Table 5. Residual compressive strength ratio estimated by ultrasonic pulse velocity test.

Year elapsed since installation (yr)		1	2	3	9	16
Mean compressive strength (MPa)	Damaged	22.8±6.5	24.0±5.7	23.8±3.1	14.5±7.2	9.7±3.4
	Undamaged	27.3±4.5	27.9±6.4	25.0±6.8	19.8±14.6	12.9±5.9
Residual compressive strength ratio (%)		83.6	86.1	95.3	73.2	74.8