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

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.