Finiteelement modeling of UHPC hybrid bridge deck connections
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Abstract
In recent years, linked bridge deck elements have gained popularity for facilitating more durable components in bridge decks, but these components require fieldapplied connections for constructing the entire bridge. Ultrahighperformance concrete (UHPC) is started to be a major material for closure pours in bridges and various Department of Transportations have been developing guidelines. UHPC is known by its superior quality than conventional concrete in terms of constructability, strength and durability. So far, very limited data are available on the finiteelement modeling (FEM) of hybrid bridge deck connections. In this study, FEMs have been presented to define the crucial factors affecting the response of bridge hybrid deck panel system under monotonic loads. The commercial software ABAQUS was used to validate the modes and to generate the data presented herein and the concrete damage plasticity was used to simulate both conventional concrete and UHPC. Numerical results were validated using available experimental data. The key parameters studied were the mesh size, the dilation angle, reinforcement type, concrete models, steel properties, and the contact behavior between the UHPC and the conventional concrete. The models were found to capture the load–deflection response of experimental results, failure modes, crack patterns and ductility indices show satisfactorily response. A sensitivity test was also conducted by considering various key parameters such as concrete and steel constitutive models and their associated parameters, mesh size, and contact behavior. It is perceived that increasing the dilation angle leads to an increase in the initial stiffness of the model. The damage in concrete under monotonic loading is found higher in normal concrete than UHPC with no signs of debonding between the two materials. Changing the dilation angle from 20° to 40° results in an increase of 7.81% in ultimate load for the panel with straight reinforcing bars, whereas for the panel with headed bars, the increase in ultimate load was found 8.56%.
Keywords
Nonlinear static analysis Ultrahighperformance concrete (UHPC) Bridge deck connections Sensitivity analysis Accelerated constructionIntroduction
The ASCE 2017 report card listed that about 9% of bridges in the USA are classified structurally deficient and each year more than 3000 new bridges are being constructed (Bhide 2008). It has been always a challenge for the bridge engineers to find new ways to build better bridges with reduced construction time. So far, significant efforts have been provided in developing innovative ways to increase the longterm structural performance, and currently, the use of UHPC has becoming more popular in the construction industry for its superior properties such as its early very high strength that might reach 96 MPa (14,000 psi) in 3 days, its promising toughness, and longterm steadiness. The term UHPC is classified as innovative cementitious composite materials, where groundbreaking technology of cement and concrete industry grouped together (Graybeal 2010).
In fact, the concept of using UHPC for connection between precast concrete panels started in the mid 90s. At that time, a building was being constructed at Aalborg University using UHPC as a closure pour material, and additional project was completed, where UHPC was used for slabcolumn connections and its bond characteristics, (Aarup et al. 2009; Hansen and Jensen 1999; Nielsen et al. 1996; Aarup and Jensen 1998). Additional research, was completed at Chalmers University focusing on the application of UHPC as closure material (Broo and Broo 1997; Harryson 1999, 2000). Simplicity in construction and outstanding performance made UHPC connection more popular than conventional modularcomponent connections, where conventional concrete connections require posttensioning, complex confinement reinforcement, large volume of concrete, etc., (Graybeal 2010). An ample amount of studies has been conducted to investigate the bond strength between UHPC and various materials. Perry and Seibert (2012) reported on applications related to precast joints of UHPC. The bond characteristics between timber and UHPC were studied by Schäfers and Seim (2011). The interfacial behavior of hollow glass fiberreinforced plastic beams having a UHPC filled compressive zone was examined by ElHacha and Chen (2012). Graybeal and Swenty (2012) investigated the performances of precast deck joints with variable cross sections. More research has been conducted on developing analytical models to predict that the compressive and tensile strength are also conducted. As this is not always feasible to conduct largescale test of UHPC connection of bridge deck elements, a need for developing dependable 3D finiteelement model is now time worthy. Graybeal (2006a, b, 2008, 2009a, b), performed comprehensive experimental tests on UHPC characterization, fullscale flexural and shear of Igirders, and pigirders.
Numerical modeling of UHPC connected deck panels has been always challenging due to nonavailability of postpeak behavior of UHPC either under compression or tension loads. The postpeak behavior is very important to predict the damage parameters needed for numerical modeling (Chen and Graybeal 2012). Different modeling approaches considering diverse assumptions have been proposed, but sensitivity analyses are needed for identifying the major parameters affecting the numerical results. In this paper, an effort has been made to develop a finiteelement model that can be applied for a variety of UHPC bridge connections subjected to monotonic loading. It is needed to recognize the major factors which affects the numerical results and evaluate the sensitivity of the material input parameters on the variability and response of the models.
Experimental program
Test specimen (Graybeal 2010)
Specimen  Orientation  Depth (mm)  Reinforcement 

8H  Transverse  200  16 M (#5) headed black reinforcement with 90 mm lap length and 450 mm (top) and 180 mm (bottom) spacing 
8G  Transverse  200  16 M (#5) galvanized straight bars with 150 mm lap length and 450 mm (top) and 180 mm (bottom) spacing 
Finiteelement modeling
The modeling UHPC connected bridge deck panels under monotonic loading using computeraided program is needed to broaden the current knowledge and provide some reliable results, especially with the high cost of experimental testing of UHPC connections. The numerical simulations were conducted using the ABAQUS (ABAQUS Inc. 2016) code, which is a general FE analysis software for modeling the nonlinear material behavior, interaction between different materials, heat transfer, fluid dynamics problem, etc. Both implicit and explicit numerical methods are available in ABAQUS for solving problems associated with large deformation and multiloading environments. ABAQUS/explicit method was used for simulating the FE models as it can effectively handle severely nonlinear behavior.
Precast panels

uniaxial stress–strain constitutive relation under compressive and tensile loading;

damage parameters \( d_{\text{c}} \) and \( d_{\text{t}} \) for compressive and tensile load, respectively.
These parameters are used to identify and validate damage and crack patterns of the developed model and compare it with experimental results. Three different concrete constitutive models were adopted in this study to identify the most suitable concrete model for this study. These models were only used to predict the behavior of the precast concrete panels.
Concrete model proposed by Hsu and Hsu (1994)
Concrete model proposed by Park and Paulay (1975)
Concrete model proposed by Saenz (1964)
Concrete parameters used in the plastic damage model
Concrete strength (MPa)  Mass density (ton/mm^{3})  Young’s modulus (MPa)  Poisson’s ratio  Dilation angle ψ (°)  Eccentricity (ε)  f_{bo}/f_{co}  \( b_{\text{c}} /b_{\text{t}} \) 

