Behavior and strength of headed stud shear connectors in ultra-high performance concrete of composite bridges

Abstract

This study presents an experimental and numerical investigation on the static behavior of headed stud shear connectors in ultra-high performance concrete (UHPC) of composite bridges. Four push-out specimens were tested. It was found that no cracking, crushing or splitting was observed on the concrete slab, indicating that UHPC slab exhibited good performance and could resist the high force transferred from the headed studs. The numerical and experimental results indicated that the shear capacity is supposed to be composed of two parts stud shank shear contribution and concrete wedge block shear contribution. The stiffness increment of a stud in UHPC was at least 60% higher than that in normal strength concrete. Even if the stud height was reduced from 6d to 2d, there was no reduction in the shear strength of a stud. Short stud shear connectors with an aspect ratio as small as 2 could develop full strength in UHPC slabs. An empirical load-slip equation taking into account stud diameter was proposed to predict the load-slip response of a stud. The reliability and accuracy of the proposed load-slip equation was verified by the experimental and numerical load-slip curves.

Keywords

ultra-high performance concrete / studs / shear strength / FE analysis / push-out test

Introduction

In recent decades, steel and concrete composite bridges have been widely constructed in bridge engineering due to the economical and constructional advantages [¹–³]. Headed studs are commonly used as shear connectors of steel and concrete composite bridges to transfer longitudinal shear forces at the interface between concrete slab and steel girder [⁴–⁶].

However, cracks may appear during the construction and service stage of a bridge and could affect the shear studs mechanical performance [⁷]. To prevent the concrete slab cracking, ultra-high performance concrete (UHPC) has been introduced to the composite bridges recently [⁸–¹¹]. UHPC is a class of concrete with superior mechanical properties in terms of high strength and excellent durability [¹²,¹³]. High strength steel fibers are randomly dispersed to improve the tensile strength of the matrix [¹⁴]. As a result, concrete cracking could be effectively controlled.

Push-out tests are usually used as an efficient method to evaluate the static behavior of stud shear connectors due to its low cost and short duration. Numerous investigations had been conducted to study the static behavior of studs in normal strength and high strength concretes [⁴,⁶,¹⁵–¹⁷]. However, limited studies have been performed to investigate the static behavior of headed studs in UHPC. Rauscher and Hegger [¹⁸] investigated the static behavior of continuous shear connectors in UHPC and found that continuous shear connectors were capable of transferring high shear forces in UHPC. Based on the push-out test results, Kang et al. [¹⁹] indicated that higher concrete strength and larger number of holes increased the shear strength of perfobond shear connectors, and that higher increase rate in the shear strength was achieved by the dowel action. Luo et al. [²⁰] reported the test results of a push-out test on the performance of headed studs in a steel-fiber-reinforced cementitious composite (SFRCC) with a compressive strength of 120–150 MPa. A failure mode of stud fracture could be ensured for a SFRCC slab without any rebar. Kim et al. [¹⁰] conducted static push-out tests on headed studs in UHPC and pointed out that the aspect ratio could be reduced from 4 to 3.1 without loss of shear strength and no splitting crack occurred at the UHPC slab. Cao et al. [²¹] experimentally studied the static and fatigue behavior of short-headed studs embedded in UHPC. It was found that the fracture of the headed studs from the root caused the failure of the specimens and the UHPC layer was still in good condition. The short-headed studs therefore could develop full strength in UHPC. Wang et al. [²²] performed a series of tests on demountable headed stud shear connectors in steel-UHPC composite structures. They found that tensile failure due to UHPC breakout could occur if the stud aspect ratio was less than 1.5. It can be concluded that research in the shear behavior of headed stud shear connectors embedded in UHPC is limited and has not been investigated adequately. In fact, the deformation of studs in UHPC becomes smaller compared to those in normal strength concrete due to the relative higher elastic modulus of concrete. The stress distribution should be different for studs in UHPC and normal strength concrete. It is worth thinking whether existing specification and theoretical methods would be applicable to evaluate the shear behavior of studs embedded in UHPC. Therefore, more information is needed for the popularization and application of UHPC-steel composite bridges in engineering practice.

