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

Abstract

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.

Keywords

cable-stayed bridge composite deck system ultra-high-performance concrete perfobond strip connector mechanical performance finite element analysis model test

Introduction

Optimal match of self-weight, strength and stiffness of a deck system in the main girder is a key consideration in the design of long-span cable-stayed bridges. The inadequate strength and durability of ordinary concrete decks will lead to a detrimental cycle of using thicker members and experiencing larger internal forces under dead loads in the main girder, and deteriorate the long-term performance of the bridge (Ma et al., 2023a). Orthotropic steel deck (OSD) is a relatively preferred deck structure with a lighter self-weight and superior bearing capacity; however, unsatisfactory fatigue resistance and durability due to OSD's low stiffness are exposed during long-term operation (Fisher et al., 2016; Di et al., 2021). While a composite deck structure that combines ordinary concrete with OSD offers higher stiffness, it still faces issues with the poor cracking resistance of ordinary concrete (Walter et al., 2007; Gara et al., 2013; Dieng et al., 2013).

Thanks to its exceptional strength, toughness, and durability (Shi et al., 2015), ultra-high-performance concrete (UHPC) has been used to reinforce the local stiffness of the OSDs under concentrated wheel loads, without significantly increasing the self-weight (Dieng et al., 2013; Shao et al., 2013). Initially, UHPC was utilized as a substitute for asphalt to resurface damaged OSDs (Zhu et al., 2018; Wang et al., 2020, 2021). Later, in some newly constructed bridges in China, the UHPC layer was cast monolithically after completing steel girder construction (Xue et al., 2020). Due to its significantly higher modulus compared to traditional asphalt, the contribution of UHPC layer to the entire bridge structure should not be omitted during design and construction. Considering the UHPC layer as a permanent structural member, new structures and forming ways of these orthotropic steel–UHPC composite decks (OSUCDs) are currently under development for its more extensive application (Shao et al., 2018a; Ma et al., 2023b).

Several studies have investigated the effects of OSUCDs on the design and construction of cable-stayed bridges. Shao et al. (2018b) examined the service state of OSUCD in a super-long-span cable-stayed bridge scheme with a main span of 1480 m and found that OSUCD with sufficient anti-cracking safety could effectively reduce the life cycle cost compared to traditional OSD. Dai et al. (2022) proposed a hybrid deck system by replacing OSD under outer heavy vehicle lines with OSUCD for a cable-stayed bridge scheme with a main span of 800 m. The corresponding analysis results showed that the flutter stability of the bridge with hybrid deck was improved due to a higher basic frequency of the torsional mode.

Based on a concept of material–structure–manufacture integration, a segmentally prefabricated orthotropic steel–UHPC composite deck (POSUCD) has been proposed for the long-span cable-stayed bridges by the authors (Tan et al., 2022). This deck system was formed by precasting a UHPC layer on the OSD of a steel girder segment in the factory, and the precast UHPC layers are connected through transverse wet joints. Since the UHPC layer and OSD act jointly as the top flange of a lightweight composite girder to withstand loads during the construction and operation phases, the contribution of the UHPC layer in POSUCD to the structural performance of the bridge is expected to be greater than that of existing OSUCDs. Recently, POSUCDs were applied to the Danjiangkou Reservoir Bridge, a newly-built cable-stayed bridge spanning the headwaters of the North Water Transfer Project in Hubei, China.

The quantitative structural contribution of UHPC and the mechanical properties of wet joints between precast UHPC layers are noteworthy topics that have not yet been thoroughly researched for POSUCD. Aiming at these topics and based on the Danjiangkou Reservoir Bridge, FE analysis was performed to identify how a POSUCD system acts in a long-span cable-stayed bridge, and flexural tests on local full-scale models were conducted to verify the feasibility of POSUCD with transverse wet joints considered.

Design aspects

The Danjiangkou Reservoir Bridge was designed as a four-lane highway bridge utilizing a partially earth-anchored structural system with a hybrid girder. Figure 1 describes the elevation and real picture of the bridge. The bridge has a main span of 760 m, and two side spans of 152 m in length. Half of the 24 pairs of stay cables in each side span are anchored to earth-anchored abutment. The prestressed concrete girders in side spans are fixed with the abutments, while the axial thermal deformation of the girder system is released at mid-span by an adaptive joint structure.

Figure 1.

Danjiangkou Reservoir Bridge. (a) Elevation. (b) Real bridge.

