## 2 Materials and Methods  
### Experimental Setup  
To examine the characteristics of acoustic emission signals throughout the damage progression of reinforced concrete columns subjected to eccentric compressive loads and to delineate the signals correlating with various failure modes, experimental investigations were undertaken on RC columns involved in eccentric compression tests. Figure 1 demonstrates the geometric dimensions and reinforcement configuration of the RC column. The experiment used the Chinese standard (GB 50010-2010) to prepare reinforced concrete columns with a strength grade of C30, using ordinary Portland cement produced by Xinjiang Tianshan Cement Co., Ltd. (Urumqi, China), with a strength grade of 42.5 MPa as the cementitious material. The concrete mix proportions by weight were formulated as follows: cement/water/sand/coarse aggregate = 2.375:1:4.42:6.58; this meant that the required amounts of water, cement, sand, and stone per cubic meter were 170 kg, 404 kg, 751 kg, and 1119 kg, respectively. The specimens featured longitudinal reinforcement spaced at 150 mm intervals, symmetrically arranged with two 8 mm diameter HPB300 steel bars (Chinese standard GB 50010-2010) per side, exhibiting a tensile strength of 300 MPa. A concrete cover thickness of 25 mm was maintained throughout. The coarse aggregates featured a diameter range of 15-31 mm, while river sand was employed as the fine aggregate. The cubic specimens, measuring 150 mm, were cured under standard conditions for a duration of 28 days, achieving an axial compressive strength of 38.6 MPa. Furthermore, the RC columns were demolded 24 h post-casting and subsequently cured for 28 days under regulated environmental conditions with a relative humidity maintained at 90% \(\pm\) 5% and a temperature set at 20 \(\pm\) 2 \({}^{\circ}\)C.  
Eccentric static loading tests on the RC columns were conducted using a servohydraulic testing machine (JSKF-IV/31.5-8) with a maximal loading capacity is 60 t, manufactured by Chengdu Servo Hydraulic Equipment Co., Ltd. in Chengdu, China. The load was incrementally applied in 5 kN stages until the load-bearing capacity stabilized, after which testing proceeded under displacement control at a rate of 1 mm/min. The test was terminated when the load capacity dropped to 80% of its peak value. A 2 kN preload was applied prior to data acquisition to enhance equipment stability. Figure 2 illustrates the experimental setup and instrumentation. Dial indicators with a measurement range of 0-50 mm were positioned at points 1, 2, and 3 in the schematic diagram to monitor displacement at these locations.  
Displacement strain gauges were uniformly distributed along the mid-height region of the column. Gauges were positioned at 40 mm intervals across the width, with the central gauge aligned along the column's longitudinal axis. The strain gauge was the BX120-80 AA type with a size of 80 \(\times\) 3 mm\({}^{2}\), and the data acquisition instrument uses the DH3818Y static stress-strain testing and analysis system from Donghua Company, Jiangsu, China. According to the critical eccentricity (e = 55 mm), the eccentric load was applied at a position offset 100 mm and 50 mm from the column centerline. AE signals were monitored using sensors and a data acquisition system from Qingcheng AE (4-channel SAEU3H acoustic emission detector Manufactured by Qingcheng Sound Emission Research Co., Ltd., Guangzhou, China). The model of the acoustic emission sensor is Qingcheng AE GI150, with a resonant frequency of 150 KHz and a frequency range of 60-400 KHz. The built-in amplifier has an amplification amplitude of 40 dB. Four sensors (S1, S2, S3, S4) were mounted as shown in Figure 2a. To minimize noise interference, the AE detection threshold was set to 45 dB.  
Figure 1: Reinforcement of specimens.