Concrete, made of crushed stone, sand, calcined (kilned at high temperature) limestone, kilned clay and water is an ancient and natural building material, which however has a relatively high embodied energy and a significant global warming contribution, and thus should be used only where necessary and in limited amounts. Foundations and floor slabs are the most obvious uses for concrete, and shallow frost-protected, rubble-trench grade beam, and slab-on-grade foundations reduce the volume of concrete required.
Pound for pound, its ecological footprint is small, but concrete is used by the ton, and is the second most human-used substance on earth after water. It is used for everything from gargantuan public works projects to skyscrapers to garden figurines.
Approximately 3.7 billion tons of cement (the active ingredient in concrete) were produced globally in 2012, and there already exist 40 tons of concrete for every person on earth, with 25 billion additional tons added every year (approximately 3.5 tons per year for every man, woman and child). The amount of concrete used worldwide, ton for ton, is twice that of steel, wood, plastics, and aluminum combined.
Portland cement production has nearly quadrupled since 1970, and global output of cement may reach 5 billion tons by 2030 and 18 billion tons by 2050. The concrete industry’s share of global anthropogenic CO2 emissions is about 8%, exceeded only by the electrical generation and transportation sectors. The production of Portland cement emits nearly a ton of CO2 for every ton of cement produced.
Portland cement accounts for 7% to 15% of concrete’s total mass by weight, but almost all of its embodied energy and embodied global warming contribution, with 40% of its CO2 emissions coming from the fuel used in the combustion process and 60% from the calcination (outgassing) of the lime/clay, as limestone gives up CO2 when it’s heated.
CaCO3 + 1500°F → CaO + CO2
Calcium carbonate plus high heat produces calcium oxide and carbon dioxide.
However modern Portland cement is made from clinker, which is an excessively-heated byproduct of clay-limestone sintering, requiring kiln temperatures of 2640° F. The global warming contribution of concrete can be reduced 30% by substituting fly ash for 30% of the cement (however, see addendum below about the potential dangers of fly ash).
Other environmental effects of the widespread use of concrete include impervious surface runoff and waterway pollution, the urban heat-island effect and increased air-conditioning costs, naturally radioactive elements in concrete and long-term health effects, and concrete dust from demolition or earthquake damage and consequent air pollution and lung damage.
Wet concrete is a hazardous material, with a pH of 12-13 (14 is the most alkaline), which is similar to that of bleach, oven cleaner and lye, and can cause skin burns or permanent eye damage.
The 2008 paper, REACT: Reducing Early-Age Cracking Today, published by the Building and Fire Research Laboratory of the National Institute of Standards and Technology (NIST) states that:
“Concrete is generally viewed as a durable and long-lasting construction material. However, the long-term performance of a concrete structure can be greatly compromised by early-age cracking… with some ready-mix companies experiencing early-age issues on as many as 70% of their jobs. “
As Robert Courland details in his 2011 book, Concrete Planet, the Romans built concrete structures that lasted more than 2,000 years. Ours will last a century – at most.
- One in four bridges are either structurally deficient or structurally obsolete.
- The service life of most reinforced concrete highway bridges is 50 years, and their average age is 42 years.
- The chemicals and bacteria in sewage make it almost as corrosive as seawater, reducing the life span of the reinforced concrete used in these systems to 50 years or less.
- Most of the concrete structures built at the beginning of the 20th century have begun falling apart, and most will be, or already have been, demolished.
The American Society of Civil Engineers (ASCE) rated America’s infrastructure in its 2013 report card as D+ grade, and stated that $3.6 trillion would have to be spent by 2020 for necessary improvements.
The problem isn’t just the concrete that forms much of our nation’s infrastructure – it’s the iron and steel rebar reinforcement inside. Cracks can be fixed, but when air, moisture, and chemicals seep into reinforced concrete, the rebar rusts, expanding in diameter four or five-fold, which destroys the surrounding concrete, and ultimately destroys the nuclear reactor and waste containment structures; the coal and natural gas power plants, buildings, homes, and skyscrapers; the roads, bridges, dams, levees, water mains, barges, airport runways, sewage and water treatment plants and pipes; schools, subways, churches, canals, corn and grain silos, shipping wharves and piers; tunnels, parking garages and lots, sidewalks, shopping malls, swimming pools, and anything else made of reinforced concrete.
Shrinkage cracks occur when concrete members undergo restrained volumetric changes (shrinkage) as a result of either drying, autogenous shrinkage, or thermal effects. Restraint is provided either externally (i.e. subgrade, supports, walls, and other boundary conditions) or internally (differential drying shrinkage, steel reinforcement).
There are three primary mechanisms for initial cracking of curing concrete.
1) Plastic shrinkage is caused when water evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, and the surface concrete shrinks. Due to the restraint provided by the concrete below the drying surface layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks of varying depth.
2) Autogenous shrinkage (or drying shrinkage) is caused by chemical shrinkage, which is the reduction in volume due to the hydration reaction. Because almost all concrete is mixed with more water than is needed to hydrate the cement, much of the remaining water evaporates, causing the concrete to shrink. Restraint to drying shrinkage is the most common cause of concrete cracking. In many applications, drying shrinkage cracking is inevitable. Therefore, contraction (control) joints are placed in concrete to predetermine the location of drying shrinkage cracks, which are deeper than the plastic surface cracks.
3) Thermal cracking is due to the considerable heat generated by the cement hydration reaction. As the interior concrete increases in temperature and expands, the surface concrete may be cooling and contracting. This causes tensile stresses that may result in thermal cracks at the surface.
There is a fourth cause of concrete cracking that is often overlooked. According to Allen Face, perhaps the world’s leading authority on the design, construction, and quality control of concrete floor slabs, “continuous steel reinforcement [in floor slabs] has only two functions: 1) to induce the formation of cracks, and 2) to stabilize gaps in the concrete.”
Concrete is enormously strong in compression (2500 psi – 60,000 psi), but relatively weak in tension (300-700 psi) and similarly weak in flexural strength (400-700 psi). When steel reinforcement is placed in a slab that is supported by well-compacted subgrade and experiences only compressive forces, the steel – whether rebar or welded wire mesh – adds little to the tensile strength of the monolithic structure (typically 1% to 4%).
Because the steel does not shrink as the concrete cures, it creates an internal resistance to shrinkage movement and hence encourages cracking to the depth of the reinforcement, while also bridging those cracks to prevent displacement and further mechanical widening. Sufficient steel encourages more dispersed and hence smaller cracks and then maintains the integrity of the slab after cracking – but it does not prevent cracking, and unreinforced concrete is less likely to crack.
Durability problems in concrete structures may be due to such causes as errors in design or carelessness in detailing, use of inferior construction materials, inadequate quality control, poor workmanship, or heterogeneity of the materials. Durability issues with concrete structures are observed in the form of cracking, spalling, corrosion of reinforcing steel bars, loss of mass, long-term creep, and loss of strength. The cause of concrete deterioration can be physical, chemical and in most cases, a combination of both. The net effect of concrete deterioration processes is to weaken the integrity of the complex microstructure of concrete.
