Advances in Structural Engineering

Although the interface performance of prefabricated NC and post-cast UHPC (abbreviated as NC-UHPC) has been well investigated, only few studies focus on the interface performance of prefabricated UHPC and post-NC (abbreviated as UHPC-NC). The UHPC-NC interface appears when UHPC was prefabricated offsite and transported to the construction site as a permanent formwork to cast composite members. The present study conducts Z-shaped direct shear tests on the UHPC-NC specimens to investigate their interfacial shear resistance. The main interface treatments include smooth, chiseling, grooving, planting rebars, and combined grooving and planting rebars, among which the latter is a relatively new interfacial treatment for the UHPC-NC interface. Three typical failure modes, namely, pure interface failure, combined interface and NC shear failure, and NC splitting failure are identified from the test results. The interface roughness dominates the interfacial cracking load. The planted rebars significantly improve nominal shear resistance and ductility and show synergistic effect with grooving when the rebars are in the grooves. In accordance with failure mechanisms and existing codes, comprehensive modified calculation formulas for nominal shear resistance of the UHPC-NC interfaces are proposed, and the calculation results agree with test results well with relative errors less than 10%.

In order to improve structural fire-resistant behaviors, this paper designed a two-layer functionally graded ultra-high performance concrete (FGUHPC) structure composed of a UHPC layer and a lightweight aggregate concrete (LWAC) layer. UHPC layers are adopted to provide structural bearing capacity and protected by LWAC layers from elevated temperature. Splitting tensile tests and three-point flexural tests were conducted under ambient and elevated temperatures to evaluate interfacial bond performance and flexural bearing capacity, where two interfacial treatments were adopted and compared. The experimental results revealed that FGUHPC members exhibited good integrity during heating, no explosive spalling occurred and the maximal temperature at interfacial regions was 266°C. The interfaces showed desirable bond performance under ambient temperature while the splitting tensile strength was decreased by around 85% in the case of high temperature. Flexural test results indicated that the structural stiffness would be reduced by around 42% under elevated temperature, as a result, the maximal deflection was increased from 2.5 mm to 3.7 mm. SWM could significantly improve interfacial bond performance and prevent debonding failure of specimens at the postfire state, leading to higher structural bearing capacities. The bearing capacities of specimens with and without interfacial treatments were 42.7 kN and 38.4 kN respectively under ambient temperature, which remained about 88% after elevated temperature.

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.

With the addition of electrically conductive steel or carbon fibers, Ultra-High-Performance Concrete (UHPC) possesses an intrinsic self-sensing capability. This opens up the possibility of combining the resilience and sustainability of UHPC with the development of self-sensing solutions for structural applications. In this study, the self-sensing behaviour of a proprietary nano-engineered UHPC material subjected to tension was investigated. To assess the self-sensing performance of the material, bulk resistivity measurements were used on direct tension and pure flexure tests, while a novel wireless approach that operates on was used on out-of-plane bending tests. The wireless approach used alternate current (AC) measurements while the bulk resistivity methods were performed through direct current (DC) and the four-probe method. In both methods, the fractional change in resistance was correlated to the state of deformation. The disposition of the actual strain field was evaluated using Digital Image Correlation (DIC). It was found that in the case of direct tension and pure flexure, the fractional change of resistance was initially decreased up to the onset of strain localization, while it gradually increased in the post-peak range, where the separation of the localized crack gradually increased. In the case of the wireless approach using AC, the onset of cracking was successfully predicted with an abrupt increase in resistivity. The wireless strain-sensing approach also captured the Poisson’s effect due to the loading.

The design of end zones to prevent severe cracking is a key requirement toward durable prestressed girders. Traditional approaches employ large amounts of steel reinforcement to attain this requirement, which may result in steel congestion. This paper investigates a hybrid girder concept that uses Ultra-High-Performance Concrete (UHPC) and Carbon Fiber Reinforced Polymer (CFRP) bars to enhance crack control and long-term durability in the end zones of prestressed girders. The objectives of this study are to quantify the UHPC zone lengths needed to restrain end zone cracks, eliminate or reduce the required amount of steel reinforcement, and investigate its replacement with CFRP bars. A finite element modeling (FEM) approach was developed for this purpose and its fidelity was validated with experimental data. The FEM results showed that UHPC zones with lengths below or equal to half of the girder depth can significantly reduce end zone cracking. CFRP bars can be added in the UHPC zones as a conservative measure to crack control, to ensure the long-term corrosion resistance of the girders.

