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Using ultrasonic pulse velocity (UPV) testing has long been regarded as an effective approach for evaluating concrete strength; however, because of the influence of numerous factors (e.g., mixture proportion, type of coarse aggregate, pozzolanic admixture, and age of the concrete), the relationship between UPV and concrete strength cannot be conclusively determined, thus hindering the application of this approach. The objective of this paper is to develop a unified equation for prediction of concrete strength at various ages using the ultrasonic pulse velocity. Firstly, this study investigated the relationship between the UPV evolution index and the strength evolution index of normal concrete with various mixture proportions, coarse aggregate types, and ages. Subsequently, the influence of adding pozzolanic materials on this relationship was considered. The experimental results showed that excellent correlation between the UPV evolution index and the strength evolution index was found and a unified equation was established in this study through regression analysis to represent the relationship between the UPV and the strength evolution index. Finally, the applicability of the unified equation was verified by data obtained from different studies. The results indicated that for all concrete specimens more than 3 days old, the error of the strength estimation decreased to within ±15%, demonstrating that this unified equation has substantial value for application in decision-making processes for major construction procedures (e.g., applying post tensioning or removing formwork supports). [ABSTRACT FROM AUTHOR]

A novel calibration method is developed for calibrating stereo-camera-based digital image correlation (DIC) setup to overcome the difficult challenge of 3D vibration measurement of large structures using digital imaging techniques. The calibration method employs an IMU sensor for camera orientations, a laser sensor for camera positions, and a virtual stereovision approach for intrinsic parameters. This novel stereo-camera calibration method is validated using an ideal physical calibration method and then integrated with DIC technique for field experiments. The first field experiment is performed on a light tower of 10m height. The results showed that the developed technique can measure 3D displacements of the light tower under various excitation modes. It was also consistent in terms of natural frequency estimation which is validated using an accelerometer. With this success, the developed methodology is employed in the second field experiment where 3D vibration study is carried out on a utility-scale wind turbine. The developed DIC-calibration method is able to obtain the in-plane and out-of-plane displacements of the tower and rotor blade of the turbine. The method is also able to produce accurate values of natural frequencies validated using a ground-based microwave interferometer. It can be said that the present study established a stereo-camera-based DIC technique with a novel calibration method for 3D vibration analysis of various large-sized civil and aerospace structures. [ABSTRACT FROM AUTHOR]

This study used the ultrasonic pulse velocity (UPV) technique to evaluate the in-place strength of concrete at various ages. Both UPV and strength of concrete increase with age, and the relationship between UPV and compressive strength has been investigated for decades. Previous research results have indicated that the density of concrete is linked positively to UPV and compressive strength. It has also been found that the amount of coarse aggregate and the moisture content of concrete have a significant influence on pulse velocity, but little effect on strength. In this work, a study was carried out to develop a general relationship between UPV and compressive strength of concrete in consideration of multiple variables such as age, coarse aggregate content and moisture content. The suitability of the established relationship for estimating the strength of concrete at various ages was also evaluated. The results obtained from the verification programme carried out on a concrete plate specimen show that the effect of moisture content on UPV must be taken into account for a better estimate of concrete strength with the UPV technique. [ABSTRACT FROM AUTHOR]

The severe deterioration of a cement asphalt (CA) mortar layer may lead to the movement of the upper concrete slab and impair the safety of the speedy train. In this study, a test specimen simulating the structure of high-speed rail track slabs was embedded with delaminated cracks in various lateral sizes inside the CA mortar layer. Impact–echo tests (IE) were performed above the flawed and flawless locations. In present study, the IE method is chosen to assess defects in the CA mortar layer. Both traditional IE and normalized IE are used for data interpolation. The normalized IE are the simulated transfer function of the original IE response. The peak amplitudes in the normalized amplitude spectrum and the peak frequency in the traditional amplitude spectrum for the top concrete overlay were used to develop simple indicators for identifying the integrity of the CA mortar layer. The index was based on the difference of the experimental peak amplitude and frequency of the ones calculated from previously developed formulas for plates without substrates. As a result, the technique does not require an experimental baseline for the crack assessment. A field test and analysis procedure for evaluating high-speed rail slab systems are proposed. [ABSTRACT FROM AUTHOR]