Figure 2 shows the general layout of the main span girder, which utilizes a Π-shaped cross-section comprising of a POSUCD and two steel boxes. The POSUCD with a standardized fabrication length of 16 m consists of an OSD and an 80 mm thick UHPC layer. A 40 mm thick Stone Mastic Asphalt (SMA) overlay is paved on UHPC to serve as a friction course.

Figure 2.

General layout of main span girder with a POSUCD. (a) Standard cross-section. (b) Schematic of POSUCD.

The steel panel of OSD is 12 mm thick. U ribs installed at a transverse spacing of 600 mm are 280 mm high and 8 mm thick. The primary and secondary diaphragms are 3 m and 0.8 m tall, respectively, and are staggered on the OSD at a spacing of 2 m. The OSD adopts Q345 steel with a nominal yield strength of 345 MPa, whereas the UHPC layer is designed with a compressive strength grade of 140 MPa.

The UHPC layer is reinforced with 10 mm and 12 mm diameter distributed reinforcing bars at a central spacing of 50 mm in the longitudinal and transverse directions, respectively. The clear thickness of the UHPC cover was 16 mm. Notched perfobond strip connectors (NPBLs) with a low-profile height are used to achieve a stiffer shear connection between the OSD and the UHPC layer. Herein, discrete NPBLs that measure 400 mm in length, 50 mm in height, and 10 mm in thickness are used here to avoid adverse thermal deformation caused by continuous welding of long strips. The NPBLs are spaced at standard intervals of 600 mm and 500 mm in the transverse and longitudinal directions, respectively. Four 30 mm diameter circular holes are drilled on each NPBL at 100 mm intervals and perforated with 10 mm diameter ribbed rebars. The perforating rebars can not only improve the bearing capacity and ductility of NPBLs along the longitudinal direction (Liu et al., 2023), but improve the transverse flexural performance of composite deck. The NPBLs can also support the reinforcement mesh and the notches on them can facilitate its positioning.

The crack resistance of wet joint interfaces between the prefabricated and cast-in-situ UHPC under tensile stress emerges as a critical factor determining the POSUCDs’ performance (Pan et al., 2016). Herein, a rectangular-tooth-shaped wet joint with NPBLs installed in dense spacing of 300 mm (Tan et al., 2022) is used, as shown in Figure 3. The clear width of wet joints is 800 mm, and the height and width of the rectangular teeth are 200 mm and 350 mm, respectively.

Figure 3.

Rectangular-tooth-shaped wet joints between precast UHPC layers. (a) General schematic. (b) Plane view. (c) Cross-section.

Construction aspects

Figure 4 depict the specific construction process of the POSUCD in the Danjiangkou Reservoir Bridge. In the factory, the prefabricated OSD components complete with shear connectors were assembled with two steel boxes to form a girder segment. An automatic paver was used to cast the UHPC layer onto the OSD, which was then steam-cured for 2 days at approximately 90°C to prevent early-age shrinkage cracking. Once cured, the girder segments were transported to the bridge site for assembly, where the precast UHPC layers were connected via transverse wet joints. Prior to casting the wet joints, the joint interfaces were roughened by electric picks and high-pressure water jet to expose 1/3–2/3 of the original length of the steel fibers. Steam curing with a small steam shed was also applied to the wet joints to advance their strengths. The assembly of the next girder segment was carried out simultaneously with the construction of the previous wet joint. The construction of each girder segment took an average of 4 days on site, which is comparable to the conventional construction period of an all-steel girder. Figure 4(i) shows the completed state of wet joint after curing, and visually, the boundary between the new and old UHPC is not distinct. The SMA friction course was paved after roughening the top surface of the UHPC layer by mobile shot blasting machines. Prior to traffic opening, a loading test was conducted with heavy vehicles spanning the structure to verify the working state of the completed bridge.

Figure 4.

Real pictures of bridge construction. (a) Components of OSDs. (b) Steel reinforcement preparation. (c) Prefabrication of UHPC layer. (d) Girder segment transportation. (e) Hoisting of girder segment. (f) Wet joint interfacial roughening. (g) Steel reinforcement in wet joint. (h) Wet joint casting and curing. (i) Completed state of wet joint. (j) Loading test on completed bridge.

