Why Does Concrete Need Reinforcement?

In the last video, we talked about concrete 101, and why concrete is such a great construction material. But, I didn’t mention its greatest weakness.

To understand concrete’s greatest weakness, first, we need to know a little bit about the mechanics of materials which is the fancy way of saying “How Materials Behave Under Stress.” Stress, in this case, is not referring to anxiety or existential dread but rather the internal forces of the material. There are three fundamental types of stress: compression (pushing together), tension (pulling apart), and shear (sliding along a line or plane). And, not all materials can resist each type of stress equally. It turns out that concrete is very strong in compression but very weak in tension. But, you don’t have to take my word for it. Here’s a demonstration:

These two concrete cylinders were cast from the exact same batch, and we’ll see how much load they can withstand before failure. First, the compressive test. (Hand pump gag). Under compression, the cylinder broke at a load of about 1000 lb (that’s 450 kilos). For concrete, that’s pretty low because I included a lot of water in this mix. The reason is my rig to test the tensile strength isn’t quite as sophisticated. I cast some eye bolts into this sample, and now I’m hanging it from the rafters in the shop. I filled up this bucket with gravel, but it wasn’t quite enough weight to fail the sample. So, I added another dumbbell to push it over the edge. The weight of this bucket was only about 80 lbs or 36 kilos - that’s less than 10% of the compressive strength.

All this to say, you shouldn’t make a rope out of concrete. In fact, without some way to fix this weakness to tensile stress, you shouldn’t make any kind of structural member out of concrete, because rarely does a structural member experience just compression. In reality, almost all structures experience a mixture of stresses. That’s no more clear than in a classic beam. This particular classic beam is homemade by me out of pure concrete here in my garage. Applying a force on this beam causes internal stresses to develop, and here’s what they look like: the top of the beam experiences compressive stress. And the bottom of the beam experiences tensile stress. You can probably guess where the failure is going to occur on this concrete beam as I continue to increase the load. It happens almost instantly, but you can see that the crack forms on the bottom of the beam, where tensile stress is highest and propagates upward until the beam fails.

You see what I’m getting at here: concrete, on its own, does not make a good structural material. There are just too many sources of tension that it can’t resist by itself. So, in most situations, we add reinforcement to improve its strength. Reinforcement within concrete creates a composite material, with the concrete providing strength against compressive stress while the reinforcement provides strength against tensile stress. And, the most common type of reinforcement used in concrete is deformed steel, more commonly known as rebar.

I made a new beam with a couple of steel threaded rods cast into the lower part of the concrete. These threads should act just like the deformed ridges in normal rebar to create some grip between the concrete and steel. Under the press, the first thing you notice is that this beam is much stronger than the previous one. We’re already well above the force that failed the un-reinforced sample. But the second thing you notice is that the failure happens a little bit slower. You can easily see the crack forming and propagating before the beam fails. This is actually a very important part of reinforcing concrete with steel. It changes the type of failure from a brittle mode, where there’s no warning that anything is wrong, to a ductile mode, where you see the cracks forming before a complete loss of strength. This gives you a chance to recognize a potential catastrophe and hopefully address it before it occurs.

Rebar works great for most reinforcement situations. It’s relatively cheap, well-tested, and understood. But it does have a few disadvantages, one of the major one being that it is a passive reinforcement. Steel lengthens with stress, so rebar can’t start working to help resist tension until it’s had a chance to stretch out. Often that means that the concrete has to crack before the rebar can take up any of the tensile stress of the member. Cracking of concrete isn’t necessarily bad - after all, we’re only asking the concrete to resist compressive forces, which it can do just fine with cracks. But there are some cases where you want to avoid cracks or the excessive deflection that can come from passive rebar. For those cases, you might consider going to an active reinforcement, also known as pre-stressed concrete.

Prestressing means applying a stress to the reinforcement before the concrete is placed into service. One way to do this is to put tension on the steel reinforcement tendons as the concrete is cast. Once the concrete cures, the tension will remain inside, transferring a compressive stress to the concrete through friction with the reinforcement. Most concrete bridge beams are prestressed in this way. Check out all that reinforcement in the bottom of this beam. Another way to prestress reinforcement is called post-tensioning. In this method, the stress in the reinforcement is developed after the concrete has cured. For this next sample, I cast plastic sleeves into the concrete. The steel rods can slide smoothly in these sleeves. Once the beam cured, I tightened nuts onto the rods to tension them. Under the press, this beam wasn’t any stronger than the conventionally reinforced beam, but it did take more pressure before the cracks formed. Also, this one wasn’t quite as dramatic because instead of failing the actual steel rods, it was the threads on the nuts that failed first.

I hope these demonstrations helped show why reinforcement is necessary for most applications of concrete - to add tensile strength and to change the failure mode from brittle to ductile. Just like the last video, I’m just scratching the surface of a very complicated and detailed topic. Many engineers spend their entire career studying and designing reinforced concrete structures. But, I’m having some fun playing with concrete and I hope you are finding it interesting. I’d love to continue this series on concrete, so if you have questions on the topic, post them in the comments below. Maybe I can answer them in the next video. Thank you for watching, and let me know what you think!

What is Concrete?

Concrete is as much a part of the urban landscape as trees are to a forest. It’s so ubiquitous that we rarely even give it any regard at all. But, underneath that drab grey exterior is a hidden world of complexity.

Concrete is one of the most versatile and widely-used construction materials on earth. It’s strong, durable, low maintenance, fire resistant, simple to use, and can be made to fit any size or shape - from the unfathomably massive to the humble stepping stone. However, none of those other advantages would matter without this: it’s cheap. Compared to other materials, concrete is a bargain. And, it’s easy to see why if we look at what it’s made of. Concrete has four primary ingredients: Water, sand (also called fine aggregate), gravel (aka coarse aggregate), and cement. A recipe that is not quite a paragon of sophistication. One ingredient falls from the sky, and the rest come essentially straight out of the ground. But, from these humble beginnings are born essentially the basis of the entire world’s infrastructure.

