Shear Performance of Keyway Interface Between UHPC and Normal Concrete

The joints of precast concrete segmental beams (PCSBs), which are in complex stress status and susceptible to failure, are very important parts of the structure. In this paper, a finite element model was established to study the shear performance of single‐keyed joints. The plastic damage model was used to simulate the cracking of specimens. Three types of single‐keyed joints were investigated, including the dry joint with normal concrete (NC), dry joint with steel fiber‐reinforced concrete (SFRC), and epoxied joint with NC. The cracking pattern, ultimate shear strength, and load‐displacement curve for these specimens were obtained. Based on these numerical simulation models, extended analyses in terms of low confining pressures and eccentric loads were performed. It has been found that the influence of fiber‐reinforced concrete should be considered. The ultimate shear strength of specimens reduced with the reduction of confining pressure. When an eccentric load was applied, a lower shear capacity would be obtained. Under the low confining stress, the AASHTO LRFD 2014 provision underestimated the shear strength of single‐keyed dry joints with both NC and SFRC, while the shear capacity of single‐keyed dry joints with both NC and SFRC has been overestimated under the eccentric loads.

Experimental and numerical study on shear behavior of shear pockets between ultra-high-performance and normal concrete for precast girder bridges

An experimental study was carried out to evaluate fresh properties of a moderately high-strength (high-flowing) self-compacting concrete (SCC) and to investigate shear behavior and performance of deep beams made with SCC. Fresh and hardened properties of normal concrete (NC) and SCC were evaluated. The workability and compacting ability were observed based on casting time and number of surface cavities, respectively. Four-point loading tests on four deep beams (two made with SCC and two with NC) were then conducted to investigate their shear behavior and performance. Shear behavior and performance of beams having two different web reinforcements in shear were systematically investigated in terms of crack pattern, failure mode, and load–deflection response. It was found from the tests that the SCC specimen having a normal shear reinforcement condition exhibited a slightly higher load carrying capacity than the corresponding NC specimen, while the SCC specimen having congested shear reinforcement condition showed a similar load carrying capacity to the corresponding NC specimen. In addition, a comparative study between the present experimental results and theoretical results in accordance with ACI 318 (Building Code Requirements for Reinforced Concrete (ACI 318-89) and Commentary-ACI 318R-89, 1999), Hsu–Mau’s explicit method (Hsu, Cem Concr Compos 20:419–435, 1998; Mau and Hsu, Struct J Am Concr Inst 86:516–523, 1989) and strut-and-tie model suggested by Uribe and Alcocer (2002) based on ACI 318 Appendix A (2008) was carried out to assess the applicability of the aforementioned methods to predict the shear strength of SCC specimens.

Thin-walled structures are widely used in long-span bridges such as truss composite arch bridges due to their excellent mechanical properties and cost-effectiveness. However, long-term service can lead to defects including interface delamination and concrete carbonation. This study aims to clarify the regulatory mechanism of bonded rebar diameter on the static direct shear performance at the interface of existing normal concrete (NC) thin-walled structures strengthened with ultra-high performance concrete (UHPC). Using mechanical cutting combined with high-pressure water jetting as the interface treatment method, three sets of shear specimens with rebar diameters of 6 mm, 8 mm, and 12 mm were designed. Through static shear tests and nonlinear finite element analysis, the influence of rebar diameter on the shear performance at the interface was systematically examined. Research findings indicated that as the diameter of embedded rebars increases, the primary failure mode at the interface transitions from interface bond failure to NC matrix failure. Furthermore, as the diameter increased from 6 mm to 12 mm, the ultimate shear strength rose from 5.75 MPa to 9.19 MPa, representing a 59.8% increase. The residual strength increased from 1.5 MPa to 3.45 MPa, representing a significant 130% improvement. The failure slip distance increased from 0.35 mm to 0.44 mm, indicating enhanced ductility. Additionally, the established finite element model can accurately predict the mechanical behavior at the interface under different planting rebar diameters, with an error margin of less than 10% compared to experimental results. The research findings provided a theoretical basis for the design of planting rebar parameters in UHPC-strengthened thin-walled concrete structures.

Ultra-high performance concrete (UHPC) with excellent mechanical properties and durability is a promising material for reinforcement of existing normal concrete (NC) structures. In this paper, the shear failure behavior of the NC–UHPC interface was studied by the slant shear test and the SEM (scanning electron microscope) visualization test, considering influence of the substrate strength and the interface roughed treatment. As the NC substrate and the UHPC overlay are tightly combined at the interface transition zone (ITZ), the interface exhibits good slant shear performance, and the measured interfacial shear strength could reach 19.4 MPa with C40 substrate and 21.8 MPa with C50 substrate. In addition, the microstructure and composition of the ITZ, the possible interfacial failure modes, and the load-carrying mechanism of the interface under compression–shear force are revealed. The high interface roughness and the substrate strength have positive influence on the shear strength, and greatly affect the prone failure mode and the load-slip characteristic.

