Concrete is the second most used material on the planet after water. If you convert the social and environmental costs of the material into externalities, as is the ubiquitous practice in business, the stuff becomes relatively cheap. If concrete were a country, it would be the third largest emitter of carbon dioxide behind the U.S. and China.
But we’re entering an era where externalities can no longer be considered external and, in the case of concrete, there are a lot of costs to be accounted for that requires an entirely new look at the way it has been used and whether it should be used in the future.
This article breaks down the role of concrete in global infrastructure and what the future of the material might look like.
Concrete’s Carbon Footprint
Most recent estimates cite concrete production as responsible for up to 8 percent of global carbon dioxide emissions. To understand where these emissions come from, it’s necessary to understand how concrete is made.
Roughly half of the emissions stem from the chemical process to produce cement, called calcination. When limestone is heated at temperatures of over 825 °C (1,517 °F) for about 10 hours, it breaks down into calcium oxide (lime) and carbon dioxide that is released into the atmosphere. The energy used to heat the kiln for calcination accounts for about 40 percent of the material’s total emissions.
In the case of the world’s most common cement, Portland cement, the lime is combined chemically with silicates and oxides—belite, alite, tricalcium aluminate and brownmillerite—to form a hardened substance known as “clinker.” The clinker is ground into dust so that once combined with water, it becomes liquid cement. Concrete, then, is a mixture of cement, water and aggregate, a mix of gravel and sand.
The final 5 to 10 percent of concrete’s emissions are the result of mixing and transporting the material. In this way, concrete actually has a lower embodied energy than any other construction material besides wood because the water and aggregate are available local to the build site.
With this in mind, we can determine that roughly 40 percent of concrete’s carbon footprint can be dropped down to zero if the production process is performed with 100 percent renewable technologies, such as wind and solar—though there may be ethics issues to contend with regarding the sourcing of materials for those technologies as well.
To address the 50 percent associated with calcination, there is the possibility of improving the recycling standards for concrete, discussed further below. Otherwise, researchers and companies are exploring technologies that will have a less negative or even positive impact on carbon dioxide emissions. These include the development of concrete that absorbs carbon dioxide during the curing process or once hardened. These technologies, however, are still very much in their nascent stages, and their overall efficacy has not yet been proven.
Other techniques for reducing the amount of cement in concrete mixtures involve incorporating a large percentage of fly ash, cinders produced in coal fire plants, or even plant matter, such as carrots, beets and bamboo.
In 2012, concrete production represented about 9 percent of industrial water usage worldwide. It is anticipated that by 2050, 75 percent of water demand for making concrete will occur in regions experiencing water stress due to climate change.
There may be ways to reduce some of this usage by relying on recycled water, though there are limitations with respect to the clarity of the water that can be used in order to ensure the production of quality concrete. It is also possible that more efficient processing of raw materials could cut some of the water used. There are concrete manufacturers that claim their products use significantly less water than traditional Portland cement does.
While the impacts of limestone mining may be gleaned from the general effects of mining—damaging fertile topsoil and local habitats, as well as possibly contaminating local water supplies—the way that concrete structures shape the surrounding environment may be more surprising.
The resistant nature of concrete is both a strength and disadvantage. While concrete sea walls can protect cities amidst rising sea levels, it also deflects stormwater, exacerbating runoff problems in urban environments. Instead of absorbing rainwater, as natural vegetation does, concrete deflects water, allowing it to pick up gasoline, heavy metals, trash, motor oil and other contaminants associated with urbanized living. According to the United States National Research Council, due to impervious surfaces like pavement and roofing, a typical city block “generates more than five times more runoff than a woodland area of the same size.”
Pervious concrete allows water to run through it to replenish groundwater sources and reduce runoff. (Image courtesy of the National Ready Mixed Concrete Association.)
This issue can be mitigated in part by using pervious concrete—porous concrete made using large rather than fine aggregates—in light traffic areas, such as residential streets, sidewalks and parking areas. Pervious concrete simultaneously addresses two problems, both giving stormwater somewhere to go, as well as replenishing groundwater, another issue escalating with climate change. Moreover, when designed properly, it can be used to filter contaminants.
New resident roadways and sidewalks can be designed with pervious concrete, but that still leaves an issue with large-scale concrete buildings where pores would impact the structural integrity of the building.
Urban Heat Island
The expansion of urban environments has seen cities pave over natural vegetation with concrete and asphalt, leading to greater heat absorption. Due to the fact that most U.S. cities are covered in 30 to 40 percent pavement, this urban heat island (UHI) effect sees cities becoming between 2°C to 12°C (4°Fto 22°F) hotter than surrounding rural areas. On hot summer days, surface temperatures can reach up to 60 °C (140 °F).
