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Book Excerpt: Concrete—From Ancient Origins to a Problematic Future

Mary Soderstrom examines the promise—and challenges—of making low-carbon concrete.

The following excerpt is taken from Concrete: From Ancient Origins to a Problematic Future  by Mary Soderstrom (University of Regina Press, 2020). The book can be purchased here.

Green Concrete

As I said, the air above the McInnis plant was wonderfully clear when I visited. A lot of that had to do with the fact that the plant was still being put into service, but it is supposed to be extremely clean, nearly “green.” Spokesperson Maryse Tremblay stressed that the factory will operate to norms that are as much as fifteen times more strict than existing Canadian regulations and that it will produce up more than 90 percent less sulphur dioxide, 70 per less nitrous oxide, and 65 percent less dust than other cement plants in Quebec.

What couldn’t be seen, of course, was the CO2, but Tremblay says the company is serious about reducing the carbon footprint by, among other things, using biomass from the surrounding woods as fuel. A year after the plant’s start-up, Tremblay told me in an e-mail that studies were underway which could lead to using 100,000 tons of dry wood and forest residue for fuel, thus supplying about 30 percent of the plant’s energy requirements and decreasing its CO2 emissions by 150,000 tons a year. This is predicated on the assumption that wood is carbon neutral, that over time second-growth trees will sequester as much CO2 as is emitted by the fires that make the cement. It’s estimated that the process will take at least twenty years, but as noted before, some forest scientists are skeptical about just how efficient the rebirth process will be.[1]

Whether or not the assumptions are valid, they are used in figuring carbon footprints. Therefore using wood left over from logging operations on the Gaspé Peninsula or grown specifically to be fuel would make it easier for McInnis to meet its obligations under Quebec’s cap and trade carbon market scheme.

My visit to the McInnis plant was in late May when, in the cool climate of the Gaspé Peninsula, nature was waking up, taking in carbon dioxide, breathing out oxygen. The fields along Highway 132 to the west of Port Daniel were brilliantly green, and in places the leaves on the poplars and other trees shone as if illuminated from within. Even the mountain of overburden that had been removed to build the plant was coming to life as grasses planted to stabilize the slopes started to grow. A stunningly green landscape . . .

Photo by Derek Torsani on Unsplash

Concrete, in contrast, is not green, usually not in colour, and usually not in impact on the world. The difference was something that haunted me on the trip. How to reconcile despoiling this verdant landscape to make the material essential to the world as we know it; how to somehow mitigate its impacts on the planet, and protect whatever future we have left?

There are attempts going on, I have been glad to learn. Some of them could be qualified as green hype, but others are sincere undertakings which will have—maybe even are having—a positive effect on the air, that basic constituent of the Roman world and of ours.

One North American initiative, the Calera project on the central coast of California, got immense publicity in the early part of the twenty-first century for proposing to turn CO2 into a chemical cousin of limestone and then into cement by bubbling it through seawater. The recipient of several government grants aimed at providing alternatives to Portland cement, the Calera process “mimics the same chemistry that natural processes use to make strong and tough structures,” according to the firm’s website.[2] Calera founder Brent Constantz—who calls himself “a serial entrepreneur”—is no longer involved, but is now CEO and cofounder of Blue Planet. This company has had some success in producing aggregate from a process that also changes CO2 into calcium carbonate, but through a process that is supposedly less energy intensive. It currently sells a line of products including roofing granules, solar-reflective pigments, and purified CO2 for other uses. “Until recently the only method for offsetting the carbon footprint of your building was achieved by planting trees. With Blue Planet sack concrete the highest CO2 footprint building material can now be carbon neutral or carbon negative,” says its promotional material.[3]

Three other projects designed to attack the problem of CO2 in cement were among the ten finalists in a four-year competition designed to discover and support start-ups that will turn CO2 into useful products, the NRG COSIA Carbon XPrize. Funded by firms whose carbon footprint is far from neutral—COSIA stands for Canada’s Oil Sands Innovation Alliance and NRG is an integrated power company that uses both coal and natural gas for electrical power generation—the C$20 million prize has garnered a lot of publicity, and possibly will point the way to more successful ways to reduce CO2. Each of the finalists will test their processes by converting CO2 from either gas-fired plants or coal-fired ones into useful material. CarbonCure, based in Dartmouth, Nova Scotia, aims to retrofit cement plants so they can produce concrete with nano-size mineral carbonate obtained from CO2 emissions. The CarbonUpcycling UCLA team is producing building materials that absorb CO2 during the production process to replace concrete.[4] Montreal-based Carbicrete transforms slag from steel manufacturing into precast concrete by curing the material in CO2 chambers, but pulled out of the competition in 2019 in order to concentrate on actually producing the project in a pilot plant, according to CEO Chris Stern.[5]