Actually, of the 4, cement is the only ingredient in concrete with any complexity at all. The most common type used in concrete is known as Portland cement. It’s made by putting quarried materials (mainly limestone) into a kiln, then grinding them into a fine powder with a few extra herbs and spices. Cement is a key constituent in a whole host of construction materials, including grout, mortar, stucco, and of course, concrete. A lot of people don’t know this, but every time you say cement when you were actually talking about concrete, a civil engineer’s calculator runs out of batteries.

I’m just kidding of course, and you an hardly be blamed for not knowing the difference if you’ve never mixed up a batch of concrete before. Even if you have mixed some concrete, good chance it was in a ready-mixed bag where all the ingredients were already portioned together. But, each ingredient in concrete has a specific role to play, and cement’s role is to turn the concrete from a liquid to a solid. Portland cement cures not through drying or evaporation of the water, but through a chemical reaction called hydration. The water actually becomes a part of the cured concrete. This is why you shouldn’t let the concrete dry out while it’s curing. Lack of water can prematurely stop the hydration process, preventing the concrete from reaching its full strength. In fact, as long as you avoid washing out the cement, concrete made with Portland cement can be placed and cured completely under water. It will set and harden just as well (and maybe even better) as if it were placed in the dry.

But, you may be wondering, “If water plus cement equals hard, what’s the need for the aggregate?” To answer that question, let’s take a closer look by cutting this sample through with a diamond blade. Under a macro lens, it starts to become obvious how the individual constituents contribute to the concrete. Notice how the cement paste filled the gaps between the fine and coarse aggregate. It serves as a binder, holding the other ingredients together. You don’t build structures from pure cement the same way you don’t build furniture exclusively out of wood glue. Instead, we use cheaper filler materials - gravel and sand - to make up the bulk of concrete’s volume. This saves cost, but the aggregates also improve the structural properties of the concrete by increasing the strength and reducing the amount of shrinkage as the concrete cures.

The reason that civil engineers and concrete professionals need to be pedantic about the difference between cement and concrete is this: even though the fundamental recipe for concrete is fairly simple with its four ingredients, there is a tremendous amount of complexity involved in selecting the exact quantities and characteristics of those ingredients. In fact, the process of developing a specific concrete formula is called mix design. And I love that terminology because it communicates just how much effort can go into developing a concrete formula that has the traits and characteristics needed for a specific application. One of the most obvious knobs that you can turn on a mix design is how much water is included. Obviously, the more water you add to your concrete, the easier it flows into the forms. This can make a big difference to the people who are placing it. But, this added workability comes at a cost to the concrete’s strength.

To demonstrate this balancing act, I’m mixing up some ready-mix concrete with different amounts of water. For the first sample, I’m using just enough water to wet the mix. You can see it’s extremely dry. A mix like this is certainly not going to flow very easily into any forms, but you can compact it into place. In fact, dry concrete mixes like this are used in roller-compacted concrete which is a common material in the construction of dams. For the next three samples, I used increasing amounts of water up to what is pretty much concrete soup. After the concrete has had a week to cure, I cut the samples out of the molds. It’s time to see how strong it is.

This is actually more or less how concrete is tested for compressive strength in construction projects. Obviously, I’m not running a testing lab here in my garage, but I think this will give us good enough results to illustrate how water content affects concrete strength, plus these cylinders look like they might attack at any time, and we need to deal with them. I made three cylinders of each mix, and I’ll break each one, watching how much pressure the cylinder was applying at the moment of failure. And this experiment was too cool not to invite my neighbors over to help.

We started with the samples that used the most water. It was no surprise that it took almost no pressure at all to break them, on average about 700 psi or 5 MPa. You can see how crumbly the concrete is even after having a week to cure. All that water just diluted the cement paste too much. The next two samples used the range of water suggested on the premixed concrete bag. These were much stronger, breaking at an average of 1600 psi and 2200 psi or 11 MPa and 15 MPa for the high and low end of the water content range. And you can really see the difference in how the concrete breaks. Finally, we broke the samples with the least water added to the mix. You can see how rough these samples were because there wasn’t enough water for the concrete to flow smoothly into the molds. But, despite looking the worst of the four, these were the strongest samples of all, breaking at an average of around 3,000 psi or 20 MPa. On this shot, you can even see the crack propagating through the cylinder before it fails. It just goes to show how important mix design can be to the properties of concrete. Even varying the water content by a small amount can have a major impact on strength, not to mention the workability, and even the finished appearance of the concrete.

It’s impossible to state how much I am just scratching the surface here. There is so much complexity to the topic of concrete partly because it has so many applications: from skyscrapers to canoes and everything in between. In fact, no matter where you are, you’re rarely more than a few feet from concrete - a fact that is inexplicably a source of great comfort to me. But, I took less than 10 minutes to describe what is literally the foundation of our modern society. So I’m dedicating at least the next few videos to dive deeper into the topic of concrete. The next video will be about its greatest weakness. If you’ve got questions about concrete, put them down below in the comments and maybe I can get them incorporated into the next videos. Thank you for watching, and let me know what you think!

Why do Bridges Move?

We often think of civil engineers as designers of static structures, or things that don’t move. That would be nice, but the reality is that everything moves for one reason or another, and one of those reasons happens to be temperature.

Whether you realize or not, you’re probably already familiar with thermal expansion, which is the property of materials to change their volume depending on temperature. If you’ve used a glass thermometer, you’ve even taken advantage of it. The liquid in a thermometer, usually mercury or alcohol, increases in volume as it heats up. Since we can characterize this expansion, we can put it to use as a measuring device. Maybe you’ve had some experience with a less useful application of thermal expansion. If you’ve ever put glass dishware on a burner or poured cold water on a dish that just came out of the oven, you know it doesn’t go so well. If it’s allowed to expand and contract evenly, stresses in the material don’t build up. But, if you heat or cool it unevenly, certain parts of the glass will fight against each other as they change size. Glass isn’t flexible, so instead of bending, it just shatters.

