Pre-tensioned concrete railroad ties provide an inherent corrosion resistance of the restressing steel since it is in direct with the high-pH concrete during casting. This high-pH environment causes a thin oxide layer to form on the prestressing steel which, in turn, serves to passivate the steel and prevent further corrosion. This passive layer will then protect the concrete for many years unless it is destroyed by the ingress of chemicals (salts, sulfates, etc.) or a lowering of the pH.
One limiting aspect of these pre-tensioned ties, however, is their lack of rotational capacity. In areas of poor ballast maintenance, ties are forced to bend under train loadings to conform to the ballast geometry. When this occurs, the prestressing tendons in these ties experience high stress levels at cracking locations which, in-turn, will reduce the service life and can result in pre-mature failures.
One alternative to the pretensioned concrete tie that has gained traction in recent years is the post-tensioned concrete tie. Post-tensioned ties are fabricated by purposefully shielding the steel prestressing tendons from the surrounding concrete during fabrication to allow the tensioning process to take place after the concrete has cured. This fabrication approach has several advantages including a reduced initial setup costs for manufacturing and increased rotational capacity of the overall ties, since the induction of flexural cracking does not significantly increase the prestressing tendon stress.
However, this fabrication method leaves the prestressing tendons susceptible to corrosion since the passivation layer is never formed as the tenons do not have direct contact with the high-pH concrete. The Hybrid Tie being investigated in this study serves to utilize the best combinations from both the pre-tensioned and post-tensioned concrete tie approaches. The hybrid concrete tie is a pre-tensioned concrete tie with un-bonded prestressing tendons in the central portion. This provides the durability and corrosion protection of pre-tensioned systems along with the increased rotation capacity of post-tensioned systems.