Initial setting time In Short: Time delay from point of water addition to the point when the cement paste begins to stiffen considerably in other words, it is that time period between the time water is added to cement and time at which 1 mm square section needle fails to penetrate the cement paste (1:3), placed in the Vicat’s mould 5 mm to 7 mm from the bottom of the mould. Final setting time In Short: Time delay from point of water addition to the point When the cement paste has hardened to the point at which it can sustain some load in other words, it is the time period between the time of water addition to cement and the time at which 1 mm needle makes an impression on the paste in the mould but 5 mm attachment does not make any impression. For all the cement in India except (Masonry cement & Railway Sleeper cement), BIS Specifications are: Initial Setting Time ...
Plastic shrinkage in concrete occurs immediately after concrete has been placed due to settlement of large solid particles by gravity action. Due to this, water in the concrete rises to the surface. This process is also called bleeding of concrete. Bleeding in concrete continues till the layer of water on the surface of concrete has set. As long as the rate of evaporation is lower than the rate of bleeding, there is a continuous layer of water at the surface known as “water sheen”, and shrinkage does not occur. When the concrete surface loses water faster than the bleeding action bring it to the top, shrinkage of top layer takes place, and since the concrete in plastic state can’t resist any tension, cracks develops on the surface. These cracks are common in slabs. The extent of plastic shrinkage depends on: Exposure to the heat from sun radiation, Relative humidity of ambient air and velocity of wind Rate of Initial heat hydration of cement Water absor...