Ultrasonic Time-Frequency Response Characteristics for Damage Evolution of Back-Filling Concrete under Staged Loading Conditions

Table 2

.Characteristic stress value in the process of graded loading of concrete sample.

Specimen	σc (MPa)	σcc (MPa)	σcd stress ratio (%)	σcd (MPa)	σcc stress ratio (%)
D-1	47.88	6.30	13.16	39.35	82.18
D-2	41.30	6.57	15.90	33.38	80.82
D-3	38.16	5.96	26.20	27.48	71.99
D-4	39.04	6.92	17.73	30.36	77.76