Accurate unbiased assessment of transfer length for prestressed concrete railroad ties requires detailed knowledge of the longitudinal variation of geometrical cross-section parameters responsible for establishing the resulting surface strain profile. This is because the complex cross-sectional shape produces a non-uniform strain plateau region, which makes the accurate evaluation of transfer length more difficult. In particular, human judgment of a “plateau region” for assessment of the average maximum strain becomes subject to large uncertainty, and clearly this procedure cannot be used in any type of automated in-plant transfer length diagnostics. The important geometrical tie parameters include the cross-sectional area, centroid, moment of inertia, and the eccentricity of the prestressing wires. If a CAD drawing is available, this information can be digitally extracted from the CAD model representation of the crosstie. In fact, this digital extraction has been done and has already been in use for some time in assessing transfer length for one of the common crosstie manufacturer designs. However, current research efforts are investigating the characteristics of existing crossties which have been in track for many years, for which CAD drawings of the original designs are unlikely to be available.
The objective of the current research is to develop a comprehensive understanding of the material characteristics that have caused splitting failures in prestressed concrete railroad ties, and those characteristics that have resulted in ties that have performed well after many years in track. As part of this effort, a three-dimensional (3D) Optical Scanning System is being used to accurately scan and quantify the surface geometry of previously manufactured ties that have been in service, so as to produce an accurate 3D CAD model for later analysis associated with the above long-term research objectives.
For the initial phase of this work, a sample from the CXT crossties of known geometrical characteristics that were subjected to representative long-term loading at the TTCI Facility in Pueblo Colorado, was scanned so as to accurately map out detailed 3D tie surface geometry. These ties were cast using the same concrete materials but with different prestressing wires, and were all subjected to the same extreme in-track loading for a period of several years. A commercially-available 3D Laser-Based Optical Scanning System, having a maximum spatial resolution of approximately 0.1mm, was used to perform the surface scanning operations presented in this paper. The CXT tie provides a useful initial evaluation of the accuracy and general feature capture capability of the scanning procedure, since a 3D CAD model for this tie has been provided by the manufacturer. A detailed qualitative and quantitative analysis is presented which compares the 3D CXT CAD model geometry with the 3D geometry of the experimentally scanned ties. Illustrations as to how this 3D technique can reveal such features as abrasion and wear, along with the longitudinal variation of the above mentioned cross-section parameters associated with longitudinal surface strain and transfer length assessment, are included in this paper.