1. Pre-stressed concrete uses steel tendons that are tensioned before or after the concrete is poured to put the concrete in compression and improve its strength. 2. There are two main types: pre-tensioned concrete, where tendons are tensioned before the concrete is poured, and post-tensioned concrete, where ducts are cast in and tendons are tensioned after the concrete cures. 3. Advantages of pre-stressed concrete include increased strength, reduced cracking and corrosion, higher span-to-depth ratios, and economic benefits. However, it requires experienced engineers and builders and sections can be brittle.