Usage of FBG fiber optic sensors to monitor the structural behaviour of a geopolymer concrete structural member

Abstract

Geopolymer concrete, a relatively new building material has been suggested as an alternative to Ordinary Portland Cement (OPC) based concretes for within the building sector, due to it having a relatively low CO2 emission during production. An issue with geopolymer concrete however, is that high amounts of early age creep, shrinkage and decrease in elastic modulus over time has been observed which in conjunction with the existing research on geopolymer concrete having been predominantly conducted in laboratory settings, leads to questions about the applicability of these findings and thus the use of geopolymer concrete as a replacement for OPC based concretes in concrete structures. To address this issue, this study makes use of Fiber Bragg Grating (FBG) fiber optic sensors, placing them within reinforced steel which in turn are placed inside geopolymer concrete to obtain data that is more applicable to real world settings compared to prior findings observed within laboratory settings.
Four precast prestressed geopolymer concrete girders are produced and rebars containing FBG fiber optic sensors are placed prior to casting. Measurements before and after the release of prestressing are analysed to determine the apparent transmission length of the prestressing strands and compared to methods defined by Eurocode EN 1992-1-1. The applicability of these guidelines is evaluated, as these are intended for use with conventional concretes. The changes in strain over time, and elastic modulus changes during curing of the precast girder is monitored and analysed, focusing on the effect of time dependent changes of the concrete to the effectiveness of the prestressing forces. The girders are combined with a geopolymer concrete topping to form two beams and one slab. The beams are subject to a flexural test and a shear test respectively. The slab undergoes a cyclic loading test before being tested in two critical point load locations in two separate tests. The FBG measurements gained during the tests are evaluated using analytical methods.
The results show that the FBG fiber optic sensors integrated into steel rebar can deliver strain measurements in locations conventional strain sensors cannot be applied. The FBG data suggests that conventional concrete transmission length guidelines are applicable to this geopolymer concrete mixture. As the FBG-integrated rebars were all placed in the same location in the precast girder cross section, the strains measured by these FBGs are only representative of concrete near the core of the girder, where the concrete is not exposed to drying conditions. And because this geopolymer concrete is susceptible to drying, the difference between measurements and expected values such as the elastic modulus and the drying shrinkage can be explained by the low amount of drying that the concrete in the core of the girder experiences. Due to the positioning of rebar in the cross section of the precast girder in this study, crack initiation during the flexural test was not detected. The occurrence of cracks is detected in the measured strains as it diverged from the linear elastic model during the crack propagation phase. This shows that the cross sectional position of the sensors is important for the desired function and role of concrete in-situ FBG monitoring.
Overall, the data that the (rebar-integrated) FBG strain sensors can provide can be of great use towards the larger task of bringing geopolymer concrete structures to a real life application. In-situ FBG strain data of the precast prestressed girders provide additional information into the application of this SCGC mixture in larger scale structural elements. This data is a useful addition to the data gained through conventional methods, and results from lab testing. FBG monitoring can provide strain data from the core of structural elements, giving an insight of the behaviour during curing and (destructive) testing that is otherwise difficult to obtain.