The rate at which a concrete structure may deteriorate is mainly dependent on the permeability of the concrete as well as how the concrete is placed, compacted, cured, and allowed to sustain load, cover depth of reinforcement and quality of cover concrete. Contact with aggressive chemical ions, such as chlorides, sulphides, acids, carbon dioxide, and even water, causes the deterioration of the concrete. Such deterioration involves either leaching of material from the surface by dissolution or expansion of material inside the concrete (rebar oxidation or alkali-silica reaction of aggregate).
Exposure conditions vary over a wide range including hot and dry desert ambient air, wind, and rain or snow and frost. Higher ambient air temperature may accelerate the chemical reactions within concrete leading to faster deterioration, while sub-freezing temperatures can lead to freeze-thaw damage. Furthermore, the concrete quality degradation mechanism may be either a physical effect such as shrinkage, creep, erosion, fire exfoliation, spalling or seismic stress; or a chemical reaction such as sulphate attack, reinforcement corrosion, alkali-silica reaction, or carbonation.
On January 21, 2008 the History Channel aired a two-hour pilot for the documentary series that premiered April 21, 2009, called Life Without People, in which scientists, structural engineers, and other experts speculate about what might become of Earth should humanity instantly disappear and maintenance of buildings and urban infrastructure comes to a halt.
Given that the United States is now perhaps half a century behind on the care and maintenance of its infrastructure, the thought experiment is really just a dramatic extrapolation of current affairs.
A Detailed History of Concrete
Burnt Lime and Civilization [All historical dates are annotated BCE – before the common era – or CE – common era – rather than the Christocentric BC – before Christ – and AD – anno domini (year of our lord).]
While the Roman Empire was known for its magnificent and widespread use of lime cement mortar for floors, walls and aqueducts, there is now archeological evidence that the earliest uses of burnt lime predates civilization and began in Paleolithic times among hunter-gatherer peoples – not the sedentary agriculturalists of the Tigris-Euphrates and Nile deltas as commonly thought.
Göbekli Tepe (“Potbelly Hill”) is an archaeological site at the top of a mountain ridge in the Southeastern Anatolia Region of Turkey, near the headwaters of the Euphrates River. It was excavated by a German archaeological team under the direction of Klaus Schmidt from 1996 until his death in 2014.
The site includes two phases of ritual use dating back to the 10th-8th millennium BCE. During the first phase, 20 circles of more than 200 massive T-shaped stone pillars were erected, with each pillar having a height of up to 20 feet and a weight of up to 20 tons. They are fitted into sockets that were hewn out of the bedrock, pre-dating Stonehenge by 6,000 years. In the second phase, the erected pillars are smaller and stood in rectangular rooms with floors of polished lime, indicating that they learned how to burn limestone at very high temperatures.
Many of the pillars are decorated with abstract, enigmatic pictograms and carved animal reliefs, including lions, bulls, boars, foxes, gazelles, donkeys, snakes, crocodiles, arthropods and birds, particularly vultures. Some of the pillars were clearly anthropomorphic, having carved arms and loincloths.
The site shows no traces of domesticated plants or animals and very little evidence of residential use, suggesting a purely ritual function. The structures not only predate pottery, metallurgy, and the invention of writing or the wheel, they were built before the Neolithic Revolution, which was the beginning of agriculture and animal husbandry around 9,000 BCE. It appears to have functioned as a spiritual center by 11,000 BCE or even earlier, and is the oldest religious site yet discovered anywhere. Schmidt believed that what he called this “cathedral on a hill” was a pilgrimage destination attracting worshippers up to 100 miles distant. Butchered bones found in large numbers from local game such as deer, gazelle, pigs, and geese have been identified as refuse from food hunted and cooked or otherwise prepared for the pilgrims.
Nearby Göbekli Tepe is found Nevalı Çori, an early Neolithic settlement on the Euphrates c. 8600 BCE, which was also pre-agricultural with limestone carving and lime products, and fired clay figures (but no pottery). Another neighboring village is Çayönü c. 8000 BCE, which was partly agricultural, with limestone carving and lime products, fired clay figures (no pottery), copper metallurgy, and woven cloth.
Farther west in Anatolia is Çatalhöyük, a very large Neolithic proto-city settlement which existed from approximately 7500 BCE to 5700 BCE, and flourished around 7000 BCE. It is the largest and best-preserved Neolithic site found to date, and was agricultural, with limestone carving and lime products, fired clay pottery, copper metallurgy, woven cloth, stone-ground wheat and baked bread.
Bedouins Didn’t Just Ride Camels
The first concrete-like structures were built by the Nabataea traders or Bedouins who occupied and controlled a series of oases and developed a small empire in the regions of southern Syria and northern Jordan around 6500 BCE. They later discovered the advantages of hydraulic lime — that is, cement that hardens underwater — and by 700 BCE, they were building kilns to supply mortar for the construction of rubble-wall houses, concrete floors, and underground waterproof cisterns. The cisterns were kept secret and were one of the reasons the Nabataea were able to thrive in the desert.
In making concrete, the Nabataea understood the need to keep the mix as dry or low-slump as possible, as excess water introduces voids and weaknesses into the concrete. Their building practices included tamping the freshly placed concrete with special tools. The tamping process produced more gel, which is the bonding material produced by the chemical reactions that take place during hydration, which bond the particulates and aggregate together.
By about 5600 BCE along the Danube River in the area of the former country of Yugoslavia, homes were built using a type of concrete for floors.
The Great Pyramid at Giza, built between 2560 and 2540 BCE, required about 500,000 tons of lime mortar, which was used as a bedding material for the casing stones that formed the visible surface of the finished pyramid.
About the same time, the northern Chinese used a form of cement in boat-building and in building the Great Wall. Spectrometer testing has confirmed that a key ingredient in the mortar used in the Great Wall and other ancient Chinese structures was glutenous, sticky rice.
By 600 BCE, the Greeks had discovered a natural pozzolan material that developed hydraulic properties when mixed with lime, but the Greeks were nowhere near as prolific in building with concrete as the Romans. By 200 BCE, the Romans were building very successfully using concrete, but it wasn’t like the concrete we use today. It was not a plastic, flowing material poured into forms, but more like cemented rubble.
What we know of the evolution and use of concrete technology in the Roman Empire comes largely from books by Cato the Elder (about 200 BCE) and the architect Marcus Vitruvius two centuries later (about 15 BCE), and from an encyclopedia by Pliny the Elder published in 78 CE.
The first detailed description of the manufacture and use of concrete is found in Marcus Porcius Cato’s On Farming. Cato wrote about using lime mortar for masonry walls and for a simulated stone floor, sometimes with pottery shards embedded. He described the building and use of a lime kiln of the “Burgundy bottle” form. “Build the limekiln ten feet across, twenty feet from top to bottom, sloping the sides in to a width of three feet at the top”. This shape allowed for the maximum loading of limestone using the minimal amount of fuel. Still, it required perhaps a couple dozen or more cords of wood for each firing.
That Cato suggested mixing sand with lime in a 2:1 ratio suggests that he had only poor-quality sand, likely mixed with dirt, available. A more common ratio with good, sharp, clean sand would be 3:1 or 4:1.
The Roman walls built at the time of Cato were constructed by laying two courses of mortared stone parallel and about 20 inches apart. Once the courses were 2 or 3 feet high, a layer of rock aggregate was filled in between them and then relatively dry lime mortar dumped on top and tamped to fill the voids. This process was repeated until the desired wall height was reached, and the outer masonry protected the lime mortar from the elements.
