I've been to places in Europe where concrete structures built by the Romans still stand in spite of the centuries. The same hold true in the Americas with structures built by the Aztecs, Mayans, and Incans. Volcanic ash? Makes a lot of sense for many places, including Mexico where huge volcanoes exist. But Yucatan and Peru? Should be interesting what they discover. Read the story @ http://www.dailymail.co.uk/sciencet...tml?ITO=1490&ns_mchannel=rss&ns_campaign=1490
Some structures are just made so they will never collapse such as the pyramids, which are extremely resilient since they have little complexity. Roman achitecture however was much more complex, it took accurate calculations of structural weights in order to make the most durable.
They are still trying to find out how Roman concrete was made. It has stood the test of time so I would have to say Roman concrete was better. I looked into it when I was building my outside pond.
If old concrete structures still stand it's due to luck , historical preservation and good engineering, roman concrete is not better than concrete we use today, they had no secret knowledge that modern structural engineers have not surpassed...the largest roman concrete structure surviving the Pantheon did collapse once and the romans rebuilt it, sound engineering and church protection is the reason it survived all these centuries...
Concrete has an interesting chemistry that they still do not fully understand. Basically, it seems that silicon-oxygen (and to a lesser extent aluminum-oxygen) linkages hold the whole thing together. One of the main ingredients is "lime" (calcium oxide), which is very alkaline and partially breaks down the silica, allowing it to later reform. The lime, together with gypsum, also acts as a dehydrating agent, eventually getting rid of any excess water, and encouraging the hydrated silica/silicates to fuse together and solidify. The dehydrated silicates and aluminates form an intermolecular network, while the calcium ions help hold things together. The presence of water (in solid hydrated form) also adds strength. This may seem counterintuitive, but on a small scale molecules of water really have a large amount of "stickiness". This is basically the same reason one can make sand castles out of wet sand.
I saw a program on one of the Discovery channels and a concrete expert thought the same thing. So they reproduced the formula for Roman concrete as best as they could and it had a higher crush than modern concrete, They used the volcanic sand, lime, clay, and pig blood. It could be that the Roman concrete is better because of the "junk" they put into the modern stuff to save money. Their concrete last for centuries and modern concrete last about fifty years.
Cost effectivness is a factor, as are engineering requirements... the Pantheon is an amazing structure that used a good quality concrete but I'd give more credit to the engineer who built it, he knew what he was doing...better concrete is more expensive and not required for every structure...if the job requirement calls for 1000psi concrete why pay more for 4000psi...if emperor who commisioned the Pantheon wanted the best it wasn't wise to dissapoint him with substandard materials that would fail....there are concrete structures from ww2 that will be around for centuries, I've seen them and there were no signs of decay, they were built to a higher structural standards... The romans like people everwhere used materials they had at hand, they had and endless supply of volcanic ash ...the hagia sophia another domed roman structure collapsed so engineering and building materials did not match the pantheon...both buildings are on my bucket list...
Most of the changes in concrete today are twofold, for real reasons. First of all, in modern concrete structures they tend to use iron to stiffen it. 2,000 years ago engineering was on a massive scale, and things were grossly over-engineered because of this. They generally did not use composite materials, things like iron or other metal skeletons inside of a structure to give it resistance to movement and flexing. Just pile on more and more concrete, make the base very thick and the upper parts very thin. The concept of load balancing on an internal skeleton would not develop for almost 2,000 years. And when you add things like iron or steel, you then add another weakness. In modern structures one of the problems is that you have to keep the iron completely dry, or it rusts. One problem we are now seeing in 100 year old structures is that the iron is rusting inside of the concrete, making it bulge and crack. So it is not the concrete that is failing, but what we are putting the concrete around to make it stronger. Then the other issue with modern concrete, curing times. Roman concrete often took months or years to cure, much less then the hours or days of more modern mixtures. Could you imagine a modern engineer saying "Yes, we will have the shell of your structure up in 6 months, but you have to wait another 6 months before we can go in and complete the structure"? The famous aqueducts normally were allowed to cure for a year or more before they were put into use, could you imagine the uproar of a modern bridge builder telling people that their bridge is complete, but they have to wait a year to use it? Durability is a known trade-off in relationship to curing times. Curing hardens the structure, and allows water and air to be forced out of the mixture. Longer curing, stronger concrete. Faster curing, quicker to be able to use the resulting structure. And guess what everybody chooses when comparing these two factors?
I also read that the Romans packed their concrete rather than just pouring it. Their is a big dam built (somewhere) that all the concrete is being packed in. It is supposed to make it stronger. And yes...I did also read that steel reinforcement can make concrete crack because the steel expands as it rusts.
Aggregates are also what gives concrete it's strength...which is different than mortar which doesn't contain aggregate...
Dry time is crucial, too quick and the concrete can spall, once that happens deterioration accelerates...keep it damp as long as possible is the rule I learned from concrete workers...if the surface is drying too quicly they'll sprinkle water on it and shield it from sunlight using poly...it's not the simple process us laymen assume it is there's a wee bit of black magic involved(chemistry/physics)... I think there's a misconception romans used a lot of concrete, they employed dry stone base for many large structures that required stability like bridges and aqueducts but ordinary bricks and mortar for most of their structures...the difference between mortar and concrete being the water (bonding)ratio and lack of aggregate....and they weren't above using a dry rubble fill to save time and expense... - - - Updated - - - Rammed earth technique perhaps?