Analysis on structural performance

Static stress state

The nominal stresses in POSUCD system in the Danjiangkou Reservoir Bridge were analyzed using the finite element (FE) approach. For design convenience, the stress state of OSUCD can be decomposed in three systems: the deck as part of the main girder (i.e., the girder system noted as System I), the stiffened composite deck supported by diaphragms (noted as System II), and the composite panel acting between longitudinal ribs (noted as System III) (Luo et al., 2019).

Herein, the stress state of the System I was determined using the full bridge model developed by Midas Civil, a specialized FE analysis package for bridge structures, as shown in Figure 5. The pylons were modeled using three-dimensional (3D) beam elements and the cables were simulated by link elements considering the sag effect. The composite girder was modeled as an assembly of parallel 3D beam elements (Pedro et al., 2010), which simulated the steel girder and UHPC layer, respectively, and were linked by multi-row flexible couplings with longitudinal and transverse shear stiffnesses k_x and k_y to simulate the connection. Herein, neither uplift nor relative rotation of the UHPC layer to the OSD is allowed. The values of k_x and k_y define the ratios of shear capacity of a connector to its equivalent slip, correspondingly, with regard to the longitudinal and transverse directions. Neglecting the effect of notches, the shear bearing capacities V_ux and V_uy of NPBLs can be predicted using the existing models (Oguejiofor et al., 1997; Kang et al., 2014; Zhao et al., 2017). For a single-hole NPBL, the average longitudinal shear stiffness amounts to 113.2 kN/mm with an average shear capacity of 113.2 kN at the equivalent slip of 1.0 mm, while the average transverse shear stiffness is 318.5 kN/mm with an average shear capacity of 223.0 kN at the equivalent slip of 0.7 mm.

Figure 5.

FE model of the full bridge. (a) Model at different stages. (b) Parallel beam elements. (c) Model verification.

The elastic modulus, linear thermal expansion coefficient, and Poisson's ratio of the materials are entered following the specific elastic material type. Table 1 provides the relevant values for the two main materials of POSUCD. The designed material properties of POSUCD listed in Table 1 and the following loading conditions are specified in the relevant specifications and literature (Ministry of Transport of the People’s Republic of China, 2015a, 2015b, 2018a; Shao, 2016). The global traffic consists of six lanes. In each lane, a combination of a uniformly distributed load of 10.5 kN/m and a concentrated load of 360 kN is applied. The maximum lateral wind load is 25 m/s when combined with the traffic loads. The temperature effect includes a uniform environmental temperature change and local gradient heating for the main girder. The uniform temperature change is assumed to be ±30°C for the steel stay cables and the composite girder with POSUCD, and ±20°C for the other members.

Table 1.

Designed material properties.

Material	Strength grade/MPa	Density/(kg/m³)	Elastic modulus/GPa	Poisson ratio	Thermal expansion coefficient/(/°C)	Characteristic value of axial compressive strength/MPa	Characteristic value of axial tensile strength/MPa
UHPC	140	2600	42	0.2	1.1 × 10⁻⁵	98	9
Q345	345	7850	206	0.3	1.2 × 10⁻⁵	345	345

The accuracy of the FE model was verified by the measured deflection response of the main girder under design traffic loads obtained in the completed bridge loading test shown in Figure 4(h).

Figure 6 shows the geometry design of comparative deck schemes. The traditional OSD scheme entails a 75 mm thick asphalt overlay, while the OSUCD scheme involves a 55 mm thick UHPC layer that is cast as an overlay on site subsequent to the closing of the main girder (Wang et al., 2021). However, the POSUCD scheme suggests utilizing a thicker UHPC layer with an equal thickness instead, since it is responsible for the dead load in System I and facilitates segmentation prefabrication. The design of the steel structures remains consistent across all schemes. Note that Ministry of Transport of the People’s Republic of China (2015b) has mandated a minimum thickness of 14 mm for the steel panels in the OSDs paved with asphalt since 2015. Herein, to analyze how the UHPC layer impacts the stress state of steel decks, the traditional OSD scheme still employs a 12 mm thick steel panel that was commonly used in the last decade.

Figure 6.

Geometry design of comparative deck schemes. (a) Traditional OSD. (b) OSUCD. (c) POSUCD.

Figure 7 depicts the stresses of the decks in the System I. Upon completion of the bridge, owing to the axial force exerted by the stay cables, the UHPC layer in the POSUCD is compressed with a maximum compressive stress of −19.9 MPa near the pylons. During the service phase, the UHPC layer in the POSUCD is subjected to a maximum longitudinal tensile stress of 7.1 MPa in the non-stayed cable zone at mid-span and a maximum longitudinal compressive stress of −32.0 MPa near pylons under the combined action of dead loads, global traffic, and varying temperatures.