This method was used from the late Republic to the end of the Roman Empire. But, around the time of Cato, the Romans began using wooden plank molds to pour concrete and then facing that with cut stone or, later, fired brick.
The Romans called their lime mortar arenatum (“sandy stuff”) since sand comprised the bulk of the mixture, and called their lime concrete caementis (“rocky stuff”) because it was mostly stone aggregate. Oddly, the modern terms have been inverted, with “cement” used to describe the “glue” made of kilned lime and clay with or without pozzolans, and “concrete” used to describe the cement-sand-stone mixture. The term “concrete”, though derived from the Latin concretus, meaning brought together or congealed, was never used by the Romans.
The hydraulic concrete that is now called Roman concrete, did not apparently have a name in Roman times, but the hydraulic additive – the pozzolan – was called pulvis puteolis, Puteoli being the ancient Latin name for the modern Italian city of Pozzuoli near Mount Vesuvius (hence the modern term “pozzolan” for the volcanic soil used to make lime concrete hydraulic).
Hydraulic concrete is a pozzolanic mixture which can set under water and remains relatively impervious to water. It allowed the building of sea wharfs and breakwaters as well as making walls and other structures more durable.
The earliest reference to Roman, or hydraulic, concrete is in De Architectura (On Architecture) by the renowned Roman architect, Marcus Vitruvius Pollio, a collection of ten books on building everything from war machines to public works. Vitruvius began his architectural career as artillery specialist to Julius Caesar. It may be that the use of Roman concrete contributed to Rome’s military success and certainly to the maintenance of its empire.
On Architecture is considered one of the most influential books on the subject ever written, as it is the only such detailed manuscript which survives from the Greco-Roman period, and influenced much Renaissance architecture, including that of Andrea Palladio.
Vitruvius described in detail the qualities of materials necessary for Roman concrete, including the types of sand and the advantages of adding ground potsherds to increase strength, particularly when poor (rounded) sand was all that was available.
Vitruvius describes the earliest known reference to hydrated (slaked) lime, and advises that it should be aged in ceramic amphora or wooden barrels for several years before use. And he describes a “kind of powder (pulvis) that naturally produces admirable results” and is found in the area around Mount Vesuzius, mentioning that when mixed with lime and rubble creates a concrete that hardens under water and is suitable for the construction of piers. He proposed a 2:1 mixture of pozzolan and lime (with no sand) which could be rammed into caissons sunk into water.
During his time, the first use of poured and formed hydraulic concrete is evidenced in surviving monolithic ceiling vaults and domes over bathhouses. Such barrel vaults and domes became commonplace in Rome, though they were new enough at the time that Vitruvius did not mention them.
What Vitruvius was likely aware of but also failed to mention in his treatise, was the use of hydraulic concrete in what remained the most massive poured structure until the 20th century: the Harbor of Caesarea built by King Herod in Judea.
The Greatest Engineering Project
The Hellenized King of Judea, friend to Augustus’ right-hand man Marcus Agrippa, was famous for rebuilding the Jerusalem temple, but perhaps moreso for the most ambitious engineering project of Roman times. He wanted to build a port city that rivaled Alexandria on a desert seacoast without limestone, trees or hard rock. Agrippa arranged for the 850,000 cubic feet (32,0000 tons) of pozzolan, and 425,000 cubic feet (25,000 tons) of lime to be manufactured and shipped to the Levant. Additionally, 100,000 to 200,000 oak trees would be needed just to kiln the lime, not to mention the trees necessary to bake the ceramic amphora or build the wooden barrels to contain the lime, the ships to transport it, and the caissons to sink into the sea to form the jetties. Aggripa extracted this price from the Roman provinces and friendly kingdoms of Moesia and Thrace, bordering the Danube River (since the Mediterranean coast was already stripped of forest).
During the eight years of construction, three different forming methods were employed, using sunken forms as much as 38 by 50 feet in size. The harbor was inaugurated in 15 BCE, and almost two millennia would pass before another harbor rivaling its size was built anywhere in the world. But this great engineering achievement was vulnerable to earthquakes in 130 and 363 CE as well as seabed subsidence and siltation, which by the sixth century left the harbor under water. It was restored by the Byzantines in 505 CE but lost to the Mamluks following the Crusades.
Ninety years after the publication of De Architectura, Pliny the Elder compiled his encyclopedia shortly before he died venturing too close to Mount Vesuvius during the eruption which buried Pompeii. He briefly mentioned concrete in his description of the cargo ship that was filled with caementis and sunk at Ostia to form a jetty. But he also waxes eloquent about the magical properties of lime: “It is something truly marvelous, that quick-lime, after the stone has been subjected to fire, should ignite on the application of water.”
This was the last reference to concrete in Roman literature, aside from inconsequential repetitions of Vitruvius. But an important development of Pliny’s time was the inclusion of crushed fired pottery (which Vitruvius had mentioned as a component of waterproof stucco). The red powder had properties similar to pozzolans, and fired clay is an important ingredient of modern concrete.
Nero Fiddled While Rome Burned
In 64 CE, the emperor Nero apparently engineered The Great Fire which destroyed much of central Rome, killed or injured thousands of residents, and left perhaps 100,000 homeless – all to free up the nearly 200 acres he needed to build a new castle complex, called the Domus Aurea, or Golden House.
The castle used brick-clad concrete walls veneered in marble and ivory, with lots of gold leaf flourishes, and elaborate frescoes and stucco embedded with jewels. Nero planted forests, stocked an artificial lake with fish, planted a vineyard, and created gently rolling hill pastures for sheep – so that he could “live like a human being”.
The use of brick cladding became common following the Great Fire, as unlike stone and concrete which exfoliates under high heat, brick is fireproof. The vulnerability of concrete to fire would not be relearned until the 20th century.
Another feature of the Golden House was an octagonal vaulted ceiling dome that was 44 feet wide and had a 20 foot oculus at the apex. The vault was painted to look like the sky and dotted with crystal gems for stars. This vault became the prototype for larger domes in later basilicas and public bathhouses.
Nero committed assisted suicide (he had a slave cut his throat) when the populous rebelled against his excesses.
The Roman Coliseum
Nero’s successor, the more restrained and practical Flavius Vespasian, opened much of Nero’s estate to the public and set aside land next to the lake for a massive amphitheater like no other on earth. The amphitheater is known today as the Roman Coliseum, though that name originally referred to the Colossus that stood nearby, an oversized bronze representation of Nero nearly 100 feet tall. Nero’s face was altered and a flaming halo added to dedicate the Colossus to the sun god Helios.
The Flavian amphitheater became the template for every subsequent stadium to the present day, as it was designed for maximum viewing area and optimum ingress and egress. Work began in 72 CE and Vespasian’s son Titus opened it to the public in 80 CE.
Its original elliptical dimensions were 616 feet by 512 feet, with outer walls 164 feet high and a perimeter of 1,788 feet, comprised of three tiers of 80 arches each. The actual arena was a sand-covered wooden floor 250 by 150 feet in size. There were 80 numbered entrances, with four set aside in the cardinal directions for VIPs (the true north entrance for the emperor and his guests). Visitors were given a ticket with entry door, row and seat number and all rows and seats were similarly numbered. The “nosebleed” seats at the top were for the lower classes, but everyone gained free admission and the stands could accommodate 70,000.
