How Fluid Concrete Works

The Three Gorges Dam in China is the biggest hydroelectric dam in the world, and it was constructed with fluid concrete.
Stuart Dee/Photographer's Choice/Getty Images

If you're like most people, you've probably never thought long and hard about concrete. Sure, its gray presence is all around us -- in our roads, bridges, parking lots and driveways -- but there can't be that much to it, right? A truck pulls up, dumps it in a defined area, and it hardens. Voila. A sidewalk is born. But you might be surprised how many ways boring-old-concrete can be mixed and cast, even for use in your own home. In fact, one "recipe" has become so popular in the last half century that you'll find it in everything from kitchen countertops to China's Three Gorges Dam, the largest hydroelectric project in the world. What is this incredibly versatile, uber-strong building material? The answer is fluid concrete.

All concrete starts out as a mixture of water, cement and some type of aggregate (like sand and gravel) and hardens thanks to a chemical reaction that occurs between the water and the cement (which we'll discuss on the next page). Ideally, concrete should have just enough water in it to react completely with the cement -- any more or less water will weaken the end product. The result is concrete with a low slump, which is an engineer's fancy way of saying that it has a very thick consistency when wet. While concrete with a low slump makes for strong hardened concrete, it's difficult to work with when it's wet because it's not thin enough to ooze into tight spaces. Fortunately there are admixtures, or chemical additives, that can thin the concrete without weakening it (the way adding more water to the mixture would). One such admixture, called a superplasticizer, is used to make fluid concrete. The superplasticizer's superpower is that it lends concrete a high slump without stealing its strength. Concrete with superplasticizers in it is very fluid when wet and very strong when hardened.

While natural concrete has been around for 7,000 years and modern portland cement concrete has been used for about 200 years, fluid concrete is a much newer invention. Ancient experimenters tried thinning concrete with organic materials like milk, blood (yes, blood), and lard with little success. It wasn't until the late 1950s and early 1960s that engineers in Germany and Japan first developed superplasticizers. The substance hit the market in 1964 and was first made available in the United States in the mid-1970s. By 1984 an estimated 90 to 100 million cubic yards (1.5 to 2.3 million cubic meters) of fluid concrete had been produced [source: Mielenz]. Today its use is widespread. Contractors love the fact that it pumps easily, fills in narrow spaces, and requires little or no compaction to remove air bubbles, all while retaining its high strength.

But of course they have science to thank for these miracle benefits. How do superplasticizers do their job?

The Science Behind Fluid Concrete

At first, the water causes the cement particles to attract one another. When superplasticizers come on the scene, they break that attraction. Eventually, however, even though the cement particles are no longer drawn to one another, they become hydrated by the water, and crystals form.
At first, the water causes the cement particles to attract one another. When superplasticizers come on the scene, they break that attraction. Eventually, however, even though the cement particles are no longer drawn to one another, they become hydrated by the water, and crystals form.

If you think about it, fluid concrete is pretty amazing (yes, we just said that concrete is amazing). It appears wetter and soupier than standard concrete, even though it has the same amount of water in it. How's this possible? Give a round of applause to a surprisingly simple principle you probably learned about in science class -- electromagnetism.

To understand how fluid concrete works, you have to understand how superplasticizers work, since this chemical additive is essentially what differentiates fluid concrete from its standard counterpart. Superplasticizers are made from either sodium salts or calcium salts and are classified in one of four ways: sulfonated melamine-formaldehyde condensates, sulfonated naphthalene-formaldehyde condensates, modified lignosulfonates, and others such as sulfonic-acid esters and carbohydrate esters. Those terms are definitely a mouthful, but the way they work is pretty simple. When concrete is first mixed, the presence of water causes cement particles to be drawn to one another, which thickens the batch. Superplasticizer molecules, which are made of long chains and links, attach themselves to the cement particles and give them a highly negative charge. As a result, they repel each other, the same way that the negative pole of one magnet will push against the negative pole of another. Because the cement particles are no longer attracted to the others, the concrete remains fluid.

However, the effect of the superplasticizers is only temporary. The cement particles, though repulsed by one another, slowly become hydrated as they react with the water in the concrete. This process causes crystals made of calcium hydroxide and calcium silicate hydrate to form on the round surface of the cement, creating a spiky ball shaped kind of like a sweetgum fruit. The crystals slowly engulf the superplasticizers so that they're no longer able to function. Without the negative charge to keep the cement particles apart, the concrete thickens again, usually after about 30 to 45 minutes. More superplasticizers can be added to thin the concrete again, but as the crystals grow and begin to interlock, increasingly higher doses are needed to keep the cement particles apart.

