Finite element analysis of contributing factors to the horizontal splitting cracks in concrete crossties pretensioned with seven-wire strands.
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Finite element analysis of contributing factors to the horizontal splitting cracks in concrete crossties pretensioned with seven-wire strands.

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    • Abstract:
      This paper employs the finite element (FE) modeling

      method to investigate the contributing factors to the “horizontal”

      splitting cracks observed in the upper strand plane in some

      concrete crossties made with seven-wire strands. The concrete

      tie is modeled as a concrete matrix embedded with prestressing

      steel strands. A damaged plasticity model that can predict the

      onset and propagation of tensile degradation is applied to the

      concrete material. An elasto-plastic bond model developed inhouse

      is applied to the steel-concrete interface to account for the

      interface bond-slip mechanisms and particularly the dilatational

      effects that can produce the splitting forces. The pretension

      release process is simulated statically, followed by the dynamic

      simulations of cyclic rail seat loading. The concrete compressive

      strength at which the pretension in the strands is released, or

      release strength, affects both the concrete behavior and the bond

      characteristics. Three concrete release strengths, 3500, 4500 and

      6000 psi, are considered in the simulations. Concrete tie models

      without and with a fastening system are developed and simulated

      to examine the effect of embedded fastener shoulders and

      fastener installation. The fastener shoulders are seated relatively

      deeply reaching between the two rows of strands.

      There is instant concrete material degradation adjacent to

      the strand interfaces near the tie ends upon pretension release.

      Without the fastening system in the model, the 3500 psi release

      strength leads to a high degree of degradation that is coalesced

      and continuous in the upper and lower strand planes,

      respectively. The damage profiles with the higher release

      strengths are more discrete and disconnected. Dynamic loading

      appears to increase the degree of degradation over time. In all

      cases, the upper strand plane is not dominant in the degree or the

      extent of material degradation, in contrast to the field

      observations that the horizontal splitting occurred in the upper

      strand plane only.

      Further simulations with the fastener model at 3500 psi

      concrete release strength indicate that the fastener installation

      process does not worsen the damage profile. However, the

      presence of fastener shoulders in the concrete matrix changes the

      stress distribution and redirects more concrete damages to the

      upper strand plane, while leaving disconnected damages in the

      lower strand plane. Under repeated dynamic rail loading, this

      potentially reproduces the exact upper strand plane, horizontal

      cracking pattern observed in the field. Subjected to further

      experimental verification, the FE analyses identify three

      contributing factors to the horizontal macro-cracks occurring at

      the specific upper strand level: (1) relatively low concrete release

      strength during production, (2) embedded fastener shoulders that

      redistribute concrete damages to the upper strand plane, and (3)

      a sufficiently large number of dynamic rail loading cycles for the

      microscopic damages to develop into macro-cracks. The number

      of dynamic loading cycles needed to produce macro-cracks

      should increase with the increased concrete release strength.

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