Distributed Sensing for Shrinkage and Tension Stiffening Measurement
by Matthew B. Davis, Neil A. Hoult, Sanchit Bajaj, and Evan C. Bentz
American Concrete Institute ACI Structural Journal May/June 2017
Newswise — Reinforced concrete is one of the most widely used and flexible building materials in the world. However the design of new complex structures and the assessment of existing structures is a challenge because it requires an accurate picture of how the structure responds to loading. This can be difficult to estimate because the properties of concrete change with time. For example, shrinkage, the overall decrease in the size of a concrete member due to drying of the concrete, varies depending on a number of factors including moisture conditions. The load response of reinforced concrete structures is also affected by the behaviour at cracks in the concrete, which develop due to the low tensile strength of concrete and are bridged by the reinforcement. The interaction between the reinforcement and the concrete at these cracks, known as tension stiffening, can significantly influence the structure’s response to load.
Until now the measurement of the effects of shrinkage and tension stiffening has been onerous and expensive and so has been limited to a few lab tests. Yet there is a need to obtain data about these behaviors so that models can be more accurately calibrated and ideally so that the specific performance of in-service structures can be measured. The recent development of fiber optic distributed strain sensors may provide the solution to these problems. Rayleigh backscatter based fiber optic sensors enable strain to be measured with strain gauge accuracy and over similar gauge lengths but along the full length of a fiber optic cable using fibers that cost just pennies a metre.
To assess whether this potentially revolutionary strain measurement technology can be used to measure the effects of shrinkage and tension stiffening in reinforced concrete, a series of shrinkage and tension stiffening experiments were undertaken as seen in Figure 1. Each specimen was instrumented with fiber optic sensors on the reinforcement and strains were measured with increasing time in the case of the shrinkage specimens and increasing load for the tension stiffening specimens.
The results of both sets of experiments illustrated the insights in to reinforced concrete behavior that are possible through the use of fibre optic sensors. In the case of the shrinkage measurements, the variation in shrinkage along the full length of the member could be measured, as seen in Figure 2, and the importance of understanding the effects of temperature was also clear. In the case of the tension stiffening specimens, the full strain profile was measured, thus eliminating the need for thousands of dollars in strain gauges. The importance of measuring the shrinkage strains was also highlighted when the results of these experiments were compared to a tension stiffening model.
This research has illustrated the potential value of fiber optic distributed strain sensors for reinforced concrete research and the potential for assessing the performance of new and existing structures. The use of this technology could enable the development of structures that can sense changes in performance and relay that information back to engineers when action needs to be taken.
The research can be found in a paper titled “Distributed Sensing for Shrinkage and Tension Stiffening Measurement,” published by ACI Structural Journal.
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