Ancient concrete - Modern concrete is made of cement, water, sand and gravel; the main component of ancient cement is quicklime, which is made by heating limestone. In 2500 BC, there were lime kilns, but the earliest known building with strong concrete was built in West Asia about 700 BC. On a 262-meter-long aqueduct bridge that has been preserved in Zewin, Iraq, a 0.9-meter-thick concrete layer was laid along the waterway. From 200 BC to 400 AD, the ancient Romans also used concrete materials to build the domes of the emperor's baths, the large domes of temples, underground channels, etc.
Reinforced concrete - the earliest user was the French florist Monier. In 1867, he used cement-covered angle wire to create water basins and flower pots. He later applied this method to making beams, floor slabs, pipes and bridges, and then obtained the patent for placing vertical and horizontal iron bars in concrete. The iron bars are in tension and the concrete is in compression. This method is still used today.
Prestressed machine concrete - German architect Docherin invented prestressed concrete in 1886, and France's Freycinet further conducted research in this area from 1940 onwards. Freisinet's idea was to stretch the steel bars while the concrete was still wet, so that the steel bars would bear the tension; after the concrete solidified, the tension would be relaxed; in this way, the areas of the concrete that would be in tension under normal loads would be subject to pre-stressing due to compression. pressure. If the preloaded pressure is greater than the tension from the weight, the concrete will only be stressed. A prestressed concrete beam can use less steel and less concrete than a reinforced concrete beam that can bear the same pressure.