128 publications from this institution
This study aims to investigate the flexural performance of ultra-high-performance concrete (UHPC) wet joints subjected to vibration load during the early curing period. The parameters investigated included vibration amplitude (1 mm, 3 mm, and 5 mm) and vibration stage (pouring—final setting, pouring—initial setting, and initial setting—final setting). A novel simulated vibration test set-up was developed to reproduce the actual vibration conditions of the joints. The actuator’s reaction force time-history curves for the UHPC joint indicate that the reaction force is stable during the initial setting stage, and it increases linearly with time from the initial setting to the final setting, trending toward stability after 16 h of casting. Under the vibration of 3 Hz-5 mm, cracks measuring 14 cm × 0.2 mm emerge in the UHPC joint. It occurs during the stage from the initial setting to the final setting. The flexural performance of wet joint specimens after vibration was evaluated by the four-point flexural test, focusing on failure modes, load-deflection curves, and the interface opening. The results show that all specimens with joints exhibited bending failure, with cracks predominantly concentrated at the interfaces and the sides of the NC precast segment. The interfacial bond strength was reduced by vibrations of higher amplitude and frequency. Compared with the specimens without vibration, the flexural strength of specimens subjected to the vibration at 3 Hz-3 mm and 3 Hz-5 mm were decreased by 8% and 19%, respectively. However, as the amplitude and frequency decreased, the flexural strength of the specimens showed an increasing trend, as this type of vibration enhanced the compactness of the concrete. Additionally, the calculation model for the flexural strength of UHPC joints has been established, taking into account the impact of live-load vibration. The average ratio of theoretical calculation values to experimental values is 1.01, and the standard deviation is 0.04, the theoretical calculation value is relatively precise.
The assembly construction of prefabricated UHPC elements can well balance quality reliability and construction convenience, thus it has excellent application prospects in bridge engineering. The joints between prefabricated elements are the key to ensuring the overall force performance of the structure, which directly determine the load-bearing capacity and the life of structure. To clarify the bending behavior of epoxy adhesive joints between prefabricated UHPC elements, four groups of 12 bending tests were carried out with different interface treatment forms as parameters. The failure modes, load-deflection curves, and ultimate bending strength of the interface were investigated. The results reveal that the interfacial failure modes mainly include the interfacial stripping failure of epoxy-UHPC surface, steel fibers and fine aggregates into UHPC surface by pulling out, and tensile damage of UHPC at the root of key teeth on the side of the keyway interface. The load-deflection curves of all specimens exhibit the two-fold lines form. The load tends to rise linearly during the loading phase, and there is no yielding phase before the failure. The load-carrying capacity of the specimen is lost immediately after the failure, and no reliable residual strength is available except for the keyway interface. In addition, the bending strength of rough interface, groove interface, and keyway interface are respectively improved by −24.02, 2.34, and 4.64%, compared with the natural interface. So it is recommended that the joint between prefabricated UHPC elements take the form of keyway interface. Finally, a simplified force model of the keytooth adhesive joint is proposed, and a calculation formula for the flexural bearing capacity is established based on the principal of Mohr’s circle, based on the experimental results and theoretical analysis. The mean ratio of the proposed adhesive joint calculation equation to the experimental results was 0.925 with a standard deviation of 0.065.
This study employed magnetic field induction and steel fiber hybridization methods to prepare Hybrid Aligned Steel Fiber-Reinforced Cementitious Composites (HASFRCCs). The direct tensile performance of specimens with aligned and randomly dispersed steel fibers was compared under different hybrid coarse-to-fine fiber ratios (3:1, 2:1, 1:1, 1:2, and 1:3). The fiber pullout tests were conducted to determine the bond-stress-slip relationship between steel fibers and matrix for both coarse and fine steel fibers. Based on these results, an analytical model for the tensile behavior of HASFRCC was then developed based on the composite mechanics theory and modified according to the hybrid fiber effect. The results demonstrated a significant increase in the fiber orientation coefficient ηθ of the HASFRCC by 21.1% to 26.9% compared with the random specimens. Additionally, the tensile strength and energy absorption capacity substantially improved by 43.8% to 64.1% and 58.5% to 71.4%, respectively, compared to the specimens with random steel fiber. In addition, the modified model of HASFRCC quantitatively reveals the role of the two kinds of fibers in the tensile process, and the difference between the model-predicted and experimental results was less than 10%.
In precision forestry, tree species identification is key to evaluating the role of forest ecosystems in the provision of ecosystem services, such as carbon sequestration and assessing their effects on climate regulation and climate change. In this study, we investigated the effectiveness of tree species classification of urban forests using aerial-based HyMap hyperspectral imagery and light detection and ranging (LiDAR) data. First, we conducted an object-based image analysis (OBIA) to segment individual tree crowns present in LiDAR-derived Canopy Height Models (CHMs). Then, hyperspectral values for individual trees were extracted from HyMap data for band reduction through Minimum Noise Fraction (MNF) transformation which allowed us to reduce the data to 20 significant bands out of 118 bands acquired. Finally, we compared several different classifications using Random Forest (RF) and Multi Class Classifier (MCC) methods. Seven tree species were classified using all 118 bands which resulted in 46.3% overall classification accuracy for RF versus 79.6% for MCC. Using only the 20 optimal bands extracted through MNF, both RF and MCC achieved an increase in overall accuracy to 87.0% and 88.9%, respectively. Thus, the MNF band selection process is a preferable approach for tree species classification when using hyperspectral data. Further, our work also suggests that RF is heavily disadvantaged by the high-dimensionality and noise present in hyperspectral data, while MCC is more robust when handling high-dimensional datasets with small sample sizes. Our overall results indicated that individual tree species identification in urban forests can be accomplished with the fusion of object-based LiDAR segmentation of crowns and hyperspectral characterization.
A large number of early hollow slab beams suffer from inadequate load carrying capacity, stiffness and durability, resulting from the overloading, environmental deterioration mechanism and poor maintenance. Most of the existing studies on the reinforcement of pre-damaged reinforced concrete beam carried out in scale-down models. However, the artificial pre-damage (mechanical damage and accelerated corrosion) alters the deterioration mechanism of reinforced concrete beam. The scale effect leads to disparities in force ratios acting on scale-down models and full-scale structures. To solve these problems, three 15-year-old full-scale hollow slab beams (taken from a serving bridge in Guangxi, China, with 15.96 m long) were respectively strengthened with steel-reinforced UHPC layer, GFRP-reinforced UHPC layer and CFRP layer. The flexural behaviors of the strengthened beams and the control beam were investigated by field four-point bending test and numerical simulation. The experimental results indicate that the ultimate flexural load capacity and stiffness of hollow slab beams after strengthened are respectively increased by 17.6%-47.1% and 4.7%-17.9%, and the deflection under service load is decreased by 7.6%-19.1%. Meanwhile, UHPC is beneficial to improve the energy absorption capacity of hollow slab beams. Moreover, the main bearer of load is the tensile steel bars in the hollow slab beams and the reinforcement layers played an auxiliary role in load bearing. Finally, the accurate prediction formulas for the ultimate flexural load of strengthened hollow slab beams with a maximum error of only 3.5% were proposed.