Trap doors in the floor of the arena opened to winch-driven elevators from the hypogeum, an underground labyrinth of limestone chambers and passages. The gladiators who fought there for public entertainment were the superstars of the day.
As much as 700,000 tons of Roman concrete were used in the building of the Coliseum, with perhaps 80% of that used in the foundation alone. The rest was brick-clad concrete and 130,000 cubic yards of travertine limestone. Much of the labor was done by the 100,000 Judean slaves taken back to Rome after the conquest of the region.
Titus celebrated the inauguration of the Coliseum by staging a massive wild animal hunt, and 9,000 creatures were killed by sword, lance, trident or arrow during the 100 days of games. There is also reputed to have been a naumachia (from the Greek word for naval battle) staged in the amphitheater, which would have required using the hydraulically mortared stone aqueduct to flood the theater and later release the bloody water into the polluted Tiber River.
The last show was held in 523 CE under Theodoric, King of the Ostrogoths, and consisted only of animal hunts, as gladiator fighting had been abolished in 438 CE, thus bringing to an end more than four centuries of uninterrupted use.
Lightning strikes, at least two earthquakes (in the 8th and 14th centuries) and the quarrying of stone from the structure for a millennium took their toll on the Coliseum; and by the 19th century, the base was buried under silt from regular floods, while much of the structure was covered with plant growth. The Coliseum was not restored until 1871.
The Pantheon of Rome was commissioned by Marcus Agrippa during the reign of Augustus (27 BCE – 14 CE) and rebuilt by the emperor Hadrian about 126 CE.
The building is circular with a portico of 16 large granite Corinthian columns under a pediment, the gray-blue granite for the 5 foot diameter by 37 foot high columns imported from Egypt. A rectangular vestibule links the porch to the rotunda, which is under a coffered concrete dome, with a central opening (oculus) to the sky. Almost two thousand years after it was built, the Pantheon’s dome is still the world’s largest unreinforced concrete dome. The height to the oculus and the diameter of the interior circle are the same, 142 feet, so the whole interior would fit exactly within a cube, and if the dome were a sphere it would just touch the floor.
The 5,000 ton weight of the concrete dome is concentrated on a ring 30 feet in diameter that forms the oculus, while the downward thrust of the dome is carried by eight barrel vaults in the 21 foot thick drum wall and into eight pairs of columns. The thickness of the dome varies from 21 feet at the base of the dome to 3.9 feet around the oculus.
Similar ancient concrete from Roman ruins in Libya show a compressive strength of 2,900 psi, which would result in a tensile strength of 213 psi. An analysis of the structure found a maximum tensile stress of only 18.5 psi at the point where the dome joins the raised outer wall.
The stresses in the dome were found to be substantially reduced by the use of successively less dense aggregate stones, such as small pots or pieces of pumice, in higher layers of the dome. If normal weight concrete had been used throughout, the stresses in the arch would have been 80% greater. Hidden chambers engineered within the rotunda form a sophisticated honeycomb structure which reduced the weight of the roof, as did the elimination of the apex by means of the oculus.
The top of the rotunda wall features a series of brick relieving arches, visible on the outside and built into the mass of the brickwork, and hidden by marble facing on the interior.
The assassination of Emperor Domitian in 96 CE inaugurated the “Golden Age” of Rome, in which the next five emperors were selected for their personal integrity rather than granted hereditary rights to secession.
Marcus Agrippa, King Herod’s patron, had built a temple to all the gods – a pantheon – in Rome in 27 CE, which fire destroyed in 80 CE. The third of the “good emperors”, Hadrian, wanted to erect a domed structure that would be the envy of the world, an expression of his power, and more beautiful than masonry would allow – and he used the site of Agrippa’s pantheon.
The shuttered dome was the most aesthetically-sophisticated form until modern times, and the use of graded aggregate was not again fully appreciated until recently. Nor was the benefit of concrete tamping rediscovered until the 1980s. The dome was formed with 4 tiers of 28 4-stepped coffers, each perfectly proportioned and placed in the massive formwork. And the sun beam through the oculus acts as a sundial to mark the seasons.
Hadrian, rather than take credit for his magnificent edifice, stamped the portico M. AGRIPPA L. F. COS. TERIUM FECIT (Marcus Agrippa, son of Lucius, built this in his third consulship). This would soon confuse Roman historians who assumed it was built 150 years earlier. It was not until the early 20th century that bricks were examined and found stamped with dates from 118 to 125 CE, about six years into Hadrian’s reign.
Concrete vaults and domes were popular in the Roman Empire during the second, third and fourth centuries, though none ever achieved the size of the Pantheon, which was also more than double the size of any previous dome.
Before the fall of the Roman Empire, the formula for Roman concrete was lost and concrete was not rediscovered for 1500 years, and it was another century or two before we realized we were making all the mistakes that the Romans had long ago learned to avoid, leaving the planet covered in concrete structures destined to deteriorate.
In the 1300s, fragments of Vatruvius’ treatise began to appear, but it was not until 1414 that a complete edition was discovered in a Swiss monastery. Few, however, could translate the technical Greek language, much of which was corrupted by transcription errors. A Franciscan monk named Fra Giovanni Giocondo, a classical scholar, archeologist and architect became royal adviser to French King Louis XII. The king wanted a bridge across the Seine near Notre Dame Cathedral. Giocondo, for the first time in 1,000 years, used the Roman recipe for hydraulic cement to build the piers for the Pont Notre Dame. In 1511, he published the Vitruvius’ On Architecture, thoroughly annotated and richly illustrated, which would remain the definitive edition for centuries. Oddly, however, those next centuries saw no further use of Roman concrete.
It wasn’t until a serendipitous discovery in Germany in the 16th century that hydraulic cement was reborn. Along the Rhine river, the hills were eroded volcanic cinder cones and the rock, called trass, was used to make millstones. Some enterprising mason used some of the leftover chips to mix in his mortar and found that it not only made the mortar harder and more durable, but also allowed it to set under water.
In the 17th century, word of this trass reached the Dutch, who quickly realized the commercial potential and used it at home for dykes, canals and levies and then sold it to the British and French for marine projects, renaming it “terras”.
The Eddystone Light
In the second half of the 18th century, engineer John Smeaton of Leeds England made scientific instruments, introduced the term horsepower, improved the efficiency of the steam engine as well as watermills and windmills. His formula for wind power was later used by the Wright brothers in the construction of their airplane, and he coined the term “civil engineer”, but he is most famous for the construction of the Eddystone Light.
The first Eddystone Light, built of stone around a wood substructure in 1698 in Plymouth Harbor, was blown down by a gale after five years. A second wooden lighthouse burned down by its own flame in 1755. Weeks later, Smeaton was commissioned by the Royal Society to build a replacement. He insisted on granite blocks that were dovetailed at their joints and experimented with hydraulic mortars, testing limestone from a variety of sources.
He found that the stone from Aberthaw on the south coast of Wales had very good hydraulic qualities and he discovered that it contained 11% clay. Smeaton rediscovered natural cement, used since Neolithic times and more recently by the Mayans in Mesoamerica. Smeaton further enhanced the natural hydraulic qualities by adding trass and found that Italian pozzolan was even better.
The new Eddystone Light was completed in 1759. Smeaton, who never took a patent on his inventions, died in 1792, but his lighthouse remained in service until 1877 when the rock it was built upon began to erode. The granite lighthouse was stronger than its foundation. Since the lighthouse was so loved by the locals, it was laboriously disassembled and rebuilt on a square in Plymouth, where it remains today.
