Knowledge of transfer length is critical for maintaining continuous production quality in the modern manufacture of prestressed concrete railroad ties. Traditional manual laboratory methods for measuring transfer length are simply not suitable for production operation. They are much too time-consuming to implement, and typically require extensive surface preparation in order to obtain the required surface strain measurements needed for determining the transfer length. In addition, the traditional 95% Average Maximum Strain (95% AMS) method of assessing transfer length from the measured surface strain profile has been shown to possess bias. The accuracy of transfer length assessment using this method is generally influenced by individual operator judgment; therefore, making it unsuitable for use as a reliable production quality-control parameter.
This paper presents recent in-plant testing of a newly developed prototype multi-camera non-contact transfer length measurement system, representing a major improvement over the traditional manual methods. The testing was conducted on concrete railroad ties at a manufacturing facility in North America. Concrete ties tested included those which were manufactured using indented prestressing wire as well as with 7-wire strand. The new device represents a next generation version of the previously successful Laser-Speckle Imaging (LSI) system developed by the authors. The multi-camera system is shown to provide nearly real-time surface strain profile measurements (subsequent to de-tensioning), with surface strain accuracy comparable to the mechanical Whittemore gage device, yet with little or no required surface preparation. Furthermore, with the previously demonstrated Zhao-Lee (ZL) transfer length processing algorithm built into a LabVIEW data acquisition and control interface, the multi-camera system is shown to provide assessments of transfer length within a nominal tolerance of +/− 1.5 inches using as few as six uniformly spaced surface strain measurements. This brings within reach the ultimate goal of providing the railroad tie manufacturer with the ability to measure the transfer length of every tie produced prior to leaving the plant, thereby ensuring that they are within an acceptable tolerance, and providing the means to quickly identify the need to modify production (e.g., concrete mix) if transfer length specifications fall out of desired range.