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Publication Number: FHWAHRT10079
Date: November 2010 

Finite Element Analysis of UHPC:Structural Performance of an AASHTO Type II Girder and a 2ndGeneration PiGirderFHWA Contact: Ben Graybeal, PDF Version (701 KB)
PDF files can be viewed with the Acrobat® Reader® This document is a technical summary of the unpublished Federal Highway Administration (FHWA) report, Finite Element Analysis of UltraHigh Performance Concrete: Modeling Structural Performance of an AASHTO Type II Girder and a 2nd Generation PiGirder, available only through the National Technical Information Service, www.ntis.gov. OBJECTIVEThis TechBrief highlights the results of a research program that developed finite element analysis modeling techniques applicable to ultrahigh performance concrete (UHPC) structural components. INTRODUCTIONUHPC is an advanced cementitious composite material that tends to exhibit superior properties such as exceptional durability, increased strength, and longterm stability.^{(1–3)} This research program is aimed at developing general finite element concepts within a commercially available finite element package to facilitate the development of UHPC structural systems. This investigation focused on calibrating the proposed finite element models to a series of completed fullscale structural tests on existing UHPC structural components, including a prestressed UHPC American Association of State Highway Transportation Officials (AASHTO) Type II girder and a prestressed UHPC secondgeneration pigirder. FINITE ELEMENT MODELS AND UHPCTable 1 presents example mechanical properties for the type of UHPC investigated in this study. The properties far surpass those normally associated with concrete. The concrete damaged plasticity (CDP) model was primarily employed to model the constitutive behaviors of UHPC.^{(4,5)} It assumes isotropic damage elasticity combined with isotropic tensile and compressive plasticity to represent the inelastic behavior of concrete. Formation of tensile microcracks is represented macroscopically with a softening stressstrain relationship, and the compressive plastic response is represented by stress hardening followed by strain softening beyond the ultimate compressive strength. Table 1. UHPC Material Properties. This research program is aimed at developing general finite element concepts within a commercially available finite element package to facilitate the development of UHPC structural systems. This investigation focused on calibrating the proposed finite element models to a series of completed fullscale structural tests on existing UHPC structural components, including a prestressed UHPC American Association of State Highway Transportation Officials (AASHTO) Type II girder and a prestressed UHPC secondgeneration pigirder. Table 1. UHPC Material Properties.
Figure 1 depicts the typical assumed uniaxial stressstrain relationship of UHPC. The CDP parameters were calibrated through comparison to experimental structural test results, including three on an Igirder and a series on the secondgeneration pigirder. The threedimensional (3D) finite element models of the Igirder and pigirder test specimens are illustrated in figure 2 and figure 3. Figure 1. UHPC Uniaxial Material Model. Figure 2. 3D Finite Element Models of IGirders 80F, 24S, and 14S. Figure 3. 3D Finite Element Models of PiGirder and PiGirder with Joint. In the pigirder models, nonlinear springs replaced actual diaphragms and linear springs replaced elastomeric pads in order to facilitate modeling. Some idealized scenarios were also investigated to complement the experimental results and to suggest potential future optimizations. Parametric studies were presented to address issues such as mesh sensitivity, concrete smeared cracking model, different tension stiffening definitions, grouting material, and contact interaction. Finite element modelpredicted results were compared with experimentally captured measurements. Figure 4 and figure 5 present a comparison of midspan deflection and strain responses of the I girder 80F. Figure 4. IGirder 80F: Deflection at Midspan. Figure 5. IGirder 80F: Longitudinal Strains at Midspan. Figure 6 and figure 7 present the predicted deflections of the Igirders 24S and 14S along six instrumentation lines spaced in the longitudinal direction in comparison with the experimental measurements that were modified by excluding possible linear elastic deformation of the test supporting systems. In figure 7, the slippage of the prestressing strands accounts for the larger nonlinear deflection observed in the experiment. Figure 6. IGirder 24S: Deflection Along Specified Instrumentation Lines. Figure 7. IGirder 14S: Deflection Along Specified Instrumentation Lines. Figure 8 through figure 13 present the experimental and finite element results on deflection, longitudinal strain, leg spreading at midspan, and diaphragm force for the pigirder. Figure 14 through figure 19 show the experimental and finite element results for the pigirder with joint. Figure 8. PiGirder: Deflection of Bulb at Midspan. Figure 9. PiGirder: Deflection of Middeck at Midspan. Figure 10. PiGirder: Longitudinal Strain on Bulb Bottom Surface at Midspan. Figure 11. PiGirder: Longitudinal Strain on Deck Surface Immediately Above Web at Midspan. Figure 12. PiGirder: Bulb Lateral Spreading at Midspan. Figure 13. PiGirder: Diaphragm Force. Figure 14. PiGirder with Joint: Deflection of Bulbs at Midspan. Figure 15. PiGirder with Joint: Deflection of Deck Near Joint at Midspan. Figure 16. PiGirder with Joint: Longitudinal Strain on Bulb Bottom Surface at Midspan. Figure 17. PiGirder with Joint: Longitudinal Strain on Deck Top Surface Immediately Above Webs at Midspan Figure 18. PiGirder with Joint: Bulb Lateral Spreading at Midspan. Figure 19. PiGirder with Joint: Diaphragm Force. Figure 20 presents the finite elementpredicted maximum principal stress contours of the pi girder and pigirder with joint at midspan cross section in deformed shapes under applied loads of 340 kips (1,512 kN) and 428 kips (1,904 kN), respectively. Figure 20. Stress Contours from Modeled PiGirder Test Simulations. The results show that CDP models using appropriate parameters in any of the three types of tension stiffening definitions can capture both linear and nonlinear behaviors of the Igirders and pigirders reasonably well. The assumed elasticperfectlyplastic tensile stressstrain relationship for UHPC used in the CDP models is reasonable. CONCLUSIONSThe CDP model replicates the observed responses better than the concrete smeared cracking model in the prestressed UHPC Igirders and secondgeneration pigirders. The CDP model, using appropriate parameters in any of three types of tension stiffening definitions, can capture both linear and nonlinear behaviors of the modeled tests. The proposed modeling techniques, including nonlinear spring diaphragms, linear spring pads, automatic stabilization, and contact interaction, were demonstrated to be effective. The failure mechanics in the physical tests have been investigated with additional information provided by the models. FUTURE RESEARCHThe research completed in this study has led to the initiation of a number of related studies. A family of UHPC pigirder cross sections applicable to a range of span lengths and configurations is under development. Combined effects of discrete and fiber reinforcements on UHPC are under investigation. Other fullscale UHPC structural component tests are being modeled in order to gain a greater understanding of the performance of precast UHPC components and fieldcast UHPC connections. REFERENCES