Just before his death, Smeaton published in 1791 “A Narrative of the Building and a Description of the Construction of the Eddystone Lighthouse with Stone”, in which he described in detail his experiments with hydraulic cements and his discovery of natural cement.
James Parker, ostensibly a clergyman and civil engineer, claims to have accidentally discovered natural cement and obtained a patent in 1796 of what he called Roman Cement, though it was not. Apparently, he stole Smeaton’s idea, claimed it as his own, patenting a naturally-occurring rock, and then moved to America.
Samuel and Charles Wyatt renamed the London cement works Parker and Wyatt Cement and Stucco Manufacturers, publicizing that they owned the Parker patent, and were so successful that all subsequent natural cement was known as Roman cement. Their product was used to build bridges, harbor works and an arched masonry aqueduct in North Wales that resembled those of ancient Rome.
Because the lime and clay were kilned together, creating a stronger molecular bond, this material was actually somewhat better than ancient Roman cement, and became the standard in Britain for 60 years.
A profusion of patents for “new and better” cement was filed in 19th century Britain. A short-lived experiment with “oil stucco”, incorporating linseed oil, proved to be waterproof but lacked sufficient adhesion.
A company by the name of its principles, Francis & White, took advantage of the expiration of Parker’s patent and manufactured the preferred cement of the time. A Frenchman named Marc Isambard Brunel, who escaped the depredations of the French Revolution to become New York’s chief engineer, later moved to England to contribute to the burgeoning industrial revolution, inventing new machines for mass production.
But Brunel’s fame was vested in his construction of the world’s first tunnel under a navigable river, the Thames – the largest use of hydraulic cement since Roman times. Two attempts failed before Brunel came forward with a unique invention called a tunnel-shield, made of cast iron, and patented in 1818, which allowed workers a safe platform to slowly advance as the tunnel shaft was bricked in behind them. It took from 1825 though 1843, including several inundations of water and mud and a number of worker deaths, to complete.
Brunel is said to have been inspired in his design by the shell of the shipworm Teredo navalis, a mollusc whose efficiency at boring through submerged timber he observed while working in a shipyard.
The Thames Tunnel, encased with five wythes of brick mortared with hydraulic cement, measures 35 feet wide by 20 feet high and 1,300 feet long, running at a depth of 75 feet below the river surface. Marc Isambard Brunel and his son Isambard Kingdom Brunel were the engineers.
The tunnel was originally designed for, but never used by, horse-drawn carriages. It now forms part of the London Overground railway network.
John Smeaton of Leeds was the first, perhaps, to note that his natural cement formed an artificial rock that resembled Portland stone from the Isle of Portland in Dorset, England. A bricklayer from Leeds named Joseph Aspdin was granted a patent in 1824 for a hydraulic mortar/stucco he called Portland cement and is often credited with its invention, though he almost certainly had not discovered the essential secret of this now-widely used material.
Aspdin’s son William, who was so corrupt that his father disowned him and announced his severance from the family business, and who had to move repeatedly following his many swindles, nevertheless seems to have stumbled upon the secret. His father had developed a slurry method of mixing lime and clay and allowing it to dry before kilning. Others used this method as well and often discarded the overly-baked and vitrified chunks called “clinker”, which were very difficult to pulverize. But, once rendered into a powder, these clinkers made a cement which was nearly twice as strong as any Roman cement.
William Aspdin went to lengths to keep his process secret, but it was discovered by others, including Isaac Charles Johnson who worked for John Bazley White of Francis & White fame, possibly through an act of industrial espionage. Aspdin’s last business partner, Edward Fewer, who also tossed William out, later sold his company and it grew to become one of the world’s major manufacturers of Portland cement.
Archeologists in 2008, however, discovered the concrete floor of a shipyard built by Isambard Kingdom Brunel, Sir Marc’s son and partner on the Thames River tunnel project, that seems to have been made with Portland cement.
In truth, a number of people were experimenting with slurry and clinker techniques, including James Frost who studied in France under Louis Vicat, whose careful investigations led to the optimum admixture of clay to lime at 15-20%, and who built the world’s first concrete bridge in Souillac France. Frost had developed the double-burning technique described in Joseph Aspdin’s patent sometime earlier; and Edgar Dobbs, William Jessop and others contributed to the contemporaneous development of better cements.
Following the development of slurrying, double-burning and clinkering, the next major innovation was the use of cement for castings rather than just mortar or stucco coatings. But few dared use it to cast walls and floors until the introduction of steel reinforcement, which was both a blessing and a curse.
At the same time that William Aspdin was setting up the Portland cement plant outside of Hamburg, another factory in the Prussian city of Stettin (now Szcecin, Poland) had already been in operation for more than two years, thanks to the efforts of a chemist named Hermann Bleibtreu. After studying chemistry at the Royal College in London and becoming impressed with the British cement industry, Bleibtreu scouted his home region for sources of lime and clay, experimented with various mixtures and built a factory in 1855, followed by several more.
Bleibtreu’s cement took first prize at the International Industrial Exposition in London and went on to win several more gold metals, setting the gold standard for quality Portland cement. The British companies which began the Portland industry became complacent and concentrated more on buying up competitors and forging a commercial trust, the Associated Portland Cement Manufacturers (APCM), which would eventually encompass 75% of the Island’s manufacturers.
The Germans, and later the Belgian and French cement companies continued to engage in R&D and quickly took over the international markets, even of America and British Commonwealth countries. It took two world wars to restore Britain’s position in the world’s cement trade.
What innovation did occur in Britain was in the development of the rotary kiln, first patented in 1877 by British engineer Thomas Crampton, followed by improved units by Frederick Ransome in 1885. But it was not until the next century that an efficient and functional rotary kiln would be invented.
Meanwhile, the Germans experimented with a vertical shaft kiln, which reduced the fuel and manpower needed for making clinker, and these units are still in use today for small-scale production.
Masonry remained the building technique of choice, as it had a long and noble pedigree, while cast concrete buildings remained a novelty in the 19th century. The ostensible inventor of steel-reinforced concrete was a landscape artist by the name of Jacques Monier, initially in the form of vases and planting tubs strengthened with iron wires. He patented his technique in 1867 and later sold the rights to two Germans, Wayss and Bauchinger, and Wayss published a book about it in 1887, crediting Monier, and the approach became known as the Monier system.
However, another Frenchman in 1861, Francois Coignet, had published a treatise on concrete reinforced with iron bars, which he had patented six years earlier. Predating Coignet’s paper by more than a decade, was the work of Jean-Louis Lambot, a gentleman farmer in southern France. In 1848, Lambot constructed a concrete rowboat reinforced with iron bars and mesh. That it sprung a leak and sank into preservative pond mud to be discovered a century later, may have earned Lambot the title of father of reinforced concrete.
The first use of steel-reinforced concrete in the building trades was by an Englishman named William Boutland Wilkinson, who was granted a patent in 1854 for “improvements in the construction of fire-proof dwellings, ware-houses, and other buildings”, which describes a network of flat iron bands or disused iron cables to reinforce concrete walls and ceilings. He built a cottage of reinforced concrete in 1865 to demonstrate the technology, but he could not interest others.
