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Oct 18 2010

Pique the Geek 20101017: Concrete, the Wonder Material

Most people never give concrete a second thought.  This is a mistake.  Concrete is one of the most versatile and widely used building materials known, and it has been known for a long time.  Concrete like materials have been unearthed in ancient Egypt, and the Romans made extensive use of it.  Concrete structures over 2000 years old are still in use today.

Roman concrete is very different than modern concrete, and it did not weather well.  Thus, Roman structures were often faced with stone or brick to increase durability.  This defect has been overcome with modern materials and production techniques.

What we call concrete is a mixture of cement (usually Portland cement), fine aggregate (sand), and coarse aggregate (gravel or, in very large jobs, stones up to several inches wide).  These materials are mixed with water to form a castable or pourable mixture that has the ability to take the shape of a form or mold.

Generally, the ratio of the ingredients is around 1:3:5 cement:fine:coarse or so, with no more than 50 pounds of water for each 100 pounds of cement.  More aggregate makes a weaker concrete, which is OK for some work.  Less aggregate makes a stronger concrete, but the cement is the most costly part of the mix so for the sake of economy, as much aggregate as possible, consistent with the project, is always used.  Sometimes dishonest suppliers will substitute substandard concrete and this has lead to quite of number of engineering disasters, particularly when buildings and other freestanding structures are involved.

Most projects have specifications for the compressive strength of the final product.  Standard US practice is to make cylinders around six inches in diameter and a foot or more long.  These cylinders are then put in a press fitted with a pressure indicator and then crushed.  The pressure at which the concrete cylinder fails is the compressive strength for the concrete.  If it does not meet the specifications, the mix is modified until it does.

Concrete typically has a compressive strength of around 3000 pounds per square inch, give or take, depending on the ultimate use and how long it has cured.  Thus, a six inch diameter column (28.27 square inches) should not fail until loaded to around 85,000 pounds, a little over the maximum weight of a fully loaded tractor/trailer.  Of course, a generous safety margin is used when concrete is used for construction.  While concrete has excellent compressive strength, its tensile strength is much lower, only around 350 psi or so.  That same six inch column would thus fail with only about a 10,000 pull on it.

Enter reinforced concrete.  It turns out that steel has excellent tensile strength, but is relatively expensive compared to concrete.  However, bars or meshes of steel embedded in a much larger mass of concrete gives the combined material both excellent compressive and tensile strength, so except for the most basic of small structures, most concrete work is done with reinforced material (there is a subset of this that will be covered in a little while).  It is reinforced concrete the formed the Hoover dam, most concrete highways, and large buildings.  This makes it possible to have extremely good strength whilst minimizing the amount of expensive steel that has to be used.

Steel has another property that makes it uniquely suited to be embedded in concrete:  its coefficient of thermal expansion is almost exactly that of modern concrete.  This might sound like a big deal, but it is.  It the coefficient of steel were significantly smaller, as a structure got warmer, the “hole” where the steel is would enlarge (remember, expansion of cylindrical solids occurs radially), so the steel rod would pull away from the concrete.  If the coefficient of steel were significantly larger, as a building heats us, the steel would expand faster than the concrete, cracking it where the reinforcement rod was.  Either case would weaken the structure.  Since sound concrete is essentially waterproof, steel reinforcing rods tend not to rust, which would also cause the concrete to crack (rust has a greater volume than steel), allowing even more water to enter.  That the coefficients of thermal expansion are so close, the marriage of steel and concrete is possible.

Another kind of reinforced concrete is prestressed concrete.  In this variant, one or more steel bars with bolts and flanges are put under tension and concrete poured around them in a form.  After the concrete cures, the tension is released and as the steel bars try to shrink to their original length, the flanges distribute the force over the length of the concrete object.  This greatly increases the strength of the concrete.  This technique is often used in bridges and can reduce the bulk of them significantly.  Think about it this way.  You want to move books from one shelf to another.  If you take both hands and push on a row of books, you can move a dozen or more (if they are not too heavy) at a time, the middle books being kept in place by the compression from the end books.

Modern concrete did not appear until around the mid 1800s, and it was not very good then.  The properties of concrete are most due to the cement from which it is made, and until fairly recent times making cement was a hit or miss prospect since little was known about how and why it works.  Since cement is so important, let us take a little time to review how it is now made.  We will discuss Portland cement, the most common kind.

Portland cement got its name because the finished concrete resembled in color and texture the popular building stone quarried around Portland, England.  It is make from some source of calcium carbonate, usually limestone and a source of silicon bearing material, like clay or shale.  Lighter colored materials are preferred, but there is not a whole lot of suitable clay or shale that is light in color due to iron in it, so most Portland cement has a definite bluish tinge.  Manufacturing process vary somewhat, but the basics are all the same.

The rock is crushed until it is quite finely pulverized and the calcium and silicon materials are mixed carefully.  Usually a few per cent of gypsum is also added to retard setting to a useful time, so that it does lock up before it can be poured.  These materials are then run through a rotary kiln that is strongly heated with fuel oil, natural gas, or coal.  Often near the end of the kiln, extra powdered coal is injected to raise the temperature.  The mixed materials first dehydrate, then carbon dioxide is driven from the limestone, and finally everything melts into a glass like material, called clinker in the trade, because it has a ringing sound when discharged.  After reaching the clinker state, cold air is passed over the clinker to cool it.  Once cool, the clinker is ground until it is extremely fine, finer than flour, and sent to storage until packing and shipping.

