Elsevier

Composite Structures

Volume 188, 15 March 2018, Pages 415-424
Composite Structures

Influence of the measurement method on axial strains of FRP-confined concrete under compression

https://doi.org/10.1016/j.compstruct.2018.01.017Get rights and content

Abstract

Accurate determination of axial strains of fiber reinforced polymer (FRP)-confined concrete is of significant importance in determining the axial compressive behavior of the material. This is challenging, however, because of the fact that axial strains are often influenced by the methods used in their measurement. Two measurement methods, involving the use of linear variable differential transformers (LVDTs) either along the full- or mid-height region of the specimens, have been extensively used to measure the axial strains of FRP-confined concretes. This paper presents a targeted study investigating how the axial strains obtained from these two measurement methods compared. A complete database of FRP-confined normal- and high-strength concrete (NSC and HSC) cylinders containing both full- and mid-height LVDT (i.e. FLVDT and MLVDT) axial strain data was assembled. The analysis results of database show that axial strains obtained from FLVDT and MLVDT are close to each other in the case of NSC specimens. However, significant differences are seen in the axial strains of the HSC specimens obtained from these two measurement methods, indicating that the axial strains of HSC are highly sensitive to the instrumentation arrangement used in their measurement. It is shown that, in addition to the significant influence of the unconfined concrete strength, the lateral stiffness of FRP confinement influences the relationship between the axial strains obtained by FLVDT and MLVDT methods. Finally, with the aim of establishing a unified framework for future design and modeling efforts, an expression is developed to describe the relationship between the axial strains of FRP-confined concrete obtained by the two measurement methods.

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.

References (41)

Cited by (21)

  • Three-dimensional finite element modeling and theoretical analysis of concrete confined with FRP rings

    2021, Engineering Structures
    Citation Excerpt :

    For model of FRCC in the FE analysis, the FRP ring/tie is tied to the concrete, as has been widely adopted in previous studies (e.g. [41–45]). The assumption that no slide between FRP and concrete occurs during the loading process is based on the following facts: i) the overlapping zone in FRP jacket/ring ensures that there is no slide between adjacent layers in the FRP jacket/ring [65]; ii) observations based on experimental studies have shown that FRP-confined concrete in circular columns dilates uniformly in the radial direction, and no slide or debonding can be seen between the FRP and the concrete [54–58]. As expected, the predicted axial stress-strain curves agreed reasonably well with the testing results (see Fig. 6).

  • Investigation of the compressive behavior and failure modes of unconfined and FRP-confined concrete using digital image correlation

    2020, Composite Structures
    Citation Excerpt :

    In addition, the confinement level is able to change the transition point (f’c1, ɛc1) and slope of second branch of axial stress-strain curve [12,14,19,21,30–37]. As discussed previously [38], the measurement methods used in experimental testing can influence the recorded axial stress-strain curve, and the use of DIC to measure the axial and lateral strain development and variation of hoop rupture strains can lead to a better understanding of the mechanical behavior compared to that can be derived through the use of contact methods [9,15,39]. The DIC based studies showed the variation of hoop strain at rupture (ɛh,rup) over specimens’ surface.

View all citing articles on Scopus
View full text