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It is well known that concrete fails at a stress smaller than its static strength, when subjected to sustained or repeated load. But the information on failure of concrete under these loads is meager, and the information on ability of concrete to resist these loads has never been established to the extent which it can be accepted without question.In this paper, the stress-strain relation for concrete beam subjected to flexure was studied and the following conclusions were obtained.(1) The relationship between flexural stress and tensile strain, plotted on logarithmic graph, is rectlinear up to a certain point, and at this point, the slope of stress-strain relation varies abruptly.(2) Test Results show that the strain corresponding to the first point at which the slope of stress-strain relation varies represents maximum deformation which concrete is capable of developing without undesirable effects.Then, the author named it "deformation limit" of concrete.

This study was firstly undertaken to ascertain the formation of microcracks in cement paste (hereafter called paste cracks) under compressive load, and secondly to obtain the information concerning these cracks.this test were normal portland cement and river sand, and they were mixed in the proportion of 1 to 2.09 with a water-cement ratio of 0.5. The cylinders were cured for one day in mold, then placed in water for 28 days and finally stored in the room atmosphere. The age of the specimens at test was about three months old.The cylinders were loaded statically until they received the strains of 0.6, 3.0, 6.8, 8.5, hours to stain the cracks and then dried in air. The same treatment was also given to broken specimens. Thin slices were made from the stained near-surface portion of the specimens andThe results obtained are as follows:(1) The paste cracks appear even by application of very small stress.(2) The paste cracks can be divided into two types, viz., the hair-shaped cracks and the void-shaped cracks.(3) The magnitude of strain has little effect on the number and length of the hair-shaped cracks.(4) On the contrary, the void-shaped cracks propagate with increasing strain, and good agreement exists between the strain beyond which the rate of deformation increases rapidly and the strain beyond which the total area of void-shaped cracks increases rapidly.

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