In this paper, an experimental and numerical study was carried out to investigate the static behavior of stud shear connectors embedded in UHPC. Four push-out specimens were tested. The shear strength equations in specifications were compared with the test results. The existing load-slip formulas were used to predict the push-out response of the test specimens and an empirical load-slip equation taking into account stud diameter was proposed to evaluate the static behavior of a stud. To further understand the effect of main parameters on the static behavior of headed stud shear connectors in UHPC, a parametric study was performed using the proposed finite element model.

Experimental Programs

UHPC mixture and properties

The mix design as well as the investigation of including coarse aggregates without reducing the mechanical properties had been completed by our cooperative research group in the State Key Laboratory of High Performance Civil Engineering Materials, Nanjing, China [²³]. Table 1 presents the ingredients of the optimized UHPC. Portland cement, silica fume and high active admixture “SBT^®-HDC(V) UHPC” which was developed by Sobute New Materials Company Limited were included as fine aggregate. Different from traditional UHPC, broken stone with the diameter ranged from 5 to 8 mm and the specific surface area of 2800 kg/m³ was included as coarse aggregate in order to increase the stiffness and decrease the shrinkage. River sand with 5 mm maximum particle size and 2.6 fineness modulus was used in the research. Hybrid high strength steel fibers with a volume fraction of 2% were dispersed randomly to improve the strength and ductility of UHPC matrix. The length and diameter of the steel fibers were 13 mm and 0.2 mm, respectively. Type I was straight fiber and Type II was end-hooked fiber.

Ten 100 mm × 100 mm × 100 mm cubic specimens and six 100 mm × 100 mm × 300 mm prism specimens were cast simultaneously as well as the push-out test specimens to evaluate material properties. The compressive strength of natural curing specimens was 146.7±3.6 MPa while the compressive strength of steam curing specimens was 165.7±3.7 MPa. The modulus of elasticity of natural curing specimens was 49.3±0.4 GPa while the modulus of elasticity of natural curing specimens was 50.4±0.2 GPa.

Specimens details

Four push-out specimens with identical geometrical dimension were fabricated and tested, as shown in Fig. 1. Two of the specimens (TX1 and TX2) were natural cured and another two specimens (TC1 and TC2) were steam cured. Hot-rolled I-shaped steel was used as the steel beam according to Chinese code GB/T11263-2010 [²⁴] and four studs were welded at each face because the specimens were prepared for a two-face push-out test. The thickness, width and height of concrete slab were 150, 600, and 650 mm. The concrete slab was reinforced by HRB400 steel rebar, which was commonly available in China market with a nominal yield strength of 400 MPa, in both longitudinal and lateral directions. The steel rebar had a diameter of 8 mm. The studs used in this study had a diameter of 22 mm and a height of 100 mm, resulting in a height-to-diameter ratio of 4.5.

Test setup and procedure

The prepared specimens were loaded using an electrohydraulic servo pressure testing machine with a load capacity of 3000 kN. The test setup for the push-out tests is illustrated in Fig. 2. Prior to testing, the specimen was loaded to a preloading of 10% ultimate load to examine the working performance of the machine. During testing, the load was applied with an increment of 10 kN until failure occurred. The longitudinal slip between the concrete slab and steel beam was measured and cracks, if any, were recorded at each loading step.

Test results and discussions

Mode of failure and load-slip curves

The studs were sheared off from the steel plate for all specimens and shank failure was observed in this study. Figure 3 shows the typical shank failure of the specimen regarding the steel surface and concrete surface. It can be seen that the headed studs were sheared off near the stud root and were still embedded in the concrete slab. It should be noted that no cracking, crushing or splitting was observed on the concrete slab. Only concrete spalling was observed within a small area around the headed stud root (Fig. 3). It can be concluded that UHPC slab exhibited good performance and could resist the high force transferred from the headed studs.

The load-slip curves of the test specimens are plotted in Fig. 4. Figure 4 reveals that the load-slip curves in each batch are quite close to each other and the curing method has no significant influence on the load-slip response. Three different stages are included in the load-slip curves: linear elastic stage, plastic stage and ultimate stage. The slip was very small in the elastic stage and the load-slip response remained a linear behavior. However, the slip increased fast in the plastic stage, and the stud shear stiffness decreased continuously. At the ultimate stage, the slip increased rapidly with a slight increment of the applied load and the studs were fractured at last. To satisfy ductility requirement, Eurocode 4 [²⁵] recommends the characteristic relative slip to be at least 6 mm in a push-out test. However, the ultimate slips in this study were not realized in all cases. Therefore, ductility improvement methods should be developed to satisfy the ductility requirement in the future study.

