The largest unreinforced concrete dome in world is on the Pantheon. It’s not a modern marvel, but rather an ancient Roman temple built almost two thousand years ago. So, if concrete structures from the western Roman Empire can last for thousands of years, why does modern infrastructure look like this after only a couple of decades? Hey I’m Grady and this is Practical Engineering. In today’s episode, we’re taking a look at the factors that affect the design life of concrete.
If you haven’t seen the previous videos in this series about concrete, here’s a quick synopsis. We’ve talked about how concrete’s made, why it often needs reinforcement, and how that reinforcement can sometimes lead to deterioration. Concrete reinforced with steel bars is the foundation of our modern society. The reinforcement is required to give the concrete strength against tensile stress. We use steel as reinforcement because of its strength, its similar thermal behavior, its availability, and low cost. But steel has an important weakness: it rusts. Not only does this corrosion reduce the strength of the reinforcement itself, but its by-product, iron oxide, expands. This expansion creates stresses in the concrete that lead to cracking, spalling, and eventually the complete loss of serviceability - i.e. failure. In fact, corrosion of embedded steel reinforcement is the most common form of concrete deterioration. But it hasn’t always been that way.
The Romans got around this problem in a very clever way: they didn’t put steel in their concrete. Simple enough, right? They harnessed the power of a few clever structural engineering tricks like the arch and the dome to make sure sure that their concrete was always resisting compression and never tension, minimizing the need for reinforcement. One of those clever tricks was just making their structures massive, and I mean that literally, because the simplest way to keep concrete in compression is to put heavy stuff on top of it, for example, more concrete. We use this trick in the modern age as well. Most large concrete dams are gravity or arch structures that rely on their own weight and geometry for stability. In both gravity and arch dams, the shape of the structures are carefully designed to withstand the water pressure using their own weight. You can see how they get larger, the deeper you go. So, even with the tremendous pressure of the water behind the structure, there are no tensile stresses in the concrete, and thus no need for reinforcement.
But lack of steel reinforcement isn’t the potential only reason Roman concrete structures have lasted for so long. One of the other commonly-cited suggestions for the supremacy of Roman concrete is its chemistry. Maybe they just had a better recipe for their concrete that somehow got lost over time, and now those of us in the modern era are fated to live with substandard infrastructure. In fact, in 2017, scientists found that indeed the combination of seawater and volcanic ash used in ancient roman concrete structures can create extremely durable minerals that aren’t normally found in modern concrete. But that’s not to say that we can’t make resilient concrete in this modern age. In fact, the science of concrete recipes, also known as mix design, has advanced to levels a Roman engineer could only dream of.
One of most basic, but also most important factors in concrete’s chemistry is the ratio of water to cement. I did an experiment in a previous video that showed how concrete’s strength goes down as you add more water. Extra water dilutes the cement paste in the mix and weakens the concrete as it cures. The Romans knew about the importance of this water to cement ratio. In historical manuscripts, Roman architects described their process of mixing concrete to have as little water as possible, then pounding it into place using special tamping tools. Interestingly enough, we have a modern process that closely mimics that of the ancient Romans. Roller Compacted Concrete uses similar ingredients to conventional concrete, but with much less water, creating a very dry mix. Rather than flowing into place like a liquid, RCC is handled using earth moving equipment, then compacted into place using vibratory rollers like pavement. RCC mixes also usually include ash, another similarity to Roman concrete. It’s a very common construction material for large gravity and arch dams because of its high strength and low cost. Again, these are usually unreinforced structures that rely on their weight and geometry for strength.
But, not everything can be so massive that it doesn’t experience any tensile stress. Modern structures like highway overpasses and skyscrapers would be impossible without reinforced concrete. So, generally we like our concrete to be more viscous or soupy. It’s easier to work with. It flows through pumps and into the complex formwork and around the reinforcement so much more easily. So, one way we get around this water content problem in the modern age is through chemical admixtures, special substances that can be added to a concrete mix to affect its properties. Water reducing admixtures, sometimes called superplasticizers, decrease the viscosity of the concrete mix. This allows concrete to remain workable with much lower water content, avoiding dilution of the cement so that the concrete can cure much stronger. I mixed up three batches of concrete to demonstrate how this works.
In this first one, I’m using the recommended amount of water for a standard mix. Notice how the concrete flows nicely into the mold without the need for much agitation or compaction. After a week of curing, I put the sample under the hydraulic press to see how much pressure it can withstand before breaking. This is a fairly standard test for concrete strength, but I’m not running a testing lab in my garage so take these numbers with a grain of salt. The sample breaks at around 2000 psi or 14 MPa, a relatively average compressive strength for 7-day-old concrete. For the next batch, I added a lot less water. You can see that this mix is much less workable. It doesn’t flow at all. It takes a lot of work to compact it into the mold. However, after a week of curing, the sample is much stronger than the first mix. It didn’t break until I had almost maxed out my press at 3000 psi or 21 MPa. For this final batch, I used the exact same amount of water as the previous mix. You can see that it doesn’t flow at all. It would be impossible to use this in any complicated formwork or around reinforcement. But watch what happens when I add the superplasticizer. Just a tiny amount of this powder is all it takes, and all of a sudden, the concrete flows easily in my hand. In many cases, you can get a workable concrete mix with 25% less water using chemical admixtures. But most importantly, under the press, this sample held just as much force as batch 2 despite being just as viscous as batch 1.
The miracle of modern chemistry has given us a wide variety of admixtures like superplasticizers to improve the characteristics of concrete beyond a Roman engineer’s wildest dreams. So why does it seem that our concrete doesn’t last nearly as long as it should. It’s a complicated question, but one answer is economics. There’s a famous quote that says “Anyone can design a bridge that stands. It takes an engineer to build one that barely stands.” Just like the sculptors job is to chip away all the parts of the marble that don’t look like the subject, a structural engineer’s job is to take away all the extraneous parts of a structure that aren’t necessary to meet the design requirements. And, lifespan is just one of the many criteria engineers must consider when designing concrete structures. Most infrastructure is paid for by taxes, and the cost of building to Roman standards is rarely impossible, but often beyond what the public would consider reasonable. But, as we discussed, the technology of concrete continues to advance. Maybe today’s concrete will outlast that of the Romans. We’ll have to wait 2000 years before we know for sure. Thank you for watching, and let me know what you think!