bridge P-0962 in Dallas County, MO (Table 1). Table 1. Bridge P-0962 in Dallas County, MO. Side View of the Bridge Approach to the Bridge RC Bent, Girders and Slab BRIDGE FEATURES • Total length is 127 ft. Height of the bridge is 18 ft. Width of deck is 23 ft. • Deck consists of three RC T-beams spaced 9 ft on centers. All spans have three transverse beams • The thickness of the slab is 6 in. • Concrete is in good condition. No spalling detected. Abutments and piers are in good condition. • Load Posting: trucks over 18 ton, 15 mph on the bridge The five bridges are distributed over three Missouri Department of Transportation (MoDOT) Districts. The candidate structures were selected in consultation with the respective District Offices. None of the bridges to be strengthened lie on the same route. Four structures, including P-0962, are of the T-beam type and one is of the solid slab type. Of the four T-beam bridges, one has a four-girder supported deck, while the others, including P0962, have a three-girder supported deck. In this group, P-0962 is a more recent structure in terms of age (1956 construction). Four FRP system technologies and a steel fiber reinforced polymer (SRP) system were used for the flexural strengthening of the five structures. Shear strengthening, when necessary, was always in the form of externally bonded laminates installed by manual lay-up. The four FRP technologies were as follows: • Externally bonded FRP laminates installed by manual lay-up • Adhered pre-cured FRP laminates • Near surface mounted FRP bars • Mechanically fastened FRP laminates This investigation covered aspects such as measurement of concrete surface roughness, evaluation of bond properties, investigation of fiber alignment, and detection of delaminations and concrete cracking. For this purpose, some strengthening was conducted with intentional defects at non-critical locations. Additionally, in-situ load testing prior to and after the strengthening was implemented on each of the five bridges. The intention of these load tests is to determine possible degradation of stiffness over the time. The overall performance of a strengthened bridge superstructure is evaluated by comparing the behavior before and immediately after the strengthening to that over a reasonably long period of time. The information and data collected during all phases of the upgrade, that is: initial assessment, design, construction, inspection and monitoring are to be used for the development of specifications and guidelines written in technical language for future strengthening projects. In the following section, a brief introduction to FRP materials is provided. After that, critical parameters related to an FRP strengthening project are described. This paper is closely related to the following five companion publications presented at this conference: • Experimental Nondestructive Testing of FRP Materials, Installation, and Performance • Nondestructive Testing Of Dallas County Bridge • Experimental Nondestructive Testing of FRP Materials, Installation, and Performance • Use of the Total Station for Serviceability Monitoring of Bridges with Limited Access • Embedded Fiber Optic Sensing For Bridge Rehabilitation FRP Material: Fiber reinforced polymer materials are composites consisting of millions of thin and high strength fibers embedded in a polymeric resin (Figure 1) [3]. Fibers in an FRP composite are the load –carrying elements, while the resin maintains the fibers together and aligned as a compact unit, also protecting them against the environment and possible damage. Among commercially available fibers, carbon fibers exhibit the highest strength and stiffness when compared with steel. The type of fiber is selected based on mechanical properties and durability requirements, while the type of resin depends upon environmental and constructability needs. Perhaps the most relevant property of CFRP composites for construction use is their resistance to corrosion that allows having them installed on the concrete surface.