Precast Ultra-high Performance Concrete Research Articles

Comparative behavior of circular precast UHPC columns with hoops and spiral reinforcement

Ultra-high performance concrete (UHPC) offers outstanding mechanical properties that can greatly influence the future of our structures. Despite the remarkable durability and high tensile and compressive strength of UHPC, its widespread use in large structural applications remains limited. This limitation is attributed to several factors such as the relatively high cost of the material and the slow development of large-scale production technology and procedures. In light of these challenges, the objective of this research is two-fold: first, demonstrate the successful fabrication of four full precast UHPC columns using economic mixture and construction-scale production in a precast plant setting, and next, compare the axial behavior of the fabricated circular columns with different transverse reinforcement detailing. The study carefully considers the choice of lateral confinement and spacing along the height of the column (spiral and hoop reinforcement with 1.5 in and 3 in spacing). The paper presents the global and local behavior that include load capacity, stiffness, axial and reinforcement strains, and ductility. The spirally confined column with 1.5 in spacing showed 26 % higher ductility than the counter hoop reinforced column, while both configurations showed a significant increase in ductility of 34 % when the transverse reinforcement spacing is reduced from 3 to 1.5 in. The research shows that the ACI design reduction factor for conventional concrete (0.65) and the accidental eccentricity reduction factor (0.8) for hoops are unnecessarily conservative and might be increased to 0.75 and 0.85, respectively as for the spiral reinforcement in the case of UHPC columns.

Retrofitting performance of precast UHPC jackets and NSM GFRP bars in pre-damaged RC columns with deficient lap splices

Fiber-reinforced polymer (FRP) jackets and the near surface-mounted (NSM) technique have been widely used to improve the seismic performance of intact reinforced concrete (RC) columns with deficient lap splices. For damaged lap-spliced RC columns, use of ultra-high-performance concrete (UHPC) jackets is more powerful in eliminating the bond splitting failure and recovering the stiffness than FRP jackets. This study proposed an L-shape precast steel-reinforced UHPC (PR-UHPC) jacket to improve the retrofit performance of conventional cast-in-place (CIP) and precast UHPC jackets. Further, PR-UHPC jackets were combined with NSM GFRP bars to develop an innovative method. Cyclic loading tests were performed on three three-fourths scaled deficient lap-spliced RC columns. One intact column was directly retrofitted with PR-UHPC jackets, while the other two were first damaged and then retrofitted with PR-UHPC jackets and the combination of PR-UHPC jackets and NSM GFRP bars, respectively. To investigate the effects of UHPC details and NSM bar types, a previous study on lap-spliced RC columns retrofitted with CIP plain UHPC jackets and NSM steel bars was incorporated. The test results indicate that the retrofitted columns failed with rebar fracture, and plastic hinge shifting was prevented. The peak load, effective yield stiffness, and ultimate drift ratio of the retrofitted columns were 29∼50 %, 37∼39 %, and 126∼165 % higher than those of the unretrofitted columns, respectively. NSM GFRP bars outperformed steel bars in terms of residual drift ratio control. The numerical model well predicted the cyclic behavior by addressing the pre-damage effect and the actual confinement of PR-UHPC jackets. The parametric study revealed that the increase of PR-UHPC jacket width effectively improved the strength and stiffness, while the pre-damage degree had a minor effect. The recommended UHPC jacket height for the present columns was 2.5 times the plastic hinge length. Finally, a design method was developed for the UHPC jacket width.