This paper focuses on the design as well as on the experimental and numerical validations of the mechanical behavior of a precast ultra-high performance fibre reinforced concrete (UHPFRC) retaining walls. The design, made in accordance with the Canadian Highway Bridge Design Code (CSA S6, 2019), led to the fabrication of a full-scale UHPFRC retaining wall with 3% fibre content which had dimensions of 3 m in height, 2 m in length, 2 m in width, with two vertical and horizontal stiffeners, and very thin vertical and horizontal panels of 40 and 65 mm, respectively. The experimental tests showed that the UHPFRC retaining wall exceeded by 42% the ultimate limit state (ULS) design factored bending moment and showed a very ductile behavior under flexural loading. At service limit state (SLS), the retaining wall had maximum crack opening between 0.15 and 0.28 mm, and a maximum lateral displacement of 4 mm. The finite-element model developed for the application captured accurately the flexural behavior of the UHPFRC retaining wall and was used later in parametric studies to optimize the design. The retaining wall optimal design includes UHPFRC with 3% fibre content and stiffeners with variable cross-section which allows volume reductions of 73% for concrete and 86% for rebars in comparison to the conventional reinforced concrete design.

This paper investigates the effectiveness of ultra-high performance concrete (UHPC) collars in strengthening reinforced concrete (RC) bridge piers against heavy tractor-trailer collisions through finite element (FE) analysis. First, validated FE models of UHPC and a heavy tractor-trailer were provided. Then, FE analyses were conducted to evaluate the strengthening performance of the UHPC collar. The effectiveness of UHPC collar was compared with conventional RC collar, and the effects of varying UHPC collar thickness, height, and collar reinforcement were investigated. The results showed that the most severe damage observed on bridge piers due to heavy vehicle collisions primarily occurred below a height of approximately 2000 mm, manifested as diagonal shear cracks and plastic hinges. Therefore, the recommended minimum collar height is 2000 mm. The comparison between UHPC collar and RC collar strengthening demonstrated the superior effectiveness of UHPC collars. A 130-mm UHPC collar exhibited a similar strengthening effect as an 180-mm RC collar. Among the three investigated parameters of UHPC collar thickness, height, and collar reinforcement, the study found that collar thickness had the most significant influence on the effectiveness of the UHPC collar in terms of damage pattern, energy absorption, and maximum deflection. While collar height primarily influenced deflection, a larger collar height was beneficial in reducing pier deflection at the end of the strengthened segment. Adding a small amount of collar reinforcement improved the performance of piers; however, this improvement was limited. The findings of this study address the lack of research on using UHPC for strengthening full-scale bridge piers against heavy tractor-trailer collisions and provide valuable references for future designs with similar applications.

Developments in concrete technologies have allowed engineers to design lightweight, slender, and aesthetically attractive structures. The application of new concrete types in real projects, however, is uncommon. The lack of regulation, uncertainty in material performance, and the absence of successfully implemented projects hinders the use of modern concretes in everyday design projects. The present paper examines the application of two specific concrete types in a prototype footbridge: ultra-high-performance concrete (UHPC), and biological self-healing concrete (BSHC). The material properties of UHPC were selected and tailored specifically for the prototype structure, applying the principles of performance-based design. To evaluate the efficiency of self-healing under real environmental conditions, BSHC beams were designed as a structural part of the bridge. The step-by step presentation of the bridge development demonstrates the specifics in material design and a structural analysis of the prototype structure. The prototype structure serves as demonstrative example of the use of BSH and UHP concretes, encouraging engineers towards the wider application of advanced materials in construction projects.

Ultra-High Performance Concrete (UHPC) is a new type of engineering material with high compressive strength, high tensile strength, and high fracture toughness. Its bending failure mechanism is different from that of traditional concrete beams, which requires a new computational model to describe the bending failure phenomena of the prestressed ultra-high performance concrete - reinforced concrete (UHPC-RC) beam without web reinforcement. Therefore, this paper, through full-scale tests on a 30m prestressed UHPC-RC beam without web reinforcement, captures unique bending failure phenomena, including initial cracking, development of local cracks, and rupture of prestressed steel strands. Considering the tension-compression constitutive relationship of UHPC material, an innovative computational model for bending bearing capacity is proposed. Based on this model, a study on the minimum reinforcement ratio of full prestressed-ordinary steel bars is conducted. The results show that in the bending failure of the prestressed UHPC-RC beam without web reinforcement, excessive tensile strain of steel strands will occur at the local crack location. At this time, the structure does not satisfy the assumption of plane sections, and the introduction of the calculation model of the limit state of external prestressed tendons can effectively match this model, which is highly consistent with the experimental results. The minimum reinforcement ratio of full prestressed-ordinary steel bars is revised to the auxiliary reinforcement ratio of full prestressed-ordinary steel bars, quantifying the minimum reinforcement requirements of ordinary steel bars. The research results of this paper can provide reference for the next step of theoretical research.

