Finite Element Modeling of Prestressed Concrete Crossties with Ballast and Subgrade Support.
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Finite Element Modeling of Prestressed Concrete Crossties with Ballast and Subgrade Support.

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    • Abstract:
      With the first major installation in North American

      railroads during the 1960’s, concrete ties were believed to last

      longer than timber ties and have the potential for reduced life

      cycle costs. However, their characteristic response to initial

      pretension release as well as dynamic track loading is not well

      understood. In North America, concrete ties have been found

      vulnerable to rail seat deterioration (RSD), but the mechanisms

      contributing to RSD failures are not well understood. To

      improve such understanding, a comprehensive computational

      study of the tie response to dynamic track forces is needed.

      This paper presents an initial research effort in this direction

      that models concrete crossties as heterogeneous media in threedimensional

      finite element analyses, i.e., the prestressing

      strands, concrete matrix and the strand-concrete interfaces are

      represented explicitly. Damaged plasticity models are

      employed for the concrete material, and linear elastic bond-slip

      relations, followed by damage initiation and evolution, are

      adopted for the strand-concrete interfaces. Further, the ballast

      is modeled with an Extended Drucker-Prager plasticity model,

      and the subgrade is modeled as an elastic half space. All

      material parameters are obtained from the open literature.

      Currently the rail fastening systems are not included in

      modeling.

      Two loading scenarios are simulated: pretension release

      and direct rail seat loading. The modeling approach is able to

      predict the deformed tie shape, initial interface deterioration,

      the compressive stress state in concrete and residual tension in

      the strands upon pretension release. The transfer lengths of the

      prestressing strands can be readily calculated from the analysis

      results. Further predicted are the rail seat force-displacement

      characteristics and the potential failure mode of a concrete

      crosstie under direct rail seat loading. The responses of two

      railroad concrete crossties with 8-strand and 24-wire

      reinforcements, respectively, are studied using the presented

      modeling framework. The analyses indicate a potential failure

      mode of tensile cracking at the tie base below the rail seats.

      The results show that the 24-wire tie is better able to retain the

      pretension in the reinforcements than the 8-strand tie, resulting

      in slightly stronger rail seat force-displacement characteristics

      and higher failure load. The effects of the load application

      method and the subgrade modeling on the predicted tie

      response are further studied.

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