Seismic performance of precast UHPC pipe pile with two pile-cap beam connection types: An experimental and numerical study

To develop an effective pile foundation scheme for earthquake-prone regions, this study introduces a novel pile structure that integrates ultra-high-performance concrete (UHPC) with traditional prestressed high-strength concrete (PHC) pipe piles. The research focuses on assessing the impact of various connection forms between the pile and cap beam on the seismic performance of bridge substructures. Two 1/3-scale specimens were meticulously designed and tested: one featuring a cast-in-place (CIP) connection and the other incorporating precast assembly (PA) connection between the pipe pile and cap beam. Cyclic loading tests were conducted to evaluate the failure mode, lateral capacity, ductility, energy dissipation ability, residual displacement, rebar strain, curvature distribution and rotation of UHPC pipe piles with the two connection forms. The results indicate that the specimen with a CIP connection exhibits a higher horizontal load capacity and stronger energy dissipation ability, while the specimen with the PA connection displays superior self-centering ability, increased ductility, and causes less damage to the cap beam. Finally, finite element models were developed to analyze the effects of design parameters on the seismic performance of the pile connected by the two methods. This research may provide valuable design guidance for incorporating UHPC in pile foundations. To facilitate its practical implementation in engineering projects, further theoretical and experimental research is recommended in this paper.

Seismic rehabilitation of RC frames with innovative precast U-shaped UHPC jackets: Experimental evaluation and computational simulation

The rehabilitation of post-earthquake reinforced concrete (RC) frames is crucial for both restoring structural integrity and enhancing seismic resilience against future seismic events. Ultra-high-performance concrete (UHPC) is distinguished by its exceptional strength, crack-control capabilities, ductility, and durability. Despite numerous studies on its use for structural retrofitting, its application in rehabilitating damaged RC frames remains limited. This study introduced a novel precast U-shaped UHPC jacket designed for the post-earthquake rehabilitation of RC frames. The jacketing method promotes composite action in rehabilitated members through lap splicing and dowel bars, complemented by cast-in-place connections using UHPC to minimize splice length and strengthen potential weak areas under external loads. The efficacy of this jacketing approach was rigorously evaluated through cyclic loading experiments on two RC frame specimens. Results indicated that the UHPC rehabilitation method not only recovered but significantly enhanced the seismic performance of the previously damaged frame, improving initial stiffness, yield strength, peak strength, displacement ductility, and energy dissipation capacity. However, the rehabilitated frame showed a reduced yield drift and increased residual displacement compared to the as-built frame. The experimental results also revealed the necessity of accounting for bond slip in structural rehabilitation analysis and design to prevent overestimating lateral stiffness. Furthermore, the study developed a computational model that incorporates flexure, bond slip, and shear responses, aiming to effectively simulate the load-displacement behavior of rehabilitated RC frames with reasonable accuracy.

Analysis of Different Early Strength Agents on the Performance of Prefabricated UHPC.

Precast ultra-high-performance concrete (UHPC) has emerged as indispensable in the engineering sector due to its cost-effectiveness and superior performance. Currently, precast UHPC grapples with challenges pertaining to slow setting times and insufficient early strength, largely attributed to its high water-reducing agent content. Effective utilization of early strength agents to augment UHPC's early strength is pivotal in addressing this issue. This study investigates the efficacy of two distinct concrete early strength agents, namely calcium formate (Ca(HCO2)2) and aluminum sulfate (Al2(SO4)3). A UHPC system with a water/cement ratio of 0.17 was used; both single and compound doping experiments were conducted using varied dosages of the aforementioned early strength agents. Our results show that both early strength agents significantly reduce setting time and enhance early strength at appropriate dosages. Specifically, the addition of 0.3% Ca(HCO2)2 led to a 33.07% decrease in setting time for UHPC. Moreover, the incorporation of 0.3% Ca(HCO2)2 and 0.5% Al2(SO4)3 resulted in a strength of 81.9 MPa at 1.5 days, representing a remarkable increase of 118.4%. It is noteworthy that excessive use of Ca(HCO2)2 inhibits the hydration process, whereas an abundance of Al2(SO4)3 diminishes the early strength effect. Simultaneously, this article provides recommendations regarding the dosage of two distinct early strength agents, offering a novel solution for expediting the production of prefabricated UHPC with a low water/cement ratio and high water-reducing agent content.

