SUMMARY In this study, the effects of high‐amplitude initial conditions on the accuracy of modal parameters, identified from output‐only vibration data, are investigated. The influence on the sample output correlation function, which is the basis of most time‐domain operational modal analysis techniques, is analyzed first. Then, a numerical simulation is performed to quantify the effect of nonzero initial conditions on the relative accuracy of natural frequencies, damping ratios, and mode shapes. It is shown that, when all identification assumptions are satisfied, high‐amplitude initial conditions can significantly reduce the estimation errors, especially for short data records. Finally, a full‐scale application is presented where the modal parameters of a six‐span high‐speed railway bridge are determined from output‐only data. The results obtained with two different data sets are compared: The first one consists of the bridge's response to ambient data only, whereas the second one also contains the free vibration recorded immediately after a train passage. Although for most modes the results are similar, it is possible to identify some additional bending and torsion modes from the free vibration data with good accuracy. Copyright © 2013 John Wiley & Sons, Ltd.

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Nano silicate SiO2 (NS) in the high-performance concrete (HPC) mixture is becoming great interest because it improves the mechanical properties of concrete. As the ultra-fine SiO2 particles with a very high ratio of surface area to mass, NS can tightening the structure and produces better quality gel products and thus concrete strength increases. However, the optimal rate of NS in high-performance concrete has not been investigated. This study experimentally evaluates the influence of NS on compressive strength and flexure strength of HPC of which nominal strength is 70MPa. Seven mixtures of HPC at NS rate of 0%, 0.5%, 1%, 1.5%, 2%, 2.5% and 3% over the total weight of the adhesive were conducted. Each mixture provides six samples for compressive tests and six samples for flexure tests. Total samples are 336 samples for four ages of concrete. Reliability of test results was carried out and results shown that at the early stage of HPC, an NS ratio of 1.5% shows the optimal value to improve the performance of concrete. This result also helps to apply NS in HPC in quality control in site application.

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