Figure 7.

Nominal stresses of the deck in the System I. (a) Completed state of OSD. (b) Completed state of UHPC layer. (c) Service state of OSD. (d) Service state of UHPC layer.

Compared to the traditional OSD and OSUCD schemes, the maximum compressive stress in the System I for the steel deck of POSUCD is decreased by 21% and 15%, respectively. The primary reason is that the UHPC layer in POSUCD shares 20%–25% of the dead axial force in the main girder near pylons. Furthermore, the region where the UHPC layer experiences tensile stress in the System I is reduced by about 75% compared to the OSUCD. Only 13% of the total number of wet joints in the POSUCD will be subjected to longitudinal tensile stress in the System I.

The stresses in the POSUCD in the Systems II and III under the vehicular wheel loads were determined using the local FE model, which is a 1/2 girder segment established by ANSYS, as illustrated in Figure 8. The steel structure and UHPC layer were simulated by Shell63 and Solid65 elements, respectively. The boundary conditions of the local FE model are as follows. The nodes at the longitudinal ends towards the pylon side were restricted from longitudinal translation and rotation around vertical axle. Symmetric constraints were applied to the nodes on the transverse symmetric plane. Nodes situated in close proximity to the anchorages of stay cables were supported by rigid vertical supports (Shao et al., 2018a). The analysis results of the local FE model with such boundary conditions are anticipated to be conservative for the stresses in UHPC layer. For the designed static stress state, three standard vehicles specified in Chinese code (Ministry of Transport of the People’s Republic of China, 2015a), each equipped with five axles and having a total weight of 550 kN, were exerted to the deck.

Figure 8.

FE model of girder segment. (a) Segmental girder model. (b) Plan view of standard vehicle. (c) Elevation of standard vehicle. (d) Transverse location of standard vehicle. (e) Longitudinal location of standard vehicle.

Table 2 summarizes the maximum static stresses in the UHPC layer under the combined action of dead loads, global traffic, temperature and local vehicular wheel loads by superimposing the calculated results of the two FE models. The results reveal that the design of the UHPC layer in the POSUCD is governed by the longitudinal compressive stress and transverse tensile stress with corresponding maximum values of −48.0 MPa and 14.1 MPa, respectively. To avoid nonlinear creep, the compressive stress in concrete members is generally required to be less than 0.5f_ck, where f_ck is the characteristic value of compressive strength of the concrete (Ministry of Transport of the People’s Republic of China, 2018b). Herein, the maximum compressive stress of UHPC reaches 98.2% of 0.5f_ck.

Table 2.

Static stresses in UHPC layer.

Deck scheme	Maximum nominal stress/MPa	UHPC layer
		Monolithic part		Wet joint
		Transverse	Longitudinal	Transverse	Longitudinal
OSUCD	Tensile	15.1	14.5	——	——
OSUCD	Compressive	−22.7	−27.5	——	——
POSUCD	Tensile	14.2	10.2	10.1	6.2
POSUCD	Compressive	−21.0	−48.0	−13.3	−41.3

Considering that the maximum tensile stress in the UHPC layer is beyond the characteristic value of tensile strength of the UHPC f_ct, the crack widths need to be verified. Herein, the nominal tensile strength corresponding to the maximum allowable crack width at the top surface is used to evaluate the crack resistance of the UHPC layer. Since cracks narrower than 0.05 mm are difficult to be observed with the naked eye (Makita et al., 2014; Zhang et al., 2020) and have hardly any detriment to the durability of UHPC (Rafiee, 2012), the value of 0.05 mm is regarded as the initial visible crack width and is usually used as an allowable crack width in the design of OSUCDs for highway bridges in China (Luo et al., 2019). For POSUCD with the UHPC layer designed as a permanent member, the requirements for crack resistance should not be reduced to prevent harmful cracks at wet joint interfaces in long-term service. Fang et al. (2022) reported the crack resistance of the POSUCD in the transverse direction. The nominal cracking strength of the UHPC layer at a maximum crack width of 0.05 mm is 18 MPa with the presence of local transverse wet joint interfaces. As such, the maximum transverse tensile stress in the UHPC layer reaches 78.3% of the nominal cracking strength. Compared to the existing OSUCD scheme, the POSUCD features a prefabricated UHPC layer that helps resist the dead load of the main girder rather than just serving as an overlay for the deck system to withstand vehicular wheel loads. Therefore, a satisfactory utilization of the compressive and tensile strengths of the UHPC is expected for the POSUCD scheme.