In the Americas, concrete was used in the construction of the Erie Canal (1817-1825), led by engineer Canvass White, who traveled to England to study the canals and aqueducts, and returned to patent his own cement in 1820.
Fifty years later, America’s first experiment with reinforced concrete, just six years after Wilkinson built his little cottage, was a house built by William Ward in Port Chester New York, which still exists as Ward’s Castle. It was the largest reinforced concrete structure in the world at that time.
American engineer Thaddeus Hyatt conducted tests of the strength of reinforced concrete slabs, beams and columns. Hyatt also discovered that the thermal expansion rates as well as elongation properties of concrete and iron under load were very similar.
But still few were willing to use the technique until a contractor named Ernest L. Ransome of San Francisco, the England-born son of one of the troubled inventors of the rotary kiln. Ernest traveled to California while still in his teens and took a job with the Pacific Stone Company, and in 1875 founded his own company.
Using his father’s no-kiln cement recipe of powdered limestone, silicate of soda and a bath of calcium chloride, Ransome solved the problem of concrete shrinkage cracking by adding control joints, now an almost universal practice. In 1882, Ransome switched to Portland cement, which was then being produced locally, probably because the calcium chloride was not compatible with iron reinforcement.
Ransome was not happy with the iron barrel bands that were commonly used to reinforce concrete, so he experimented with square rods. When he got a commission for paving blocks for a Masonic Hall in Stockton, Ransome tried twisting his square bars for better grip to the concrete, and discovered that cold-twisting also increased the iron’s tensile strength. In 1884, he patented the “Ransome system”, and his “rebar” was used widely for three decades.
In 1884, Ransome built the first large commercial structure of reinforced concrete – a fireproof warehouse for the Arctic Oil Company Works of San Francisco, and then the Alvord Lake Bridge in Golden Gate Park in 1887, which still stands and was designated a civil engineering landmark by the American Society of Civil Engineers. Subsequently, he built a winery building in 1888 and the California Academy of Sciences display hall and office in San Francisco in 1889, using his rebar concrete for the floors and ceilings.
Ransome built the completely concrete Sweeney Observatory in Golden Gate Park in 1891, cast to look like a sandstone masonry structure. It was so popular as an observation point that a second, more enclosed storey was built the following year. Unfortunately, it was destroyed in the earthquake of 1906.
In the late 1880s, Ransome partnered with Francis Marion Smith, whose “20 Mule Team Borax” had cornered the market, and created the nation’s leading concrete construction firm, as well as producing his patented concrete mixers that would soon dominate the industry. His two reinforced concrete borax refineries – in California and New Jersey – proved the applicability of the technology in commercial structures.
The world’s first sky scraper (a name that came from the tallest mast on a sailing ship), was the brick masonry Monadnock Building in Chicago, built in 1891 to 214 feet. But it was also the last of its kind, as cheaper and stronger steel from Carnegie’s factories and cheaper cement from the first commercially viable rotary kiln in Pennsylvania, made such tall masonry buildings obsolete.
Ransome also took advantage of the stronger steel alloys for his rebar, and his reinforced concrete was less expensive than a steel-framed building. But at that time, the tallest reinforced concrete buildings in the US were four stories, all of them built by Ransome.
In 1901, the opportunity came to demonstrate the qualities of reinforced concrete when the railroad magnate Melville Ezra Ingalls decided to commission an office building in Cincinnati. His design firm wanted to try concrete, and after two years of wrangling with the Cincinnati Planning Commission, this new breed of skyscraper commenced construction. The builder, the Ferro-Concrete Construction Company had licensed Ransome’s rebar and used his concrete mixers.
Completed in 1903 after eight months of construction, the sixteen-storey building measured 50 by 100 feet at its base and 210 feet tall. The exterior concrete walls are eight inches thick in unbroken slabs 16 feet square with a veneer 4 to 6 inches thick. The Beaux Arts Classical exterior is covered on the first three stories with white marble, on the next eleven stories with glazed gray brick, and on the top floor and cornice with glazed white terra cotta.
Still in use today, the building was designated a National Historic Civil Engineering Landmark in 1974 by the American Society of Civil Engineers. In 1975, it was added to the National Register of Historic Places.
Though Ransome did not build it, it used his patented technology and vindicated his methods. A factory that he did build for the United Shoe Company in Beverley Massachusetts in 1906 was considered the most advanced of its kind in the world and exerted a strong influence on the young German architect, Walter Gropius.
Trade union resistance to concrete buildings dissipated in the wake of the 1906 San Francisco earthquake and fire, in which such structures were claimed to have proved their durability and fire resistance (see the reality below).
A Stubborn Genius
Ironically, the one inventive genius who should have left concrete well enough alone was Thomas Edison. He was known to be a stubborn man who did not easily adopt new ideas that he did not arrive at by his own trial and error methods. This was most starkly seen in his disinterest in the alternating current ideas of a far greater genius, the Serbian Nicola Tesla (called “the man who invented the 20th century). Tesla then joined with George Westinghouse in creating an AC generation system that has become the global standard, leaving Edison’s DC generators in the dust.
In 1906, Thomas Edison told a New York City dinner reception that he would offer the world durable and very inexpensive concrete homes (this was just after the devastating San Francisco earthquake and fire). He was then constructing the world’s largest cement plant in New Jersey, when his iron ore processing machinery became obsolete as the price of new high quality ore plummeted. Serendipitously, Edison realized he could use his invention to crush limestone efficiently, and then constructed a 150 foot-long rotary kiln, almost double the size of any in existence.
In 1911, the inventor claimed that “men are lunatics” to keep building with bricks and steel, rather than concrete laced with steel reinforcing bars. “A reinforced concrete building will stand practically forever” he said. He also predicted that by 1941, “all construction will be of reinforced concrete, from the finest mansions to the tallest skyscrapers”. This, as we now know, did not come to pass, though some did use his ideas, including the former partner he nearly bankrupted, to cast a small number of “Edison concept” concrete homes and rowhouses.
However, after a few experiments and failures in creating large-scale cast iron molds for single-pour homes, Edison switched to making concrete furniture (which wasn’t well received) and eventually declared that he had “shown the way” and that it was up to others to “fulfill the promise”.
The Architect Who Established Modernity
When Edison announced his unfulfilled intention in 1906, a 38-year-old architect in Oak Park Illinois was already designing buildings of reinforced concrete, but stunning custom homes rather than low-cost projects.
Until the arrival of Frank Lloyd Wright, concrete buildings were built in the same style as more conventional structures. Wright, however, was the first architect since Roman times to realize that reinforced concrete allowed the creation of completely new forms, and he used the extraordinary tensile strength of the material to create cantilevered structures.
Wright’s mother had determined that her infant son would become an architect, and it was fortunate that Frank’s inclinations followed her insistence. Wright also possessed an eidetic mind (much like Nicola Tesla), which could imagine a fully-developed creation in every intricate detail.
In fact, his most famous house design, known as Fallingwater, was drafted for an impatient client in two hours, with Wright’s staff trying desperately to keep him supplied with sharpened pencils, as it was already fully designed in Wright’s mind.
Frank Wright went to Chicago to join a couple of the architecture firms of the “Chicago School”, which helped rebuild the city following the Great Fire of 1871. The first man he worked for, Louis Sullivan, was famous for the dictum that “form follows function” and considered the “father of modernism”. After joining an even more prestigious firm, Wright specialized in residential buildings, which kept him occupied for six decades.
