In Japan many reinforced concrete buildings suffer from concrete cracks, especially leakage of rain through crack, which come primarily from hardening and drying shrinkage. Hitherto many investigations were made to prevent shrinkage crack of concrete. It is so far known that increasing the gypsum content of cement improves the shrinkage tendency of cement paste and mortar. At the same time, increasing the gypsum content under the present cement manufacturing process is liadle to cause the false set, and so it is disagreeadle today to increase the gypsum content of cement of cement. Here the author recognized the same effect of admixing gypsum with concrete as increasing the gypsum content of cement, and in this way tried to prevent cracking of concrete due to hardening and drying shrinkage. After the dasical study in the ladoratory, reinforced concrete roof slabs of apartment houses were chosen as field test objects and it was proved that the shrinkage cracks in concrete can be actually prevented by admixing the proper amount of gypsum with the original concrete mix. The adopted gypsum admixing ratio to the cement is 4 to 5% by weight, calculating gypsum as CaSO_4 constituent.