Experimental study on mechanical behavior of precast large-cantilevered UHPC bent caps with π-shaped cross-sections

To reduce the self-weight of the precast bent caps with large cantilevers commonly used in municipal bridge construction, an innovative thin-walled bent cap scheme made of ultra-high-performance concrete (UHPC) was proposed in this paper. Six scaled bent cap specimens, characterized by a scale ratio of 1:2.5, were subjected to static loading tests, encompassing both bending-shear and bending-shear-torsion combinations. Throughout the loading process, careful observations were made regarding the progressive development of crack patterns, stress distribution within various materials, and the identification of ultimate failure modes. It was observed that for the UHPC cap beams with small shear span-to-depth ratios, both the web shear cracks and flexural cracks were fully developed, leading to a combined shear and bending failure mode. With shear span-to-depth ratio increased, the effect of bending behavior became more and more dominate, resulting in an increased ductility. When subjected to a combined bending-shear-torsion loading, the UHPC bent caps exhibited conspicuous diagonal shear failure attributes, indicating that the torsion presented in the bent caps should be limited. In summary, the adoption of a π-shaped section for the UHPC bent caps with large cantilevers results in an impressive weight reduction of 40%∼50% compared to the conventional solid cap beam design. Moreover, the proposed design not only aligns with specifications in terms of cracking strength and ultimate bearing capacity but also facilitates monolithic precast, making it exceptionally well-suited for practical engineering applications.

Bending performance of prefabricated ultra-thin UHPC unit plate reinforced orthotropic steel bridge decks

To address the challenges related to lengthy construction period, complex maintenance requirement, and the elevated risk of shrinkage cracking associated with cast-in-place UHPC reinforcement of orthotropic steel bridge decks. This paper proposes a novel solution that prefabricated ultra-high-performance concrete (UHPC) slab with epoxy bond connection is used as a reinforcement layer for orthotropic steel bridge decks. Four sets of bending tests on composite bridge deck were carried out to compare the flexural performance of composite bridge decks under different joint forms and loading patterns. The results indicate that the precast UHPC decks delaminated from the epoxy bonding layer without failure of the epoxy layer itself in all cases. The positive bending capacity of the jointless composite bridge deck is approximately 27.67 kN, while the negative bending capacity is around 16.58 kN. For the composite bridge deckwith epoxy adhesive joints (EA-J-Ln), the negative bending capacity is 2.54 kN, and the negative bending capacity of the joint area reinforced with carbon fiber cloth (EA-JC-Ln) is increased to 4.17 kN. Therefore, the use of carbon fiber cloth can significantly improve the bending resistance of the joints. Finally, numerical model of the composite deck based on Cohesive Zone Model (CZM) was established, validating the applicability of this simulation method in the novel composite bridge deck.

Retrofitting RC beam-column joint subassemblies using UHPC jackets reinforced with high-strength steel mesh

Outdated building codes in seismic zones often did not require seismic detailing on beam-column joints, leaving many existing structures vulnerable to earthquakes. To address this issue, three different ultra-high-performance concrete (UHPC) jackets were proposed for retrofitting seismically-deficient exterior beam-column joints, with variables including construction method (cast-in-place or precast) and inclusion of high-strength steel bar meshes. The effectiveness of these jackets was investigated using cyclic loading tests on beam-column joint subassemblages. The results demonstrated that all three UHPC jackets considerably improved the strength, stiffness, and energy dissipation of the beam-column joint subassemblage, as well as both repairable and collapse drifts. However, the cast-in-place UHPC jacket, when not reinforced with steel meshes, failed to effectively improve the ultimate drift capacity of the original beam-column joint subassemblage. Moreover, retrofitting with precast UHPC jackets using chemical anchors failed to generate sufficient composite action. Only the cast-in-place UHPC jacket, when reinforced with high-strength steel meshes, successfully delayed localized joint shear damage and allowed for effective utilization of beam flexural capacity. It improved the joint shear strength by 66%, initial stiffness by approximately 35%, and total energy dissipation by 140%. Finally, strength models were suggested for evaluating the joint shear strength and beam flexural capacity of UHPC-retrofitted beam-column joints.