Wright probably began experimenting with concrete around 1900, but still relied on wooden beams for his cantilevers. He cast a model for the Universal Portland Cement Company to display at the 1901 Pan-American Exposition in Buffalo New York, though no description or photograph remains.
From 1901 to 1910, Wright used concrete in exotic and traditional ways, combining cast ornamentation with Roman wall-building technique for a Unitarian temple in Oak Park in 1905. In an ironic testament to Roman construction, it took Wright three years to complete the project.
Though he never mentioned it in his own writings, Wright used Vitruvian wall building technique from 1905 to 1925, and preferred the thinner and wider Roman brick for both veneers and casings for pounded concrete walls. The Roman technique was used in his famous Prairie-style homes, with the non-traditional addition of vertical steel rebar in the wall cores.
Wright developed his “organic” design philosophy based on a structure “growing” naturally out of its surroundings and using only natural colors to achieve a “spiritual integrity”. Ironically, his first realization of this design ethic was the love nest called Taliesin, built in Wisconsin for him and the first of three mistresses as well as incorporating a design studio in 1911. The “spiritual integrity” of the house, however, was revealed perhaps as incomplete – just as the house remained forever incomplete – when in 1914 a servant killed his mistress, her children and five others with a hatchet, setting the house on fire and attempting suicide.
After a period of notoriety and isolation, Wright returned to design some of his most notable structures, including Fallingwater, the Johnson Wax headquarters and the Guggenheim Museum, which took from 1945 to 1958 to complete, after the deaths of Soloman Guggenheim and Wright – though Wright lived long enough to see the forms come off his masterpiece of modernity. The only other concrete building that rivaled Wright’s best work was built on the other side of the globe.
Sydney Opera House
The Sydney Opera House is a multi-venue performing arts center in Sydney, New South Wales, Australia.
Designed by Danish architect Jørn Utzon, the facility formally opened in 1973 after a gestation beginning with Utzon’s 1957 selection as winner of an international design competition. As was often the case with such experimental concrete buildings, the construction was held up for years by authorities and design changes and suffered significant cost overruns – ten years late and more than fourteen times over budget.
The facility features a modern expressionist design, with a series of large precast concrete “shells” (precast concrete panels supported by precast concrete ribs), each composed of sections of a sphere of 246 feet, 8.6 inch radius, forming the roofs of the structure, set on a monumental podium. The building covers 4.4 acres of land and is 600 feet long and 394 feet wide at its widest point. It is supported on 588 concrete piers sunk as much as 82 feet below sea level.
It was identified as one of the 20th century’s most distinctive buildings and became a UNESCO World Heritage Site in 2007.
Claims and Reality
In the 19th and early 20th centuries, the Portland cement industry made unsupportable claims about concrete, including its “permanence”, that it was “fireproof” and “earthquake proof”. None of these proved to be correct.
While there were only five or six concrete buildings in San Francisco at the time of the 1906 earthquake and subsequent fire, 80% of them were destroyed and – contrary to the findings of two very biased official reports – masonry buildings fared much better. Those that did not collapse in the earthquake were seriously damaged by thermal exfoliation during the fires. Bricks, born in fire, are the only truly fireproof building material, though many masonry buildings collapsed due to poor mortar and inadequate tying to interior frames.
Iron and steel reinforcement for concrete structures did not come into general use until the 19th century. Reinforced concrete bridge construction became popular for small to medium sized bridges, since it was assumed that the rebar was “sealed” by the concrete from the elements. It was also known that the high alkalinity of fresh concrete inhibited oxidation.
But the alkalinity of concrete diminishes as it cures and all concrete suffers cracks, from tiny to large, from shrinkage, stress and freeze-thaw cycles, which allow moisture and salts to infiltrate. Iron and steel – even stainless steel – has a strong tendency to oxidize.
Reinforced concrete is uniquely vulnerable in marine environments, particularly if salt water was used in the mix, as was not infrequently the case. Marine concrete generally has a life expectancy of less than 50 years. Some practitioners, such as Ransome, added salt to the mix to prevent cracking, though Vitruvius had warned about this 2,000 years earlier.
When rebar rusts, it expands as much as four times its original volume and can shatter the surrounding concrete. It also, of course, loses mass and strength. Even in non-marine environments, reinforced concrete roadways and bridges often require major reconstruction in 50 years (and most such American infrastructure is nearly that old). While “experts” often blamed poor construction practice and quality control, the best practices extend the service life of buildings to perhaps a century.
By the 1960s and 1970s, reinforced concrete structures all over the world were failing. Conditioned buildings often fared better, as the heat or desiccation of the air conditioning were protective of the envelopes. However, few that did not have historic value have survived the century. To restore historic concrete buildings typically requires many times their original construction cost in inflation-adjusted dollars.
One of the surviving buildings, the Ingalls Tower of Cincinnati, was protected by masonry cladding and is in a seismically quiet region.
Frank Lloyd Wright’s Fallingwater has survived since its 1935 debut, but its famous cantilever deck has sagged and required post-tensioned cables to prevent further creep. This, in spite of the fact that the builders secretly doubled the number of rebar runs from Wright’s specified four to eight.
The reason for the survival of a few notable reinforced concrete buildings may have to do with the use of pre-WWII cement. Just before the war, a new “high-strength” cement was introduced, but it led to more cracking and quicker rebar deterioration. The American Concrete Institute now defines high-strength concrete as concrete with a compressive strength greater than 6,000 psi.
Roads, Bridges & Dams
The first concrete roadbed in the US was built in 1891 by George Bartholomew in Bellefontaine OH, which had a compressive strength of 8,000 psi, is still in use today (though as a pedestrian walk), and won first prize for achievement in engineering technology at Chicago’s World Fair in 1893.
In 1913, the first section of highway in the United States was poured using concrete pavement. It covered 24 miles and was 5 inches thick, spanning a width of 9 feet, and was built just outside of Pine Bluff Arkansas. One year later, there were more than 2,300 miles of concrete highway in the US.
The culmination of concrete usage in the United States was the Eisenhower Interstate Highway System, authorized in 1956 and completed 35 year later. It was at that time the largest use of concrete in a civil engineering project, and was expanded subsequently. As of 2012, it had a total length of 47,714 miles, making it the world’s second longest after China’s, and sees about one-quarter of all vehicle miles driven in the country. It required 2.2 billion cubic yards of concrete.
Large bridges began to be built with reinforced concrete, beginning with the 328 foot long Risorgimento Bridge in Rome in 1911. The Panama Canal’s locks were built of reinforced concrete from 1904 to 1914, costing 5,600 lives. It was the world’s largest engineering project to that time, but the material reached its zenith in the 1930s in the construction of the Hoover (3,250,000 cubic yards) and Grand Coulee (12,000,000 cubic yards) dams, surpassed only in 2009 by the Chinese Three Gorges Dam project which required an estimated 35,500,000 cubic yards of concrete.
1927 alone saw the first concrete mixer truck with a horizontal rotating drum (US), the first pre-stressed concrete (France), and the first aggremeter – a large hopper that accurately measures and dispenses the ingredients for concrete mixing (US).
