CONCRETE: IMPROVEMENTS TO AN OLD TECHNOLOGY

Quiz time. Question:  What is the largest commodity in the world?   Answer:  water.  Question:  What is the second largest commodity in the world?  Answer:  concrete. Over 6 billion cubic meters of the stuff are poured every year.  That is a meter (over 35 cubic feet) for every person on earth.  Per year. And did you know that the concrete in the Hoover Dam is still getting stronger, even though it was completed in 1936? OK, now I know I have your attention.  First, for the layperson like me, we need to deal with the terminology: concrete and cement are not the same thing.  Concrete is what you drive on.  Cement is the grey powdery stuff you pour out of a bag, add gravel and sand to, and mix with water.  After several hours, it hardens to become concrete.  Then you can drive on it.  Pretty simple, right?  Actually, concrete is a widely studied material with billions of dollars devoted to improving its cost, service life, and ease of construction.  Where does cement come from? Well, first just a little chemistry.  To really appreciate cement’s ancient history, you’ll have to bear with me.  It turns out that all cements have common ingredients.  That is, they all have atoms of calcium (Ca), oxygen (O), silicon (Si), iron (Fe) and/or aluminum (Al).  With lots of heat, these individual atoms are combined into four basic molecules.  These molecules are largely calcium in combinations with the others.  When you add water, these molecules dissolve and reform in a “new” combination.  The primary material formed is (CaO)•(SiO2)•(H2O)(gel) + Ca(OH)2,  plus heat.  If you include gravel and sand as inert filler and wait awhile, then you can walk on it.  You have concrete.  If you didn’t add a source of silicon, you’d have “lime plaster,”  the most common kind of plaster.  The problem with plaster is that it is not stable in water.  That means that without silicon (sand), your roads would wash away when it rains! What is historically interesting is how these ingredients were found in nature and mixed together.  CaO, also known as “lime” or “burnt lime”, is made by heating limestone, which is naturally occurring and found almost everywhere.  Limestone is calcium carbonate, or CaCO3. (Limestone is fun because you can dribble acid on it and it will “fizz”, as the limestone releases carbon dioxide, or CO2.. I used to carry acid with me all the time when I went rock hunting, so I could tell when I had limestone.)  Anyway, if limestone is heated to 1,500o F,  it releases CO2 gas and leaves CaO solid behind.  When CaO mixes with water, it releases a lot of heat and forms calcium hydroxide, or Ca(OH)2.  (In the 1200s, the English apparently won a sea battle by throwing a bunch of CaO into the air so that it drifted into the eyes of the French sailors on the other side, which then, since eyes have water, caused heat—and, well, the English won).  Another source of CaCO3 rather than limestone, is sea shells. The source of silicon or aluminum is varied.  The most common source for making cement is from “clays.”  Clays primarily contain various combinations of silicon and oxygen, or aluminum and oxygen, or silicon and aluminum and oxygen, or all of these and magnesium, calcium, sodium, iron, and potassium.  There is such a blizzard of combinations, each one of which is considered a different mineral, that I’m sure introductory geology students pass or fail based on their ability to keep them all straight.  Of course clays occur all around the world, so are very accessible. So there you have the chemistry to appreciate the very long history of concrete. Plaster, which is like cement but without the silicon or aluminum, was used as a building material as far back as 8,000 B.C. in the Middle East.  It has been found in pyramids 4,000 years old, and the Aztecs paved streets with it.  Note that these are all places where it almost never rained! Apparently the first use of cement dates back to 800 B.C. with the Macedonians, but the Romans are the ones who gave cement its name and showed its real potential as a building material.  These groups were the first to realize that in order to keep cement from dissolving in water, you’ve got to add silicon.  This was a critical finding, one that has enabled ancient concrete aqueducts, roads, and cathedrals to remain standing today.   Good examples are the Coliseum, completed in 80 A.D, and the Pantheon’s dome, built in 128 A.D.   Roman cement consisted of lime (CaO), plus Pozzolana (a source of silicon and aluminum), named for a volcanic ash quarry from the Italian town of Pozzuoli, near Naples. Another source of silica and aluminum was powdered ceramic pots, since they were made of clay.  Amazingly, Roman concrete is said to have the same compressive strength as modern concrete.  That is pretty impressive all by itself, but even more so in light of the fact that it took modern engineers over a hundred years of continued testing and improvement to get to that point.  