Fatigue stress ranges

For the fatigue assessment of steel deck in POSUCD, the FE model of 1/2 girder segment with local mesh densification near the OSD was used. Figure 9 depicts the sketch of vehicular wheel load cases. A fatigue vehicle, featuring four axles with a weight of 120 kN each, was utilized (Ministry of Transport of the People’s Republic of China, 2015a). The tire contact area in each axle measures 200 × 600 mm². Owing to the fact that vehicle-induced fatigue stresses in OSD exhibit a strong local response, only the double-rear-axle tandem loads of a fatigue vehicle (see Figures 9(a) and (b)) were exerted to the bridge deck with the superimposition effect of the other axes ignored (Shao et al., 2018a).

Figure 9.

Sketch of vehicular wheel load cases for fatigue assessment. (a) Plan view of fatigue vehicle. (b) Elevation of fatigue vehicle. (c) Transverse location of fatigue vehicle.

Based on the relative position between the wheels and U-shaped ribs, three typical load cases were considered in the transverse direction as shown in Figure 9(c).

Figure 10 shows the typical fatigue-prone details in OSD and the corresponding analysis results of maximum stress ranges. The six fatigue-prone details include: detail I and II at rib-to-deck welded connection near the location of fatigue crack initiation in steel panel and U-shaped rib, respectively; detail III and IV at rib-to-diaphragm welded connection near the location of fatigue crack initiation in U-shaped rib and diaphragm, respectively; detail V at the free edge of the cut-out on the diaphragm; and detail VI at longitudinal splice joints of the U-shaped ribs (Kolstein, 2007; Hobbacher, 2016). The maximum stress range is defined as the difference between the maximum and minimum stresses during the most unfavorable stress history for each fatigue-prone detail. Herein, the hot-spot stress method based on the surface extrapolation was used for the fatigue-prone details I to IV, whereas the nominal stress method was adopted for the details V to VI (Shao et al., 2018a). The stress ranges at the six fatigue-prone details of POSUCD are 28%–90% lower than those of the traditional OSD scheme. These results are comparable to Shao's analysis results (Shao et al., 2018a) on the reduction of fatigue stress range at fatigue-prone details of orthotropic steel deck led by the UHPC layer (i.e., 13%–87% for conventional U ribs and 28%–86% for large U ribs).

Figure 10.

Analysis results of fatigue stress ranges. (a) Order of typical fatigue-prone details. (b) Corresponding fatigue stress ranges.

The significant reduction in stress ranges is achieved by a larger local stiffness under the composite action of 80 mm thick UHPC layer to the steel deck. The constant amplitude fatigue limit strength at two million cycles is 90 MPa for the fatigue-prone details I to IV and VI, and it is 125 MPa for the detail V without a welding joint (Ministry of Transport of the People’s Republic of China, 2015b; Shao et al., 2018a).

The original OSD scheme needs to be reconsidered with a focus on fatigue performance because the maximum stress ranges at the fatigue-prone details I to III exceed the corresponding fatigue strengths. However, the utilization of the UHPC layer in the POSUCD produces a notable decrease in fatigue stress ranges and preserves them within the counterpart fatigue strengths, eliminating the prominence of fatigue issues.

Shear resistance of interfacial connection

To determine the maximum shear force that the connectors will experience, each hole of NPBLs was simulated by a spring element Combine39 in the local FE model. Table 3 shows the evaluation results for the shear resistance of NPBLs. For each hole of NPBLs, the ratio of shear capacity to the corresponding maximum shear force is 4.0 and 12.3 along the longitudinal and transverse directions, respectively.

Table 3.

Evaluation result of shear resistance of NPBLs.

Shear connector	Longitudinal direction			Transverse direction
Shear connector	Maximum shear load V/kN	Shear capacity V_u/kN	V_u/V	Maximum shear load V/kN	Shear capacity V_u/kN	V_u/V
Each hole of NPBL	28.6	113.2	4.0	18.1	223.0	12.3

The average shear strength was used as an index to compare the shear resistance of NPBL-based shear connections and commonly-used short headed studs. Based on Li’s prediction models (2021), the average shear strength is 1.16 MPa for short headed studs with dimensions of 13 mm × 60 mm in diameter and height, respectively, and spaced at intervals of 200 mm × 200 mm. The average shear strengths of NPBLs are 3.02 MPa and 5.95 MPa along the longitudinal and transverse directions, representing 2.60 and 5.11 times the average shear strength of group studs, respectively. As such, the shear resistance of NPBL-based shear connections is well substantiated.

