EXCESS OF THE WATER CEMENT RATIO MAY PRODUCE PORES IN THE CEMENT PASTE .

WATER CEMENT RATIO :-- Water is used for mixing and curing shall be clean and free from injurious amounts of oils ,acids ,alkalis ,salts ,sugar and organic materials or other substances that may be deleterious to Concrete or the steel . Potable water is generally considered satisfactory for mixing of the concrete. For Complete hydration of a given amount of cement, an amount of water equal to about 25 percent of the cement by weight is needed for the chemical reaction .An additional about 10to 15 percent is added to provide mobility for the water in the cement paste during the hydration process so that it can reach the cement particles. This makes for a minimum water cement ratio of 0.35 to 0.40 percent by weight . Water cement ratio in concrete are generally considerably larger than this minimum, to provide the necessary workability of the Concrete mix .Any amount of water above the 25 percent of consumed in the chemical reaction produces pores in the cement paste .Since only the solids and not the voids resist the stress , strength increases directly as the fraction of the total volume occupied by the solids increases, thus the strength of the cement paate decreases directly with increasing the water cement ratio. The chemical process involved in the settings and hardening liberates heat , known as heat of hydration.In large concrete masses, such as dams this heat gets dissipated very slowly and therefore there will be rise in the temperature consequently increases in the volume of concrete during hydration and subsequently on cooling there will be contraction and cracking. To avoid the serious cracking and weakening,special measures must be taken and low heat Portland cement be used in the dams and the bridges .