3.1.2. Meter Placement Design

The positions of displacement meters and strain gauges are given in Figure 3. A YHD-50 displacement meter was fixed at the top of the specimen to record the deformation of the sand mat, which was used to eliminate the impact of the uneven loading plate. Four YHD-50 displacement meters were fixed on the actuator of the loading device at equal angle intervals to record the longitudinal displacement of the specimen. At the half height of the specimen, four 100 mm concrete strain gauges were fixed on the outer surface of the sleeve along the circumferential and longitudinal directions to record the circumferential and longitudinal strains of the sleeve, respectively. At the same time, at the half height of the self-stressed reinforced specimens, four 80 mm concrete strain gauges were fixed on the inner surface of the sleeve along the circumferential and longitudinal directions to record the circumferential and longitudinal strains of the filled concrete, respectively. Both the displacement and strain time histories were recorded using a TDS-530 static data collector with an acquisition frequency of 0.5 Hz, as shown in Figure 4.

**Figure 3.** The placement of displacement meters and strain gages. (**a**) Design positions of meters; (**b**) Actual positions of meters.

**Figure 4.** TDS-530 static data collector.