Shear strength and shear stiffness

Since there are no equations to calculate the shear strength of headed studs in UHPC, the equations for normal strength concrete are used here to predict the shear strength of the test specimens.

In AASHTO LRFD specifications [²⁶], the shear strength of one headed stud shear connector embedded in solid concrete is determined by

(1)P_{\mathrm{u}}={\varphi }_{\mathrm{s}\mathrm{c}}\mathrm{0.5}{A}_{\mathrm{s}}\sqrt{E_{\mathrm{c}}{f}_{\mathrm{c}}\prime }\le {\varphi }_{\mathrm{s}\mathrm{c}}{A}_{\mathrm{s}}{f}_{\mathrm{u}},

where f_sc is resistance factor for shear connectors and equals to 0.85; A_s is the cross-sectional area of the studs; E_c and f_c′ are the elastic modulus and compressive strength of concrete, respectively; f_u is the tensile strength of the studs.

According to Eurocode 4 [²⁵], the height-to-diameter ratio is taken into account and the shear strength of a headed stud shear connector shall be taken as

(2)P_{\mathrm{u}}=\mathrm{0.29}\alpha d^{\mathrm{2}}\sqrt{E_{\mathrm{c}}{f}_{\mathrm{c}}\text{'}}/{\gamma }_{\mathrm{v}}\le \mathrm{0.8}{A}_{\mathrm{s}}{f}_{\mathrm{u}}/{\gamma }_{\mathrm{v}},

where α is the factor considering the height-to-diameter ratio, α = 0.2(h/d + 1)≤1; h and d are the height and diameter of the studs, respectively; γ_v is the material partial factor and equals to 1.25.

As provided in the Chinese code GB50017-2003 [²⁷], the shear strength of a headed stud can be calculated by

(3)P_{\mathrm{u}}=\mathrm{0.43}{A}_{\mathrm{s}}\sqrt{E_{\mathrm{c}}{f}_{\mathrm{c}}}\le \mathrm{0.7}{A}_{\mathrm{s}}{f}_{\mathrm{u}},

where f_c is the compressive strength of concrete prisms.

Test observations indicated that the mechanical behavior of a headed stud shear connector in high-strength concrete is different from that in normal-strength concrete [²⁸,²⁹]. Doinghaus et al. [³⁰] pointed out that the shear strength of a headed stud was influenced by the weld collar and proposed a new design approach as expressed by Eq. (4)

(4)P_{\mathrm{u}}=(\mathrm{0.85}{A}_{\mathrm{s}}{f}_{\mathrm{u}}+\eta f_{\mathrm{c}}\text{'}{d}_{\mathrm{w}\mathrm{c}}{l}_{\mathrm{w}\mathrm{c}})/{\gamma }_{\mathrm{v}},

where η is the coefficient of shear strength improvement caused by the weld collar; d_wc and l_wc are the diameter and height of the weld collar, respectively; γ_v is the material partial factor and equals to 1.25. Based on the test results, Doinghaus et al. [³⁰] recommended the value of η be 1.5 for conventional high-strength concrete while Luo et al. [³¹] and Cao et al. [²¹] stated that the value of η should be 2.5 for high strength steel fiber reinforced concrete and UHPC. Therefore, both the two values of coefficient η (1.5 and 2.5) were evaluated. According to the Chinese code GB/T 10433-2002 [³²], the diameter and height of the weld collar are 29 and 6 mm, respectively.

Figure 5 presents test results and theoretical predictions of the shear strength of the headed studs. The predicted shear strength of a stud based on concrete failure mode is much higher than that based on stud failure mode. The shear strength predictions of the current specifications are approximately 36.6%–52.3% lower than the real shear strength obtained from the tests. This phenomenon indicates that the current design specifications over underestimate the shear strength of the headed studs embedded in UHPC, which is also confirmed by Cao et al. [²¹]. However, the shear strength predictions based Eq. (4) give a better agreement with the test results, especially when 2.5 is adopted for the coefficient η. Furthermore, it is also clearly that the predicted shear strength is still 22.4% lower than the tested shear strength even if η = 2.5. Thus, a more accurate and reliable equation needs to be proposed in the future work.

