This does not have much to do with casting concrete in molds or forms, but it is interesting. I wondered just how long concrete had been around after reading an article that claims that the pyramids are made of concrete blocks. http://www.geopolymer.org/science_archaeology/pyramids_egypt/index.html - if you want to see it. It looked interesting so I looked up Roman concrete. They where known to be using concrete and molds 2000 years ago. The cement part was the same as today. The difference was the way it was obtained. Modern cement is made from limestone in cement plants. It is crushed and ground to a fine powder, and cooked at very high temperatures. In Roman times they used volcanic ash (composed of silica and some aluminum and iron oxide) and lime. This was mixed with sand in as dry a mix as possible on site and placed over layers of rock that had been pre set. Then they pounded the concrete mix into the rocks until the spaces between the rocks where filled with the mix. Then another layer of rocks, more concrete mix, etc. Until the thickness that was wanted was reached. It was a good system, the Pantheon is still there in Rome along with a lot of other buildings made this way over 2000 years ago. Around 1980 or so this method was "rediscovered" and is often used now. With modern equipment such as mobile vibrators and vibratory rollers it is used to make Dams, airport runways, and in other places where strong concrete is needed. This is stronger than poured concrete because there are less voids. Voids cause weakness. It is also cheaper because no rebar is used. some say it is environmentally friendly, -less steel, less water and runoff, and less cement. In modern poured concrete the voids are compensated for with rebar. The romans did not have steel, and their concert didn't need it! Some of their buildings used clay bricks in the concrete and some had bronze where we would have rebar. This was not a common practice, but was used on roofs and arches. I wonder if some off our modern buildings will be around in 2000 years? Probably not. The Pantheon will be 4000 years old then, but probably be starting to show its age. For more on this search for "roller compacted concrete". Here is a link to a dam in CA. http://www.cncpc.org/pages/media.html Those of you that live downstream from a dam may be particularly interested. copyright 2005 Delmar Germyn http://www.delsmolds.com |