Clearly fluid concrete is a pretty magical concoction. But how is it put to work at construction sites?

Using Fluid Concrete on a Construction Site

Unless you live under a rock, you've seen something built with fluid concrete. (Or, who knows? Perhaps the rock you're under is a chunk of fluid concrete.) Contractors and engineers often use it on the job because its thinner consistency makes it ideal for a variety of projects.

One benefit of using fluid concrete in construction is the ease with which it can be pumped into places trucks can't go. To push the thin mixture, builders pour it into the hopper of a specially-designed pump mounted on a truck or trailer. The pump moves the mixture through pipes made of rubber or steel that either run along the ground to move concrete horizontally or are attached to a mechanical arm (resembling the one on a cherry picker) to transport it vertically. Because fluid concrete is so thin, contractors can pump it great distances or heights without having to increase the pressure to keep it flowing as they would with thicker, standard concrete. You might compare it to the difference between sucking a soda through a straw and sucking a milkshake through a straw; the thinner beverage just takes less effort to drink.

Fluid concrete can also be useful for filling forms -- structures typically made of wood or steel that help the concrete hold its shape while it's wet. You've probably seen simple forms bordering sidewalks or driveways on construction sites; they're removed when the concrete hardens. Forms that are particularly narrow (like those for walls), are heavily reinforced (with steel mesh or rods known as rebar), or have embedded items (like pipes or bolts), may be filled with fluid concrete because it's able to seep into narrow spaces and hug impediments.

Using fluid concrete also reduces the work that has to be done at the construction site. For builders attempting to pour a level foundation for a building or other structure, fluid concrete is thin enough that it can pretty much level itself, much like the water in a swimming pool when it becomes very still. Time is also saved because there's no compaction process for fluid concrete. Standard concrete must be compacted using a special tool that consists of a small gas or electric motor connected to a short hose that vibrates on the end. The vibration removes air pockets that may have become trapped in the mix when it was poured. Fluid concrete doesn't have to be compacted because air isn't as likely to become trapped in the thin, soupy mixture.

Using Fluid Concrete in Your Home

When most people think of projects involving concrete, they picture a parking lot -- perhaps one that stretches for what seems like miles outside a big box store. But fluid concrete is finding its way into some unexpected and less drab places -- specifically people's homes.

Given the self-leveling ability of fluid concrete, it would probably make a pretty good floor or patio slab in and around the home. However, given the added expense of the superplasticizers that thin the concrete, such a use probably wouldn't be cost effective. So, the most common use of fluid concrete in the home is for kitchen countertops.

That's right, kitchen countertops. You've probably seen them made from formica, tile and granite, but concrete countertops are becoming increasingly popular; they're ultra-durable and have a contemporary look. Concrete countertops are made with either fluid or stiff concrete. Stiff concrete has a very thick consistency -- almost like clay -- and typically its aggregate is sand. This means that it's very easy to apply to forms, but will wind up having little craters on the surface where air bubbles became trapped against the mold and couldn't escape through the thick puttylike concrete. Some people prefer this look -- which is reminiscent of stone -- but a smoother surface can be achieved by using fluid concrete.

When fluid concrete is used to make countertops, the aggregate is usually a bit coarser -- gravel or crushed stone -- and as with the mixes used in construction, it includes superplasticizers. The superplasticizers mean the countertop will have a smooth finish and it'll be strong and not as susceptible to shrinking and cracking. Still, this thin consistency means that forms must be more watertight than those used with stiff concrete. The fancier your kitchen is the more complicated things get. If you wanted, say, an integrated sink in your countertop, a stiff mixture could be pressed around a simple form because it holds itself in place, but a soupy fluid mixture requires a more complex form to support the concrete on all sides.

Concrete countertops can be installed in a couple of different ways. Craftsmen can fabricate them in a workshop then install them in your home, or they can be poured in place, right in your kitchen. Either way, the concrete will have to be reinforced with rebar or a wire mesh. You might assume that the final product will look grey and industrial, but that's not always true: Various pigments can be added during the mixing process to give the concrete a custom color to fit your specific tastes. When completed, these countertops can be a beautiful and long-lasting addition to your home.

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More Great Links


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