Shear stiffness is an important evaluation index for push-out test. The calculation of shear stiffness usually adopts the secant stiffness based on load-slip curve. Johnson and May [³³] suggested that the secant slop of load-slip curve at the half ultimate load could be used as shear stiffness. JSSC [³⁴] specifies the shear stiffness as the secant slop of load-slip curve when the load is one-third of the ultimate load. Eurocode 4 [²⁵] defines the shear stiffness as the secant slop of load-slip curve when the load is 70% of the ultimate load. The calculated shear stiffness by different methods of a single stud is presented in Table 2. Table 2 indicates that the calculated shear stiffness of test specimens is significantly influenced by the method adopted.

To compare the studs shear stiffness in UHPC and normal strength concrete, the load-slip equation proposed by Ollgaard et al. [³⁵] is used here to calculate the shear stiffness of a stud embedded in normal strength concrete

(5)F/P_{\mathrm{u}}={(\mathrm{1}-{\mathrm{e}}^{-\mathrm{18}S})}^{\mathrm{0.4}},

where F is the applied load; P_u is the shear strength of a single stud; S is the interfacial slip.

The shear stiffness calculation in Table 3 is based on the assumption that a headed stud embedded in normal strength concrete has the identical shear strength compared to that in UHPC. Note that the secant stiffness was used here as mentioned before. The shear stiffness of a stud in UHPC is obviously higher than that in normal strength concrete. Except for the shear stiffness prediction on TX by JSSC [³⁴], the stiffness increment of a stud in UHPC is at least 60% higher than that in normal strength concrete, in accordance with the results obtained by Cao et al. [²¹] and Kim et al. [¹⁰].

Expression of load-slip relationship

Load-slip curve is an important characteristic in typical steel-concrete composite bridges. The influence of stud shear connectors in the elastic range on the flexural behavior of composite bridges can be neglected due to full-shear connection design. However, the behavior of shear connectors in plastic region plays an important role because inconsistent load distribution occurs in shear connectors, concrete slab and steel beam induced by partial interaction behavior of the shear connectors. Several load-slip equations had been put forward to predict the static response of push-out specimens. However, most of the existing methods are established for normal strength concrete. Whether these methods could be applied on UHPC remains unknown. In the following equations, P denotes shear load of a stud, S denotes slip and P_u denotes maximum shear load.

The load-slip relationship for reloading condition of push-out specimen was proposed by Butty [³⁶]

(6){\displaystyle \frac{P}{P_{\mathrm{u}}}}={\displaystyle \frac{\mathrm{80}S}{\mathrm{1}+\mathrm{80}S}}.

For continuous loading specimens, Ollgaard et al. [³⁵] proposed an empirical formula to predict load-slip response expressed as

(7){\displaystyle \frac{P}{P_{\mathrm{u}}}}={(\mathrm{1}-{\mathrm{e}}^{-\mathrm{18}S})}^{\mathrm{0.4}}.

Using nonlinear regression, An and Cederwall [²⁸] proposed empirical formulas for normal strength concrete and high strength concrete (HSC). The expressions are given by

(8a){\displaystyle \frac{P}{P_{\mathrm{u}}}}={\displaystyle \frac{\mathrm{2.24}(S-\mathrm{0.058})}{\mathrm{1}+\mathrm{1.98}(S-\mathrm{0.058})}}\mathrm{for\; NSC\; specimens,}

(8b){\displaystyle \frac{P}{P_{\mathrm{u}}}}={\displaystyle \frac{\mathrm{4.44}(S-\mathrm{0.031})}{\mathrm{1}+\mathrm{4.24}(S-\mathrm{0.031})}}\mathrm{for\; NSC\; specimens}.

Xue et al. [²] proposed the following empirical formula based on 30 push-out specimens

(9){\displaystyle \frac{P}{P_{\mathrm{u}}}}={\displaystyle \frac{S}{\mathrm{0.5}+\mathrm{0.97}S}}.

