NEW CONCRETE CEMENTS

 

Sometimes, though less frequently than we would like, one solution appears that neatly solves two problems at once. As we have seen, the tremendous volume of clay and limestone being kilned each year pours millions of tons of CO₂ into the atmosphere, both from the burning of fossil fuels to cook the material and from the material itself. This is especially true of limestone, which generates a phenomenal amount of CO₂ when it is transformed in the kiln from calcium carbonate to calcium oxide. Likewise, the wide-scale adoption of high-strength/low-durability concrete cement in the mid-twentieth century has proven disastrous to our infrastructure. Something obviously had to be done about these troubling situations. Fortunately, a simple solution came to the fore that helped address not only both issues but a third as well.

The steel industry and coal-burning power plants have been generating a tremendous amount of solid waste products for years. The steel industry produces millions of tons of slag, and the coal-burning power plants generate an equal or greater amount of fly ash. Slag is that portion of iron ore that is left after the metal is smelted. To aid in the process of separating iron ore from mineral impurities, lime and magnesite are added, and these become components of the slag as well. Fly ash is the lighter portion of coal ash that was previously allowed to fly out of the smokestacks of power plant furnaces. With the enactment of environmental laws in the 1970s and 1980s, coal-powered power plants were forced to capture this ash with electrostatic precipitators or particle filters. In the case of both slag and fly ash, the material was either piled up in nearby heaps or assigned to landfills.

As with the millstone refuse in Andernach, Germany, some three hundred years ago, people discovered that the chemical composition of slag and fly ash made the combination ideal for producing cement. Even better, while the Andernach chips were suitable only as a pozzolanic element, slag and fly ash have both pozzolanic and cementitious components.1 In other words, they can replace not only much of the kilned clay in Portland cement but much of the kilned limestone as well. That's not all: because of the high percentage of silicates in some fly ash, it can also be used as a filler to replace some of the sand used to make the concrete. Adding gravy to this good news is that when slag or fly ash is mixed with Portland cement, the result is a high-performance product that has both high compressive strength and long durability, thus, no early cracking and premature rebar corrosion to worry about.

The only downside in this otherwise upbeat story is that most fly ash and slag suitable for cement production is still not being utilized for this purpose. Conventional Portland cement still predominates, as well as the pollution and wasted resources that come with its production. The cement industry lobbies hard to block any government legislation or Environmental Protection Agency (EPA) regulations that it feels would limit its freedom to do as it pleases.² This is often the case: industry will always lobby for or against anything that it sees as furthering or countering its perceived interests. However, narrow and short-term commercial policies often conflict with the wider public good, as in the case of concrete cements. The US government has been successful in specifying fly ash concrete cement in a number of construction projects, but such measures have had little effect on the private sector, where standard Portland cement or, worse, its old high-strength/low-durability counterpart, can still be used.³ Taking into account the proven costly and/or dangerous flaws of the latter substance, an outright ban should be seriously considered. As the old saying goes about the squeaky wheel getting greased, the public must make it plain to its elected representatives that a shift to greener cements is in everyone's best interests, including those of the industry that would produce the material. (Surely Portland cement manufacturers would not want to see a return to steel-frame or—heaven forbid!—masonry construction.)

 

NEW REINFORCEMENT BARS

 

Although the use of the green cements and stainless-steel rebar can double the life span of reinforced-concrete buildings, one basic problem remains: the inevitable corrosion of the steel that eventually compromises their structural integrity.

One method by which the corrosion of steel rebar can be—theoretically—indefinitely postponed is cathodic protection. The corrosion of iron or an iron-based alloy is an electrochemical process. Electrochemical processes pervade the natural world. (As you read this page, countless electrochemical actions are taking place in your brain.) In the case of corroding steel rebar, a small current is generated, with the corrosion patch serving as the positive (+) pole and the closest noncorroding area of the rebar serving as the negative pole (-). In other words, the action is similar to that seen in a battery, with the different portions of the rebar acting as both the anode and the cathode. If unchecked, the rusting anode corrodes and expands, shrinking the cathode until none of the latter remains (or the building fails). All that is left is pure rust. The concrete around the rebar is—especially when moist—the electrolyte, the medium that allows the flow of this current.

The electrochemical properties of rust have been known for a long time,4 as well as the ways by which it can be managed, both passively and actively. An example of the passive method is connecting another, more vulnerable (“less noble”) metal to the one being protected.⁵ If you own a standard home water heater, there is a rod inside called a sacrificial anode, usually made of aluminum or magnesium, to which the current is passively directed. This rod rusts instead of the steel of your water tank. Because of the diverse properties of the electrochemical process in various corrosion environments, different sacrificial metals are employed. One that might work well in freshwater, such as aluminum, may not do as well in saltwater, so another is used, such as zinc.