Flexural behaviors of a novel precast hollow UHPC composite beam reinforced with inverted T-shaped steel: Experimental investigation and theoretical analysis

To fully utilize the excellent mechanical properties and durability of ultra-high performance concrete (UHPC) as well as reduce the production cost, a novel UHPC encasing inverted T-steel (PUES) composite beam with hollow section is proposed in this paper. This paper aims to study the flexural behaviors of PUES beams, a bending test program consisted of one solid PUES beam and seven hollow PUES beams is carried out. The test parameters include thickness of inverted T-shape steel flange, tensile reinforcement ratio and stud spacing. The results showed that the PUES beams failed in the typical flexural failure mode and exhibited excellent ductility without obvious drop in load. Compared to composite beam with solid section, the hollow section had negligible influence on the flexural performance of PUES beams, while it saved 26 % UHPC used compared with the one with solid section. The increasing of the thickness of inverted T-shape steel flange and the reinforcement ratio effectively improved the stiffness and the flexural capacity of PUES beams, while the larger diameter of reinforcement incurred an obvious reduction of ductility. An excessively large stud spacing resulted in a poor composite behavior and a reduction of flexural capacity. Based on the analysis of the experimental results, calculating methods considering the shear connection degree between inverted T-steel and peripheral reinforced UHPC were developed to predict the ultimate flexural capacity of the hollow PUES beams. This study provides a reference for further research and practical engineering applications of this innovation composite beam.

Seismic performance of precast frame with UHPC composite beams and HSC columns under cyclic loadings

This research investigates the seismic performance of precast frame with ultra-high performance concrete (UHPC) composite beams and high-strength concrete (HSC) columns. An experimental study was conducted on two frame specimens with 1/2-scale, three-story, and two-bay: a precast UHPC-HSC composite frame (UHPC-HSC frame) and a fully cast-in-place concrete frame (RC frame). The precast UHPC-HSC frame consists of composite beams with precast U-shaped UHPC permanent beam formworks and cast-in-place beam cores, precast HSC columns, and cast-in-place UHPC connections. The results indicate that both frames exhibit a mixed sidesway mechanism with beam hinges dominating. The UHPC-HSC frame demonstrates better structural integrity and experiences smaller structural damage, making it more suitable for rapid repair after earthquakes. The hysteresis curves of both frames are plump. The load-carrying capacity of the UHPC-HSC frame is 15.78% higher than that of the RC frame. The UHPC-HSC frame exhibits better ductility and deformation capacity compared to the RC frame. The whole and interstory ductility coefficients of both frames are greater than 3.49. The energy dissipation capacity of the UHPC-HSC frame is superior to that of the RC frame. At θ = 4.5%, the cumulative energy dissipation value of the UHPC-HSC frame is 18.66% higher than that of the RC frame. The research results can provide a reference for the engineering application of precast UHPC-HSC frame with UHPC composite beams and HSC columns.

Numerical and experimental study of a segmentally prefabricated orthotropic steel–ultra-high-performance concrete composite deck for long-span cable-stayed bridges

Utilizing the high strengths, toughness, and durability of ultra-high-performance concrete (UHPC), a segmentally prefabricated orthotropic steel–UHPC composite deck (POSUCD) system has been developed for long-span cable-stayed bridges. The deck system combines an orthotropic steel deck (OSD) with a reinforced UHPC layer using notched perfobond strips (NPBLs). The UHPC layer is prefabricated onto the OSD of the girder segment in the factory to form the POSUCD. After the precast UHPC layer is cured, the girder segments are transported to the bridge site and assembled. The precast UHPC layers are then connected via transverse wet joints before tensioning stay cables. Against the background of the Danjiangkou Reservoir Bridge with a main span of 760 m, finite element (FE) analysis was conducted to examine the stress state and flexural tests were carried out on local full-scale models to verify the feasibility of POSUCD. The results show that the prefabricated UHPC layer reduces the maximum static compressive stress of the steel deck in the girder system by 21%, and the stress ranges at fatigue-prone details of the steel deck are decreased by 28%–90% compared to traditional OSDs with asphalt overlay. Since the prefabricated UHPC layer helps withstand dead axial load in the main girder, the high compressive strengths of UHPC can be utilized more effectively than in deck systems with UHPC only used as a cast-in-situ overlay to resist vehicular wheel loads. The transverse wet joint interfaces were identified as the weakest aspect of the POSUCD, whether under tension or compression; however, their nominal flexural strengths along the longitudinal direction meet the required design standards. The shear resistance of NPBLs was also well confirmed by the minimal interfacial slip on the specimen when it reached the ultimate capacity under positive bending moment.

