Finite Element Analysis of Contributing Factors to the Horizontal Splitting Cracks in Concrete Crossties Pretensioned With Seven-Wire Strands
Authors: Yu Hailing 
Published: July 2017

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 in-house 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.