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.