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Prestressing is a method of concrete construction in which internal stresses are introduced into the concrete to counteract the stresses that will develop when the structure is subjected to external loads. This pre-compression significantly improves the concrete's tensile strength, allowing it to resist cracking under tension and enabling the design of more slender and efficient structural elements.

The process typically involves embedding high-strength steel tendons or bars within the concrete. These tendons are then tensioned, either before or after the concrete hardens. If tensioned before hardening, it is called pre-tensioning. In this method, tendons are anchored to bulkheads, tensioned, and then the concrete is cast around them. Once the concrete gains sufficient strength, the external anchors are released, and the tendons transfer their tension to the concrete through bond.

If tensioned after the concrete hardens, it is called post-tensioning. Here, ducts are placed within the formwork, and concrete is cast around them. After the concrete cures, tendons are threaded through the ducts and tensioned using hydraulic jacks. The tendons are then anchored to the concrete structure, and the ducts are often grouted to protect the tendons and improve load transfer.

Prestressing is widely used in bridges, buildings, water tanks, and other structures where long spans, heavy loads, or resistance to cracking are critical. It allows for reduced material usage, increased durability, and aesthetically pleasing designs due to the ability to create thinner sections.