This report is focused on the measurement of strain distributions and crack detection in unbonded and bonded pavement overlays. The main objectives of this study are: (a) to characterize the strain sensing properties of distributed fiber optic sensors with recently developed pulse pre-pump Brillouin optical time domain analysis (PPP-BOTDA), (b) to develop an installation method for real world applications, (c) to document the performance of the PPP-BOTDA technology in unbonded/bonded pavement applications, and (d) to develop a numerical model to facilitate the analysis of mechanical behavior of unbonded pavement overlay under vehicle wheel loads. A thin concrete layer can be cast on top of a severely deteriorated pavement layer with a fabric sheet in between to rapidly and cost effectively improve the driving condition of existing roadways. Once cured, the concrete layer is divided into many panels and often referred to as the unbonded Portland cement concrete (PCC) overlay. The service life of PCC overlays can be appreciably extended by appropriate rehabilitation strategies at early stages of deterioration based on the information provided by health monitoring. The strain distribution and crack detection are of interest to engineers in this application. Minor or moderately deteriorated existing concrete pavements can also be resurfaced with a thin concrete layer to improve their driving condition. In this case, potential cracks in the existing pavement may easily penetrate through the new concrete layer. The way the potential slip at their interface develops over time is an interesting question to answer. This study reports an application of a commercial single mode optical fiber to measure strain distributions in full-scale fiber reinforced unbonded overlays. Prefabricated cementitious mortar grid instrumented with distributed fiber optic sensors, namely smart grid, was developed and proposed to address the logistics of handling delicate optical fibers, and thus facilitate the in-situ construction. The smart grids can be laid on top of the fabric sheet and embedded in concrete overlay. With the proposed method, the pavement overlays instrumented with distributed sensors were successfully constructed in Minnesota's Cold Weather Road Research Facility (MnROAD). The optical fibers were characterized on a precision load frame at room temperature. A Neubrescope was used to measure strain distributions based on the pulse pre-pump Brillouin optical time domain analysis (PPPBOTDA). The overlays were subjected to repeated truck loads and eventually cracked. Strain distributions were obtained from the distributed fiber optic sensor. Cracks were identified and localized by mapping the strain distribution in which the sharp peaks represent the cracks. The strain distribution was further investigated using a three-dimensional finite element model incorporating nonlinear boundary conditions. Opening between substrate and overlay concrete was demonstrated, and strain distributions in overlay and substrate concrete were determined with the numerical model. For the bonded concrete overlays on existing pavement, a delamination detection method was developed and implemented using the distributed fiber optic sensors. Delamination can be identified as sharp peaks in the measured strain distributions.
Y. Bao and G. Chen, "Strain Distribution and Crack Detection in Concrete Overlays with Pulse Pre-Pump Brillouin Optical Time Domain Analysis," Mid-America Transportation Center, Aug 2015.
Civil, Architectural and Environmental Engineering
Research and Innovative Technology Administration (RITA)
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01 Aug 2015