Post-tensioning is a technique where high-strength steel cables are tensioned (pulled tight) after concrete has hardened, compressing the concrete and improving its strength and crack control. The cables are installed in plastic ducts embedded in the concrete before casting. After concrete cures (typically 24 hours), the cables are tensioned using hydraulic jacks that pull the cables against the concrete. Anchors at each end of the cables hold the tension permanently after the jacks are removed. Post-tensioning reduces the amount of conventional reinforcement needed and allows longer spans and thinner sections. In floor systems, post-tensioning allows spans of 40 to 60 feet or more, compared to 20 to 30 feet for conventional reinforcement. Thinner slabs reduce building height and material cost. The technology is widely used in parking structures, bridges, and other long-span structures. Installation of ducts during concrete forming must be precise; ducts route through the concrete from one anchor point to another. Ducts may be straight, draped (following the bottom of the member for strength where needed most), or in complex patterns. The steel cables are inserted into the ducts after concrete placement and before tensioning. Grouting of the ducts after tensioning prevents corrosion of the cables. Specialized equipment and trained technicians are required for tensioning. Hydraulic jacks apply force gradually and evenly to avoid overstressing the concrete. Load cells measure tension as the cables are pulled. Records of tension applied are maintained for each cable. Post-tensioning systems have been used successfully for decades and provide excellent durability when properly designed and installed.