Note that the unit of S is inch in Eqs. (6) and (7), and the unit is mm in Eqs. (8) and (9).

Due to the significant influence of stud diameter on the static behavior of a stud, pertaining on shear strength, shear stiffness and ductility, an empirical equation was proposed to predict load-slip response of a stud as expressed by

(10){\displaystyle \frac{P}{P_{\mathrm{u}}}}={\displaystyle \frac{S/d_{\mathrm{s}\mathrm{t}\mathrm{u}\mathrm{d}}}{\mathrm{0.005}+S/d_{\mathrm{s}\mathrm{t}\mathrm{u}\mathrm{d}}}}.

The above proposed functions are compared with the typical measured load-slip curve in this experiment, as shown in Fig. 6. It can be seen that equations found in literatures underestimated the stud push-out response while the proposed equation agreed better with the test result. In addition, further verification of the proposed equation will be explicitly presented in the subsequent numerical analysis part.

Finite element analysis

Finite element analysis is an efficient tool to simulate the behavior of structures [³⁷–³⁹]. To better understand the push-out behavior of headed stud shear connectors in UHPC, numerical analysis works were conducted using the commercial software ANSYS. Before finite element (FE) investigation, it should be aware that the reliability of simulation relates to many factors such as material constitution, mesh technology, boundary condition, contact interaction, loading manner, etc. Deficient result even errors may emerge if inappropriate parameter setting or assumptions are involved. Consequently, a verification study was first executed to ensure the reliability of the FE model.

Material model and properties

Multilinear isotropic hardening model was selected for stud and concrete while bilinear isotropic hardening model was adopted for steel beam. Figure 7 shows the constitutive model of concrete. For compression, concrete was treated as an elastic-plastic material and the descending stage was neglected because it is difficult to deal with the local crushing of concrete underneath the stud shank. As a result, the stress-strain relationship of concrete can be expressed by

(11){\displaystyle \frac{{\sigma }_{\mathrm{c}}}{f_{\mathrm{c}}}}={\displaystyle \frac{k\eta -{\eta }^{\mathrm{2}}}{\mathrm{1}+\left(k-\mathrm{2}\right)\eta }},

where s_c is the stress in the concrete; f_c is the concrete compressive strength; η = ε_c/ε_c1; ε_c1 is the strain at maximum stress; k = 1.05E_c× ε_c1/f_c according to EC2 [⁴⁰]; E_c is the elastic modulus of concrete. For tension, the concrete was treated as a linear elastic material up to the tensile strength (Fig. 7). A multilinear model was used to characterize the stress-strain relationship for the stud based on experimental tensile test, as shown in Fig. 7.

FE model establishment

In light of the symmetry of the geometrical dimensions, boundary conditions, and loading mechanisms, half of the test specimens were simulated with the consideration of computational efficiencies. In the FE model, three-dimensional eight-node nonlinear element (Solid65) was used to simulate concrete while three-dimensional eight-node isoparametric element (Solid45) was introduced to simulate the steel plate and studs. Three-dimensional spar element (link8) was used to simulate the embedded reinforcements.

As shown in Fig. 8, the FE model was assembled with four parts including concrete, rebar, studs, and steel plate and was set based on mechanical symmetric regulation. The bottom concrete surface (surface 1) was restrained in all three directions. The symmetry boundary condition was applied to the steel beam web surface (surface 2), meaning that all nodes located on this surface cannot move along the X axis. The loading surface of the steel beam is also shown in Fig. 8.

To prevent a dramatic increase in the kinematic energy, the loading rate should be carefully designed and slow enough. An optimum loading rate of 0.02 mm/s was adopted as recommended by Qi et al. [³]. Relative sliding could occur at the contact surfaces between steel flanges and concrete slabs and between stud shear connectors and surrounding concrete. Contact pair algorithm was therefore used to simulate the interfacial sliding. The contact surfaces of studs and steel flanges, defined as contact elements, were simulated by conta174 element while the concrete contact surfaces, defined as target segment elements, were simulated by targe170 element. As suggested by Xu et al. [⁴¹] and Qi et al. [³], the coefficient of friction between the interlayer faces was assumed to be 0.3.

