A non-contact inspection method to determine the transfer length in pre-tensioned concrete railroad ties

Abstract

The traditional experimental method to determine the transfer length in pre-tensioned concrete members consists of measuring concrete surface strains before and after de-tensioning with a mechanical strain gage. This method is prone to significant human errors and inaccuracies. In addition, since it is a time-consuming and tedious process, transfer lengths are seldom if ever measured on a production basis for product Quality Assurance. A rapid, non-contact method for determining transfer lengths in pre-tensioned concrete railroad ties has been developed. The new method uses laser-speckle patterns that are generated and digitally recorded at various points along the pre-tensioned concrete member. A prototype was fabricated as a portable self-contained unit for field testing. It incorporates a unique modular design concept that has several preferable features. These include flexible adjustment of the gauge length, easy upgradability to automatic operation, robustness and higher accuracy. The laser speckle strain sensor was applied to transfer length measurements of typical pre- tensioned concrete railroad ties in a railroad tie plant. These prestressed concrete tie members are expected to withstand repeated axle loadings of 290 kN totaling 250 million gross tons annually occurring at speeds in excess of 110 km/h. The technique achieved a microstrain resolution comparable to what could be obtained using mechanical gauge technology. Surface strain distributions were measured on both ends of twelve ties, and their associated transfer lengths were subsequently extracted. The measurements of transfer length using the laser speckle strain sensor were unprecedented since it was the first time that the laser speckle technique has been applied to pre-tensioned concrete inspection and particularly for use in transfer length measurements of concrete railroad ties. It was also demonstrated that the technique was able to withstand the harsh manufacturing environment, making transfer length measurements possible on a production basis for the first time.