Experimental study on the interfacial shear performance between macro basalt fibers reinforced UHPC as repair material and normal concrete

Experimental study on shear performance of perfobond steel plate in ultra-high performance concrete (UHPC)-normal concrete (NC) connection

Ultra-high performance concrete(UHPC) is known for its high strength, high toughness and durability, which makes UHPC be seen as a promising repairing material for normal concrete(NC). In order to make sure the application of UHPC in reinforcement, especially in cryogenic circumstance, it is critical to characterize the bonding performance between UHPC and NC. Through 11 sets of normal concrete and ultra-high performance concrete specimens (UHPC-NC specimens) were tested by double shear tests, the shear properties of UHPC-NC specimens in normal and cryogenic environment (−60 °C) were evaluated and discussed. Different interface treatments were used, including untreated, water jetting and using retarder. The effect of interface agent was also studied. The results show that the shear strength of the interface was improved by increasing surface roughness degree. The failure mode presented brittle failure, no matter what kind of interface treatments. Cryogenic circumstance can improve the bonding strength of UHPC-NC, and the group without interfacial agent had a more significant improvement. The performance of interfacial agent in low temperature limits the improvement of interfacial bonding strength to a certain extent.

Engineered Cementitious Composite (ECC) has received much attention due to its excellent properties of tension strain-hardening and multiple-cracking. Considering the full utilization of ECC properties and optimal cost, a kind of ECC-NC (normal concrete) composite structure has been developed and has shown great application potential. In this paper, a series of bi-surface shear tests were carried out to investigate the shear performance of ECC-NC specimens, a key factor affecting the performance of ECC-NC composite structures. The types of interfaces, including flat surface, rough surface and bubble grooves, the casting sequence and concrete strength were taken as the variables in the tests. The test results showed that the NC strength correlated strongly with the shear strength of the interface, and the effect of the casting sequence was non-negligible. Simultaneously, the formula for calculating the shear bearing capacity of bubble groove interfaces between precast NC and post-cast ECC was proposed.

Interfacial shear behavior of salt freeze-thaw pre-damaged NC with ECC: Experimental and theoretical investigations

Shear performance of interface between ultra-high performance concrete and normal concrete treated with ribs

Ultra-high-performance concrete (UHPC) has been widely utilised in strengthening and rehabilitating conventional normal concrete (NC) structures due to its exceptional mechanical properties and durability. However, in cold climates, the interfacial bond between UHPC and NC is susceptible to degradation under freeze–thaw cycles, potentially compromising the composite action and long-term performance of strengthened structures. This study systematically investigated the shear behaviour of a UHPC-NC interface with planted reinforcement subjected to various freeze–thaw conditions. The experiments were conducted considering different numbers of freeze–thaw cycles (0, 20, 40, 60, 80, and 100) and salt solution concentrations (0%, 3.5%, and 5%). Direct shear tests were performed to evaluate interfacial failure modes, mass loss, and shear strength degradation. Results identified three characteristic failure modes: adhesive debonding at the interface, mixed failure involving both the interface and the NC substrate, and crushing failure within the NC substrate. Specimens exposed to 3.5% salt solution experienced the most significant deterioration, exhibiting a 35% reduction in shear strength after 100 freeze–thaw cycles. Normally, lower salt concentrations were found to induce greater interfacial damage compared to higher concentrations. The study underscores the importance of increasing the embedment depth of the planted reinforcement to alleviate stress concentration and enhance interfacial durability in freeze–thaw environments.

Strengthening concrete structures with ultra-high performance concrete (UHPC) can both improve the bearing capacity of the original normal concrete (NC) structure and prolong the service life of the structure due to the high strength and durability of UHPC. The key to the synergistic work of the UHPC-strengthened layer and the original NC structures lies in the reliable bonding of their interfaces. In this research study, the shear performance of the UHPC-NC interface was investigated by the direct shear (push-out test) test method. The effects of different interface preparation methods (smoothing, chiseling, and planting straight and hooked rebars) and different aspect ratios of planted rebars on the failure mode and shear performance of the pushed-out specimens were studied. Seven groups of push-out specimens were tested. The results show that the interface preparation method can significantly affect the failure mode of the UHPC-NC interface, which is specifically divided into interface failure, planted rebar pull-out, and NC shear failure. The critical aspect ratio for the pull-out or anchorage of planted rebars in UHPC is around 2. The interface shear strength of straight-planted rebar interface preparation is significantly improved compared with that of the chiseled and smoothened interfaces, and as the embedding length of the planted rebar becomes longer, it first increases greatly and then tends to be stable when the rebar planted in UHPC is fully anchored. The shear stiffness of UHPC-NC increases with the increase of the aspect ratio of planted rebars. A design recommendation based on the experimental results is proposed. This research study supplements the theoretical basis of the interface design of UHPC-strengthened NC structures.

Static behavior of stud shear connectors in high-strength-steel–UHPC composite beams

Investigations on the shear performance degradation of ECC-NC grooved interface under salt freeze-thaw cycles