Transfer Length Characterization of Entire Crosstie Plant Casting Bed Using Continuously Traversing Dual-Camera Non-Contact Optical Strain Sensors
Authors: Beck B. Terry , Robertson Aaron A. , Peterman Robert J. , Riding Kyle A. , Wu John 
Published: July 2017

Transfer length has been identified as a key diagnostic parameter for evaluating the load bearing capability of prestressed concrete railroad crossties. Furthermore, it has been proposed for use as a valuable quality control parameter. However, until quite recently the capability to easily and accurately measure transfer length has been limited primarily to a laboratory setting. This is especially true for measurements made in the harsh environment of a tie manufacturing plant. The development of portable non-contact optical strain sensors has opened the door to rapid in-plant transfer length measurement. The measurement capability of these devices has been repeatedly demonstrated not only in the laboratory, but more importantly also through actual testing at multiple tie manufacturing plants. The latest version of the automated Laser-Speckle Imaging (LSI) system developed by the authors offers improved optical resolution of longitudinal surface strain, with the ability to resolve longitudinal prestressed concrete crosstie surface strain without time-consuming special surface preparation. The new system is also capable of making measurements of strain in a real-time “on-the-fly” manner over the entire distance range of interest on the tie associated with transfer length development. This faster capability to capture the strain profile with high resolution makes this new technology very beneficial for field testing and in-plant diagnostics applications. It has been demonstrated to be capable of resolving minor differences in longitudinal surface strain profiles associated with ties even in adjacent cavities.
As a logical next step toward eventual implementation of transfer length as a quality control parameter, it is important to evaluate the expected variation of transfer length during the tie manufacturing process. This paper presents the results of extensive in-plant assessment of transfer length in an attempt to characterize experimentally the in-plant manufacturing variations that can occur in practice. To the best of the authors’ knowledge, this is the first time extensive real-time measurements to this extent have been attempted in an actual tie manufacturing plant with the expressed purpose of statistically characterizing the variations in transfer length that take place over an entire casting bed.
A sampling of transfer lengths from well over 50 ties was determined during the manufacturing process (corresponding to over 100 transfer length measurements). The sampled tie measurement locations were distributed at different “form” locations along the casting bed, and included samplings of ties from several different “cavities” within a given form. The entire bed was 45 forms in length, each form having 6 tie cavities, for a total bed size of 270 ties. The statistical distribution of overall transfer length measurement results is presented, along with what may be typical variations in strain profile and resulting transfer length as a result of variations that took place in the manufacturing process. The overall range of transfer length observed, along with an investigation of possible bias due to position within the casting bed, and apparent variations of transfer length within a given form, are identified and discussed.