Automated Optical Surface Strain Measurement System to Determine the Transfer Length in Pretensioned Concrete Railroad Ties

Abstract

This report documents the advances that have been made to determine the transfer length of pretensioned concrete railroad ties using non-contact surface displacement measurements by digital image correlation. The work has culminated with two fully-functional devices that address specific needs of the industry. The first device utilizes a multi-camera method for measuring the surface strain profile on a railroad tie and determining the associated transfer length to within +/- 1.5 in. with as few as 5 independent measurements of surface strain. The work represents a practical step towards the continuous monitoring of in-plant prestressed railroad tie production, using transfer length as a quality control parameter. The second device is capable of making measurements of strain in a real-time continuously scanning/traversing (CST) manner over the entire distance range of interest on the tie associated with transfer-length development. It was shown to be capable of a strain measurement resolution of nominally about ± 20 microstrain, at traversing speeds of up to several inches per second.