Performance and life cycle of ultra-high performance concrete mixes containing oil refinery waste catalyst and steel fibre recovered from scrap tyre

An economic solution to recycle spent equilibrium catalyst (Ecat) disposed of by petrochemical manufacturers and steel fibre recovered from scrap tyre would be their use in the production of structural concrete. In this study, the performance and life cycle of ultra-high performance concrete (UHPC) mixes containing these waste materials was examined. They were made using a total binder content of 1160 kg/m3 and also contained fly ash and silica fume in addition to Ecat and cement. Their average compressive strength at the end of 28 days was found to be above 150 MPa. The replacement of cement with Ecat reduced the CO2 emission and material cost of recycled steel fibre (RSF) reinforced UHPC samples by 15% and 6%, but there is no significant change in their compressive, flexural and split tensile strengths at the end of 28 days. Only 2% of the elastic modulus of these samples increased as a result of the addition of Ecat by weight of the binder. Ecat, an aluminosilicate material, is generally classified as a pozzolanic material, but its addition together with fly ash and silica fume in the UHPC mixture did not affect the strength gain properties. Based on our experimental findings, the use of waste materials including Ecat and RSF in the preparation of precast UHPC elements for explosion and earthquake resistant structures is recommended.

Dynamic analysis of precast ultra-high performance concrete tunnel under internal explosion

Underground structures hold great significance in the infrastructure of modern society. With the rapid construction of such facilities, the possibility of explosions occurring inside these structures due to unforeseen accidents or deliberate acts cannot be ignored. Past catastrophic events have demonstrated the necessity of implementing anti-blast design for underground structures, particularly in vulnerable locations. This promotes investigations into the behavior of underground structures subjected to internal explosions. For the first time, a thorough simulation model is developed using the multi-material Coupled Eulerian–Lagrangian approach to examine a full-scale precast ultra-high performance concrete (UHPC) tunnel under internal explosion. The precast tunnel structure closely resembles real construction configurations. The simulation model takes into account the simultaneous interaction between the tunnel and the surrounding soil. The accuracy of the suggested simulation model is validated against experimental results. For various explosive charge weights, tunnel lining thicknesses, materials, and tunnel shapes, extensive parametric simulations are conducted. Results obtained highlighted UHPC's superiority as a substitute for conventional concrete due to its strong blast-resistant capacity. The findings from this research also shed light on the precast UHPC tunnel's structural response to an interior explosion, that can assist designers and managers choose the best design for blast protection.

Analytical study of splitting resistance of precast prestressed ultra-high performance concrete girder end zones

Ultra-high performance concrete (UHPC) has gained popularity gradually because of its superior mechanical properties, including high compressive and tensile strengths, and exceptional durability. Many researchers have explored the possibility of using long, precast prestressed UHPC girders in buildings and bridges. To do so, the girders generally require a large number of prestressing strands, which causes concern about potential cracking in the end zones at prestress release. Currently, there are no specifications or codes on end zone designs for precast prestressed UHPC girders in the United States. Thus, this paper presents an analytical study that explores highly prestressed UHPC girders’ end zone behavior. Finite element analyses were conducted to analyze the end zones of a series of UHPC girders, including one 889-mm-deep and two 813-mm-deep I-girders, and the analysis results were validated with test data. Additional finite element models were developed for other girders with heights of 1,829 mm and 2,743 mm using various reinforcement details. Bursting stress variations in the reinforcement and UHPC were reported based upon the analytical and test results. An equation for splitting resistance attributable to the reinforcement and UHPC was proposed for highly prestressed girders.

