Weldable Fiber Optic Sensors Measure the Strain of Reinforced Concrete Structure

A group of researchers from the Higher Technical School of Building of the Polytechnic University of Madrid (UPM), directed byJulián García Díaz, Nieves Navarro Cano and Edelmiro Rúa Álvarez, conducted tests designed to detect cracks in the concrete and measure the strain of the steel after applying different load steps.
In this way, they determined the exact moment at which a beam started to crack.

Problem

A group of researchers from the Polytechnic University of Madrid (UPM) were given the challenge of measuring the strains of a structure from within, not just the concrete but the steel as well. To achieve this, they needed state-of-the-art weldable fiber optic sensors and data recording technology.

Solution

The university team used a measurement solution consisting of fiber optic and temperature sensors, displacement transducers, a data acquisition (DAQ) system QuantumX MX1615, FS22 interrogators and HBM’s (now HBK’s) catman Easy software. This solution proved to be accurate, easy to connect and an ideal size for field work.

Result

The tests, carried out with the aid of  HBK products, demonstrated the suitability of the fiber optic sensors for this project, resulting in a more realistic depiction of the concrete structure’s behaviour than that obtained with traditional material strength formulae.

The UPM team used weldable fiber optic sensors based on FBG (Fiber Bragg Grating) instead of more traditional methods, such as infrared thermography, acoustic emission, digital image correlation (DIC), ultrasonic sensors, plastic optical fibers or carbon nanotube sensors.

“As the sensors can be welded to the rebar, it was possible to measure the strains of a structure from within, not only of the concrete but also of the steel itself,” said Julián García Díaz, doctoral student in Technological Innovation in Building.

Fiber optic sensors were chosen because of their characteristics, and in particular, their longevity compared to conventional electric strain gauges, long-term signal stability in unfavourable conditions, the option to place several sensors on the same cable (multiplexing) and immunity from interference (EMI/RFI). Furthermore, the cable length does not affect the measurement process. All this translates into cost advantages and improvements in performance and accuracy.

As a result, some microcracks that were invisible to the naked eye can now be detected by the fiber optics and, consequently, it should be possible to analyse the behaviour of the concrete and determine the deformation of the steel.
The tests have demonstrated that, for example, the steel strain is greater than that established in the theoretical calculations and occurs long before that indicated in previous laboratory tests.
García Díaz added that

“The results have met my expectations, as throughout the tests we have been able to achieve much more realistic behaviour of structures than that obtained with the traditional material strength formulae.”

For all these reasons, the UPM research group is planning to continue using HBK products in their future projects on structure deformation measurement.