Thermal movement is something that has to be considered in nearly every field of engineering because there aren’t many places that don’t see fluctuations in temperature. And there are really only two options when designing for thermal expansion: The first one is to prevent the movement by constraining it, a feat that is almost always impractical. Thermal movement can generate tremendous amounts of internal stress. Watch how this wire can lift up a weight just by heating it up, then cooling it down. So, the other way to accommodate thermal movement is just to allow your design the freedom to move as it so desires. But sometimes that’s easier said than done, especially for large civil structures.

This is the formula for thermal expansion. It may look complicated, but it’s really not. It essentially says that the change in size of anything is a linear function of temperature proportional to its length. And the slope of this line is the coefficient of thermal expansion. We’ve measured this property for a whole host of materials, and you can look up tables online. Lucky for civil engineers, the thermal expansion coefficients for steel and concrete are nearly identical, which is why we can combine them into the ubiquitous construction material, reinforced concrete, without worrying too much about fluctuations in temperature pulling it apart. But, even though they expand and contract at the same rate, they still expand and contract. A perfect example of this is a sidewalk.

Let’s look back at our formula and plug in some numbers for a very typical situation. If we enter values for the average length of a city block, the average high and low temperatures in a given year, and the thermal expansion coefficient for concrete, we can see that the total movement of a sidewalk over the course of a year can be upwards of 4 cm or 1.5 inches. Obviously, you can’t leave a gap in the sidewalk that big at the end of every block, so instead, we leave small gaps spaced every so often to accommodate that movement. Most of the joints you see along a sidewalk are just to control cracking, but if you pay attention, every so often you’ll see an actual break in the concrete filled with some kind of flexible material. These are expansion joints that give the walkway the freedom to move from fluctuations in temperature.

But what about structures that are longer than a city block? Thermal movement scales with length, so engineers need to take a lot more care with linear infrastructure. Long runs of pipe, especially if they experience fluctuating temperatures, need expansions joints to prevent damage. The rails for trains can experience “sun kink” where a hot day can actually buckle the steel. One of the biggest challenges for thermal movement is on bridges. So I built a little model to show why.

Unlike sidewalks that can have periodic expansion joints, bridges only have support between spans. You can’t have an unsupported break in the bridge, so that means all of the thermal movement happens at the supports. The allowance can’t be evenly distributed across the length; it happens all in one spot. For bridges with very long spans, that can be a lot of movement. I’ve got my bridge set up with one side pinned and one side free to move. I’ll fire up the sunny day simulator and watch what happens on the dial indicator. The bridge expands along its length as it heats up. This is exactly what happens in real life. Now I’ll try to pin both sides of the bridge so that the movement is constrained. The bridge still expands as it heats up, but now it has to expand in directions it wasn’t meant to go. It’s a little bit hard to show on camera, but the entire bridge has buckled side to side. I’m using a flexible rod for this demo, but if a real bridge was constrained like this, the forces generated by thermal expansion would probably lead to failure of the structural members.

Expansion joints on bridges not only have to allow the bridge to move while still being supported, they also have to bridge the gap in the road deck so that cars can safely drive over it. So, if you look closely, you’ll see lots of creative ways engineers manage the thermal expansion. These are some photos collected from the web and sent to me by viewers of different bridge bearings that allow thermal movement. My inspiration to make this video came when I was looking through some vacation photos. Take a close look at this steel catwalk over the river. The shorter, cantilevered beams are welded directly to their anchor plates. They’re free to move because they’re only connected to the rock on one side. But look at the beams that stretch across both sides of the canyon. At first glance, it seems like they’re constrained on both sides and we know that’s bad engineering. But if you look closely, you can see that they’re bolted to the anchor plate using slotted holes to allow the beams to expand and contract. I hope this video gave you a little more insight into the dynamic nature of structures we normally consider static. Keep your eyes out and you’ll notice allowance for thermal movement everywhere you look. Thank you for watching, and let me know what you think.

How to Collapse a Pipe by Closing a Valve

In the last two videos, we’ve looked at phenomena that cause high-pressure spikes in pipes. But a lot of people pointed out that very low pressure in pipes can be just as dangerous.

If you watched the water hammer video I made a few months back, you’ll know that slamming a valve shut on a flowing pipe can cause a huge spike in pressure. That’s because the fluid inside a pipe has a lot of momentum, and fluids aren’t compressible enough to absorb sudden changes in velocity. Spikes in pressure aren’t always bad, but they can be dangerous if a pipe bursts or expensive by requiring stronger pipes with higher pressure ratings.

But in that video, I didn’t talk about what happens on the other side of the valve. So, I’m revisiting that demonstration with a few modifications so we can get the full picture. Here’s the setup: valve, clear pipe, pressure gage, more clear pipe, 50-foot garden hose, tree. The tree’s not important but I don’t want anyone to think I’m wasting this water. You won’t be surprised to learn that flowing fluid in a pipe downstream of a valve also has momentum, and that fluid also has a hard time stopping without a big fluctuation in pressure. But, unlike upstream where the momentum is carrying the fluid toward the valve, on the downstream side, the fluid is trying to flow away from it. So, the spike in pressure is negative - in other words, it creates a vacuum.

You may have noticed something different about this pressure gage. It only measures pressures that are below atmospheric - it’s a vacuum gauge. Watch what happens when I slam this valve shut. We get a very strong vacuum in the pipe, and then some fluctuations as the pressure wave propagates back and forth through the pipe. The momentum of the fluid in the water hose is pulling away from the valve. That fluid tension sharply lowers the pressure in the pipe. This trapped bubble gives a pretty good indicator of what’s happening as well. This is pretty far from a laboratory setting (no offense to the backyard scientist), but I’m seeing a peak of more than 30 inches of mercury or 100 kilo-pascals below atmospheric pressure. That’s a lot of vacuum. In fact, it’s enough to pull dissolved gas out of the water.