45  2.4E − 009  26,764.7  0.2  20, 36, 40  0.1  1.16  0.7 
UHPC
Ultrahighperformance concrete parameters used in the plastic damage model (Chen and Graybeal 2012)
Concrete strength (MPa)  Mass density (ton/mm^{3})  Young’s modulus (MPa)  Poisson’s ratio  Dilation angle ψ (°)  Eccentricity (ε)  Concrete strength (MPa)  \( b_{\text{c}} /b_{\text{t}} \) 

210  2.565E−009  53,000  0.18  15  0.1  1.16  0.7 
Reinforcing steel
Parameters of reinforcing steel
Type  Poisson’s ratio  Elastic modulus (MPa)  Mass density (ton/mm^{3})  Yield stress (MPa) 

Steel  0.3  200,000  7.85E−009  414/517 
Sensitivity analysis
Variables used for sensitivity analysis
Sensitivity test  Parameters  Straight bars  Headed bars 

Mesh size  20 mm  SBH20TE35°  HBH20TE35° 
10 mm  SBH10TE35°  HBH10TE35°  
5 mm  SBH5TE35°  HBH5TE35°  
Dilation angle  φ = 20  SBH20TE20°  HBH20TE20° 
35  SBH20TE35°  HBH20TE35°  
40  SBH20TE40°  HBH20TE40°  
Concrete model  Hsu and Hsu (1994)  SBH20TE35°  HBH20TE35° 
Saenz (1964)  SBS20TE35°  HBS20TE35°  
Park and Paulay (1975)  SBP20TE35°  HBP20TE35°  
Steel properties  Elasticperfectly plastic  SBH20TE35°  HBH20TE35° 
Bilinear  SBH20TB35°  HBH20TB35°  
Contact modeling  Perfect bond (tie)  SBP20TE35°  HBP20TE35° 
Penalty (friction)  SBP20FE35°  HBP20FE35°  
UHPC  All parameters were constant for all cases 
Effect of concrete model
Effect of dilation angle
Effect of steel properties
Convergence study
Contact modeling
Validation of the finiteelement model
Comparison between FEM and experimental results
Sensitivity test  Variables  Straight bar  A_{exp}/A_{FEM}  Ultimate load (kN)  Headed bar  A_{exp}/A_{FEM}  Ultimate load (kN) 

Mesh size  20 mm  SBH20TE35°  0.95  486.98  HBH20TE35°  0.92  469.81 
10 mm  SBH10TE35°  0.95  445.21  HBH10TE35°  N/A  N/A  
5 mm  SBH5TE35°  0.94  490.31  HBH5TE35°  N/A  N/A  
Dilation angle (φ)  20  SBS20TE20°  0.93  453.17  HBS20TE20°  1.01  442.33 
35  SBS20TE35°  0.94  473.76  HBS20TE35°  0.91  467.13  
40  SBS20TE40°  0.93  488.54  HBS20TE40°  0.98  480.21  
Concrete model  Hsu and Hsu (1994)  SBH20TE35°  0.95  486.98  HBH20TE35°  0.92  469.81 
Saenz (1964)  SBS20TE35°  0.94  473.76  HBS20TE35°  0.92  480.21  
Park and Paulay (1975)  SBP20TE35°  0.93  482.62  HBP20TE35°  0.92  486.07  
Steel properties  Elasticperfectly plastic  SBH20TE35°  0.95  486.98  HBH20TE35°  0.91  469.81 
Bilinear  SBH20TB35°  0.94  515.74  HBH20TB35°  0.97  465.59  
Contact modeling  Perfect bond (tie)  SBP20TE35°  0.93  482.62  HBP20TE35°  0.92  486.07 
Penalty (friction)  SBP20FE35°  0.95  462.05  HBP20FE35°  1.01  462.02 
Conclusions

The damage in concrete under monotonic loading is found higher in normal concrete than UHPC with no signs of debonding between the two materials.

The FE model captured the damage pattern of the composite slab deck quite satisfactorily.

The numerical model is well capable of predicting the load displacement response, though it experiences higher stiffness initially.

Changing the dilation angle from 20° to 40° results in an increase of 7.81% in ultimate load for the panel with straight reinforcing bars, whereas for panel with headed bars, the increase in ultimate load was found 8.56%.

It was fund that for the panel with straight reinforcement bars, higher dilation angle produced slightly higher initial stiffness. In case of headed bar specimens, the stiffness of the composite panel for a dilation angle of 40° was found 9.69% greater than the stiffness found for dilation angle 20°, whereas for the straight bar panel, 5.47% higher value of initial stiffness was found for 40° dilation angle.

The initial stiffness decreased 8 and 20% for panel with straight bars and panels with headed bars, respectively, which justifies that the bilinear model could predict the overall panel performance closely to the experimental results.

The energy absorption ratios of all the experimental results compared to the developed FEM models were in the range of 89–110%, which justifies that the FEM models are in good agreement with the experimental results.
Notes
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