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E-Mod Tests: Concrete Extensometer/Compressometer Issue

E-Mod Tests: Concrete Extensometer/Compressometer Issue

E-Mod Tests: Concrete Extensometer/Compressometer Issue

(OP)
Hello,

Just wondering if anyone here has ever had any issues with a Zwick/ToniTechnik extensometer for concrete cylinders and prisms? The instrument in question employs a pair of opposing transducers with lever-arm knife-edge supports. Strain is measured by the compression machine controller across the chosen gauge length.

I'm using one to determine Young's Modulus for 150mm diameter concrete cylinders to EN12390-13. Unfortunately, however, the instrument and associated controller have provided hugely-understated results for E when compared to theoretical projections (from Eurocode 2 and Model Code 2010). I have also attached resistive strain gauges to a number of cylinders in tandem with the extensometer. The strain gauge-derived values correlate perfectly with the theoretical values for Young's Modulus. Values given by the extensometer are approximately 25% smaller.

The test machine and extensometer have both been calibrated recently and are used solely by trained technicians. I have borrowed a second extensometer of the same model from a third party to test in my lab, and also tested both extensometers in another lab with another compression machine. Despite all of this, the results are consistently much lower than they should be.

Are these transducers just inherently poor and unfit for purpose? Do other researchers or engineers find that concrete extensometers provide E values close to those given by strain gauges?

RE: E-Mod Tests: Concrete Extensometer/Compressometer Issue

stick with the strain gauges.... extensiometer slip is a common problem

RE: E-Mod Tests: Concrete Extensometer/Compressometer Issue

(OP)
Unfortunately that is not an option. I have almost 2000 cylinders to test over the coming months.

RE: E-Mod Tests: Concrete Extensometer/Compressometer Issue

Have you compared the stress strain curves for the two different approaches? Are the curves consistently off or does the delta appear over time? If the delta only appears at high stress; you might be able to use the first part of the curve to calculated E.

Just a thought.

Mike Lambert

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