Modern concrete differs from ancient Roman concrete in a number of ways, which is why old concrete is actually stronger than new concrete. If you look at some of the more iconic Roman structures still standing today like the Colosseum, you’ll note that they’re still standing thousands of years later despite being made from concrete.
So how is this possible? How are ancient Roman structures built using concrete still standing today, while modern concrete structures start disintegrating in as few as 50 years?
Continue on for an in-depth breakdown of why old concrete is stronger than new concrete, and what made ancient Roman concrete so durable and long-lasting.
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Why is Old Concrete Stronger than New Concrete?
Older concrete is able to withstand far more wear and tear than new concrete is. The reason for this is fairly simple: Old concrete – specifically roman concrete – used a specific sort of volcanic ash in its formulation.
This volcanic ash contained rare earth minerals that caused it to increase in hardness over time. Interestingly, today we don’t know what specific type of volcanic ash this is, so we aren’t able to replicate the process today.
Old concrete used a mixture of volcanic ash, lime, and seawater, which were then mixed with volcanic rocks and poured into molds, and immersed in more seawater. The seawater then dissolved components of the volcanic ash, which then created a rare hydrothermal element known as aluminum tobermorite (Al-tobermorite).
This element is possible to make in a lab but is difficult to incorporate into modern concrete.
As seawater was used in the creation of old concrete, you’ll find that many ancient Roman seawalls are not only still standing, but actually get stronger and stronger over time due to the additional minerals strengthening the concrete’s chemical bonds. Modern concrete on the other hand is damaged by salt, so it won’t perform nearly as well in saltwater environments.
Why Can’t We Use the Same Concrete Today?
If ancient Roman concrete is so durable and long-lasting, then why can’t we simply use the same stuff nowadays in modern construction?
The basic concrete mix we use for most of modern construction is pretty cheap stuff. Even if you were to find a way to easily incorporate the rare mineral Al-torbernite into modern concrete, the cost would still be prohibitive.
While we have the basic formula for ancient Roman concrete, we still don’t know how to replicate the process precisely. This means for the time being we are stuck with concrete mixes that don’t have the longevity of the ancient Roman stuff.
Old Concrete vs New Concrete
Modern concrete mixes use a mixture of Portland cement, aggregate, sand, and water in various proportions. The mix is formed into a paste which is then poured into forms to harden. The aggregate, sand, and cement harden over time and cure into a rock-solid, hardened surface.
Portland cement acts like the glue that keeps the other ingredients together. It’s made from silica, clay, limestone, and chalk. As the cement dries it forms bonds between the sand and aggregate, which in turn leads to a strong final product.
Roman concrete – while it may be similar looking – uses different ingredients and a completely different process. Old concrete used volcanic ash, lime, and seawater to create the equivalent of the Portland mortar in modern concrete.
The mortar portion was then mixed with volcanic rock and seawater. This mortar would then react with the volcanic rock to form an extremely strong and dense concrete that would last for centuries.
How Strong is Modern Concrete?
Modern concrete’s strength is measured in PSI (pounds per square inch). This is a measure of the concrete’s compressive strength – which measures the concrete’s ability to handle compressive loads.
The higher the number, the more compressive strength the concrete has, and the more load it will be able to handle. The major factor influencing its strength is the ratio of cement, water, and aggregate in the mix.
Generally speaking, the higher the proportion of water to cement used, the weaker the final product will be. The ratio of coarse to fine aggregate also plays a role in the final strength, with a higher proportion of fine aggregate resulting in a weaker product.
Modern concrete varies from 2,500 to 3,000 for a standard mix to about 4,000 for a high-strength mix.
Other factors influencing modern concrete’s final strength are the additives used, rebar, the slab’s thickness, and the proper curing process.
How to Increase Modern Concrete’s Strength?
There are a number of ways you can increase the strength of modern concrete. You still won’t achieve the results that you might expect with Roman concrete, but unfortunately, that is all but impossible.
First off, using rebar will drastically increase the strength of any concrete project when compared to using plain old concrete. Rebar is what enables skyscrapers, bridges, huge slabs, and the like, as concrete will crack fairly easily if you try to build too big of a project without rebar reinforcement.
Concrete has high compressive strength as mentioned previously, but it is fairly weak in terms of tensile strength. Tensile strength is the amount of load or stress that a material can handle until it stretches and breaks under pressure.
Rebar will greatly increase tensile strength, which is what is needed when building large structures with concrete.
Wrap Up
While it sounds kind of crazy, ancient concrete is actually a superior building product to modern concrete. It’s far more durable and actually strengthens as time goes by rather than weakening. This is especially noticeable in concrete used near the ocean, as much of the ancient roman stuff is still standing today, while modern concrete cracks and breaks down quickly in a marine environment.
The secret to the strength and longevity of old concrete is the rare earth minerals contained within it. These minerals increased the strength of the bonds between the concrete’s components. Unfortunately, we can’t replicate this process today, at least not in a cost-effective manner – so we’re left with concrete mixes that must be sealed to prevent damage from excess moisture.
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