Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / B. Terry Beck / The current experimental method to determine the transfer length in prestressed concrete
members consists of measuring concrete surface strains before and after de-tensioning with a
mechanical strain gage. The 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.
A rapid, non-contact method for determining transfer lengths in prestressed concrete
members has been developed. The new method utilizes laser-speckle patterns that are generated
and digitally recorded at various points along the prestressed concrete member. User-friendly
software incorporating robust and fast digital image processing algorithms was developed by the
author to extract the surface strain information from the captured speckle patterns. Based on the
laser speckle measurement technique, four (4) successively improved generations of designs
have been made. A prototype was fabricated for each design either on an optical breadboard for
concept validation, or in a portable self-contained unit for field testing. For each design,
improvements were made based on the knowledge learned through the testing of the previous
version prototype. The most recent generation prototype, incorporating a unique modular design
concept and self-calibration function, has several preferable features. These include flexible
adjustment of the gauge length, easy expansion to two-axis strain measurement, robustness and
higher accuracy.
Extensive testing has been conducted in the laboratory environment for validation of the
sensor’s capability in concrete surface strain measurement. The experimental results from the
laboratory testing have shown that the measurement precision of this new laser speckle strain
measurement technique can easily achieve 20 microstrain. Comparison of the new sensor
measurement results with those obtained using traditional strain gauges (Whittemore gauge and
the electrical resistance strain gauge) showed excellent agreement. Furthermore, the laser
speckle strain sensor was applied to transfer length measurement of typical prestressed concrete
beams for both short term and long term monitoring. The measurement of transfer length by the
sensor was unprecedented since it appears that it was the first time that laser speckle technique
was applied to prestressed concrete inspection, and particularly for use in transfer length
measurement. In the subsequent field application of the laser speckle strain sensor in a CXT
railroad cross-tie plant, the technique reached 50 microstrain resolution, comparable to what
could be obtained using mechanical gauge technology. It was also demonstrated that the
technique was able to withstand extremely harsh manufacturing environments, making possible
transfer length measurement on a production basis for the first time.
| Identifer | oai:union.ndltd.org:KSU/oai:krex.k-state.edu:2097/19764 |
| Date | January 1900 |
| Creators | Zhao, Weixin |
| Publisher | Kansas State University |
| Source Sets | K-State Research Exchange |
| Language | en_US |
| Detected Language | English |
| Type | Dissertation |
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