In the early 1930s, “high-strength” cement was developed which cured in less time, had greater compressive strength, and allowed a wetter mix for better flow. Older concrete had a compressive strength of 3,000 psi after seven days (the standard), while the newer versions offered 4,500 to 5,400 psi in the same curing time. However, high-strength concretes also tend to crack more than “weaker” concretes, as the hydration reaction takes place more rapidly.
In 1944, the US Public Roads System (now the Federal Highway Administration) undertook an evaluation of 200 reinforced concrete bridges in several states, and found that the older ones were in better condition than those built just 14 years prior to the study. Of the pre-1930 bridges, 67% were found to be in good condition, while only 27% of those which were no more than 14 years old passed muster. These results were confirmed in two later surveys in the 1950s. But it was not until 1987 that a report by the US National Materials Advisory Board detailed the accelerated decay of reinforced concrete infrastructure. Highway construction standards, however, were not upgraded until the 1980s and 1990s.
In addition to our crumbling highway system, reinforced concrete water and sewer pipes, water-treatment plants and pumping stations are also deteriorating. The American Society of Civil Engineers rated America’s infrastructure at a D grade in 2009, estimating that billions of dollars would be necessary to improve that to a B level.
What was realized too late, and in spite of cement industry claims to the contrary, is that the reinforced concrete buildings of the 20th century had a life expectancy shorter than masonry and even quality wood structures.
Environmental Protection & Serendipity
The steel industry produces millions of tons of “waste” slag each year, coal-fired power plants produce at least as much fly ash, and the ferro-silica industry produces silica fume as a polluting waste product. With the passage of environmental laws in the 1970s and 1980s, power plants were required to capture their fly ash, which required shipping to landfills.
As with the millstone debris from German lime grinders, it was discovered that the mineral content of these “waste” products was ideal for producing or enhancing cement, as they worked for both pozzolanic hydraulicity and as a cementitious material and densifier. Thus they can replace much of the kilned clay in Portland cement and some of the kilned limestone as well. And, because of the high silica content of fly ash, it can also replace some of the sand aggregate. The result is a high-performance product that is both stronger and more impervious to water. Ironically, much of this material is still wasted and most concrete is still made with conventional Portland cement.
Another major recent advancement is the use of non-ferrous rebar, including aluminum bronze, glass-fiber reinforced polymer and carbon-fiber polymer. The polymeric rebar has double the tensile strength of steel at one quarter the weight, but it can’t be bent on site and does not have the elasticity of steel. The aluminum bronze has the strength of steel and its bendability, while being 35% less expensive than the stainless steel which is used on some critical projects.
While standard mild steel rebar might offer a service life in concrete of 75 years and stainless steel would extend that to 110 years, aluminum bronze should last 500+ years and the additional cost would pay for itself many times over.
A more radical solution might be the elimination of reinforcing bars entirely, at least in compression load applications such as roadways and floor slabs.
The unreinforced concrete Pantheon has survived for two millennia, and Bartholomew’s road is still in use after more than a century. The Hockley Viaduct in southern England, built in the late 1880s, is an elevated rail platform built like a Roman aqueduct of masonry and concrete, mostly the latter, formed into 33 arches – and still stands as mute testament to the durability of unreinforced concrete.
The concrete and masonry Aelian Bridge of Rome, built in a similar style by the emperor Hadrian in 134 CE, is doing fine after 1900 years.
What began thousands of years ago as a simple and relatively natural building material, has become a complex chemical soup, with dozens of possible admixtures to alter the properties of concrete.
Chemical admixtures include accelerators, retarders, air-entrainers, plasticizers and super-plasticizers, pigments, corrosion inhibitors, bonding agents, and pumping aids. Mineral admixtures include fly ash, ground granulated blast furnace slag, silica fume, high-reactivity metakaolin clay, and short or long steel, fiberglass or polymer fibers.
Today, architects, engineers and builders brag about structures expected to last 100 years – little more than a single lifetime. When planned obsolescence is built into our nation’s essential infrastructure, it’s difficult to claim real progress in construction technology, and a question remains whether the concrete world we leave to future generations is more a monument to short-sightedness than to the greatness of humanity.
A Tale of Unintended Consequences – Myopia in Action
Excerpted from “Is Fly Ash an Inferior Building and Structural Material”, Science In Dispute, Volume 2 (2003), by Lee Ann Paradise, David Petechuk, & Leslie Mertz:
Fly ash is an artificial pozzolan, with glassy spherical particulates that contain the active pozzolanic ingredient. However, fly ash is inferior to natural pozzolan.
For example, the hydration of fly ash causes the envelope (the membrane that covers fly ash particles) to prevent or slow down its reaction with calcium hydroxide during cement curing. This slower process may lead to the envelope breaking at a later stage and causing the delayed formation of crystals of the mineral ettringite in the concrete, which can cause an internal sulfate attack that can cause cracking and peeling in the concrete.
In addition, fly ash does not control alkali-aggregate reactions in cement as well as natural pozzolan. The fly-ash envelope slows down the reaction with calcium hydroxide, and the silicate inside the fly ash particles reacts with alkali in the cement. As a result, silica gels are formed and expand, causing cracking and differential movements in structures, as well as other problems such as a reduction in durability in areas where there are freezes and thaws, as well as reductions in compressive and tensile strength. In contrast, natural pozzolan, quickly reacts with calcium hydroxide, trapping the alkali inside the cement paste to form a denser paste with almost no alkali-aggregate reaction.
One of the most touted advantages of fly ash concrete is that high-quality fly ash can reduce the permeability of concrete at a low cost. However, the quality of fly ash varies widely, often depending on how hot a coal plant is burning, which influences the ash’s carbon content. Low nitrogen oxide (NOx) combustion technology used to burn coal in a manner that better controls pollution often increases the carbon content of the ash, resulting in low-quality fly ash with carbon content above 10%. (The American Society for Testing and Materials [ASTM] 618 standard for building codes sets a limit of 6% carbon content, and industry preferences are set at 3% or lower.) This low-quality product can actually increase permeability and interfere with the air-entrainment process, leading to unreliable pours. Many other variables also affect the quality of fly ash and its suitability for making concrete. For example, a low tri-calcium aluminate content of 1.3% and sodalite traces can result in a substantial lowering of sulfate resistance in mortar blends. Overall, fly ash is also typically linked with slower-setting concrete and low early strength.
Approximately 50 to 60 million tons of this fly ash are produced each year in the United States as a byproduct of coal combustion, and disposing of this fly ash has caused concern. Why? Because fly ash can contain any number of more than 5,000 hazardous and/or toxic elements, including arsenic, cadmium, chromium, carbon monoxide, formaldehyde, hydrochloric acid, lead, and mercury. Fly ash also includes harmful organic compounds such as polychlorinated biphenyls (PCBs), dioxins, dimethyl and monomethyl sulfate, and benzene.
Many of the substances in fly ash are known to have carcinogenic and mutagenic effects; and some, such as dioxins, are so toxic that experts cannot agree on a safe level of exposure.
Even if the fly ash were not causing immediate harm to people or the environment as part of a construction material, little is known about the leachability of materials made with fly ash. And, if the concrete in a building is a source of environmental health problems, replacement is often not an option. It is bewildering for government regulations to require industries to spend millions of dollars on anti-pollution devices to capture deadly toxins, but then allow these toxins to be used via fly ash in the construction of office buildings, houses, roads, and even playgrounds.
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