Influence of the measurement method on axial strains of FRP-confined concrete under compression
Introduction
Over the last two decades, several models have been developed to predict the mechanical behavior of fiber reinforced polymer (FRP)-confined concrete [1], [2], [3], [4], [5], [6]. Experimental test results were used to establish practical models and to validate proposed analytical and numerical models. Among the mechanical properties of FRP-confined concrete columns, axial compressive behavior has received significant attention. A large number of experimental test results exist in the literature on the axial stress-strain behavior of FRP-confined normal- and high-strength concrete (NSC and HSC) [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. However, as was shown previously [1], [7], [19], [20], [21], [22], the consistency and reliability of the test database significantly affect the overall performance of the developed model.
A review of the literature shows that axial strains of FRP-confined concrete specimens have been typically measured using two different methods: 1) unidirectional strain gauges placed on the surface of the specimen [23], [24], [25]; 2) linear variable displacement transformers (LVDTs) [26], [27], [28], [29], [30]. As was discussed previously [31], [32], among these two measurement methods, LVDTs provide more reliable measurements for the axial strain of FRP-confined concretes, as the strain gauges are only able to capture the local strains, which can vary significantly from the overall strains especially along the inelastic portion of the axial stress-strain behavior. Two different LVDT measurement methods, namely full-height and mid-height LVDT (FLVDT and MLVDT) methods, have been extensively used to measure the axial strains of FRP-confined concrete. FLVDTs are mounted at the corners between loading and supporting steel plates of the testing machine to determine the average axial strain along the entire height of the specimen, whereas MLVDTs are mounted on the surface of the specimen through the use of a cage along the mid-height region to measure the axial strains along this region.
A number of previous studies that used both MLVDT and FLVDT measurement methods [18], [31], [33], [34] have shown that, axial strains obtained from these measurement methods were similar to each other in the case of NSC (i.e. compressive strength below 50 MPa) specimens. However, significant differences were observed in the axial strain of HSC specimens obtained from these two measurement methods and the differences became more pronounced with an increase in unconfined concrete strength (f′co). These observations suggest that the axial strains of HSC specimens can be sensitive to the instrumentation arrangement used in their measurement. Therefore, development of models by the direct use of existing axial strain databases that were obtained from different measurement methods could lead to unreliable results for HSC specimens. Therefore, it is crucial that the influence of instrumentation method should be considered in the modeling to establish an accurate and reliable model, especially in the case of specimens with f′co over 50 MPa. A targeted study is also required to understand the reasons behind the differences in the axial strains obtained from different measurement methods.
As the first systematic study to date, the study presented in this paper was aimed at investigating the relationship between the axial strains of FRP-confined concrete obtained from the two most widely used measurement methods, namely FLVDT and MLVDT methods. A complete database of FRP-confined NSC and HSC cylinders containing both FLVDT and MLVDT axial strain data was assembled. The influential parameters affecting the relationship between axial strains obtained by the two measurement methods are evaluated. An expression is also developed to describe the relationship between the axial strains of FRP-confined HSC obtained by the two measurement methods with the aim of providing a unified framework for future design and modeling efforts.
Section snippets
Experimental database
Fig. 1 shows the test setup and instrumentation arrangement for the FLVDT and MLVDT measurement methods. The test database was compiled with the results from circular FRP-confined concrete specimens with unidirectional fibers in the hoop direction and a height-to-diameter ratio of 2, for which the axial strains were measured by both FLVDT and MLVDT methods [31], [32], [33], [34], [35], [36], [37], [38]. In the database, only the specimens that were confined continuously and experienced FRP
Investigation of the influence of LVDT measurement methods on axial compressive behavior of FRP-confined concrete
This section presents a discussion on different failure modes of FRP-confined NSC and HSC specimens and their axial stress-strain relationships obtained by FLVDTs and MLVDTs, which is followed by a detailed discussion on the influential parameters affecting the relationship between axial strains obtained by the two measurement methods.
Conclusions
This paper has presented the results of the first systematic study investigating the relationship between the axial strains of FRP-confined concrete obtained from the two most widely used measurement methods, namely FLVDT and MLVDT methods. A complete database of FRP-confined NSC and HSC cylinders containing both FLVDT and MLVDT axial strain data was assembled. The influential parameters affecting the relationship between the axial strains obtained by the two measurement methods were also
Acknowledgements
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China through Grant No. 51650110495 and the University of Adelaide through a Research Excellence Grant awarded to the third author.
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