Performance of a Continuously Traversing 2-Camera Non-Contact Optical Strain Sensor for In-Plant Assessment of Prestressed Concrete Railroad Crosstie Transfer Length

Abstract

Accurate knowledge of transfer length has been shown to be crucial to the goal of maintaining continuous production quality in the modern manufacture of prestressed concrete railroad ties. Traditional manual laboratory methods, such as the conventional Whittemore method which requires the use of embedded reference points, are clearly not suitable for production operation or for use in reliable production quality-control. This paper presents the results of another advance in the development of automated transfer length measurement systems for practical in-plant operation. The new device offers a significant improvement over the previously successful automated Laser-Speckle Imaging (LSI) system developed by the authors. The earlier automated LSI strain measurement system has been modified to provide significantly improved optical resolution of longitudinal surface strain, with the ability to resolve longitudinal prestressed concrete crosstie surface strain without time-consuming special surface preparation. More importantly, 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. It features both a “jog” mode of operation, similar to its predecessor in which measurements of longitudinal surface strain are automatically captured in arbitrary spatial increments over the entire range of the computer-controlled traverse, and an “on-the-fly” mode in which measurements of longitudinal surface strain are captured without the need for stopping at each measurement location. This latter mode offers the potential of a much faster capture of the strain profile and should prove to be very beneficial for field testing and in-plant diagnostic applications. The performance of this new system is first demonstrated using a new calibrated step-wise uniform strain field setup which has been developed specifically for verification of this and other automated transfer length measurement systems. This verification system produces a calibrated step change in surface deflection, effectively subjecting the automated strain measurement system to an ideal step change in longitudinal strain for a given gauge length. In addition, the new automated system is demonstrated by conducting measurements of longitudinal surface strain on prestressed concrete crossties in a manufacturing plant. For this latter experimental in-plant testing, strain measurements using the new system are also compared directly with those from the recently introduced 6-camera transfer length measurement system, as well as with the traditional Whittemore gauge measurements. The agreement between these independent measurement systems is remarkable, and it is shown to even be possible to discern differences in strain profile and associated transfer length between adjacent crossties within a given casting bed. This new automated and high-resolution device should provide a very convenient and fast diagnostic tool for the manufacturer to quickly identify the need to modify production (e.g., concrete mix) if transfer length specifications fall out of desired range.