Concrete compressive strength is important, yet difficult to quantify without direct testing. In particular, it is difficult to obtain the mature concrete strength measurements which are necessary for safe and optimal use of existing structural capacity. Reliable measurements of mature strength using nondestructive testing methods (NDT) like ultrasonic pulse velocities depend on many factors, including the inherent material variability, sampling frequency, and quality of the NDT measurements. Methods like ground penetrating radar (GPR) and concrete maturity relationships are common for investigating the early-age properties of concrete but are rarely used for mature concrete. Using a case study of a concrete pedestrian bridge where both long term temperature data from structural health monitoring (SHM) and recent GPR surveys of the bridge are available, this work compares the predicted 8-year strength using two different indirect methods. The first uses a regression model trained on laboratory GPR attributes and material properties. The second uses the maturity method to predict strength based on 28-day cylinder tests and the temperature history recorded by the bridge's SHM system. The maturity method predicts the correct relative trends in strength between the two phases and overpredicts the cylinder 28-day strength by 12% 25%. The GPR predictions do not reliably capture the relative difference between the two phases, but have similar accuracy and underpredict cylinder strength by 4% 22%. These strength comparisons from noninvasive methods motivate further improvements in GPR attribute modeling and integrating these methods with other ultrasonic models to improve spatial resolution and reliability.
Damage characterization often requires direct sensing due to the localization of the anomalous behavior near the cracks. Direct sensing, however, is expensive because of the need to deploy a dense array of individual sensors. Sensing sheets based on Large Area Electronics (LAE) and Integrated Circuits (ICs) are a novel solution to this problem. Such sensing sheets could span several square meters, with a dense array of strain sensors embedded on a polyimide substrate along with the relevant electronics allowing for direct sensing while keeping the costs low. Current studies on LAE based sensing sheets are limited to laboratory experiments. This paper explores the question of suitability of the sensing sheets as a viable option for real-life SHM based on LAE and ICs. Results of laboratory experiments on an aluminum beam are provided to demonstrate the performance of sensing sheets in ideal conditions. Then, the sensing sheets are employed on a pedestrian bridge already equipped with fiber-optic sensors. The strain measurements from the sensing sheets and the fiber-optic sensors are compared and sources of differences are discussed.
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