Moreover, some surviving Roman concrete is even more resistant to corrosion from salt than modern concrete. After the Romans, concrete technology virtually disappeared during the Dark Ages.  It next shows up in Finland the 15th century, and in 17th century we have the incredible Canal de las Doas Mars (also called the Canal du Midi).  Completed in 1681, it stretches across most of France to the Mediterranean, allowing ships to skip the month-long trip around Spain.  And the canal is still functional!  You can still row its 150-mile length today (how awesome would that be?!). The use of concrete got a really big boost in the 1700s when British engineer John Smeaton discovered that the best concrete was made of native limestone that had a particular clay content.  He closely studied the work of the great Roman builders and essentially rediscovered concrete technology.   There then followed a series of improvements that culminated in the still-famous “Portland cement.”  First, this cement was NOT made in Portland, Oregon (which is what I used to think).  It was so named because the color of the cement made it look like the highly-sought-after British Portland limestone.  The basic improvement in cement manufacture consisted of grinding the limestone, heating it, mixing it with clay,  and then mixing in water and letting it set.  The resulting concrete was heated in a furnace (kiln) and re-ground to a fine powder, which was called “Portland cement” in Joseph Aspdin’s 1824 patent.  Around this time, several methods for making cement were evolving, but Portland cement is still the most commonly used.  As you might imagine, however, modern manufacturing methods are very different from the process described in the Aspdin patent. So by the 1800s, concrete production was coming on strong.  And in the late 1800s, the greatest inventor of all time even got into the “mix” --Thomas Edison invented concrete houses that you can still see in New Jersey today.  I understand some may leak, but hey, so did homes made by Frank Lloyd Wright!  Although Edison’s concrete business was ultimately not successful, along the way he invented the 150-foot-long rotating kiln, which was almost twice the size of the standard kiln in use at the time and resulted in economies of scale that led to significant cost savings. And that brings us to the construction of the Hoover Dam, 1931-1936, which was at that time the largest concrete project the world had ever seen.  The dam was not poured as a continuous structure, but was actually composed of huge blocks of concrete that were poured in place.  If the builders had made a continuous pour, it would have taken 125 years to cool, and cracks would have formed.  Amazingly, the concrete is still “curing”, and a core sample taken in 1995 shows that as this process continues, the dam is still gaining in strength.  All told, there is enough concrete in the Hoover Dam to make a two lane road stretching from San Francisco to New York.  And just to set the record straight, there are no human bodies entombed inside the dam, even though that has been rumored from time to time. And now for roads.  We all know that a lot of roads are made of concrete, but others are made of asphalt.  Which is best? That turns out to be an unexpectedly complicated question, which requires us to answer another question first:  What is asphalt?  Asphalt is derived from oil.  It is called the “bottoms,” since it is the approximately 6% that remains after other “lighter” components have been removed from “normal” crude oil through distillation.   Asphalt can also occur naturally in deposits, such as the Canadian Tar Sands.  When mixed with gravel/rocks, it has been used for making roads since before the Roman times.  Today it is sometimes called “asphalt concrete,” not because it contains concrete, but because the engineering definition of concrete is broad enough to include a mixture of asphalt and aggregate, such as rocks and gravel.  And many of us use the name “tarmac” to refer to airplane runways, but tarmac (abbreviated from “tar macadam”) is simply another word for asphalt concrete, or just plain “asphalt.” Since asphalt is made from oil, the cost of asphalt is absolutely connected to the price of oil.  This is a negative.  However, asphalt makes for less noisy roads when compared to concrete.  This is a positive.  Asphalt also becomes deformed when it gets hot, and heavy traffic can cause troughs or ruts; it is also very sensitive to moisture and subsequent cracking.  On the other hand, concrete requires steel reinforcement, which is more sensitive to salt.  All told, asphalt roads have a life expectancy of 10-15 years, while concrete roads have an expectancy of 30-40 years and sometimes much higher.  Several concrete roads in Texas have been in place for over 70 years.  