FE model verification

The analyzed load-slip curves produced by the numerical simulation for each specimen are compared with the test results in Fig. 9. The numerical results achieved good correlation with the test results, verifying the reliability of the FE model. The small differences can be explained by the differences between the real and assumed material constitution, contact interaction and boundary condition. Therefore, it is believed that the proposed FE model has sufficient accuracy and reliability to simulate the push-out tests and to conduct the following parametric study.

Parametric study

To further understand the effect of main parameters on the static behavior of headed stud shear connectors in UHPC, a parametric study based on the specimen TC was conducted using the proposed FE model. Three parameters consisting of concrete strength, stud diameter, and stud aspect ratio were selected. Table 4 shows the detailed information of parametric study specimens. The concrete strength ranged from 80 to 200 MPa, the stud diameter was designed from 16 to 30 mm and the stud aspect ratio was set from 2 to 6.

Figure 10 illustrates the load-slip curves of every FE push-out specimens listed in Table 4. As the stud diameter increased, the shear strength and stiffness increased significantly. This could be explained by the increment in stud shear area and stud shank failure mode, in which the stud strength and stud area dominants the shear strength. The shear stiffness increased as the concrete strength increased. However, it is interesting to find that although stud shank failure occurred in specimens with different concrete strengths, the shear strength increased as the concrete strength increased, which went against the current specifications. This phenomenon indicated that there should be another shear force resistance mechanism except for stud shank shear strength contribution. It can be seen from Fig. 3 that a concrete wedge block was sheared off from the concrete slab and adhered to the stud root at failure. Based on this observation, it is believed that the mechanical mechanism on shear strength consists of two parts, namely stud shank contribution and concrete wedge block contribution. This hypothesis was also confirmed by the test results performed by Wang et al. [⁹]. Stud aspect ratio showed no obvious influence on the static behavior of the specimens. The current specifications require that the aspect ratio of a stud should not be smaller than 4 to ensure adequate anchorage of a stud [²⁵,³⁴]. However, specimens with different stud aspect ratios (2 to 6) showed similar push-out behavior. Therefore, short stud shear connectors could develop full strength in UHPC slabs, which is also confirmed by Wang et al. [⁹] and Cao et al. [²¹] previously.

Figure 11 illustrates the Von Mises stresses of all specimens listed in Table 4 at ultimate state. It can be seen that stud shank failure occurred in all specimens. As the concrete strength increased, the stress concentration moved toward stud root. The stress distribution showed no significant difference as the stud diameter increased. It should be noted that as the stud aspect ratio decreased, the stress in stud head increased dramatically. This may be the reason why can short stud with an aspect ratio as small as 2 satisfy the anchorage requirement.

To verify the feasibility and accuracy of the proposed equation, the numerical results and theoretical predictions of load-slip response were compared as shown in Fig. 12. The proposed equation reached a good agreement with the numerical load-slip curves. Therefore, the proposed load-slip equation could be used to predict the static response of a stud in UHPC.

Conclusions

The static behavior of stud shear connectors in UHPC was investigated using a push-out test on four specimens. The effect of concrete strength, stud diameter and stud aspect ratio on the load-slip response was further investigated based on a proposed finite element model. The shear strength, shear stiffness and load-slip response were analyzed. The following conclusions can be drawn from the present study:

1) Based on the test results, no cracking, crushing or splitting was observed on the concrete slab, indicating that UHPC slab exhibited good performance and could resist the high force transferred from the headed studs. The shear strength predictions of the current specifications were approximately 36.6%–52.3% lower than the test shear strength. The stiffness increment of a stud in UHPC was at least 60% higher than that in normal strength concrete.

2) An empirical load-slip equation taking into account stud diameter was proposed to predict the load-slip response of a stud. The proposed equation correlated well with the test and numerical load-slip curves.

3) According to the FE and experimental results, it was found that the shear capacity of a push-out specimen with stud shear connectors in UHPC is supposed to be composed of two parts stud shank contribution and concrete wedge block contribution.

4) Based on the FE analysis, stud diameter and concrete strength had significant influence whereas stud aspect ratio showed no obvious influence on the stud shear behavior. Even if the stud height was reduced from 6d to 2d, there was no reduction in the shear strength of a stud. Short stud shear connectors with an aspect ratio as small as 2 could develop full strength in UHPC slabs.

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