Other attempts to cut down on the CO2 footprint of cement and concrete include a UK project to introduce graphene into the concrete mix, making it much stronger and therefore requiring much less to be used.[6] More conventional additives like silica fume and slag, mentioned before, have already had an impact on how much CO2 cement and concrete account for. Then there are the projects which aim to capture the CO2 during the process of making cement. One promising possibility developed by a research consortium that includes the world’s fourth largest cement company, HeidelbergCement, and the Australian technology company Calix is now undergoing tests in Belgium. It has the lovely name of LEILAC (Low Intensity Lime and Cement) and captures the gas in an almost pure form during the calcining process.[7]

Photo by Ricardo Gomez Angel on Unsplash

Then there are a number of experiments aiming to replace concrete with wood in large-scale construction. The technical problems previously inherent in building with wood are being solved, architects and engineers in several countries assert. New techniques reduce the risk of fire, and stability issues have been drastically reduced: high-rises made of wood as tall as 18 and 20 stories are in the works in Sweden, while a developer in Vancouver is planning a 35-to-40-storey building.[8] And unlike concrete construction, where coming up with building materials leaves nothing but holes in the ground and CO2 in the air, wood is a renewable resource, at least in principle. “A forest, if managed properly is a large carbon reservoir,” notes a report on tall wood structures.[9] “A typical North American timber-frame home captures about 28 tonnes of carbon dioxide, the equivalent of seven years of driving a midsize car or about 12,500 liters of gasoline . . . . If Mass Timber building systems were to become common in the building industry, the amount of carbon stored in buildings would significantly increase.”

That sounds wonderful: our towns might become the more or less permanent solution to the question of where to put all that CO2. Indeed, it sounds almost too good to be true, and, unfortunately, it is. Trees planted to replace those cut down for lumber might sop up CO2 in the air today, but there’s a bigger problem: where would the wood to build what people want to build come from?[10]

One of the attractions of concrete in the twentieth century, you’ll remember, was that it was a good substitute for wood, which was becoming increasingly scarce for building houses. There’s an old Chinese saying that resonates with that: When’s the best time to plant a tree? The answer? Twenty years ago. That certainly wasn’t done on a large enough scale to provide lumber for a massive shift in construction techniques.

The corollary to that question is: what’s the second best time to plan a tree? Usually the answer is given as “now,” but that also is simplistic. Not only can tree planting programs fail because not enough of the seedlings survive, but also with a large enough managed forest, the temptation is to burn the wood for fuel, not to keep it as a “carbon bank,” either in the forest or in our houses.[11] No, cutting down CO2 in the atmosphere is going to require far more action on several fronts.

The gas does have some important industrial uses, among them putting the fizz in beer and soft drinks and prolonging the shelf life of packaged bread and goodies. Most of what’s used now is a by-product of making ammonia-based fertilizer, but in the summer of 2018 an unusually large number of fertilizer plants in Europe were shut down for maintenance, causing CO2 shortages. Brewers and soft drink makers in the United Kingdom had to cut production briefly because they couldn’t get enough, a near-disaster in a summer that proved to be much hotter than usual.[12]

Commercial CO2 capture from cement plants for these ends is a long way off. Nearly all technology advances have been targeted at reducing CO2 from gas- or coal-fired power plants. Among the most significant ones are a large-scale carbon capture plant in Chinese oilfields that the country announced in 2017,[13] and Petra Nova, the world’s largest postcombustion carbon capture system, opened in Texas in December, 2016, and named POWER magazine’s Plant of the Year for 2017.[14]

But the ultimate decrease will probably come from Chinese initiatives to control their air pollution problems, because they have such a huge share of the market. Three Chinese companies produced 20.1 percent of the world’s cement production in 2018, with China National Building Materials (recently merged with Sinoma) producing 11.6 percent of the total and selling it only in China.[15] Following the Paris Agreement on Climate Change, China announced its Intended National Determined Contributions (INDC)—or how much they’re committed to do in the war against climate change. The INDCs include lowering the country’s CO2 emissions per unit of GDP by 60 to 65 percent by 2030 from levels in a baseline year, 2005. To do this it is introducing a national cap and trade program and putting a cap on coal consumption by 2020.[16]

