Signal processing and analysis of structural vibration measurements are key components of structural damage detection (SDD) in structural health monitoring (SHM). The goal of signal processing is to extract subtle changes in the measured signals, which can be used to infer changes in structural parameters and damage. Time-frequency analysis is one of the most popular characterization methods for studying non-stationary vibration signals. In this article, the local time-frequency transform (LTFT) is applied and evaluated to calculate the time-domain signals because of its excellent time-frequency energy distribution properties. The LTFT matches the input data by the Fourier basis in an inverse problem framework and uses the least squares method to solve the time-varying Fourier coefficients. Subsequently, it defines the time-frequency spectrum as the calculated time-varying Fourier coefficients. While the LTFT has been used in the field of geophysics for seismic data processing, its application to structural vibration signals is novel. Both synthetic signals as well as signals collected from a large-scale laboratory test of a reinforced concrete girder were processed with the LTFT and compared with Rényi entropy for quantifying the time-frequency spectrum, the time-frequency resolution abilities of short time Fourier transform (STFT), and S transform (ST). The results show that the LTFT is superior to the traditional time-frequency analysis schemes, in that it is more effective in identifying the energy changes in the time-frequency spectrum before and after structural damage in the form of cracking has occurred. At the same time, it provides high-precision time-frequency resolution and excellent noise suppression abilities. The effectiveness and feasibility of the LTFT applied to the synthetic and experimental signals are verified.

Ultra-high-performance concrete (UHPC) is a kind of structural material with ultra-high strength, extremely low porosity, and excellent durability, which has extremely broad application prospects. In order to promote the application of UHPC constrained by carbon fiber-reinforced polymer (CFRP) sheets as strengthening material in practical engineering, a total of nine specimens were designed, and two kinds of UHPC strengthening layer thickness (35 mm and 45 mm, respectively) were designed. By changing the constraint form of the UHPC strengthening layer (longitudinal reinforcements and ordinary stirrups, longitudinal reinforcements and spiral stirrups, and CFRP sheets, respectively), the axial compression performance of the strengthened column was explored. The study shows that compared with the without strengthened column, the uplift of carrying capacity of the strengthened test column is 277–561%. The reinforcement form of the strengthening layer has little influence on the lifting capacity. Among the three different strengthening methods, the wrapped CFRP has the best improvement effect on carrying capacity and ductility, followed by longitudinal reinforcements and spiral stirrups in the strengthening layer. With the increase of CFRP layers from two to five layers, the maximum carrying capacity increases by 21.3%. The carrying capacity of three different types of UHPC-strengthened columns is theoretically calculated, and the accuracy of the theoretical calculation method is verified by comparing the test value with the theoretical value, which provides a theoretical basis for the application of UHPC-strengthened columns in the future.

By investigating the bond performance between glass-fiber-reinforced polymer (GFRP) bars and concrete, GFRP bars can be better applied to concrete structures as a building material. This paper considered the effects of three different GFRP bar surface treatments, three bonding lengths, corrosive solution, and immersion time on the bonding strength. The test results indicated that the bond strength decreases with the increase in the diameter and bond length. The bonding between GFRP bars and concrete can be improved by treating the surface of the bars in different ways. Compared with the control group, the bond strength of the specimens in the saline solution decreased by 1.3–21.4%, and the bond strength of the specimens in the alkaline solution decreased by 26.5–38.8%. In the corrosive environment, the bond properties are degraded. A bond strength calculation formula considering the surface treatment method of the GFRP bars was proposed. The prediction formula of the bond strength retention rate between the GFRP bar and concrete in the corrosive environment was established. The formula was validated with the available research data and the calculated values agreed well with the test values. The MBPE model and CMR model are modified to establish the bond-slip model of the GFRP bars and concrete in the corrosive environment. The model curve is close to the test curve. This paper provides a theoretical basis for future research on the bond-slip performance of GFRP bars and concrete.

In order to obtain the calculation model for the local bearing capacity of concrete F_l under two headed bars, six pull-out concrete specimens were prepared. The effect of the net distance between two headed bars c on F_l was mainly investigated. The test results show that the local bearing capacity of concrete would first decrease and then increase with the increase in c, and the boundary point of the two stages was c = 40 mm. It is determined that the stress transformation from the local compression state to the axial compression state in the stress distribution model is characterized by the variation rate of the vertical stress under individual headed bars, which would infinitely approach a constant value. The constant value under individual headed bars is used as the limit value. The height of the vertical stress under two headed bars is modified, and then the height of the tensile region of the specimen with different c values is determined. Combined with the experimental phenomena and the results, two stages of calculation models are established, respectively: the integral calculation model and the individual calculation model. The integral calculation model focuses on the interaction of the compression region under two headed bars. The individual calculation model mainly focuses on the interaction of the tensile region under two headed bars. The calculation equations considering the influence of the height of the tensile region are established. Two groups of similar test data regarding the local bearing capacity were collected and verified as the integral calculation model and the individual calculation model. The average value of the ratio between the test value and the calculated value is 1.057 and 1.061, the standard deviation is 0.153 and 0.091, and the coefficient of variation is 0.055 and 0.086. It is proved that the calculation model proposed in this paper has a certain accuracy. It can provide a reference for calculating the local bearing capacity of concrete under multiple headed bars.