Because steel rebar is especially vulnerable to chlorides, zinc is often used as the sacrificial anode. One excellent passive protection method is coating the rebar in zinc. This zinc coating protects the rebar from corrosion. Should corrosion begin somewhere on the rebar, the zinc continues to draw it away from the steel,6 even after much of the zinc has been “sacrificed” in the process. Of course, once the zinc coating has vanished, corrosion then begins to attack the steel, so this is not a permanent solution.

The “active” form of protection seems to offer a more permanent solution, but it is also more complicated and costly. A direct current (DC) of electricity is sent into the concrete, making the rebar a cathode. An electrical lead is connected to the rebar that draws the current away to a DC rectifier box powered by standard alternating current (AC).⁷ This method has been employed for steel pipelines and ship hulls for many years, but it is more problematic when the steel is buried in concrete. It is best employed at the time of construction, although some reinforced concrete structures can be retrofitted with the devices. The connection points and rectifier boxes must be continually monitored and maintained, just as a reinforced concrete bridge must be, but this process does add costs and an extra box to maintenance checklists. Active cathodic protection of rebar also adds about 15 percent to the cost of an average freeway bridge.⁸ For these reasons, some engineers are not especially drawn to the active form of cathodic protection.

 

NONFERROUS REBAR

 

A popular response of late to reinforced concrete's corrosion problems has been the use of stainless steel rebar. While stainless steel rebar does last longer than the standard mild steel version, perhaps adding a decade or two to the concrete's life span, it will eventually corrode as well. Again, it is the iron within the stainless steel that ultimately dooms it. Since at least the 1970s, scientists have been intensively researching methods by which the iron element can be completely eliminated in the rebar. By the late 1980s, the products of this research were beginning to come to market. One is GFRP (glassfiber reinforced polymer) rebar. In tension strength, it is stronger than steel at one-fourth its weight. It is immune to many chemicals to which steel is vulnerable, such as chlorides. Because it does not conduct electricity, GFRP rebar is obviously resistant to electrochemical corrosion as well, and so will not rust. Its nonconductivity is especially useful in some applications. For example, MRI (magnetic resonance imaging) scanners in hospitals are highly sensitive to ferrous metals, including the steel rebar in the walls. Tollbooths using radio-controlled toll-collection devices, airports with radio or compass calibration pads, and high-power voltage transformer vaults can also react with the rebar buried in the concrete.⁹ GFRP rebar helps counteract these problems. For the same reason, you may obtain better cell phone reception in a GFRP-reinforced structure than one using steel reinforcement. GFRP has been used for roadbeds and bridge decks. The initial data indicate that it will greatly extend the life of such structures, while at the same time significantly reducing maintenance costs.¹⁰ This latter advantage is important, for while the steel-reinforced concrete structures built today will last longer, they will still need regular maintenance to check for corrosion—and costly repairs once it is found. The tests so far conducted on roadbeds and bridge decks using GFRP-reinforced concrete show that it should last a very long time, certainly longer than its steel-reinforced equivalents.

The physical characteristics of GFRP rebar are different from those of steel. While its tension strength is almost two times that of steel at one-fourth its weight, it is less elastic.11 Another drawback of GFRP rebar is that it cannot be bent at the worksite to accommodate the elaborate latticework required for columns and other architectural forms. GFRP rebar can be ordered prebent for a construction project, but the small variances that can occur at the worksite may not conform to the ideal found in a blueprint. For this reason, GFRP rebar has been used where straight lengths of rebar are needed, and where the structure's component calls for compressive strength, such as the aforementioned roadbeds and bridge decks.

Likewise, the newer carbon fiber rebar now coming into the market seems to display similar virtues and drawbacks, and more testing is still needed. One application of this technology is the use of carbon fiber grids in precast concrete blocks or panels for sectionalized construction. Because of the strength and lightweight nature of carbon fiber, thinner panels of concrete can be cast, further decreasing the weight. The weight factor may be a major design consideration if, for instance, a building is planned in an area of soft soil. This virtue of being lightweight is shared by GFRP rebar, and handling either rebar is far easier for the workers than the traditional steel versions.

One material that holds much promise is aluminum bronze. Cold-drawn aluminum bronze alloys are of equivalent strength to the mild steel used in most rebar. It does not corrode away and is 35 percent cheaper than stainless steel, one of the most popular varieties of rebar now being used to fight corrosion.12 Aluminum bronze alloys have been used in the maritime industry for decades. They hold up well in seawater, which steel does not do (unless, of course, the latter is charged by an electric current to provide cathodic protection). For example, the bronze equipment and massive propellers of the RMS Titanic will likely be the ship's only metallic survivor after a thousand years have passed. Copper-based alloys like bronze develop a microfine film of corrosion that protects it from further corrosion—often seen as a green patina on the metal.¹³ Examples of this patina film can often be seen on bronze statues, some over two thousand years old, which have endured to this day because of the seemingly unlimited life span of this alloy. Classic bronze consists of copper combined with tin. Aluminum bronze alloys mostly consist of copper, combined with 5 to 11 percent aluminum and smaller amounts of nickel, manganese, and iron as well, though the corrosion properties of the latter are suppressed by the larger mass of the alloyed metals.

