It has been hypothesized that surface contaminants, such as lubricants on prestressing wires or strands, influence the resulting transfer length. However, until recently, the extent of this possible influence has only been speculation, as has been the relative influence on wire in comparison to strand. With the recent development of the ability to rapidly assess transfer length using new non-contact optical methods, it is now possible to explore hypothetical scenarios such as this with nearly real-time capability in the manufacturing plant.
This paper presents a recent attempt to determine the effect of lubricating oil on the transfer length of ties, by conducting nearly real-time in-plant transfer length measurements using a newly developed prototype multi-camera non-contact transfer length measurement system. The testing was conducted on prismatic concrete turnout ties manufactured at the Nortrak plant in Cheyenne, Wyoming. Two different types of turnout ties were investigated, one containing indented 5.32-mm-diameter wire reinforcement and the other containing 3/8-in.-diameter 7-wire strand. These ties were located near the end of the casting bed. Prior to casting, one end of the form was sprayed with a generic lubricant, literally saturating the prestressing wires or strands. The ties were then cast and de-tensioned following the normal manufacturing process. This clearly represented a highly worst-case scenario for the influence of surface contaminants.
Measurements were made using the new multi-camera system, providing a detailed profile of surface strain over several feet along each end of the last three ties in the casting beds (one for strand and one for wire) — the last tie being the one subjected to the application of oil prior to casting. Hence, the influence of oil application on adjacent ties was also revealed by these tests. For the tie end with strand reinforcement subjected to oil soaking, the maximum compressive strain only reached about 400 microstrain, far below the nominal average maximum strain level of approximately 1000 microstrain. In fact, the associated transfer length for the oil-soaked end could not be definitively measured because the strain level never achieved the plateau level of strain. In contrast, the tie end with oil-soaked indented wire exhibited a significant increase in transfer length; however, the transfer length remained well below the distance to the rail seat. From these worst-case tests, one can conclude that smooth strand is potentially highly influenced by lubricating oils, whereas the influence on indented wire is likely small by comparison.