Take a look at the spot just downstream of the valve when I slam it shut. A spontaneous cloud of fine bubbles forms as the vacuum pulls. This is dissolved gases coming out of solution with the water. When the pressure returns, the bubbles shrink, but they don’t immediately go back into solution with the water, so you can still see a light haze in the water, especially when I turn the valve back on. Very cool in this demonstration, but bad news if your pipe wasn’t designed to withstand these types of pressures. Just like positive pressure spikes from water hammer, this phenomenon has caused numerous failures of pipe systems from implosions due to vacuum.

So, how can this be avoided? If the risk of failure is significant, like for very large pipelines or costly equipment, engineers will specify vacuum relief valves that will allow air into the pipe if the pressure gets too low, reducing the vacuum to protect the equipment. But, the simplest solution is the same as discussed in the other water hammer video: avoid sudden changes in velocity. Ask any firefighter and they’ll tell you: you gotta close valves slowly. You still get a vacuum downstream, but much less of one. Hope you liked this quick follow up. Thank you for watching, and let me know what you think!

What is a Steam Hammer?

Last month we talked about the damaging effects of water hammer, but there’s another state of H2O equally if not more dangerous when put in pipes.

Unless you live in a home with an older radiator or work in certain industrial settings, you probably aren’t as familiar with pipes that carry steam as those that carry water. We don’t normally need access to steam in our everyday lives like we do to its liquid analog. That’s not to say, though, that we don’t rely on steam. In fact, it plays a critical role in our modern society. We use steam for heating, cleaning, cooking, and a vast array of industrial processes. About 90 percent of all electric power produced in the world is through the use of steam turbines.

If you didn’t see my previous video about water hammer, here are the basics: water is heavy and incompressible. If you suddenly stop water while it’s moving through a pipe, it can create a massive spike in pressure and break stuff like this pressure gauge. Unlike water, steam is compressible. It’s “springy” and can absorb sudden changes in velocity without a big change in pressure. The danger with steam is when it doesn’t want to be steam anymore. In most places on earth, water exists naturally as a liquid. Under the ambient temperature and pressure conditions we consider habitable, most steam that happens to exist will condense. In a steam pipe, the water that forms from condensation (also known as condensate) is the real danger. And I mean danger in the truest sense of the word. Many lives have been lost in tragic accidents resulting from misunderstanding or misapplication of good engineering principles for steam systems. There are several problems that condensate can create, and we’ll talk about two of them in this video. The first one is "thermal shock".

Imagine this: you open a valve allowing steam to flow into a steel pipe. As the steam comes into contact with that cold steel, it condenses. The problem is that steam takes up about 1600 times more volume than its equivalent mass as a liquid. So, when it condenses, it shrinks. In a closed container like a pipe or this glass bottle that just came out of my microwave, that collapsing steam can lead to catastrophic damage. Water rushes to fill the vacuum created by condensation, cooling the steam even further and creating a runaway situation. This can happen extremely fast, and all that water can accelerate and decelerate violently, hence the name steam hammer. If it’s violent enough, it can rupture the pipe leading to an explosion like the one that happened in New York City in 2007. Check out Nick Moore’s video linked below if you want to see this demo in slow motion.

A thermal shock is a dangerous form of a steam hammer, but it’s easy to mitigate. When starting up a steam system, engineers and operators expect condensation as the pipes warm up. So start-up procedures will include running at reduced pressure with bleed valves open to make sure that condensation can’t form a vacuum. The bigger danger happens during normal operations, but to show how it works, first, we need a steam pipe.

Condensation in a steam pipe is always occurring just from the normal transfer of heat to the outside air. And this is roughly what that might look like. I’m using compressed air here in lieu of steam for the obvious safety implications. Engineers manage this condensate by sloping steam pipes and by installing devices that can get rid of condensate from the pipes called steam traps. Steam traps are a fascinating topic on their own, but occasionally they can get clogged or malfunction, allowing condensate to build up.

When water and steam flow together in the same pipe, it’s known as biphase flow. In this situation, the velocity of the steam is usually much higher than the velocity of the flowing liquid water. If there’s only a little bit of condensate in the pipe, that’s really not a big issue. But, if condensate is accidentally allowed to pool up, things can get dangerous. The steam passing over the top of the liquid can create turbulence and waves. If those waves get high enough, the liquid can create a complete seal inside the pipe with the full pressure of the steam behind it. This seal of water becomes a slug or piston and accelerates down the pipe like a barrel of a cannon, picking up more condensate as it travels. This slug of liquid eventually slams into the end of the pipe, resulting in a dangerous pressure spike known as differential shock. Just like thermal shock, many people have tragically lost their lives in steam pipe explosions caused by this phenomenon.

Engineering of steam systems is an incredibly complex topic in mechanical and chemical engineering, and I’ve just scratched the surface in this video. Whether you realize it or not, many of our modern conveniences are a direct result of steam systems, most notably electricity. So it’s critical that engineers can design steam systems to be safe from dangerous phenomena, including thermal and differential shock, also known as a steam hammer. Thank you for watching, and let me know what you think.

What is a Water Hammer?

You might know that most liquids are incompressible (or least barely-compressible), which means no matter how much pressure you apply, their volume doesn’t change. This can be useful, like in hydraulic cylinders, but that lack of “springiness” can also lead to catastrophic failure of pipe systems.

It’s easy to forget how heavy water is since we hardly ever carry more than a few ounces at a time. But if you add up the water in the pipelines of your city or even the pipes in your house, it makes up quite a bit of mass. And, when all that water is moving through a pipe, it has quite a bit of momentum. If you suddenly stop that movement—for example, by quickly closing a valve—all that momentum has nowhere to go. Since water isn’t compressible or springy, it can’t soften the blow. You might as well be slamming concrete into the back of the valve and the walls of your pipe. Instead of being absorbed, that sudden change in momentum creates a spike in pressure that travels as a shockwave through the pipe. Sometimes, you’ll even hear this shockwave as banging in your walls when you close a faucet or run the washing machine, hence the superhero-esque nickname, Water Hammer.