The tests performed on this alloy have proved very promising, but there doesn't seem to be much interest in the material, even though it offers the potential of providing a “forever” rebar that can also be bent on the worksite.

One argument against aluminum bronze rebar is that its price would rise as demand increases, since copper is less common in nature than iron (aluminum is at least as abundant as iron). Assuming this would be the case, and that the price for the alloy rises and is one day as expensive as stainless steel, let us do some cost comparisons, since calculations have already been performed comparing stainless-steel and standard (mild-steel) rebars. The construction costs shown are an arbitrarily chosen average; some bridges would be far less expensive, and others, far more expensive.

These are conservative estimates based on the increased life span of the new fly ash concrete cements. The 19-percent-higher construction cost for the bridge when using stainless-steel rebar is amply returned by the structure's increased life span. However, this gain is relatively small in comparison to that offered by aluminum bronze rebar. Actually, we do not know how long the third bridge would last—it might be two millennia or more. In any case, it would be a very, very long time. The concrete might crack, and perhaps small chunks would fall off during the centuries, but these would likely be cosmetic deformities that could easily be patched at minimal expense. Assuming that the replacement costs for these bridges would be the same as the construction costs (we will ignore adjustments for an unknown inflation rate), one sees the enormous savings accrued over the following centuries. Not calculated are the enormous amounts of pollution generated to manufacture the cement and the tremendous waste of resources entailed in rebuilding that same bridge over and over again.

 

DO WE REALLY NEED REINFORCEMENT FOR ALL CONCRETE STRUCTURES?

 

Perhaps the most controversial solution to the problems presented by reinforced concrete is to simply eliminate the reinforcement completely. I am on dangerous ground here. While I am not necessarily advocating such measures, there are enough examples—both ancient and recent—of this kind of construction to allow me to play the role of devil's advocate.

As noted in chapter 3, the Pantheon offers a perfect paradigm for the durability of unreinforced concrete, and there are other instances nearer at hand and time. George Bartholomew's unreinforced concrete street in Bellefontaine, Ohio, has lasted over a century, during which time it required less maintenance than other nearby streets. It is now a pedestrian zone, but this change was made primarily to preserve the original concrete surface that would have been otherwise obscured by a fresh layer of modern concrete poured on top that would keep the roadway up to spec. Since concrete has enormous compressive strength, why is reinforcement needed for a street or highway, particularly if either is well enough bedded so that fractures do not lead to lateral displacement of their parts? In our world of steel-reinforced concrete, cracks are feared—and rightly so. A crack can allow the ingress of air, water, and salts, and this can lead to the corrosion of the steel rebar, endangering the structural integrity of the roadway. However, cracks in unreinforced concrete are usually benign. This is not to say that unreinforced concrete streets and highways will not need to be patched or resurfaced every so often, but the costs of these measures are far less than the expense of roadway replacement.

The open-minded engineer would say at this point, “You might have a point there, but the use of unreinforced concrete for other applications would not be suitable. A bridge, for instance, would require the tensile strength of some kind of reinforcement, whether that be steel or some other material, such as aluminum bronze alloys or polymer-carbon fiber composites.” Yes, reinforced concrete would be preferred for most construction work, but perhaps concrete bridge building does not exclusively require such reinforcement.

In southern England there is a remarkable structure, the importance of which has not been widely recognized. The Hockley Viaduct is an elevated rail platform that was part of a line connecting Didcot, Newbury, Winchester, and Southampton. Completed in 1891, it provided a second independent line to the Southampton Docks in order to break the monopoly then held by the London & South Western Railway. Like some Roman bridges, it is combination of masonry and concrete, mostly the latter. In fact, it looks very much like a Roman aqueduct and has thirty-three arches. During both world wars, the viaduct was extensively used to transport military personnel and equipment to Southampton, the main embarkation point for France. The viaduct provided an especially vital link during World War II, when it was completely closed to passenger traffic to allow the transport of the mountains of war material sent to Southampton for the Normandy Invasion. In the year prior to D-Day (June 6,1944), sixteen thousand train cars traveled across the viaduct, many of them carrying heavy tanks and artillery pieces.14 The transport of this equipment no doubt exceeded the load capacities envisioned by the viaduct's nineteenth-century builders. The Hockley Viaduct was closed under the “Beeching Axe,” the informal name given to the British government's reorganization of the country's railways under the direction of Dr. Richard Beeching in the 1960s. The reorganization closed many lines deemed “unproductive,” including the one to which the Hockley Viaduct belonged. What makes this brick-clad concrete structure so interesting is that it has no reinforcement. The beautiful viaduct stands as a testimonial to the strength of ancient building methods applied in the Industrial Age. It is easily the most important concrete structure to have survived from the nineteenth century, not only for its beauty, but also for the lessons learned from its construction methods and its remarkable durability. Unfortunately, this splendid viaduct has been subject to much abuse since the 1960s. In some places, the adjoining walls have been demolished to pilfer the bricks. Since English Heritage refused to grant the Hockley Viaduct landmark status to ensure its preservation, a small group of local volunteers must continually paint over the scrawls of vandals, repair the damaged spots, replace stolen copings, and pull weeds to prevent them from taking root in the mortar seams and causing further injury.

