Hard core: The secret inner life of concrete

Text by Michele Regenold, Photos and illustrations by Alison Weidemann

Step outside and you’ll probably see it—in sidewalks, curbs, streets, bridges, parking lots, airport runways. Concrete is practically everywhere.

The basic ingredients are simple:

• Cement (which is dry)
• Water (to mix with the cement to make a paste)
• Aggregate (sand and rocks)

Dry cement makes up about 9–15% of the total volume of concrete. Water is another 15–16%. Aggregate makes up the bulk, 70–75%.

Rocky roads

Rocks by themselves can make a hard road surface. The problem is they don’t stay put. Traffic dislodges them, so aggregate roads have to be regularly maintained. For rural roads with low traffic, workers can keep up, but rocks alone just aren’t practical for high-speed, high-traffic highways.

Aggregate is a crucial ingredient in concrete and is what makes the final product strong and long-lasting. Since strength is so prized, you might think that using mostly large pieces of aggregate would be the way to go. However, larger rocks leave larger spaces between the rocks (the spaces between are called voids). These voids must be filled with cement paste, which isn’t as stable as aggregate.

So what about using mostly smaller aggregate? Wouldn’t smaller aggregate mean smaller voids? Yes, but you’d have more voids, so you’d need more cement paste.

Let’s say you have 2 beakers of the same size filled with rocks. One beaker is filled with larger rocks all about the same size, and one is filled with smaller rocks all about the same size. Both beakers have approximately the same volume of voids! When all the rocks in a mixture are of uniform size, whether large or small, the total amount of voids will be relatively high.

The way to reduce the total volume of voids is to include a range of aggregate sizes: large, small, and in between. That way the smaller pieces can fill in the voids around larger pieces. Then only enough cement paste is needed to coat each piece of aggregate, not to fill in lots of voids. The paste-coated aggregate pieces fit together like a gigantic, 3-dimensional jigsaw puzzle.

Cement—isn’t that the same thing as concrete?

Nope. Cement is an ingredient in concrete.

The most common cement used in concrete construction is called portland cement. (The man who invented it back in 1824 thought it looked like the grayish white stone found near Portland, England.)

Cement has just a few basic ingredients. Shale and limestone, two kinds of sedimentary rock, are heated to 2,500° F (1,400° C). This heating process causes chemical reactions that yield a product called clinker—hard balls about 1 inch in diameter. Clinker is then ground up with the mineral gypsum to make cement.

Cement under the microscope: Some weird stuff going on

If you were to actually sit in a lawn chair and watch a new, wet concrete sidewalk harden, you’d be missing the real excitement because the weird stuff happens at the microscopic level. The thick, wet concrete doesn’t just dry out; the chemical composition of the cement paste coating the aggregate actually changes.

When water and cement are mixed, a chemical reaction called hydration occurs. Hydration is the central process in transforming a mixture of water, cement, and aggregate into the product we know as concrete.

For the first few hours after mixing, the silicates in the cement are just slowly dissolving in the water. During this time, the chemical reactions are in a dormant phase. The concrete is still wet (or “plastic” as professionals call it).

Construction workers take this opportunity, while the concrete mixture is plastic, to transport it in a dump truck or ready-mix truck, pour or “place” it at the construction site, and finish the surface. For streets and roads, finishing basically means smoothing the surface. Workers have to accomplish these tasks before the real fun starts.

When the water becomes super-saturated with calcium ions from dissolving cement silicates, serious chemical reactions take off quickly. Elements in the water and cement combine in new ways to produce new solid compounds. The chemical reactions (hydration) release a lot of heat. The concrete actually gets hot for several hours during this phase.

At the microscopic level, the new compounds look like either crystals or fibrous fingers. They interweave and mesh together around the aggregate, locking the aggregate pieces together and making the concrete stiffen. By this time, the concrete can no longer be poured or finished.

As the new solid compounds replace the cement paste, the concrete hardens and gains strength. The fingers and the crystals keep growing, filling in much of the space between pieces of aggregate. As the compounds grow, they also start to interfere with the remaining water and cement particles that haven’t dissolved. As a result, hydration slows down and the concrete starts to cool.

After several hours, the concrete will be strong enough to walk on, but it will take about 72 hours before the concrete will be strong enough for traffic.

Even after the concrete looks completely hard and feels rock solid, hydration continues, just more slowly. Those fingers and crystals keeping growing and getting tighter, making the concrete denser and stronger over time. As long as water and cement can come into contact, hydration can go on for years.

About the jobs

Some of the transportation-related jobs dealing with concrete include road construction workers, paving equipment manufacturers and operators, cement/concrete plant workers, concrete materials quality-control technicians, road construction inspectors, and engineers specializing in concrete or construction. Also managers and workers at rock quarries. And chemists who finetune special additives that enhance certain concrete qualities.

Concrete road construction is physically demanding work. In parts of the country that experience significant cold weather, road construction is seasonal, which allows workers to pursue other interests during the winter. Important qualifications are reliability and the willingness to work hard. Many of the specific skills needed can be learned on the job.

For jobs requiring more technical skills, take a look at community colleges with programs in construction, engineering, and related fields. They may also offer short certification courses that give high school students a leg up on summer construction jobs.

A 4-year degree in engineering is required for those interested in designing and overseeing whole construction projects. If you’re fascinated with the chemistry of concrete, you might want to study it in graduate school, after finishing your bachelor’s degree. University researchers study concrete in detail, looking for ways to improve cements, other materials, concrete mixtures, and construction practices.