Photo by Edwin Chin on Unsplash

Critics, of course, question China’s use of 2005 as a baseline year, since its expansion was well under way by then, and suggest that an earlier baseline would be more appropriate. Others, as mentioned earlier, assert that if China’s CO2 emissions have grown while those of Europe have dropped, it’s partly because developed countries have in effect exported much energy-intensive production, with its accompanying CO2 debt, to developing countries.

China may be preparing to do the same as it embarks on its Belt and Road Initiative. The country, through its government-supported and -controlled industry, is moving toward deep economic involvement in projects elsewhere in the world. An example is the key role Chinese firms and investment is playing in building in the new capital for Egypt. So, while China actually began cutting back on its domestic cement producing capacity in 2018—the goal supposedly is to eliminate 280 million tons of capacity at home—facilities owned by Chinese companies elsewhere in Asia and in Africa will be increasing production, according to the industry magazine World Cement.[17]

Of course, the quickest way to cut down on CO2 emissions from cement would be to stop building. How effective that would be was hinted at in the first months of 2020 when concerns about the spread of COVID-19, the new coronavirus, led to the quarantine of several Chinese cities and the shut down of industry for weeks. As noted before, Chinese business goes on holiday during the Lunar New Year celebrations, but in 2020 the shutdowns were so widespread and prolonged that China’s CO2 emissions in early 2020 were about 25 percent lower than they were during the same period in 2019.[18]

It seems unlikely that slowdown will continue once the COVID-19 epidemic is mastered because the Chinese are determined to increase the country’s development. As Charles C. Mann, who has written extensively on carbon capture, says, “the Chinese government faces twin imperatives: lifting people out of poverty and avoiding the worst consequences of industrialization.”[19]

Another option for the Chinese would be to switch to the kind of low-rise housing for which Indian architect Doshi and Chilean architect Aravena have been celebrated recently. That is unlikely too: the high-rise “tower in the park” model seems too well entrenched. But building better quality might have long-term positive effects, largely because the housing would last longer and so the future demand for cement and concrete would be reduced.

Observers like Wade Shepard who have studied China’s current city-building binge say that housing built in the first wave of construction in the 1980s and 1990s from the get-go wasn’t designed to last. The result has been an enormous amount of demolition, with the resulting concrete debris, some of which can be recycled, but a lot of which can’t. Whether China’s new housing will also be short-lived is an open question. But the same could be asked about buildings going up in North America and Europe.

Agencies like Fannie Mae, the U.S. Federal National Mortgage Association, have looked closely at the expected life of buildings and drawn up detailed check sheets for particular components. For multifamily structures, the only elements that can be expected to last for fifty years are the concrete ones: foundations, concrete slab roofs, and precast concrete panel exterior walls can be expected to last forty-five to fifty years.[20] But actual wearing-out of a building is not the only reason for demolishing it: “area redevelopment” (34.8 percent) was given more frequently than “physical condition” (30.8 percent) as the reason a building had been torn down in one study of 227 buildings in the United States and Canada.[21]

Photo by Patrick Robert Doyle on Unsplash

This brings up myriad questions about the kinds of decisions cities are making—or allowing developers to make—about the way cities develop and evolve. While working on this book, I spent a week in Toronto, which you’ll remember has more skyscrapers than any other North American city but New York, many of which date back to the 1960s and 1970s. What was striking was not that these older, mostly residential, buildings were being upgraded: few were that I could see. No, what amazed me was that in the heart of the city some seemingly perfectly good buildings were being demolished to be replaced by more elaborate, taller ones. At that point there were eighty-one skyscrapers higher than 150 metres either under construction or in the planning stages.[22] Housing is expensive and in short supply in Toronto, but a legitimate question is whether this kind of demolition followed by new construction is the best way to fill that need in Toronto or anywhere.

Then there are the sports stadiums, which seem to have a “best before” date of no longer than twenty or so years after construction. Between the late 1990s and 2010, ten were demolished, some of which were imploded while cheering crowds watched.[23] The destruction continued in the new century, but sometimes the old buildings resisted: it took two tries to bring down Detroit’s Pontiac Silverdome in 2017. Aside from general decrepitude, the reason usually given for doing this is that the old stadium wasn’t equipped for modern technology like super screens or luxury touches.