Local full-scale model test

Experiment program

To confirm the practicability of the design and construction process for transverse wet joints connecting precast UHPC layers, a local full-scale POSUCD model was fabricated and tested in the factory. Figure 11 presents the geometry of the POSUCD model, which is 1/4 of the deck in a standard girder segment.

Figure 11.

Geometric layout of test models (unit: mm). (a) Cross-section of local full-scale model. (b) Plan view of strip specimen. (c) Elevation of strip specimen. (d) Cross-section of strip specimen.

The width and length of this POSUCD model are 13.4 m and 8 m, respectively. The model was manufactured using the same materials, equipment and fabrication processes as the actual bridge project. After the model was completed, it was cut into strip specimens along the driving direction. Table 4 provides the test parameters of strip specimens.

Table 4.

Parameters of strip specimens.

Specimen order	Loading mode	Steel panel thickness/mm	Height/thickness of U rib/mm	Diaphragm spacing/mm	Thickness of UHPC/mm	Diameter/spacing of longitudinal reinforcement/mm
I	Negative bending moment	12	280/8	2000	80	10/50
II	Positive bending moment	12	280/8	2000	80	10/50

The strip specimens were tested under a four-point bending load. Herein, longitudinal flexural tests were carried out to examine the strengths of transverse wet joints. Figure 12 exhibits the flexural test setup. Some strip specimens were subjected to the negative bending moment, while others were flipped to experience the positive bending moment. A force sensor was used to obtain real-time loads. The deflection and relative slip at the interface between the steel panel and the UHPC layer were measured by linear variable displacement transducers (LVDTs). Crack widths on the UHPC layer were measured using a specialized crack observation instrument. The flexural tests were performed using a force-displacement controlled procedure.

Figure 12.

Flexural test setup. (a) Loading with negative bending moment. (b) Loading with positive bending moment.

Material properties

The UHPC adopted in both the local full-scale model and the actual bridge deck is a commercially available product with a compressive strength grade of 140 MPa. Table 5 lists the mixing proportion of the UHPC. Two types of fine copper-plated steel fibers were added to the UHPC. One was straight fiber in diameter and length of 0.2 and 13 mm, respectively. The other was hook-ended fiber in diameter and length of 0.22 mm and 14 mm, respectively. The volume fractions of straight fibers and hook-ended fibers are 1.5% and 1.0%, respectively, resulting in a total volume fraction of 2.5%.

Table 5.

Mixing proportion of the UHPC.

Concrete type	Relative weight ratio of each component to cement							Volume fraction of steel fibers (%)
Concrete type	Cement	Silica fume	Mineral powder	Quartz sand	Fly ash	Water reducer	Water	Volume fraction of steel fibers (%)
UHPC	1.000	0.100	0.075	1.388	0.050	0.009	0.225	2.5

To ensure that the UHPC produced by the above mixing proportion possesses the design-required mechanical performance specified in Table 1, a series of material tests were conducted. The cubic compressive strength f_cu was obtained through 100 mm cube samples, whereas the prismatic compressive strength f_c and the compressive elastic modulus E_c were determined using prisms with sizes of 100 × 100 × 300 mm³. Small beams with dimensions of 100 × 100 × 400 mm³ were subjected to four-point bending tests to measure the flexural strength f_ft. The axial tensile behavior of the UHPC was identified by tensile tests on dog-bone shaped specimens (Wang et al., 2017) as shown in Figure 13. All the material samples were prepared during the fabrication process of the local full-scale POSUCD model and cured in the identical environment. Table 6 provides the mean values of measured properties of the UHPC. The mean cubic compressive strength of the UHPC is 149.2 MPa with a coefficient of variation of 0.05, which meets the requirements for evaluating concrete strength grade in the Chinese standard (Ministry of Housing and Urban-Rural Development of the People’s Republic of China, 2010). The flexural strength and axial tensile strength of the UHPC are 28.1 and 11.2 MPa, respectively. These results indicate that the UHPC used in the model test and the real bridge have the mechanical properties not inferior to those used in the design analysis.

Figure 13.

Axial tensile test. (a) Geometry of specimen (unit: mm). (b) Test setup.

