Durability and Inspection of Hybrid Composite Beams

About the report:

The ability to inspect reasonably and ensure the durability of hybrid composite beams (HCBs) is of concern to transportation agencies.  In case of infiltration of chloride-laden water through the fiber-reinforced polymer (FRP) shell of an HCB, tension reinforcement strands in the bottom flange will be subject to corrosion, thus severely affecting the service life of the beams.  Test specimens from HCBs, which were load tested to failure at Virginia Tech for a 2012 study, and specimens manufactured for testing by HCB, Inc., were used in this study.  A selected number of specimens were subjected to accelerated exposure in the laboratory by exposure to ultraviolet radiation and condensation cycles.  In addition, to compare the influence of actual field exposure, specimens were placed in two selected field locations with different exposure conditions.  All specimens were characterized before and, periodically, during the exposure using nondestructive and destructive methods to understand the change over time.  The specimens were photographed periodically during the study to identify any progress of deterioration.

Ground penetrating radar was found to be reliable in detecting moisture in the interior of HCBs.  Fourier transform infrared spectroscopy was used to understand the changes in the chemical structure of the exposed specimens.  Spectrocolorimetry was usedto understand the natural changes in the surface conditions of the FRP shell as the exposure duration progressed.  The literature review provided extensive information on the aging behavior of the FRP shell specimens. 

The greatest degree of damage was found at the field exposure site with the highest recorded UV index values.  Based on the preliminary results, HCB specimens with no preexisting blemishes have exhibited only minor degradation in the course of laboratory and field exposures.  The specimens with preexisting damages such as delaminations, scratches, and other blemishes have degraded more rapidly than have the sound specimens.  None of the beam components exhibited complete failure after 1 year and 8 months of outdoor atmospheric exposure.

Scanning electron microscopy and X-ray computed tomography provided excellent visualization of the surface and the volume, respectively, of the FRP material to demonstrate the fiber layout.  Tensile testing of the FRP specimens indicated the importance of the fiber content in the FRP specimens.  Resin degradation because of natural ultraviolet and freeze-thaw degradation and accidental damages because of flooding events should be repaired to protect the load-carrying fiber content.  At this stage, regarding findings related to durability, the Virginia Department of Transportation should pause further use of HCBs, since there are still many unknowns about the late-stage deterioration mechanism of FRP, and also because neither visual nor nondestructive methods can fully assess the condition of the beams at this time.  Further environmental exposure is recommended for a reliable sense of HCB durability.

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