and supporting steel plates of the compression test machine to
measure the axial deformations of the specimens. The average
axial strains along the height of the specimens were calculated
by using the recorded deformations. Additional four LVDTs were
placed at the mid-height at a gauge length of 175 mm at 90 spacing
along the circumference of specimens to measure the axial
deformation of the middle portion of the specimens.
Furthermore, the specimens were instrumented at the mid-height
with two unidirectional strain gauges to measure axial strains,
which were used to validate LVDT measurements at the early
stages of loading. Hoop strains recorded on FRP tubes were
obtained from 9 unidirectional strain gauges with a 5-mm gauge
length that were spaced equally around the perimeter at the
mid-height of the specimen outside the overlap region.
The specimens were tested under axial compression using a
5000-kN capacity universal testing machine. The loading was initially
applied with load control at 3 kN per second, whereas displacement
control was used at approximately 0.003 mm per
second beyond the initial softening until specimen failure. Prior
to testing, all specimens were ground at both ends to ensure uniform
distribution of the applied pressure, and the load was applied
directly to the core of concrete through 150-mm diameter precision
cut high-strength steel discs placed at both ends of the specimen.