Noncontact Inspection Method to Determine the Transfer Length in Pretensioned Concrete Railroad Ties

Abstract

8 9The traditional experimental method to determine the transfer length in pre-tensioned concrete 10 members consists of measuring concrete surface strains before and after de-tensioning with a 11 mechanical strain gage. This method is prone to significant human errors and inaccuracies. In 12 addition, since it is a time-consuming and tedious process, transfer lengths are seldom if ever 13 measured on a production basis for product Quality Assurance. 14 A rapid, non-contact method for determining transfer lengths in pre-tensioned concrete railroad 15 ties has been developed. The new method uses laser-speckle patterns that are generated and 16 digitally recorded at various points along the pre-tensioned concrete member. A prototype was 17 fabricated as a portable self-contained unit for field testing. It incorporates a unique modular 18 design concept that has several preferable features. These include flexible adjustment of the 19 gauge length, easy upgradability to automatic operation, robustness and higher accuracy. 20The laser speckle strain sensor was applied to transfer length measurements of typical pre-21 tensioned concrete railroad ties in a railroad tie plant. These prestressed concrete tie members are 27The measurements of transfer length using the laser speckle strain sensor were unprecedented 28 since it was the first time that the laser speckle technique has been applied to pre-tensioned 3

A portable modular optical sensor capable of measuring complex multi-axis strain fields

“…A novel alternative device to measure the transfer length, called a Laser Speckle Imaging (LSI) sensor, was developed previously by the authors 6,7,8,9,10 . It is based on a previously developed 5-axis (five degree of freedom) surface displacement measurement technique 11 and its operation has been verified through extensive manual testing.…”

Reliable Transfer Length Assessment for Real-Time Monitoring of Railroad Crosstie Production

“…The crossties were manufactured with many different prestressing wire types, resulting in a wide range of transfer lengths. The surface strain measurements were obtained using the previously developed automated LSI system [7,8], shown in Figure 3. Bias in the measurement of crosstie transfer length due to the non-prismatic behavior is also investigated in this paper, including the effect of the length of the strain measurement region and the importance of thermal strain compensation.…”

“…As an additional example of the significance of this thermal offset, consider the measured crosstie strain profile shown in Figure 9. The dashed line corresponds to the modified (generalized) ZL transfer length algorithm with no thermal offset, while the solid red curve includes a thermal offset parameter that is extracted during the minimization process associated with Equation (8). Without thermal offset, the resulting transfer length is only 6.3 in, whereas with thermal strain included the resulting transfer length is 24.9 in.…”

“…The transfer length determination procedure consists of two steps. First, the surface strain distribution along the pretensioned concrete railroad ties is measured by using various mechanical, electronic or optical sensors [6,7,8,9]. The surface strain profile is then plotted and the transfer length value is extracted by using some prescribed computational algorithm.…”

“…These bumps, similar to those shown in the "plateau region of Figure 1, result from the varying crosssectional area and prestressing wire eccentricity, and preclude accurate and reliable estimation of a so-called average maximum strain (AMS). The development of the automated Laser-Speckle Imaging (LSI) sensor [7,8] has for the first time opened up the real possibility of in-plant assessment of transfer length for each and every manufactured crosstie. An example of such an automated system is shown schematically in Figure 2.…”