Full-Scale Experimental Study on Flexural Performance of the New Precast UHPC Diaphragm Slab in Utility Tunnels

In this paper, a bending test of a precast ultra-high-performance concrete (UHPC) diaphragm slab was carried out. The test revealed that the flexural failure process of specimens under the action of a positive bending moment can be divided into three stages: the elastic, crack-propagation, and yield stages. The first stiffness reduction of the structure was caused by cracks at the bottom of the diaphragm slab, while the second stiffness drop resulted from the yielding of the bottom longitudinal rebars. During the loading process, the ultimate bearing capacity was 3.75 times higher than the design load value (150 kN vs. 40 kN). Additionally, a nonlinear finite element model was established using Abaqus software validated by the test and exploiting parameter analysis. Based on this model, the initial crack stress of the actual slab was determined to be 5.12 MPa. Parameter analysis indicated that the shear strength of the diaphragm slab was stronger than the flexural strength, and the diaphragm slab’s bearing capacity could be improved by increasing the ratio of bottom longitudinal reinforcement. This research confirmed that the new UHPC diaphragm slab used in Guangzhou Smart City is safe, and it also helped the design of similar UHPC slabs for utility tunnels.

A novel seismic strengthening method for RC frames: Precast ultra-high performance concrete braces

A novel bracing method to strengthen the seismic behavior of reinforced concrete (RC) frames was developed in the study. The brace was made using steel reinforced ultra-high performance concrete (UHPC). UHPC is a class of tensile strain-hardening cementitious composites and has ultra-high compressive strength of greater than 120 MPa. To utilize the capacity of the UHPC brace prior to the onset of major inelastic response of the RC frame, a design method for the UHPC brace was established. In the developed strengthening method, the in-situ construction process was simplified by conveniently installing the precast UHPC brace using the UHPC connections that was constructed in-situ to link the UHPC brace to the frame. The performance of the UHPC bracing method was evaluated using cyclic loading tests on two frame specimens, i.e., a bare RC frame and a UHPC brace-strengthened RC frame. The test results showed that the UHPC brace effectively enhanced the initial stiffness, peak strength, and energy dissipation capacity of the original frame. Moreover, it successfully preserved the capacity of the columns when the UHPC brace was active without magnifying the shear demand in the columns. As a result, the strengthening method allowed the original columns to utilize their full force resisting capacity after the capacity of the UHPC brace was depleted. In addition to the experimental study, an analytical model, which was derived based on equilibrium, compatibility, and suitable material constitutive laws, was developed to reasonably evaluate the load-displacement response of RC frames strengthened with UHPC braces.

Mechanistic understanding of precast UHPC segmental beams with external tendons and epoxy joints subject to combined bending and shear

Precast ultra-high-performance concrete (UHPC) segmental bridges (PUSBs) have become a competitive bridge type in modern civil engineering. To establish a thorough mechanistic understanding of PUSBs, seven specimens were fabricated and experimentally investigated. These specimens were designed to have different longitudinal reinforcements, construction methods, joint types, shear span-to-depth ratios (λ), stirrup ratios, and numbers of shear keys. Two failure modes were observed in the tests: flexural failure with crushing of the UHPC at the joints, and shear failure with shear tension sliding and concrete spalling of the web. Additionally, it was found that although the ultimate capacity, stiffness, and cracking load of the segmental beams were slightly lower than those of the monolithic beams, the segmental beams tended to exhibit better ductility and deformation capacity. As λ increased, the ultimate capacity and stiffness of the segmental beams decreased, whereas the growth rates of the deflection and tendon stress increased. Moreover, the stirrups showed a positive effect on the cracking control ability and shear strength of the beams. The three-keyed specimen exhibited better structural behavior than the single-keyed specimen. Finally, calculation methods were proposed to predict the cracking strength and ultimate capacity of PUSBs.

