Application of ultra-high-performance concrete (UHPC) in joints can improve the impact resistance, crack resistance, and durability of structures. In this paper, the direct shear performance of ultra-high-performance concrete (UHPC) adhesive joints was experimentally studied. Twenty-four direct shear loading tests of UHPC adhesive joints were carried out considering different interface types and constraint states. The failure modes and load-slip curves of different interfaces were studied. Results indicated that passive confinement could enhance the strength and ductility of the interface; the average ultimate bearing capacity of the smooth, rough, grooved, and keyway specimens with passive restraint were, respectively, increased by 11.92%, 8.91%, 11.93%, and 17.766% compared with the unrestrained ones. The passive constraint force changes with the loading and finally tends to be stable. The epoxy adhesive has high reliability as a coating for the UHPC interface. The adhesive layer is not cracked before the failure of the specimen, which is also different from the common failure mode of adhesive joints. Failure of all specimens occurred in the UHPC layer, and the convex part of the groove interface shows the UHPC matrix peeling failure; the keyway interface is the shear damage of the key-tooth root, and the rest of the keyway showed UHPC surface peeling failure. According to the failure mode, the shear capacity of UHPC keyway adhesive joints under passive restraint is mainly provided by the shear resistance of key teeth, the friction force of the joint surface, and the bonding force of the UHPC surface. The friction coefficient was determined based on the test results, and the high-precision fitting formula between the shear strength of the UHPC surface and the passive constraint force was established. According to the Mohr stress circle theory, the proposed formula for direct shear strength of UHPC bonded joints under passive constraint was established. The average ratio of the proposed UHPC adhesive joint calculation formula to the test results was 0.99, and the standard deviation was 0.027.
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.
In this study, a test beam model of negative moment region was designed to study its deflection characteristics under over-limit static load and variable amplitude cyclic load scenarios. (1) According to the deflection test results of the test beam under over-limit static load, the load-deflection curve under static load before and after 10000 times of over-limit fatigue load tended to sharply increase in the first place and then slow down afterwards. At the ultimate load, the maximum deflection value of the two was 8.625 mm and 8.76 mm, respectively. The maximum deflection value after 10000 times of over-limit fatigue load increased by 1.57% compared with that before. (2) The deflection test results of the test beam under variable amplitude cyclic load showed that both the total deflection and the residual deflection percentage increased firstly and decreased with the increasing fatigue load times. The total deflection ranged from 4.07–4.3 mm, and the residual deflection percentage was 0.1–1.65%; The residual deflection percentage reached a maximum of 1.65% when the fatigue load times reached 2 million. (3) The deflection test results of the test beam under over-limit variable amplitude fatigue load showed that the maximum residual deflection reached 0.08 mm (maximum residual deflection percentage 1.75%), 0.11 mm (maximum residual deflection percentage 2.30%), 0.16 mm (maximum residual deflection percentage 2.65%) after 1.5, 2 and 3 times over-limit variable amplitude fatigue load was imposed respectively. The maximum deflection did not fluctuate obviously, indicating that the test beam was had sound working performance. (4) In this study, the shear stiffness and fatigue load times were fully considered, the mean value of the ratio of the deformation value to the measured value of the test beam was 0.89, and the standard deviation was 4.86%. When the fatigue load times exceeded 2.8 million, the ratio of the calculated value to the measured value was less than 0.85. In conclusion, the model had its own limitations.
In this study, a model of test beam in negative moment area is designed, and the slip characteristics of test beam under overlimit static load and variable amplitude cyclic load are studied, respectively. By constructing the relationship between shear stiffness and slip parameters of the test beam, the deformation calculation method of the test beam under different fatigue cycles is derived, and the accuracy of the calculation model is verified by experiments. The results show that the maximum slip value (0.038 mm) of static load after 10,000 times of limited fatigue load is increased by 58.3% compared with that before overlimit fatigue load is applied (0.024 mm). When the fatigue cycle is 0-2 million times, the total slip is between 0.019 and 0.026 mm and the residual slip percentage is between 4.17 and 8.33%. The maximum residual values are 0.022 mm, 0.027 mm, and 0.028 mm after 1.5 times, 2 times, and 3 times of overload and variable amplitude fatigue loads, respectively, and the slip values have no obvious fluctuation, all of which show good working performance. The average value of the ratio between the deformation value and the measured value of composite beams is 0.89, and the standard deviation is 4.86%. When the fatigue loading times are more than 2.8 million, the ratio between the calculated value and the measured value is less than 0.85, so the adaptability of the calculation model has certain limitations. On the whole, the calculation model proposed in this study fully considers the factors such as the stiffness and fatigue loading times of composite beams, and the error between the calculated results and the measured results is within 5%, with high accuracy, which can be used as a reference for the actual design.
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