The research team discovered that while modern concrete is made to be inert, the Roman version interacts with the environment. When seawater interacts with the mixture, it forms rare minerals aluminous
Roman concrete has 'self-healing' properties that have helped it to last for millennia. Researchers analysed 2,000-year-old Roman concrete samples and found lumps of lime in the mortar — you might have spotted these distinctive bright white clumps of calcium carbonate on your latest trip to the agora.
Their findings suggest that the lime clasts can dissolve into cracks and recrystallize after exposure to water, healing cracks created by weathering before they spread. The researchers said this self-healing potential could pave the way to producing more long-lasting, and thus more sustainable, modern concrete.
Based on the team's spectroscopic examination, it seems like Roman concrete was probably made by mixing the calcium carbonate with the pozzolanic material and water at very high temperatures, a process called 'hot mixing'. The team had now concluded that 'hot mixing' was the key to the concrete's super-strong nature.
Concrete is typically believed to last forever. While it may have ancient durability, its life span doesn't usually exceed 100 years. Architects recognize concrete as a stone-like, homogeneous material, a mix of limestone and other rock.
There's also a load-bearing issue. "Ancient" is the key word in these Roman structures, which took a long, long time to develop their strength from seawater. Young cement built using a Roman recipe would probably not have the compressive strength to handle modern use — at least not initially.
Does concrete take 100 years to cure? No, this is a bit of a myth with the concrete industry. While concrete does continue to harden indefinitely, pore moisture has to drop below a certain level at some point and this isn't typically 100 years.
Its disadvantages include the longer drying time and somewhat lower strength than modern concrete, despite its greater durability. It is no coincidence that the walls of Roman buildings are thicker than those of modern buildings.
The Roman recipe – a mix of volcanic ash, lime (calcium oxide), seawater and lumps of volcanic rock – held together piers, breakwaters and harbours. Moreover, in contrast to modern materials, the ancient water-based structures became stronger over time.
There are also other chemical processes such as sulphate attack, lime leaching and alkali-aggregate expansion all of which degrade modern concrete. Some submerged concrete objects may last only 10 years; meanwhile, 2000-year old concrete constructed during the Roman Empire is still going strong (Figure 1).
Researchers have been trying for decades to re-create the Roman recipe for concrete longevity — but with little success. The idea that hot mixing was the key was an educated guess. Masic and colleagues had pored over texts by Roman architect Vitruvius and historian Pliny, which offered some clues as to how to proceed.
It turns out the ancient Romans had the perfect recipe for water-resistant concrete. The material, called opus caementicium by the Romans, is made from a hydraulic cement, meaning it can set underwater or in wet conditions.
Ancient Roman concrete is known as some of the strongest in history, and a new study finally explains why. MIT researchers studied the self-healing properties of the concrete mix. Extreme temperatures while mixing created quicklime, meaning hot mixing leads to self-healing.
According to the American Concrete Institute, concrete gains 90% of its strength within the first 28 days of curing. However, the concrete continues to gain strength over time, with some concrete structures becoming stronger even after 50-100 years.
It was a formidable tool of Roman engineering know-how. Yet, for all its advantages, concrete had one major defect: it was unsightly. Once the wooden formwork was removed, it showed an ugly surface. In the beginning, its use was mainly restricted to substructures where noone would see it.
Curing times for concrete
Technically, concrete never stops curing. In fact, concrete gets stronger and stronger as time goes on. But, as far as we're concerned, to reach a practical strength, most industrial concrete mixes have a 28 day curing period.
Incredible facts about Roman concrete
Reinforced concrete (reinforced with steel rebar) did not exist. As a result, buildings lasted longer as they did not suffer from steel corrosion. Pozzolana (derived from volcanic dust) made the concrete more resistant to saltwater than our modern-day concrete.
One of the main reasons so many concrete structures from the Roman era are still intact today is that concrete has the ability to last potentially more than a thousand years (Taylor 4).
The loss of 'Institutional Memory' (also referred to as corporate or organizational memory) is a perennial issue for organizations and even whole civilizations. Humans forgot how to make concrete for over a thousand years. The Romans perfected the recipe around 150 BCE.
The use of seawater as a mixing and curing agent has proven to be a secret recipe for Roman concrete's unique strength and durability that gets better with age. Roman concrete is not only solid but durable due to the way it sets and cures. The hydration process of Roman concrete undergoes centuries.
Materials, Methods and Innovations
because it was more durable. Also, its off-white color made it an acceptable substitute for marble.
Steel construction requires 70% fewer working hours than concrete. The rapid work rate means that facilities can be commissioned faster. In addition, savings in materials themselves should also be noted. A steel structure is 30-60% lighter than a similar one made from concrete.
Concrete that is not moist-cured at all dries too rapidly, and reaches less than half its potential design strength. It will also have a greater number of shrinkage cracks.
The answer is that concrete never cures completely. It is always hardening a little bit more each day. The way concrete hardens is a function of the cement particles reacting with the water it is mixed with. As the cement bonds with the water molecules, the concrete gets harder.
Curing time of concrete is typically 24-48 hours, at which point it's safe for normal foot traffic. After one week, concrete is typically cured enough to handle continued construction including heavy machinery. Concrete is recognized to have reached full strength 28 days after placement.