A diagram of UHI effect. (Image courtesy of CSIR.)
The resulting temperature increase can lead to greater smog formation, more demand for water and energy usage, lower ability to work, and illness or death in sensitive populations.
The best proven strategy for addressing the UHI effect is adding more vegetation to urban environments, including more parks and wooded areas, as well as incorporating plants into traditional urban structures like roadways, parking lots and roofs. Further gains can be made through the use of high-albedo building materials, such as white or reflective pavement.
According to a 2015 report, air pollution at the 19 largest construction sites in Delhi exceeded safe levels by at least three times, and the dust from stocks and mixtures represented 10 percent of the particulate matter in the city. Building demolition and natural disasters can worsen the problem significantly, as a study of the Great Hanshin-Awaji Earthquake revealed.
The dust from construction has its largest impact on construction workers themselves. While lime causes irritation, silica can lead to asthma, tuberculosis, obstructive pulmonary disorder, kidney disease, silicosis and even lung cancer.
Waste from construction and demolition (C&D) represents about 25 percent of the solid waste annually in the U.S. In 2015, about 70 percent of that C&D waste was made up of concrete.
Construction companies are coming to increasingly value recycled concrete. Recycling rates vary around the world, with just 1 percent in Brazil, 10 percent in China and 90 percent in Japan. In most cases, the material is downcycled as filler for road repair and other low-value applications.
To use recycled waste as a replacement for new concrete, it’s necessary to separate the aggregate from the cement and metal rebar. A precise method for doing this is currently in the research phase. One method being explored by the Concrete Technology Group in Germany is the application of electricity to concrete blocks to break them into their constituent parts.
Less arduous is performing construction with recycling in mind. Unfortunately, pre-used concrete doesn’t come with very detailed supply chain information, making it difficult to determine the composition of the material, including what the aggregate is made from and the exact proportion of ingredients. To ensure that recycled concrete is acceptable for structural projects, better supply chain tracking will need to be put into place.
Once this and other issues are addressed, a more sustainable method for concrete use can be established. Dutch think tank Metabolic has proposed a circular economy for cities like Charlotte, N.C., in which unused concrete and demolition concrete are taken to a local recycler that can crush the material and validate its use for subsequent construction projects. Builders would need be active members of the circular economy and accept the reusable material for future construction use.
Replacing concrete would be a difficult task. The material is strong, comparatively cheap, moldable in liquid form and quick to set. Aggregate ingredients are in plentiful supply worldwide.
One potential alternative may be cross-laminated timber (CLT), a treated form of wood that addresses the anisotropic properties of traditional timber. With CLT, stacks of wood are glued together, each layer perpendicular to the next so that it becomes as strong as concrete but without the same environmental drawbacks.
A diagram describing the orientation of wood panels in CLT. (Image courtesy of Chemical Materials.)
Because wood is generally thought of as a renewable resource, timber can be harvested without the same emissions as concrete, and CLT can be made without relying on fossil fuels. The Nordic countries are leaders in the CLT space, and Stora Enso, Europe’s largest supplier, plants two to three trees for every tree used. While Sweden has doubled its rate of forest cover in the past 100 years through these types of forestry practices, it must be mentioned that new managed forests, such as those created by Stora Enso, do not serve the same ecological functions that old-growth forests do.
CLT floors and walls can be prefabricated, potentially saving time and money, while also ensuring accuracy in that that prefab components can be placed under tighter quality control than items made on-site. The material is also very light. Foundations made from CLT do not need to be as large, and the machines required for on-site construction are smaller than with concrete structures. The weight and modularity of CLT also allows for quicker, easier installation than concrete poured onsite.
The disadvantages lie in the fact that CLT is a relatively new material, with CLT construction only really taking off in the early 2000s and large-scale projects beginning even more recently. This means that the cost of CLT is still greater than concrete. One estimate finds that CLT is approximately 16 to 29 percent more than cast-in-place reinforced concrete. Additionally, the track record of the material is still being established, though much technical research has been performed.
An illustration of the W350 building in Japan, which is set to be about 10 percent steel and 90 percent wood. (Image courtesy of Sumitomo Forestry Co.)
Up until now, CLT has been limited to buildings up to 10-stories tall, but new technology has enabled construction companies to push this limit. In Norway, the Mjøstårnet building is aiming for 18 stories. In Japan, the W350 building is planned to reach 70 stories.
While a number of solutions and mitigation strategies have been discussed here, actually driving society to make changes is a different story altogether. In its massive series on concrete, The Guardian has published opinion pieces on how to incentivize business to address its concrete usage through a concrete tax and how individuals can influence the materials used in their built environment using such tactics as divestment.