The stadium built for the 1976 Montreal Olympics is an exception, despite its many structural problems. It was supposed to have a retractable roof suspended from a sloping mast that can be seen for kilometres around, but its many post-tension cables and its fabric roof have been plagued with problems since shortly after it was built.[24] Why it’s been allowed to remain is a question I won’t go into here, but in part it has to do with two things: Montreal and Quebec politicians like the idea of having a monumental tower that can be compared to the Eiffel Tower in Paris, and to bring it down safely would be horrifically expensive because of the way it’s built and its location in a residential area. In the meantime, it draws more than a million visitors a year, and crews are constantly at work maintaining it, including painting the mast, which for most concrete structures is not usually done.

When it comes to skyscrapers, older ones may be more robust than the elegant new super-tall buildings. Take, for example, the difference between the Empire State Building and the Burj Khalifa in Dubai, currently the world’s tallest building. Fourteen people died when a B-25 airplane crashed into the Empire State Building in 1945, but the building, completed in 1931, reopened for business a few days later. “Back in the early 20th Century they were still calculating everything by hand, so they always added extra steel just in case,” structural engineer Roma Agrawal told the BBC. Because of this, he said, the Empire State Building, which is less than half the height of the Burj Khalifa, weighs two-thirds as much.[25]

That’s over-engineering, but there are also flagrant examples of under-engineering that sometimes come with tragic results. One occurred in Bangladesh in 2013 when the Rana Plaza, an eight-storey commercial building, collapsed, killing 1,134 people. Most of the victims were garment workers who couldn’t get out of the sweatshops in time when cracks in the structure, noticed the day before, suddenly began to widen. Subsequent investigation showed that poor quality materials, a foundation laid on a filled-in pond, and the addition of three floors more than had been approved contributed to the collapse.[26]

Photo by Taylor Smith on Unsplash

But, just as McGill’s Saeed Mirza says that the rapid deterioration of the Champlain Bridge shouldn’t condemn concrete as a material to be used for big public works projects, the Rana Plaza disaster shouldn’t be an argument against concrete being the ubiquitous building material of our time. What is at issue here is the practice Mirza decried of “Design it, build it, forget it,” its first corollary, “Don’t ever check to see if the rules are being followed,” and their opposite, “Maintain the damn thing.”

Maintenance is the key to making modern concrete the material that is truly the “Rock of Ages” it was thought by many to be in the early and mid twentieth century. Maintenance is no more sexy than cleaning bathrooms, but like housework it can make the difference between a liveable place and a slum.

As art historian Adrian Forty writes:

“The early pioneers of concrete thought they had found an everlasting material; in this they were to be sadly disappointed, for . . . it is not as stable a substance as they had supposed. Although it may last for a very long time, it does undergo changes and according to the precise combinations of ingredients and local atmospheric conditions, may lose its strength or otherwise deteriorate over time. Among concrete experts, the view is that all concrete structures will sooner or later need radical repair—a difficult and costly process.”[27]

Maintenance also seems to have helped some of the Roman concrete structures like the Pantheon weather centuries rather well. The excellent concrete used to form its dome is partly responsible, but also important was what happened after the fall of Rome. The building became a Christian church in 609 CE, which meant that it was not abandoned for any length of time, not plundered, and looked after rather well over the centuries. In addition, the original wisdom of its architects can be seen in the way that fissures that have developed over time seem to have had little effect on the soundness of the building.[28] It has aged, but the damage is nothing compared to what many more recent concrete structures have experienced, even though rain and snow can enter from the opening at the top of the Pantheon’s dome, the oculus.

In contrast, “time and weather, which give mellowness to brick and stone, make untreated concrete more and more dirty, dark, and untidy and rapidly lower its initially low power of reflecting light,” warned the Royal Institute of British Architects after World War II when plans were being drawn up to reconstruct the war-damaged country.[29] Yet ugly stains may be nothing more than blemishes, with real damage to reinforced concrete not being immediately apparent. That occurs when water enters and attacks the steel inside, silently weakening the structure.