Table 6.

Mechanical properties of the UHPC.

Item	Cubic compressive strength f_cu/MPa	Prismatic compressive strength f_c/MPa	Compressive elastic modulus E_c/GPa	Flexural tensile strength f_ft/MPa	Peak tensile strength f_ct/MPa
UHPC	149.2	142.1	51.2	28.1	11.2

Test results and discussion

Load–deflection response

Figure 14 shows the relationship curves between the applied load and mid-span deflection. Under the negative bending moment, the deflection of the specimen increased linearly with the applied vertical load before the UHPC layer cracked. The cracks initiated at the outer interfaces of rectangular-tooth-shaped wet joint at 110.0 kN. When the load increased to 120.0 kN, fine cracks were also observed in the precast UHPC layer near the diaphragms and a slight nonlinearity could be found in the load–deflection relationship curve. With a further increase in the applied load, the stiffness of the specimen decreased with the propagation of cracks. When the load was close to the yield load (200.0 kN) that should be associated with the compressive yield developed at the bottom flange of the U-shaped rib, the number of cracks remained constant whereas the crack widths increased faster. After the yield load, an obvious yield platform could be observed.

Figure 14.

Load–deflection relationship curve. (a) Under negative bending moment. (b) Under positive bending moment.

The load–deflection response of the specimen under positive bending moment also started with an approximate linear elastic behavior. As the load increased to 255.0 kN, the specimen entered the elastic-plastic phase due to the gradual tensile yield of the bottom flange of the U-shaped rib. The load peaked at 510.9 kN with the local crushing of UHPC near the wet joint interfaces where the maximum surficial compressive strain of UHPC was measured at −5326.4 με, as monitored in real-time.

Furthermore, the elastic response obtained from the ANSYS model of strip specimens based on the localized FE modeling method shows good agreement with the experimental results, indicating the effectiveness of this modeling method for elastic stress analysis during the design of POSUCD.

Load–crack width response

Figure 15 illustrates the development of maximum crack width in the UHPC layer under increasing vertical load leading to negative bending moment. Two stages of crack width development can be observed. In the first stage, cracks propagated on the top surface of the UHPC layer and their maximum crack width grew in an approximate linear manner as the applied load increased. As the load increased further, the cracking state entered the second stage with the steel fibers pulled out at the critical cracks of which the crack width exhibited rapid growth. Compared to the monolithic UHPC layer in the pure bending segment, the crack width at the wet joint interface grew faster due to the lower post-cracking tensile strength of the local UHPC, which is attributed to the weakened bridging action of interfacial steel fibers (Pan et al., 2016; Tan et al., 2022).

Figure 15.

Load–maximum crack width relationship curve.

Damage mode of UHPC layer

Figure 16 specifies the damage mode of UHPC layer corresponding to the ultimate state of the specimens. For the specimen loaded with negative bending moment, Figure 16(a) shows the damage state of UHPC layer after yield load. Numerous fine cracks were found on the top surface of the UHPC layer, indicating an impressive crack resistance of the UHPC. Certain wide cracks extended from the top surface of the UHPC layer to the steel panel. The widest crack occurred at the outer wet joint interfaces. However, the difference in crack widths between the widest crack and other cracks was insignificant and hardly recognizable to the naked eye. The reason for this is that the steel fibers and dense steel reinforcement caused the cracks to close.

Figure 16.

Damage mode of UHPC layer. (a) Under negative bending moment. (b) Under positive bending moment.

For the specimen loaded with positive bending moment, Figure 16(b) illustrates the damage state of UHPC layer after peak load. The UHPC layer was crushed near the wet joint interfaces, indicating that the presence of wet joint also led to a decrease in the compressive strength of the local UHPC. This phenomenon could be attributed to the localized internal damage and reduced steel fiber content (Xiao et al., 2021) resulting from interfacial roughening. The aforementioned test results identify the wet joint interfaces as the weakest aspect in the UHPC layer of POSUCD, whether subjected to tension or compression.

Interfacial slip

Figure 17 demonstrates the test results of interfacial slip between the steel panel and the UHPC layer under the positive bending moment. At the peak load, the maximum interfacial slip is merely 0.098 mm, which is far less than the limited value of 0.2 mm in the relevant Chinese standard (Ministry of Transport of the People’s Republic of China, 2015c) for the maximum steel–concrete interfacial slip under the serviceability limit state. These results also demonstrate the shear safety of NPBLs in the proposed POSUCD.

