抄録
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.
The tests were made on cylindrical mortar specimens of 5×10cm. The materials used in 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, 12.3 or 15.8×10-4. After removal of the load, each specimen was soaked in red ink for 24 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 and microcracks were examined with a microscope using 150×(sometimes 600×) magnification.
The 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.