To address problems with modern concrete, the Getty Conservation Institute has undertaken a huge project aimed at conserving some of the twentieth century’s most remarkable concrete structures. In 2017 three museums designed by Le Corbusier, two in India and one in Japan, received grants to explore ways to ensure that the buildings remain structurally sound and as beautiful as Le Corbusier would have liked.[30] As the project’s mission statement says:

Reinforced concrete was the material of choice for many architects of the modern era, and they exploited the material in a multitude of creative and innovative ways . . . . The current state of decay of many significant reinforced concrete structures arises from the novelty of the material and construction techniques used at the time of their construction, and the fact that architects and engineers experimenting with reinforced concrete often pushed the limits of the material structurally and architecturally.

Although there are many well-constructed, carefully crafted concrete buildings of this time, there are also many buildings suffering rapid deterioration due to poor quality materials or construction. This is often the result of building at a time when materials were scarce, under pressure for accelerated construction, and with little quality control . . . . Moreover, these buildings often suffer from the mistaken belief that reinforced concrete was a maintenance free, extremely durable material. The result is a large stock of culturally significant reinforced concrete buildings with deterioration that ranges in scale from local to general.[31]

However, it is possible that the human spirit, that other component of air, can provide solutions for these problems. Certainly, it has long used concrete as a medium for expressing wonderful and extravagant thoughts.

 

[1] Oliver, et al., “Carbon, Fossil Fuel, and Biodiversity Mitigation.”

[2] “Calera: Proven, Economical, and Better for the Planet,” Calera, accessed March 5, 2020, http://www.calera.com/beneficial-reuse-of-co2/science.html.

[3] “Technology,” Blue Planet, accessed March 5, 2020, http://www.blueplanet-ltd.com/#technology.

[4] “How the oil sands and XPRIZE could reinvent carbon,” XPRIZE, accessed March 5, 2020, https://carbon.xprize.org/press-release/ten-teams-five-countries-advance-finals-of-20m-nrg-cosia-carbon-xprize?language=en.

[5] Chris Stern, email message to author, July 23, 2019.

[6] Kate Baggaley, “ ‘Green’ Concrete Could Be Game-Changer for Construction Industry Microscopic Flakes of Graphene Add Strength and Durability—But Also Raise Cost and Safety Concerns,” NBC News, May 2, 2018, https://www.nbcnews.com/mach/science/new-green-concrete-could-be-game-changer-construction-industry-ncna870371.

[7] Low Emissions Intensity Lime and Cement, accessed March 3, 2020, www.leilac.org.uk.

[8] We Don’t Have Time, “Concrete Ways to Solve Our Concrete Problem,” Medium, April 9, 2019, https://medium.com/wedonthavetime/concrete-ways-to-solve-our-concrete-problem-ae307d64d2fe; and Aaron McArthur, “Vancouver Developer Proposes World’s Tallest Wood Tower,” Global News, April 25, 2019, https://globalnews.ca/news/5201857/vancouver-tallest-wood-tower/.

[9] mgb ARCHITECTURE + DESIGN, Equilibrium Consulting, LMDG Ltd., and BTY Group, “The Case for Tall Wood Buildings,” 26.

[10] Gabriel Popkin, “How Much Can Forests Fight Climate Change? Trees Are Supposed to Slow Global Warming, But Growing Evidence Suggests They Might Not Always Be Climate Saviours,” Nature, January 15, 2019, https://www.nature.com/articles/d41586-019-00122-z.

[11] Bill McKibben, “Don’t Burn Trees to Fight Climate Change—Let Them Grow,” The New Yorker, August 15, 2019, https://www.newyorker.com/news/daily-comment/dont-burn-trees-to-fight-climate-changelet-them-grow.

[12] Sarah Zhang, “Europe Is Running Low on CO2: The Shortfall Has Sparked Fears of Shortages of Beer, Meat, and Crumpets. (Crumpets!),” The Atlantic, July 6, 2018, https://www.theatlantic.com/science/archive/2018/07/europes-co2-shortage/564458/.

[13] “China Shows Climate Change Prowess as Large-Scale CCUS Facility Enters Construction,” Markets Insider, March 29, 2017 https://markets.businessinsider.com/news/stocks/china-shows-climate-change-prowess-as-large-scale-ccus-facility-enters-construction-1001879901.