As you can see, cement manufacture is extraordinary energy intensive.  Fuel costs are by far the largest costs associated with a cement plant.  Some plants supplement their fuel supply with hazardous waste that otherwise go to an incinerator for disposal.  One of the reasons that construction costs have risen is directly due to the cost of fuel for making cement.  In addition, cement plants are extremely efficient producers of carbon dioxide, both from burning enormous amounts of fuel AND the carbon dioxide that is released from the limestone as it is decomposed into calcium oxide and carbon dioxide.  Thus, the environmental footprint of concrete construction is larger than most people realize.  Fortunately, cement is the minor ingredient in concrete, since only around 10%, give or take, of finished concrete is cement.

When cement and aggregate is mixed with water, the chemistry is extremely complicated.  As the ground clinker rehydrates, hundreds of new compounds are formed, many of them high polymers of silicates.  These polymers grow around the aggregate, enmeshing them in what becomes an extremely hard, rigid material.  In a cement truck, keeping the wet cement turning keeps the length of the polymer chains fairly short by shearing forces, so the concrete in the truck will stay pourable longer than if were kept still.  However, there is a limit to it, and it is possible for concrete to harden in the truck.

As concrete cures, it gets hot.  For backyard projects this is hardly noticeable, but for big jobs it can cause severe problems.  Special cements that have a lower heat output have been developed to minimize this tendency, and large dams, for example, are usually constructed of low heat concrete.  The problem with heating is that it tends to evaporate the water in the mix, causing cracks and voids.  Low heat concrete is not usually quite as strong as standard Portland concrete in small applications, but since the final pour has more integrity in large jobs with low heat concrete, the structure is stronger.  On the other hand, special cements have been developed to release significantly MORE heat than standard Portland mixes.

The setting speed of concrete is a function of temperature.  Like most chemical reactions, the rate just about doubles with every 10 degrees Celsius temperature rise.  Thus, the hotter the environment, the faster it sets.  At near freezing temperatures, the rate of setting becomes extremely slow.  High heat concrete helps to compensate for low temperature work because of its internal temperature.  We shall get back to this in a minute.

Setting and curing are two very different things.  Setting refers to the time required for concrete to harden such that it will keep its shape.  Curing refers to the time required for concrete to reach its specified strength.  Standard concretes take 28 days to cure (by the ASTM standard), and the strength can more than double from when it was only set.  Actually, good concrete slowly gains strength essentially forever, because even cured concrete slowly absorbs carbon dioxide from the air, becoming even stronger.  Like setting, curing is also a function of temperature.  There is some speculation that the concrete used on the Deepwater Horizon well had not fully cured when the wellhead gave way, and you have to remember that the temperature 5000 feet below the surface of the Gulf of Mexico is near freezing.  The low temperature also caused the problem with the methane hydrates clogging the original cap.

Concrete is remarkably resistant to environmental assaults.  The only things that really damage it chemically are acids and certain alkali waters and soils.  Concrete tanks for acid service must have liners to protect them, but a special cement is available to protect concrete from alkali soils and waters.  One thing that does damage concrete badly is freezing and thawing, and this a exacerbated with the use of ice melting salts.  I am sure that you have seen sidewalks that have deteriorated badly from repeated freezing and thawing.

There is a remedy for that, however.  Air entrained concrete has a material added to it (the nature of the particular ingredient varies) that causes tiny bubbles to form while the concrete is being mixed.  If you try to make it yourself, better rent a proper mixer since it is hard to get enough agitation with a hoe and wheelbarrow to entrain enough air to do the job.  From three to seven percent air does the job.  Air entrained concrete has some other advantages, so it is used in other situations than just freeze thaw conditions.

The Romans used a neat trick when they built the Pantheon, the largest domed structure built until very recent times.  They used high strength, heavy aggregate concrete at the bottom to bear the load, but for the top of the dome they switched to pumice for the aggregate to save weight.  Without that trick, the top of the dome would  have been too heavy to support itself.  That was pretty good engineering, since it has stood for nearly 2000 years.

In the early 20th century concrete was used more and more for residential and commercial construction projects, and concrete buildings are common today.  One advantage is that they can be built relatively quickly, and with the proper reinforcement and design such buildings can withstand tornadoes and earthquakes.  Concrete is so vastly superior to brick or block as far as strength goes that there is really no comparison.  Frank Lloyd Wright pretty much pioneered the use of cast in place concrete for construction, just another one of his visionary insights.

There are many more facets to concrete, much too broad to cover here.  I do hope that the next time you look at a concrete structure that you have a little better appreciation as to what goes into such a common, but such a remarkable, material.  Because of its combination of properties, its adaptability to a wide variety of conditions, and its relatively low cost, concrete is truly the wonder material.

Well, you have done it again.  You have wasted many more perfectly good einsteins of photons reading this rocky post.  And even though Carl Palidino stops those “horsey” emails when he reads me say it, I always learn much more than I could possibly hope to teach by writing this series.  Please keep comments, questions, corrections, and other items coming.  Remember, no scientific of technology issue is off topic here.

Warmest regards,

Doc

Featured at TheStarsHollowGazette.  Crossposted at Docudharma.com

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