Banging pipes inside your walls can sound a bit spooky, but for large diameter pipelines that can be hundreds of kilometers long, that surge in pressure from a change in momentum can cause major damage. Let’s do a quick calculation: if you have pipeline carrying water that is 1 meter in diameter and runs for 100 kilometers (a fairly average-sized pipeline), the mass of water in the pipe is about 80 million kilograms. That’s a lot of kilograms. In fact, that's about 10 freight trains. Imagine you’re an operator at the end of this pipeline in charge of closing a valve. If you close it in a short amount of time, you’ve basically slammed those trains into a brick wall. And the pressure spike that results from such a sudden change in momentum can rupture the pipe or cause serious damage to other parts of the system. There’s actually another term for when a large spike in pressure ruptures a sealed container: a bomb. And water hammer can be equally dangerous. So, how do engineers design pipe systems to avoid this condition? Let’s build a model pipeline and find out. [Construction montage].

Here’s my setup. I’ve got about 100 feet (30 meters) of PVC pipe connected to the water on one end and a valve on the other. I also have an analog and digital gauge so we can see how the pressure changes and a clear section of pipe in case anything exciting happens in there. I mean civil-engineering-exciting, not like actual exciting. Watch what happens when I close this valve. It doesn’t look like much from the outside but let's look at the data from the pressure gauge. The pressure spikes to over 2,000 kilopascals or 300 psi. That’s about 5 times the static water pressure. It’s not enough to break the pipe, but way more than enough to break this pressure gage. You can see why designing a pipeline or pipe network can be a little more complicated than it seems. These spikes in pressure can travel through a system in complicated ways. But we can use this simple demonstration to show a few ways that engineers mitigate the potential damage from water hammer.

This is the equation for the pressure profile of a water hammer pulse. We’re not going to do any calculus here, but the terms of this equation show the parameters that can be adjusted to dial back these damaging forces. And, the first one is obvious: it’s the speed at which the fluid is moving through the pipe. Reducing this is one of the simplest ways to reduce the effect of water hammer. Velocity is a function of the flow rate and the size of the pipe. If you’re designing a pipeline, the flow rate might be fixed, so you can increase the size of your pipe to reduce the velocity. A smaller pipe may be less expensive, but the flow velocity will be higher which may cause issues with water hammer. In this case, my pipe size is fixed, but I can reduce the flow rate to limit the velocity. Each time I reduce the velocity and close the valve, the resulting spike in pressure decreases.

Next, you can increase the time over which the change in momentum occurs. One common example of this is adding flywheels to pumps so they spin down more slowly rather than stopping suddenly. Another example is to close valves more slowly. If I gently shut the valve rather than allowing it to snap shut, the pressure changes are more subtle. On large pipelines, engineers design the components and develop the requirements for operation of the equipment. This will almost always include rules for how quickly valves can be opened or closed to avoid issues with water hammer.

The final parameter we can adjust is the speed of sound through the fluid, also known as the wave celerity. This describes how quickly a pressure wave can propagate through the pipe. The wave celerity is an indirect measure of the elasticity of the system, and it can depend on the compressibility of the fluid, the material of the pipe and even if it’s buried in the ground. In a very rigid system, pressure waves can reflect easily without much attenuation. I’ve got flexible PVC pipe sitting on the ground free to move which is already helping reduce force of the water hammer. I can increase the flexibility even more by adding an anti-surge device. This has an air bladder that can absorb some of the shocks and reduce the pressure spike even further. Anti-surge devices are very common in pipe systems, and they can be as simple as a spring-loaded valve that opens up if the pressure gets too high. In water distribution systems for urban areas, water towers help with surge control by allowing the free surface to move up and down, absorbing sudden changes in pressure.

Plumbing is one of the under-acknowledged innovations that has made our modern society possible. When you harness the power of water by putting it in pipes, it’s easy to forget about that power. Water can be as hard as concrete when confined, and if you bang two hard things together, eventually something’s going to break. If you’re an engineer, your job is to make sure it’s not the expensive infrastructure you designed. Part of that means being able to predict surges in pressure due to water hammer and design systems that can mitigate any potential damage that might result. Thank you for watching, and let me know what you think!

How Soil Destroys Buildings

When most people think of property damage, they think about natural disasters. But what if I told you, there’s a slow-moving geologic phenomenon that causes more damage in the United States than earthquakes, floods, hurricanes, and tornadoes combined.

If you’ve ever been to a place where the ground looks like this, or if you’ve been in a building that looks like this or this, there’s a good chance you were in a place that had expansive soils. Just like these dinosaur toys, certain types of clay soils change their volume depending on moisture content. They swell when they get wet, and shrink as they dry. This is a microscopic mechanism where the shape and arrangement of the molecules actually change according to the amount of water mixed in. And, large portions of the U.S. Gulf coast and great plains have these kinds of soils. If you’re starting a foundation repair or road paving business, this is an important map for one very important reason: expansive soils break stuff.

Movement on its own and especially very slow movement is usually not a problem for structures. This is why we can lift buildings and even move them to new locations. What causes damage is differential movement. This is where certain parts of a structured move relative to each other. Differential movement leads to sticking doors and windows, cracked walls, and just general out-of-plumpness. And this is why expansive soils are so insidious because they don’t expand and contract evenly. For example, if your house sits on a concrete slab and you haven’t had any rain, the soils around the edges of the slab that are more exposed will dry out and shrink while the interior remains moist. Now you’ve got a foundation with no support around the edges. This breaks one of the fundamental laws of civil engineering, which says, and I quote, “You gotta have dirt underneath your concrete.”

Expansive clay isn’t just an issue for buildings. All kinds of infrastructure are at risk of damage from a shifting foundation. Leaking pipes can cause swelling of the soil, pulling apart joints and eventually leading to issues like sinkholes. Rainwater infiltrating through the cracks in roadways causes localized areas of swelling. This makes the road bumpy and uneven. Not even sidewalks, and by proxy rollerbladers, are spared. When designing to account for expansive clays, engineers not only have to know how much the soil can change in volume but also how hard it can push on anything sitting above, also known as swell pressure. So I’ve rigged up a little test so that we can see not only how soil swells, but also how much pressure it can exert. This apparatus called an odometer. It’s similar to a hydraulic cylinder, except I’m using dirt instead of oil, and I’ll use a dial indicator to measure how far the sample is able to move the piston. If you work in a soil laboratory, I’ll just apologize now for the rest of this video.

