Genda Chen, Missouri DOT
Missouri University of Science & Technology, Rolla
Fiber Bragg grating sensors, Mechanistic-Empirical Pavement Design Guide, Monitoring, Optical fibers, pavement design, Pavement performance, Portland cement concrete, Sensors, Strain (Mechanics), Time domain analysis
Unbonded Portland cement concrete overlays have recently received increasing attention in new highway constructions and existing pavement rehabilitations. The AASHTO Mechanistic-empirical Pavement Design Guide (MEPDG) provides an analysis and evaluation methodology for such multilayer systems. However, interaction between the overlays and the underlying layers cannot be fully taken into account in the current design methodology, which was the motivation for the on-going NCHRP 01-51 study led by Dr. Lev Khazanovich from the University of Minnesota (UMN). For example, the time-varying friction or bonding between the new and existing layers, and its dependence on the underlying base material were not considered in the AASHTO Guide. The influences of construction practices and bonding behaviors on pavement performances were not sufficiently accounted for. For the same reason, Minnesota Department of Transportation (MnDOT) is currently leading a multi-state pool funded study with Dr. Lev Khazanovich as Project Director to understand and quantify the above-mentioned factors on pavement performance tailored into the state specific applications in field condition. Missouri University of Science and Technology (Missouri S&T or formerly University of Missouri-Rolla) recently developed a cost-effective, integrated global-local monitoring system with optical fiber sensors for large-scale transportation infrastructure. In the compact instrumentation system, the grating technology (e.g. fiber Bragg grating for local strain and temperature measurement) was integrated with the Brillouin scattering technology (e.g. single mode optical fiber for global strain and temperature measurement). One of the original developers of the monitoring system is Dr. Ying Huang, an assistant professor from North Dakota State University (NDSU) and a former Ph.D. student from Missouri S&T under the supervision of Dr. Genda Chen. More recently, NeubreScope NBX-7020 model was manufactured by Neubrex Co. Ltd., Japan. NBX-7020 represents the state-of-the-art, long-distance optical measurement technology, combining advantages of both Brillouin and Rayleigh backscattering phenomena. The Brillouin sub-system employs the latest optical measurement breakthrough technology with Pulse Pre-Pump Brillouin Optical Time Domain Analysis (PPP-BOTDA) while the Rayleigh sub-system uses the Tunable Wavelength Coherent Optical Time Domain Reflectometry (TW-COTDR). With a single-mode optical fiber, the hybrid measurement system provides non-proportional frequency shifts for Brillouin and Rayleigh scatterings and, as such, allows a separation of the measured strain and temperature at every point of the optical fiber. The strain and temperature accuracies can be as low as 10 ë_ëµ and 0.5 å¼C, respectively, over a length of as short as 2 cm. In this study, an integrated grating and Brillouin scattering optical fiber sensing system is proposed and applied to the strain monitoring of concrete pavements. The new sensing system combines multiple fiber Bragg grating (FBG) sensors and a Brillouin optical time domain Analysis (BOTDA) sensor into one single optical fiber, thus referred to as an FBG-BOTDA sensing system. The FBG sensors will be applied for local strain measurements while the BOTDA sensor will provide strain distributions over a large area. The integration of grating and Brillouin scattering sensors also allow for temperature compensation of individual sensors. The more recently introduced PPP-BOTDA/TW-COTDR hybrid technology further allows the measurement of strains and temperatures along the length of a single-mode optical fiber without deploying any grating. This project will take advantage of the pool-funded field study on the performance of concrete overlays on existing pavement in the state of Minnesota with contributions of both MnDOT and UMN.