Shear performance of high-strength friction-grip bolted shear connector in prefabricated steel–UHPC composite beams: Finite element modelling and parametric study

As a competitive alternative for accelerated bridge construction (ABC), prefabricated steel–ultra-high-performance concrete (UHPC) composite beams containing high-strength friction-grip bolt (HSFGB) shear connectors offer numerous advantages, including reduced on-site construction time and ease of replacing/removing deteriorated components. However, the failure mechanism of HSFGBs in UHPC remains unclear due to the lack of internal stress analysis, which hinders the design of these innovative composite beams. To clarify the shear performance of HSFGBs in prefabricated steel–UHPC composite beams, an effective finite element model (FEM) considering the non-linearities of materials and geometry was developed through ABAQUS. Based on the experimentally verified model, the internal stress transfer mechanisms of HSFGBs and the failure mechanism of precast UHPC were revealed. According to the extension parametric analysis results, a stronger HSFGB presented better shear performances in terms of ultimate shear strength, initial shear stiffness and slip capacity. Adopting oversized holes with appropriate bolt-to-hole clearance can improve the constructional efficiency without considerable strength and stiffness reduction. HSFGBs with low bolt pretension exhibited unfavorable initial shear stiffness, while smaller slip capacity in high bolt pretension conditions. A smaller ductility was observed as the steel beam tensile strength and slab concrete strength increased. Additionally, the ACI 318–19, Eurocode 3, and AASHTO LRFD specifications underestimated the shear strength of HSFGBs, whereas the Eurocode 4 presented acceptable predictions in determining the ultimate shear capacity of HSFGBs in prefabricated steel–UHPC composite beams.

Effects of stud aspect ratio and cover thickness on push-out performance of thin full-depth precast UHPC slabs with grouped short studs: Experimental evaluation and design considerations

Steel-precast ultra-high-performance concrete (UHPC) composite beams containing grouped short studs in thin full-depth slabs are an economical and eco-friendly alternative for accelerated bridge construction (ABC). Nevertheless, there is limited information on the push-out performance of thin full-depth precast UHPC slabs with grouped short studs. To this end, this study examined the impacts of stud heights disturbing from 35 mm to 85 mm, stud diameters ranging from 16 mm to 22 mm, and cover thicknesses varying from 15 mm to 65 mm on the push-out performances of such members. The test results indicated that increases in stud height yielded insignificant enhancements in the shear stiffness, strength, and ductility of grouped short studs embedded in thin full-depth precast UHPC slabs. Similarly, variations in the stud height and cover thickness exerted negligible influence on the structural performance. Notwithstanding, larger cover thicknesses or larger stud diameters allow the stud to exhibit attractive shear behaviors. Based on these results, a lower limit of 1.84 for stud aspect ratios (i.e. the ratio of stud height to stud diameter) and 15 mm for cover thickness were recommended. Finally, more accurate formulas were developed to predict the shear capacity of studs by capturing weld collar contributions and to simulate the load-slip relationship of grouped short studs embedded in thin full-depth precast UHPC slabs by considering the effects of slab thickness.

Longitudinal shear behavior and design method of UHPC connection for prefabricated slabs in composite beams

Ultra-high-performance concrete (UHPC) has advantages in mechanical properties and bond properties, which was available to connect the precast members of prefabricated composite structures. In this paper, some kinds of wet connections with post-cast UHPC applied in the composite beam with prefabricated components (slab and steel beam) were designed, and the longitudinal force-slip behaviors for the above UHPC connections were investigated through a series of push-out tests. In the case of the specimens with the post-cast UHPC rectangular connection, which failed in the interface between the precast slab and UHPC connection instead of the concrete region around the stud connectors, the increase in interface reinforcement ratio was beneficial for enhancing the interface shear resistance. The arrangement of two layers of steel bars out of the precast slab side could make full use of the plastic properties of bottom interface reinforcements to strengthen the ultimate shear strength and ductility of the interface. Compared with the rectangular connection, the post-cast UHPC connection with T-section presented a better interface performance by increasing the bond area, while the increase of the transverse width only worked in a limited range due to the interface shear lag effect. Based on the experimental and theoretical analyses, the formulas corresponding to different types of post-cast UHPC connections in prefabricated composite beams were proposed to calculate the longitudinal shear capacity, then the composition of equations and the reduction factors corresponding to different conditions were recommended.