For my first test, I’ve got some soil straight from my own backyard. After all, there’s no place like a geologic unit containing abundant clay with high swelling potential. I put this in the oven to dry it out first, don’t tell my wife. Just kidding she knows whom she married. Now let’s put it in the apparatus and watch what happens. As it saturates, the soil expands over time, eventually reaching a 10% increase in volume over its dry state. Trust me, that’s enough to put a crack in the drywall. But, it’s really not that dramatic on video. So, to help illustrate these concepts a little better, I’ve got a bag of the instant viral video. That’s right I’m talking about Superabsorbent Polymer Beads, also known as Orbeez. These beads behave very similarly to expansive soils, except they’re way cooler than dirt in almost every way, even for a civil engineer.

First I tested these with no confining pressure and went a bit overboard. You can imagine if you built a house on this, you might get motion sickness every time it rains. It would wreak havoc on your structure. I tried it again with fewer orbeez, but it was still too much. This is an exaggerated view of what happens as water penetrates the subsurface and saturates an expansive soil. It’s hard to imagine anything that could avoid damage in this environment. So, let’s add some weight - and fewer orbeez this time so I don’t max out the range of my dial indicator. You can see that these fishing weights hardly make a difference. And that makes sense, right? A house probably puts more pressure on the ground below it than a few fishing weights. What about ten times that weight? It takes them a lot longer, but the orbeez are still able to swell to their full dimensions under this 20lb barbell, which is about the most my little acrylic oedometer can handle.

This is not just the case for orbeez by the way. Some clay soils have swell pressures on the order of megapascals (that’s hundreds of pounds per square inch). So you can see how big of a challenge these expansive soils can pose. There are lots of ways that engineers try to mitigate damage from these kinds of soils. You can simply remove all the expansive clay and bring in better soils for your project. You can grade the site so that water drains away from your structure, keeping moisture fluctuations down. You mix chemicals into the soil that limit its ability to absorb water. Finally, you can simply to build heavy enough to counteract the swell pressure and keep the soil from expanding. But as we saw in the demonstration, even a small amount of soil or in this case a colorful soil surrogate can lift a lot of weight.

I’m leaving out the simplest solution, which is simply to avoid expansive soils because it’s generally not feasible. It may be true in the parable that the wise man built his house on rock, but some civil engineer had to build a road to that guy’s house, and the engineer didn’t get to choose what kind of soil was on the way. Expansive soils are not a particularly newsworthy or exciting hazard (unless you’re the type of person who makes videos about dirt in your garage), but they still cause a tremendous amount of damage to buildings and the public infrastructure we rely on every day. They are one of the many factors taken into account when designing civil structures and the subject of ongoing research to find cost-effective and sustainable practices for mitigating the damage they cause. Thank you for watching, and let me know what you think!

How Quicksand Causes Dam Failures

When you think about quicksand, you probably don’t feel much fear or uncertainty. What was once a popular plot device in island and jungle movies of the 50s through the 80s has begun to fade from our collective curiosity. But, in civil engineering, quicksand is more than just a puddle of mud. It has a specific, and potentially more catastrophic, definition that can affect all kinds of infrastructure.

If you ever tried to build a dam in a creek or even in the storm gutter on your street, you know how hard it is. Water is not only powerful, but it’s elusive too. It seems to always find a way through. Like all fluids, water flows from areas of high pressure to low pressure, and a dam is a structure that separates those two conditions. This is a prime circumstance to induce flow, whether it’s through the dam itself, or underneath (through its foundation). Water flowing through soil is called seepage, and it follows some interesting and somewhat unexpected rules. So, to give you a better understanding, I’m building this demonstration out of clear acrylic sheets and some plumbing fixtures. This will essentially let us see a cross section through a dam so we can observe how the seepage behaves.

All dams have some seepage, so engineers need to be able to characterize it. Flow through soil follows Darcy’s Law, which is simple to understand, but complicated to calculate in two dimensions. Before computer models, engineers used an analytical tool called a flow net to estimate the behavior of seepage. Now software can do the work much more quickly and accurately. Darcy’s law says that the rate of seepage depends on the length of the flow path and the difference in pressure across it. The combination of these two factors is called the gradient. If the difference in pressure is small, like in this example, the seepage will be very slow. The bigger the difference in pressure, the greater the seepage flow rate will be. But, if you increase the difference by too much, eventually some strange things start to happen.

What’s happening here? I’ve talked about shear strength of soils in a previous video. Check that out if you want more detail, but here’s the gist: Soil is a granular material that has one main way of holding itself together: friction. Gravity pushes the soil particles together, creating friction which gives the soil strength. Seepage is the enemy of friction. The water gets between the soil particles and pushes them away from each other, reducing the friction and thus reducing the strength. This is important, because shear strength is really the only thing separating a solid from a fluid. In fact, when water pressure within the soil gets high enough to eliminate its shear strength all together, it’s called liquefaction, otherwise known as the “quick condition”. The soil is literally behaving as a liquid rather than a solid… it’s quicksand!

You can see why seepage is bad for dams and levees: engineers generally try to avoid building civil structures out of liquids. Soil liquefaction can be the starting point for a specific type of erosion called piping. As soil is carried away from the dam’s foundation, the seepage path gets shorter. Remember that the seepage depends on the gradient, which is a function of the length of the seepage path and the difference in pressure. If the pressures stay the same, but the path gets shorter, the gradient goes up, creating more seepage. This is a dangerous positive feedback loop. The erosion shortens the seepage path while the increasing seepage creates more erosion. Eventually the erosion pipe reaches the reservoir, leading to catastrophic failure of the dam’s foundation. This isn’t just speculation; piping is the number one cause of failure for earthen levees and dams.

