This paper describes two sets of results of long-term creep tests by bending of rock beams, one for large granite beams (215×12.5×6.8cm) loaded since 1957 and the other for small beams of granite (21×2.5×2.0cm) and gabbro (16×2.0×1.5cm) loaded since 1974. The results during last 27 years for the large beams and 10 years for the small beams are summarized as follows.
(1) The creep curve of each specimen showed an undulatory pattern accompanying “turn-backs” in deflection. The most remarkable turn-back gave a reduction of over 10% on the total deflection in the duration of about 650 days.
(2) The initial and the delayed deflections due to elasticity changed depending upon specimens. With the small specimens, the delayed deflection lasted for 200 to 300 days, reaching 10 to 45% of the initial deflection.
(3) Owing to reconstruction of the laboratory buildings, the large specimens were obliged to move twice after 10 years and 24 years since the start of the experiments. The creep curve for the unloaded beam (the maximum bending stress of 12.8kg/cm
2) showed a discontinuous increase in deflection after the first move and a discontinuous decrease after the second move, surprisingly with no change in the creep rate in the general trend. On the other hand, the creep curve for the center-loaded beam (the maximum bending stress of 24.8kg/cm
2) was not affected by these two moves.
(4) Taking the general trend as the secondary creep, the viscosity of the tested specimens was calculated by neglecting the yield stress. It was 3×10
20 to 6×10
20 poise for granite and 0.9×10
21 to 6×10
21 poise for gabbro.
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