We are so attached to steel and steel-reinforced concrete construction that the idea of building an unreinforced roadway or bridge is inconceivable to most engineers today, yet such structures undeniably possess much longer life spans and lower maintenance costs than our corroding modern structures. The concrete and masonry Aelian Bridge (now called the Ponte Sant'Angelo) in Rome, built in 134 CE by the emperor Hadrian, is doing just fine after nineteen hundred years, and, if given minimum protection, the Hockley Viaduct in England should also last as long. Though such construction methods may rarely be employed today, they should at least remain on the table as an option.

 

 

THE WORLD WE HAVE BUILT

 

I once heard an engineer who, while talking about his involvement in constructing a bridge that will have a one-hundred-year life span, concluded his remarks by saying, “By the time that thing fails, I'll be long dead.” Considering that most existing bridges have a service life of fifty years, he was proud to have built something that would endure twice as long. I experienced a similar sense of the despondency when reading about the construction of the Pentagon Memorial dedicated to the 184 victims killed at the Pentagon and on American Airlines Flight 77 on 9/11. Its builders confidently predicted that it would last over a century.15 Think about that: a memorial that will last only a little longer than the life span of a healthy person. What is the point of a memorial that will mostly be viewed by contemporaries who already have firm recollections of the tragic event it memorializes? Compare this to the many bronze and granite memorials in our nation's capital built in the nineteenth and early twentieth centuries that will, like their Roman and Greek predecessors, probably endure millennia. Such are the values of this world we have created, one in which we have come to accept the short life expectancy of not only our infrastructure but of our memorials as well.

We have built a disposable world, and we pride ourselves on being able to extend its existence a bit more, rather than seeking ways to make it permanent. We can always tear down a “permanent” structure if it stands in the way of an important public development, or, given a revival of a now largely vanished sense of aesthetics, to replace it with something more beautiful.

One altruistic belief beloved by Americans is that the world we leave to our children should be better than the one we found. Apparently, we have confused “better technology” with a “better world,” for we have done a sorry job with everything else. Our principal legacy for our descendents is a soaring national debt and a corroding infrastructure. And the two are not entirely unconnected.

Let's go back and look at that comparison of the various bridges built with different rebar. Take the savings accrued by building a permanent bridge (aluminum bronze rebar does not necessarily have to be involved) and compare it to the “extended life” of a stainless-steel-reinforced concrete bridge, and multiply that by six hundred thousand (the number of rail and highway bridges in the United States). The savings in bridge construction over this five-century period in the United States alone would be just under $120 trillion, over three times the total current public debt of the entire planet (approximately $39 trillion as this book goes to press).

The concept of nonpermanent construction is a recent one. Before the advent of reinforced concrete, major buildings and bridges were built to last a very long time. One can walk around many European cities—particularly those that had not been subject to Allied or German bombing during World War II—and find oneself surrounded by buildings constructed centuries ago that will likely last centuries more. Look at the extraordinary beautiful ancient structures in Prague in the Czech Republic, particularly the splendid Charles Bridge (Karluv most) designed and built by Peter Parler in 1357. The old builders of Prague would have been struck dumb with amazement if they had been asked to construct buildings that could only last a century.

We do not need to go back to masonry construction to achieve the same durability for our buildings that our ancestors simply took for granted. We have the tools to do it now with reinforced—or unreinforced— concrete. A good first step would be to put into place a transition period from steel to nonferrous rebar for most construction work. In the meantime, we can begin utilizing GFRP rebar for roadbeds and bridge decks, and aluminum bronze rebar for other purposes. Since copper is a finite resource, something similar to the “X Prize” (the “X Prize” was awarded for the first successful privately financed space vehicle) should be put forward to encourage the development of a strong, enduring artificial rebar that can be bent at the worksite.

We can do this. In fact, we cannot afford not to do this. Depending on which course we take, our descendents will either thank us or curse us.