How do engineers deal with seepage? One common ways is called a cutoff wall, a subsurface wall constructed in the foundation of dam. This is some footage from a previous demonstration I built that shows a cutoff wall in action. Notice the length of the seepage flow paths. They’re longer, which means the gradient is lower. That means less seepage and less chance for erosion. Thank you for watching, and let me know what you think!

How to Super-Size the Eclipse - Sun Funnel

If you live in North America, you know that this upcoming Monday is a big day for aficionados of astronomical phenomena: it’s a solar eclipse. And if you haven’t already, you need to get some of these glasses that let you look at the sun. But, the sun’s angular diameter in our sky is only half a degree. It’s smaller than your thumbnail at arm's length, and that’s pretty small. I won’t be in the path of totality this time, but I will be at an elementary school here in San Antonio enjoying the eclipse with students. And as usual when I talk to kids, I’m feeling compelled to try and make this experience a bit more visceral. Like mythbusters with explosions, I figure the best way to increase the excitement of a solar eclipse is to make it bigger.

The one job of a telescope is to gather a large amount of light and concentrate it so that you can see astronomical objects more clearly. That’s perfect for faint subjects like planets or nebulae, but not ideal for our nearest star, the sun. Even looking at the sun with no magnification can damage your vision, and magnifying those harmful rays is going to make the damage worse. So how can we get a bigger view of the sun safely? There’s two common ways: first you can use a solar filter that works just like eclipse glasses, blocking out the majority of the light so that you can safely see the sun through your telescope. But this way has a disadvantage: only one person can see at a time. Not ideal if you’re trying to share the excitement with a crowd. The second way is to use the telescope to project an image of the sun on a screen, and that’s the basis of the sun funnel.

Telescopes and binoculars use lenses or curved mirrors to concentrate light. The optics are usually aligned to focus the light onto the retina in your eye. But, if you move your eye out of the way, the light keeps going. Put a screen in front of it, and now you’ve got a projection. It really is as simple as holding up a screen in front of the eyepiece, but the sun funnel solves two practical problems: (1) it automatically keeps the projection screen in the correct orientation to the eyepiece even as you move your telescope around to follow the sun, and (2) it keeps anyone who doesn't know better from looking through the telescope at the sun and accidentally blinding themselves.

My design is based on a guide published on NASA’s website that I’ll link down below. They used a plastic funnel, but I decided to make my own using this thin-gauge aluminum sheet. I used pop rivets to fasten the edges. You can size the funnel based on your telescope, eyepiece, and how large you want the sun to be using a fairly simple formula. The eyepiece to my telescope slipped into the small end of the funnel and I connected it with a hose clamp. On the other end, I stretched a piece of vinyl shower curtain to act as a rear-projection screen. The last part of the project is to stop down the aperture on my telescope. Most reflector telescopes aren’t meant to focus the strong rays of the sun , and I didn’t want to overheat my secondary mirror. I’ll also be covering up the aperture altogether at regular intervals to make sure I don’t damage my telescope.

Here’s a test I did in my backyard. The sun is plenty bright enough to see, and big too! I think the kids are going to be impressed. I’ll post some pictures of the real eclipse in the sun funnel on my instagram next week but for now I can show a simulation of what it will probably look like. Even if you’re not looking at an eclipse the sun funnel is a fascinating way to observe an actual star up close and personal. I hope you liked this quick project, and for those of you in North America, I hope you get a chance to see the eclipse next week. Thank you for watching, and let me know what you think.

https://eclipse2017.nasa.gov/make-sun-funnel

How does land surveying work?

We use all kinds of smaller measuring tools in our everyday lives such as rulers, protractors, and tape measures. These tools work fine for home projects, but what if we need to layout something large like a road, bridge, dam, or pipeline?

Surveying is the science of taking big measurements. You’ve probably seen these guys on the side of the road looking through fancy equipment on a tripod. Almost any civil engineering project starts with a survey. This is to determine the legal boundaries between parcels of property. Surveying is also used to determine the location of existing infrastructure, and the topography and slopes of the land. Humans have always had a penchant for building big stuff. This means surveying is a career full of history and tradition. Behind every wonder of the ancient world was an ancient geometry nerd who laid out the angles and alignments during construction. Surveying is also how we created accurate maps of the continents like the Great Trigonometrical Survey of India. This took almost 70 years to complete. Everyone should aspire to accomplish something in your life that can be prefixed with the words “great trigonometrical.”

The ubiquitous tool for a survey is called a theodolite, and it’s one job is to measure the horizontal and vertical angles between points. Combine those angles with distances from a chain or tape measure, and you can triangulate the location of any point using trigonometry. Modern theodolites, called total stations, cannot only measure angles, but distance as well, and they have onboard computers to do the calculations and record the data for later use. When you see a surveyor peering through a funny telescope, it’s probably a total station, and he or she is probably sighting a reflector to record the location of a point. For long distances, these measurements have to be corrected for variations in earth’s gravity, refraction by the atmosphere, and yep, even the curvature of the earth. But don’t tell the flat-earthers. We’re sworn to secrecy along with NASA employees and airline pilots.

That’s just scratching the surface of sophistication with modern surveying equipment. With GPS and unmanned aircraft, surveying can get a lot more complicated. But I’ve got a few ways you can do your own topographic survey with fairly basic and inexpensive tools. Maybe you’ve got a drainage issue on your land or you’re planning a landscaping project. Or maybe you just want to exercise your God-given right to take measurements of stuff and write those measurements down on a clipboard. That’s my kind of recreational activity. My goal is to perform a leveling survey of my front and backyard, which is just a way to get the relative topography for an area. I laid out a grid of points on a map of my house and then transferred those points to real life using pin flags. Now I just need to pick my datum or base point and measure the relative difference in height between that point and all the others. I tried a few ways to do this and there are no sines, cosines, or tangents required.

First, a sight level which is essentially a combination of a telescope and a spirit level. To use it, first get a buddy or a willing spouse to hold a surveying rod on the point of interest. Now, look through the sight at a surveying rod and raise or lower the end until the bubble is centered on the line. Once it’s centered you know that you’re looking at a point that is exactly level to your eyes. Simply subtract the height of your eye-line with the height measured on the rod and that’s your elevation. It’s not a precision technique, but it is cheap and simple which the most you can usually hope for in any part of a home improvement project.