[14] Sonal Patel, “Capturing Carbon and Seizing Innovation: Petra Nova Is POWER’s Plant of the Year,” POWER, August 1, 2017, http://www.powermag.com/capturing-carbon-and-seizing-innovation-petra-nova-is-powers-plant-of-the-year/?pagenum=1.

[15] “Top 10 Cement Producer Profiles,” Global Cement Magazine, July–August 2018, 7, https://globalcement.com/magazine/articles/1072-top-10-cement-producer-profiles. The number two producer, at 8 percent, is LafargeHolcim, which operates around the world from its European base. Number three is the Chinese firm Anhhui Conch (7.5 percent), which sells primarily in China, Southeast Asia, and Russia.

[16] Natural Resources Defense Council “The Road from Paris: China’s Progress toward Its Climate Pledge,” Issue brief 17-11-D, November 2017, https://www.nrdc.org/sites/default/files/paris-climate-conference-China-IB.pdf.

[17] Jonathan Rowland, “China Drags on Global Cement Consumption Growth,” World Cement, October 17, 2017, https://www.worldcement.com/special-reports/17102017/china-drags-on-global-cement-consumption-growth/.

[18] Brad Plumer, Nadja Popovich, and Shola Lawal, “The Coronavirus and Carbon Emissions,” The New York Times, February 26, 2020, https://www.nytimes.com/2020/02/26/climate/nyt-climate-newsletter-coronavirus.html.

[19] Charles C. Mann, “Renewables Aren’t Enough: Clean Coal Is the Future” Wired, March 25, 2014, https://www.wired.com/2014/03/clean-coal/.

[20] “Instructions for Performing a Multifamily Property Condition Assessment (Version 2.0)” Appendix F, Fannie Mae, 2014, https://www.fannimae.com/content/guide_form/4099f.pdf.

[21] Jennifer O’Connor, “Survey on Actual Service Lives for North American Buildings” (paper presented at Wood Frame Housing Durability and Disaster Issues Conference, Las Vegas, NV, October, 2004), http://cwc.ca/wp-content/uploads/2013/12/DurabilityService_Life_E.pdf.

[22] Daniel Tencer, “Toronto Skyline Evolution: Video Shows How City Is Transforming Amid Skyscraper Boom,” Huffington Post, April 4, 2019, https://www.huffingtonpost.ca/2019/04/04/toronto-skyline-evolution_a_23706558/. Among these skyscrapers is Monde, a 44-storey luxury condo development designed in part by Habitat 67 architect Moshe Safdie. It features many units with gardens but is far from “affordable housing.” One-bedroom units are listed beginning at $574,990. “Monde Residential Development,” SafdieArchitects, accessed March 3, 2020, https://www.safdiearchitects.com/projects/monde-residential-development.

[23] See ten of them: “Top 10 U.S. Stadium/Arena Demolitions,” YouTube video, 5:37, posted by Richard S. Dargan, March 15, 2015, https://www.youtube.com/watch?v=7X7hWcctXLw.

[24] Among the problems: a concrete chunk weighing fifty-five tons fell off in 1991, making the Montreal Expos, who used the “Big O” as their home field, finish the season playing all their games away. Seven years later heavy snow caused the roof to tear; that time the Rolling Stones had to cancel two concerts.

See “Roger Taillibert défend la conception du Stade Olympique,” Radio-Canada, July 12, 2016, https://ici.radio-canada.ca/nouvelle/792320/stade-olympique-architecte.

[25] Zaria Gorvett, “Will the Skyscrapers Out Last the Pyramids?,” BBC, August 9, 2016, http://www.bbc.com/future/story/20160808-will-the-skyscrapers-outlast-the-pyramids.

[26] Tansy Hoskins, “Reliving the Rana Plaza Factory Collapse,” The Guardian, April 23, 2015, https://www.theguardian.com/cities/2015/apr/23/rana-plaza-factory-collapse-history-cities-50-buildings.

[27] Forty, Concrete and Culture, 76.

[28] Courtney Humphries, “Why We Should Let the Pantheon Crack: Modern Architects Have a Lot to Learn from the Sound Engineering of the Ancients,” Nautilus, May 21, 2015, nautil.us/issue/24/error/why-we-should-let-the-pantheon-crack.

[29] Forty, Concrete and Culture, 52.

[30] “Concrete Conservation,” The Getty Conservation Institute, November 2017, http://www.getty.edu/conservation/our_projects/field_projects/concrete/overview.html.

[31] Ibid.

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