However, around 95% of all concrete roads in the U.S. have an asphalt “topping,” so even if you think you’re driving on an asphalt road, it may really be concrete underneath. Another innovation regarding concrete and roads is “rebar,”—you know, those steel bars crisscrossing the base of concrete highways.  Or bridges.  Or any other concrete structure that needs to “flex.”  These materials are a great match!  While concrete is weak under tension, steel is not.  So in the areas where the concrete will bend, they add rebar to help keep the inevitable cracks very small.  In addition, the universe has conferred an amazing physical property on steel rebar—it has the same expansion/contraction properties as concrete.  So when a road freezes, for example, there is no separation/cracking around the rebar because it shrinks just as much as the concrete does.  However, when salt seeps down inside concrete, it can cause corrosion when it makes contact with the rebar.  Fortunately the steel in concrete takes much longer to corrode than steel in the open air because the chemicals in the concrete form a protective layer around the rebar.  For all of these reasons, rebar and concrete are intertwined in modern construction. Now you might think that with a building material that is thousands of years old, the chances of making any new improvements would be slim.  You’d assume, reasonably enough, that there is not much room for innovation.  But you would be wrong. Here at OSU, improvements in concrete technology are coming fast and furious.  Our concrete inventor here is Dr. Tyler Ley, Associate Professor in Engineering.  He had a thing or two to say about this blog.  All good, of course. Take corrosion.  As mentioned above, the rebar in concrete can corrode, and losing the strength of rebar in concrete is not a good thing.  Think collapsing bridges and crumbling buildings.  Inventors at OSU have developed a silver-dollar sized corrosion detector made with a low powered circuit that mimics an RFID chip (the same kind of chip Wal-Mart embeds in clothing to detect if a garment has been removed from the store illegally).  If salts enter the concrete, then they will corrode the sensor’s external detection wires, and the sensor will change its response.  A remote detector can be used to read the sensor and thus detect the salts.  All of this can be done without any wires or batteries, and for about $30 per sensor.  OSU has actually embedded these sensors in several bridges in Oklahoma, two of them on I-35 near Stillwater.  Pretty cool, no?  Of course we have filed for a patent on this. Then there is concrete “curing.” When a highway is poured, especially under the brutally hot conditions here in the south, the concrete can lose water too rapidly.  This can cause cracking and for the concrete not to gain strength.  What we want to do in this situation is slow down the evaporation and control the temperatures in the concrete, which allows more time for the various chemical reactions to take place and results in stronger concrete.  A common practice is to spread wet burlap bags over the concrete to slow the evaporation.  In looking for a better solution to this problem, inventors at OSU have developed a product that is 95% recycled materials, the largest ingredient being recycled paper pulp.  When sprayed on new, wet concrete, this mixture will hold moisture three times longer than conventional burlap bags.  Further, “wetness” dyes can be added to the pulp to indicate the moisture of the covering.  If it dries too quickly, then the material can be re-wetted.   As soon as the concrete has reacted enough, the pulp can simply be blown off and recycled.  This system was tested with great success last summer on a bridge in Woodward, Oklahoma.  We’ve filed for a patent on this technology, too. Freezing and thawing are hard on concrete, but the problems can be alleviated if small pores are allowed to form within concrete by adding soaps while mixing.  In fact there are standards dictating the size and number of these “void” spaces.  The function of the void is to provide an escape path for water that soaks into the concrete, so that upon freezing, the water can expand into the voids and not crack the concrete.  The problem is that currently these small void spaces can only be detected AFTER the concrete has cured, at which time a core of concrete is removed and examined under a microscope.  If it turns out that the voids are not the right size and number—you guessed it, the concrete has to be broken out and re-poured.  Ouch.  So inventors at OSU have developed a device that measures the size and number of the voids in WET concrete by using the response of the material to air pressure.  If the concrete doesn’t have the correct voids, then more soap can be added and mixed into the concrete while it is still on the truck.  Do you think we’ve filed for a patent on this one?  Absolutely. A point I continually make is that the universe seems to be infinitely amenable to more and more innovation.  Even in the oldest systems.  And so it is with concrete.