The next way I tried is a water level which is literally just a length of clear vinyl tubing filled with a liquid. As long as there are no bubbles or kinks in the line, the free surface at each end of the tube will self-level. I kept one end at my datum a fixed height and measure the height of the water at the other end as I carry it around to each of my points. It’s a little more unwieldy but it does have a distinct advantage, no line of sight required. You can use this method around corners or behind trees with no problem, and again, it’s a cheap and simple solution.

The third method to take a level survey worked best for me: my laser level. Here’s the thing: I really like lasers. I relish any chance I get to use them in a constructive way, and this is perfect. The laser level creates a perfect horizontal line that can be used to line up cabinets or tile, but it is also super easy to read on a surveying rod. You don’t need a helper, but you do probably need to wait until dusk unless your laser is really bright, or you have these sweet laser enhancement glasses. This isn’t the cheapest solution for a DIY land survey, but it is the fastest one I tried, and it’s a tool a lot of people already have.

Surveying is one of the oldest careers in the world, and also one of the most important. Why? Because land is important. If you own some, it’s probably your most valuable asset, and even if you don’t, you're pretty much stuck to it no matter where you go. As a career, surveying is a fascinating mix of legal knowledge, fieldwork, and technical challenges. And since most civil structures are too big for conventional measurement tools, the surveyor is one of the most important companions for the civil engineer. Thank you for watching, and let me know what you think!

How Do Sinkholes Form?

Some of the most complex civil engineering problems stem from the interaction of water and the ground. It sounds mundane but, there’s a good chance you’ve seen sinkhole on the news. How is it possible for the ground to simply open up and indiscriminately swallow anything or anyone that happens to be around?

We all know about erosion. This is the process that removes soil and rock from the earth’s crust and moves it somewhere else. And there’s a lot of ways this can happen: wind, landslides, abrasion, and scour. But here’s the thing, none of it compares to just the movement of water. Water is the great eroder. If you ever find yourself wondering how did this particular feature of the earth come to be here, or why is the ground shaped like so, or just why are things the way that they are, more often than not, the the answer is pretty much just water.

The ability of water to move soil or rock depends on several factors. The faster and more turbulent the flow, the more erosive it is. Larger particles like gravel and more resistant to erosion than small particles like silt or clay. Another important soil property is cohesion, or the ability of individual particles to stick to one another. Clay soils have more cohesion than sands, so they are more resistant to erosion. However, some clay soils are dispersive, which means they naturally wash away with water, making them particularly vulnerable to erosion. I love the standard test for dispersive soils, which is literally just to drop a clod of soil into a cup of water and see what happens. Finally, rather than physical erosion, some materials are soluble in water, just like sugar or salt, and can be eroded just by dissolving into the groundwater over time.

Most of us think about erosion on the surface of the earth, but erosion can occur in the subsurface as well. In fact, scientist and engineers have a very creative name for just such a process: internal erosion. If just the right factors come together in the subsurface, some very interesting things can occur, including sinkholes. But let’s look at a non-erosive example of groundwater movement first. This is a from a video I made before the channel was even called Practical Engineering. Water is flowing from the left side of the demo under an obstruction and over to the right. Notice two important things: first, the movement of water is slow. There’s not a lot of open space between all that sand, so it takes time for water to flow through it. Second, the sand is confined. Even if it wanted to move, there would be nowhere for it to go.

If those two conditions go away, that’s when sinkholes happen. Most natural sinkholes happen in areas with large deposits of carbonate rocks, like limestone. Over long periods of time, groundwater flowing through the subsurface can dissolve the rock, creating voids and open tunnels. In fact, this is how most caves are formed. These tunnels and voids create a significant change the character of groundwater flow. First, they allow water to flow quickly just like it would through a pipe, making it more erosive. Second, they create a space for soil to wash away. With those two conditions, any soil overlying a dissolution feature runs the risk of eroding away from the inside, eventually leading to a sinkhole.

But not every sink holes is formed through natural processes. In fact, many of the most famous sinkholes in recent times were human made. Just like a cave dissolved into the bedrock can act like a pipe and allow groundwater to carry away soil, an actual pipe can do the same thing. And actual pipes aren't limited to areas with a specific geology. If you could take a look into the subsurface of any urban area, you'd see miles and miles of water, sewer, and storm water drainage pipes. Unfortunately we can't see into the ground, so I built this demonstration so we can see for ourselves how this works.

All it takes is a little bit of settlement or shifting to create an opening in one of these pipes and allow internal erosion to start. Water moving through the pipe is able to dislodge the adjacent soil and carry it away. Notice that there's no signal on the surface that anything is awry. As more soil is washed away, the subsurface void grows. Depending on the type of soil and the speed of erosion, this process can take days to years before anyone notices. Many of our subsurface utilities are placed directly below roadways, and the paving often acts as a final bridge above the sinkhole, hiding the void below. It's only a matter of time before anything above is swallowed up.

Sinkholes aren’t the only problem caused by internal erosion. A specific type of internal erosion called piping is the most common cause of failure for earthen levees and dams, including Teton Dam in Idaho which killed 11 people and caused billions of dollars of damage when it failed in 1976. Maybe I’ll build a piping demonstration someday for a separate video. Internal erosion can be a natural process, but sometimes sinkholes can form to bad decisions, bad construction, or just bad luck with human made infrastructure as well. It’s just one of the complex failure modes that civil engineers must consider when designing a structure that might interact with water, the great eroder. Thank you for watching, and let me know what you think!

What's that Infrastructure? - Subsurface Utilities (Episode 4)

When you run a new utility line, whether it be electrical, gas, water, sewer, or communications, you basically have two choices for where to put it: overhead strung across poles, or below the ground. Today we’re talking about that second one, subsurface utilities. What’s that Infrastructure is a series where we divulge and discover the manmade world around us, and below us too.

Always call 811 before you dig (or the equivalent outside the US).