Figure 17.

Load–interfacial slip relationship curve.

Nominal cracking strength

The nominal cracking strength of UHPC layer under negative bending moment can be formulated based on transformed section method (Pan et al., 2016) as follows:

σ_{n} = \frac{M y_{ct}}{n_{E} I_{0 t}}

(1)

where M denotes the applied bending moment (in N·mm); n_E denotes the elastic modulus ratio of steel to UHPC; y_ct denotes the distance from the top surface of UHPC layer to the centroid axis of transformed section (in mm); and I_0t denotes the moment of inertia of transformed steel section (in mm⁴).

Table 7 presents the test results for the nominal cracking strengths of the UHPC layer under negative bending moment. The strengths of the monolithic UHPC layer at initial cracking and the maximum crack width of 0.05 mm are 13.0 MPa and 18.2 MPa, respectively. For the wet joint interfaces, these two strengths are 11.5 MPa and 16.7 MPa, representing 88.5% and 91.8% of the corresponding values for the monolithic UHPC layer, respectively.

Table 7.

Test results of nominal cracking strengths of UHPC layer.

Cracking state	Monolithic part		Wet joint interface		Strength ratio
Cracking state	At initial cracking σ_e/MPa	At maximum crack width of 0.05 mm σ_nc/MPa	At initial cracking σ_e,j/MPa	At maximum crack width of 0.05 mm σ_nc,j/MPa	σ_e,j/σ_e	σ_nc,j/σ_nc
Nominal strength	13.0	18.2	11.5	16.7	0.885	0.918

Highlighting the maximum crack width of 0.05 mm, the monolithic UHPC layer and wet joint interfaces demonstrate nominal cracking strengths of 1.78 and 2.69 times the corresponding maximum longitudinal tensile stresses caused by the design loads (10.2 MPa and 6.2 MPa), respectively. Therefore, the test results indicate that the UHPC layer in the POSUCD can fulfill the design requirement for a crack resistance safety factor of at least 1.5, even with the presence of transverse wet joints.

To furtherly examine the actual functionalities of the POSUCD, more topics on its mechanical behavior under local impact and different temperature fields, as well as the long-term performance of wet joints under shrinkage and creep, still need to be studied in the field.

Conclusions

This paper presents a numerical and experimental study on the mechanical performance of a segmentally prefabricated orthotropic steel–UHPC composite deck (POSUCD) for long-span cable-stayed bridges. Based on the current investigation, the following conclusions can be drawn:

(1) The POSUCD features a prefabricated UHPC layer that helps resist the dead load of the main girder rather than merely serving as an overlay for the deck system to withstand vehicular wheel loads. As such, the maximum static stresses in the UHPC layer reach 78.3% of the tensile limit and 98.2% of the compressive limit, indicating a satisfactory material utilization.

(2) Thanks to the presence of prefabricated UHPC layer, the maximum static compressive stress of the steel deck in the girder system is decreased by 21%, and the stress ranges at fatigue-prone details of steel deck are decreased by 28%–90% compared to traditional OSDs with asphalt overlay.

(3) The flexural test results show that the wet joint interfaces are the weakest links in the UHPC layer of POSUCD, either under tension or compression. The nominal strengths at initial cracking and the maximum crack width of 0.05 mm for wet joint interfaces are 11.5% and 8.2% lower than the corresponding values for monolithic UHPC layer, respectively. Despite the presence of wet joints, the crack resistance of the UHPC layer can meet the design requirements.

(4) The integration of the UHPC layer with the steel deck through the use of notched perfobond strips (NPBLs) is deemed reliable, evidenced by merely 0.1 mm measured slip at the steel–UHPC interface when the POSUCD specimen reached its flexural capacity under the positive bending moment.

(5) The present POSUCD was applied to the Danjiangkou Reservoir Bridge that has been completed and opened for traffic in December 2022. To date, the bridge is not only the world's largest partially earth-anchored cable-stayed bridge, but the world's largest cable-stayed bridge using POSUCD. It provides a case for certain similar applications, particularly for future cable-stayed bridges with a kilometer-scale span in the world.

Footnotes

Acknowledgements

The authors are grateful to the relevant institutions responsible for the design and construction of the Danjiangkou Reservoir Bridge for their supports in this study.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is sponsored by the National Natural Science Foundation of China (Grant No. 51938012).

ORCID iD

Zhi Fang

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