Читать книгу Monument Future - Siegfried Siegesmund - Страница 271

To monitor the AE and strain, specimens were formed into a cylindrical shape with a diameter of 50 mm and a length of 100 mm. Before the test, each specimen was washed using water to remove contaminants and dried for 10 days in a vacuum desiccator. The specimen was installed in a temperature-controlled chamber after setting up equipment for AE, strain, and air and rock surface temperature measurement. The AE and strain data were recorded using a laptop computer.

The AE equipment employed during the test consisted of an amplifier and a piezoelectric sensor (Fig. 1). In this study, peak amplitude, which is an important parameter in the test because it determines AE signal detectability, was continuously monitored during the entire test period at 1/100 s. 181The sensor was placed on an axis face of the specimen. Notably, vaseline was smeared in the contact area of the sensor and specimen to ensure their coupling effect; then, a c-clamp was used to fix the sensor on the specimen.

Figure 1: A schematic diagram of the AE and strain monitoring system.

Self-temperature-compensated strain gauges (10 mm in length) were installed on the center of the specimen in the axial and lateral directions using a three-wire system to reduce thermally induced apparent strain. A dedicated adhesive was used to glue strain gauges to the specimens. The specimen strain was continuously recorded using a measuring unit.

The surface temperature of the specimens was monitored at a 1-s interval using a thermocouple sheet and logger. Air temperature in the chamber was also recorded by a logger at a 10-s interval.

The chamber was programed with a heating–cooling range of 4–84 °C and an RTC of ±2 °C/min based on field measurements. Namely, Peel (1974) reported a maximum rock surface temperature (dark sandstone) of 79.3 °C in the Tibesti Mountains. This temperature is thought to be the highest rock surface temperature ever recorded. Meanwhile, Waragai (2019) reported the results of field measurements conducted during the dry season at Cambodia. The range of the surface temperature of the sandstone specimen varied from a 1.50 °C/min increasing rate to a −1.88 °C/min decreasing rate. As possible temperatures due to insolation, the heating–cooling range and RTC were therefore set inside the chamber: the specimens were heated from 4 °C to 84 °C over 40 min after cooling from room temperature to 4 °C over 8 h and 8 min. Then, the specimens were maintained at 84 °C for 4 h and then cooled to the initial temperature of 4 °C over 40 min. In the test, the temperature change of 4–84 °C was repeated four times. After that, the P-wave velocity of the specimen was measured using a TICO.