1975 年 24 巻 261 号 p. 534-539
When a concrete specimen is subjected to repeated loading or sustained loading, its strength does not always decrease but sometimes increases. It seems likely that the increase or decrease of strength of concrete results from its internal microcracking. The main purpose of the present paper is to verify this presumption.
The compressive strength of concrete specimens after a repeated loading, and the microcracks developed due to the repeated loading were studied. The major repeated load levels studied were 44, 59, and 74% of the ultimate strength, The numbers of load cycles were 0, 2, 4, and 15.
The microcracks developed in the concrete by the repeated loading were divided into four types, viz., the paste, bond, aggregate, and mortar cracks. The paste cracks were tiny microcracks which developed in cement matrix. They were examined with a microscope using 150X. The three microcracks other than the paste cracks were investigated with a stereomicroscope using 10X.
The test results obtained are as follows. (1) With increasing number of load cycles, the strength of concrete fluctuates rhythmically, (2) Whether the strength increases or decreases after the repeated loading depends on the microcrackings, and (3) The paste cracks raise the strength of concrete but the other microcracks, i.e., bond, aggregate, and mortar cracks, lower it.