This study investigates the response of small-scale, reinforced ultra high performance concrete beams. Seven specimens were cast and experimentally tested. Steel fiber volume fraction, amount of longitudinal and transverse steel reinforcement, and casting direction varied between specimens. High longitudinal reinforcement ratios of 4.1% and 9.5% were expected to drive relatively large shear demands and low steel fiber volume fractions of 0.5% and 1% were expected to decrease the UHPC’s ability to resist tension, shear, and compression. Specimens were either cast at one end or mid-span. All specimens failed in shear, as expected. R/UHPC beams with only 0.5% fibers and a stirrup spacing equal to half the beam’s depth carried more ultimate load than beams with 1% fibers and twice the stirrup spacing. At a 0.5% fiber volume fraction, UHPC performed well, exhibiting fiber bridging in tension and resisting spalling in compression. As longitudinal reinforcement ratio increased from 4.1% to 9.5%, load carrying capacity generally increased, but not proportional to the increase in steel. Placement method did not influence fiber orientation in the R/UHPC beams when measured in 2D planes transverse or longitudinal to the specimens’ major axis. The longitudinal and transverse reinforcement interrupted fiber flow and prevented significant fiber alignment; combined with the short span of the beams, these factors mitigated the influence of placement method.

The use of ultra-high-performance concrete (UHPC) allows for much smaller cross-sections compared to conventional reinforced concrete columns, which may make reinforced UHPC (R-UHPC) columns more susceptible to slenderness effects. Currently, there is no guideline in design standards for the slenderness limit of R-UHPC columns. This paper, therefore, attempts to develop a design provision for determining the slenderness limit of R-UHPC columns. Firstly, a numerical analytical model was proposed for predicting the load-deflection of R-UHPC columns under eccentric loading, which was validated by comparing its predictions with available experimental results from the available literature. Based on the validated model, a parametric study was then conducted to determine the key parameters affecting the slenderness limit of R-UHPC columns. It was found that the slenderness limit corresponding to the 5% strength reduction was sensitive to the ultimate compressive strain of UHPC, the tensile strength of UHPC, and the reinforcement ratio. On this basis, a design equation for the slenderness limit of R-UHPC columns in single curvature was statistically derived. Additionally, the slenderness limit for R-UHPC columns in non-sway frames was also proposed in a convenient form for design procedures.

The main objective of the transverse joint between prefabricated full-depth precast concrete deck panels is to prevent the relative vertical movement between the panels and to transfer the loads between adjacent panels without cracking. The most common failure mode for these types of joints during their service life are the interface cracks. Such cracks serve as a conduit for ingress of water into the superstructure, leading to further deterioration requiring frequent maintenance. UHPC is known for its high tensile and bond strengths and is often considered as a joint fill material that has the potential to make this connection more durable. This research has been initiated to investigate if closure joints cast using UHPC material can compete with the performance of monolithic deck systems. The experimental program consisted of two full-scale concrete bridge decks that were statically tested until failure. One bridge deck was a jointed panel deck (JPD) consisting of two half-size panels connected by UHPC closure joint, while the control deck was a full panel deck (FPD) cast as one monolithic specimen. Both decks were connected to the steel girders with UHPC shear pockets that contained four to six shear studs welded to the girder flange. The JPD had rectangular tapered shear pockets and the FPD had circular shaped pockets. The results indicated that the JPD and FPD reached the same ultimate loads, and both failed via concrete punching at the load point situated adjacent to the closure joint of JPD. The results demonstrated that using UHPC closure joint resulted in similar performance and behaviour under loading as the monolithic deck. The interfacial failure was not indicated even at ultimate loads. This confirms the ability of the UHPC closure joint to transfer the load to the adjacent panel. The results also indicated the use of circular or rectangular pockets led to similar behaviours.

In order to study the failure phenomenon and shear resistance performance of UHPC-NC beams without web reinforcement, shear resistance tests were carried out on 15 UHPC-NC beams without web reinforcement. The variation parameters were shear span ratio, web thickness, steel fiber content, and compressive stress level. The load-deflection curve, failure phenomenon and crack development of the test beam are analyzed. The test results show that the joint between the top plate and the bottom plate of UHPC-NC I-beam without web reinforcement is the weakest, and the cracked web is easy to be “cut off” at the joint; The “banding crack area” of web is closely related to the shear span ratio, and the width of ribbon area increases with the increase of shear span ratio. The shear capacity of UHPC-NC I-beam without web reinforcement decreases with the increase of shear-span ratio. The shear capacity increases with the increase of web thickness and compressive stress level. The shear capacity has little relationship with steel fiber content, and the initial stiffness of the structure can be improved by applying prestress. Finally, considering the influence of the top slab compression-shear zone and the bottom slab tension-shear zone on the shear capacity of UHPC-NC prestressed composite beams without web reinforcement, the corresponding calculation method of shear capacity is given based on the modified compression field theory and the idea of sub-item superposition, and the applicability of the calculation method is verified by the test results.