According to the recent Economist article “Cracks in the Surface: Why Grey Firms Will Have to Go Green,” cement poses a worldwide challenge. Though its production is the third largest industrial source of pollution, according to the U.S. Environmental Protection Agency (EPA), its manufacturers have done little to address environmental concerns. To date, the $250 billion-a-year sector has evaded the wrath of activists. However, recent commitments by the 195 signatories of the 2015 United Nations Paris Climate Conference (also known as COP21, the 21st Conference of the Parties) will necessitate a significant and prompt reduction in the material’s carbon footprint. And those ecological advocates are watching.
Concrete faces not only a problem in the production of a key ingredient, but also one of longevity. Steel-reinforced concrete, the most widely used building product in the world, is inherently flawed. The reason? Unprotected steel corrodes. Standard practice dictates a shielding the steel rebar or welded wire fabric with a layer of concrete to safeguard the metal from the oxidation and degradation that would occur if exposed to the elements. However, engineers are finding this method is inadequate, as evidenced by the number of deteriorating bridges and roadways in this country, designed for decades of use, that are now threatened by the premature failure of the reinforcing.
These problems at both ends of the concrete life cycle represent a critical compound challenge: How do we significantly improve the environmental and physical performance of this ubiquitous material?
Improving Environmental Performance
According to the EPA, the cement industry generates “more than 500,000 tons per year of sulfur dioxide, nitrogen oxide, and carbon monoxide,” and, per the Economist article, contributing 5 percent of anthropomorphic carbon dioxide emissions annually. The world’s appetite for the material continues to grow unchecked, at the rate of approximately 9 million metric tons per year. Although cement’s contribution to GHG emissions has remained about the same in recent years, it is essential to remember that the absolute amount continues to increase. In a July 28 GreenBiz article, Robert Hutchinson, the Rocky Mountain Institute’s managing director for research and consulting activities, writes: “[R]evolutionary thinking and significant disruption is [sic] needed—a Tesla for cement, as it were.”
The industry has many strategies underway, but change is slow. One approach is to improve the efficiency of the kilns used to create clinker, the nodule-like predecessor of cement powder that accounts for the greatest proportion of its embodied energy. Another is to find alternative sources of energy, such as biomass, to fire the kilns. A third idea is to couple cement production with the carbon dioxide output from other industries. For example, San Jose, Calif.–based Blue Planet produces cementitious products via a carbon-capture process inspired by “blue carbon,” the carbon sequestered naturally in marine environments. By mimicking natural biological and geological processes of mineralization to produce carbon-negative materials, the company reports a 40 percent reduction in operating costs.
Another tactic revises the composition of concrete by reducing or eliminating cement. Supplementary cementing materials (SCMs), like slag or flyash, are popular substitutes although their availability is diminishing. Watershed Materials, in Napa, Calif., makes an alternative concrete masonry unit with geopolymers derived from natural minerals, reducing the amount of cement used by half and embodied energy by 65 percent, compared to a conventional concrete masonry unit. Meanwhile, MIT’s Functionally Graded Printing project seeks to optimize the performance of the ultimate concrete structure by emulating the structure of bone and gradually adjusting the mixture’s density to deliver the highest utility from the available material.
Preventing Premature Deterioration
For new construction projects, architects should specify improved reinforcing methods to help their projects achieve their life expectancy. Steel rebar can be protected through hot-dip galvanizing, epoxy coating, or zinc-phosphate coating; stainless steel can be specified too. Unfortunately, these enhanced products are more expensive than conventional untreated steel, but they would represent an investment in structural longevity. Another option is fiber-reinforced polymer (FRP), which is not only impervious to corrosion and many chemicals but also exhibits 1.5 to 2 times the tensile strength of steel. Similar to FRP, HollowRebar, developed by Composite Rebar in Madison, Wis., consists of glass fiber-reinforced vinyl ester resin surrounding a hollow interior that eliminates the potential of corrosion, is 25 percent the weight of steel, and can be used as a chase for conduit. Other rebar alternatives include basalt fibers and treated bamboo.
For existing structures, continual assessment and maintenance are crucial to life span although some deterioration will be inevitable. According to Robert Courland, author of Concrete Planet: The Strange and Fascinating Story of the World's Most Common Man-Made Material (Prometheus Books, 2011), “virtually all the concrete structures one sees today will eventually need to be replaced, costing us trillions of dollars … in the process.”
In his 2002 report “Evaluation and Rehabilitation of Concrete Structures,” Jay Paul, a structural engineer with Chicago-based Klein and Hoffman, agrees: "The extent of deterioration to concrete structures globally is occurring at an alarming rate.” He recommends a systematic method for condition assessment that includes a visual inspection and structural analysis of deteriorated areas. Repair may require rapid-setting cements, rapid strength concrete such as magnesium phosphate, or polymer-impregnated concrete (PIC) for enhanced durability. Furthermore, additional surface protection may include the application of penetrating sealers, polymeric membranes, and cathodic protection, the latter of which involves connecting the steel to an external electricity source or sacrificial metal.
Combining Strategies
These two fundamental and coinciding challenges can seem daunting, but there is hope in consolidating efforts. One potential solution, from the University of Michigan Center for Sustainable Systems, would be to use highly ductile concrete with non-steel reinforcing and SCMs to create a greener product that far outlasts conventional reinforced concrete—particularly in high-demand applications. Concrete recycling, while historically difficult, is also improving. Scientists at Germany’s Fraunhofer Institute for Building Physics have developed an electric impulse-based method for separating existing concrete into its original components of hardened cement and aggregate. Netherlands-based StoneCycling creates new masonry units from recombined scrapyard materials such as brick, mortar, and other cementitious substances.
Today’s architects can also create enduring solutions simply by looking to yesterday’s teachers. As Courland notes in his Economist article: “If the Romans had used steel-reinforced concrete—which they did not have—to build their beautiful bridge in Alcántara, Spain, the bridge would have to have been rebuilt at least 16 times by now.”
