Abstract
Although the consistency of concrete can be increased by adding vibration, the effect of vibration is continually lowering in the setting process of concrete, and the rate increase rapidly according as the hydration of cement actively. The hydration process of cement is divided into dormant period and acceleratory period according to its characteristics. In this paper we have discussed that the expiration of dormant period can be clearly specified by the characteristics of the change with time of the dynamic shear modulus of cement paste and mortar, and that the properties of revibrated concrete show the remarkable change according to the time when the vibration is added;that is dormant period or acceleratory period. Thus the boundary time was defined as the initial set. It is cleared that the initial set is affected by a water-cement ratio of mixtures;i. e. the lower the water-cement ratio, the later the time of initial set, and that the initial set is independent of the content of aggregate. Furthermore we have discussed on the standard method of test for the time of setting of concrete mixtures by penetration resistance, then following were cleared. The penetration resistance is insensible to hydration of cement (as is estimated from the fact that the penetration resistance is independent of water-cement ratio of mixtures). Therefore the progress schedule of concrete works may not be controlled well by a fixed value of penetration resistance. To provide a practical usage for penetration resistance test method, the penetration resistance for initial set must be classified according to the mix of concrete.
