Chemical prestressing
In prestressed concrete, normally tendons are stretched and are retained in a stretched condition by concrete. Consequently, concrete comes under compression. In the method of chemical prestressing or self-stressing, special types of cements, called 'Expanding cements', are used in concrete. A typical expanding cement may consist of around 75% Portland cement, 15% High-alumina cement and 10% gypsum. Using this cement, a linear expansion of around 3-4% may be achieved in concrete. When concrete expands, the tendons also expand in length and tensile prestresses are introduced in tendons. These tensile stresses are transferred to concrete in the form of compressive stresses.