Environmental impact of concrete
The environmental impact of concrete, its manufacture, and its applications, are complex, driven in part by direct impacts of construction and infrastructure, as well as by CO2 emissions; between 4-8% of total global CO2 emissions come from concrete.[1] Many depend on circumstances. A major component is cement, which has its own environmental and social impacts and contributes largely to those of concrete.
The cement industry is one of the main producers of carbon dioxide, a greenhouse gas.[2] Concrete causes damage to the most fertile layer of the earth, the topsoil. Concrete is used to create hard surfaces which contribute to surface runoff that may cause soil erosion, water pollution and flooding. Conversely, concrete is one of the most powerful tools for proper flood control, by means of damming, diversion, and deflection of flood waters, mud flows, and the like. Light-colored concrete can reduce the urban heat island effect, due to its higher albedo.[3] However, original vegetation results in even greater benefit. Concrete dust released by building demolition and natural disasters can be a major source of dangerous air pollution. The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns due to toxicity and (usually naturally occurring) radioactivity.[4] Wet concrete is highly alkaline and should always be handled with proper protective equipment. Concrete recycling is increasing in response to improved environmental awareness, legislation, and economic considerations. Conversely, the use of concrete mitigates the use of alternative building materials such as wood, which is a natural form of carbon sequestering.
Carbon dioxide emissions and climate change[edit]
The cement industry is one of the two largest producers of carbon dioxide (CO2), creating up to 5% of worldwide man-made emissions of this gas, of which 50% is from the chemical process and 40% from burning fuel.[2][13] The CO2 produced for the manufacture of structural concrete (using ~14% cement) is estimated at 410 kg/m3 (~180 kg/tonne @ density of 2.3 g/cm3) (reduced to 290 kg/m3 with 30% fly ash replacement of cement).[14] The CO2 emission from the concrete production is directly proportional to the cement content used in the concrete mix; 900 kg of CO2 are emitted for the fabrication of every ton of cement, accounting for 88% of the emissions associated with the average concrete mix.[15][16] Cement manufacture contributes greenhouse gases both directly through the production of carbon dioxide when calcium carbonate is thermally decomposed, producing lime and carbon dioxide,[17] and also through the use of energy, particularly from the combustion of fossil fuels.
One area of the concrete life cycle worth noting is its very low embodied energy per unit mass. This is primarily because the materials used in concrete construction, such as aggregates, pozzolans, and water, are relatively plentiful and can often be drawn from local sources.[18] This means that transportation only accounts for 7% of the embodied energy of concrete, while cement production accounts for 70%. Concrete has a total embodied energy of 1.69 GJ/tonne, lower per unit mass than most common building materials besides wood. However, concrete structures often have high masses, so this comparison is not always directly relevant to decision making. Additionally, this value is based only on mix proportions of up to 20% fly ash. It is estimated that a 1% replacement of cement with fly ash represents a 0.7% reduction in energy consumption. With some proposed mixes containing as much as 80% fly ash, this could represent a considerable energy saving.[16]
A 2022 report from the Boston Consulting Group found that investments in greener forms of cement would lead to greater greenhouse gas reductions, per dollar, than investments in many other green technologies—though investments in plant-based meat alternatives would reap considerably greater reductions than even this.[19]
Mitigation[edit]
Design improvements[edit]
There is a growing interest in reducing carbon emissions related to concrete from both the academic and industrial sectors, especially with the possibility of future carbon tax implementation. Several approaches to reducing emissions have been suggested.
Surface runoff[edit]
Surface runoff, when water runs off impervious surfaces, such as non-porous concrete, can cause severe soil erosion and flooding. Urban runoff tends to pick up gasoline, motor oil, heavy metals, trash and other pollutants from sidewalks, roadways and parking lots.[52][53] Without attenuation, the impervious cover in a typical urban area limits groundwater percolation and causes five times the amount of runoff generated by a typical woodland of the same size.[54] A 2008 report by the United States National Research Council identified urban runoff as a leading source of water quality problems.[55]
In an attempt to counteract the negative effects of impervious concrete, many new paving projects have begun to use pervious concrete, which provides a level of automatic stormwater management. Pervious concrete is created by careful laying of concrete with specifically designed aggregate proportions, which allows for surface runoff to seep through and return to the groundwater. This both prevents flooding and contributes to groundwater replenishment.[56] If designed and layered properly, pervious concrete and other discreetly paved areas can also function as an automatic water filter by preventing certain harmful substances like oils and other chemicals from passing through.[57] Unfortunately there are still downsides to large scale applications of pervious concrete: its reduced strength relative to conventional concrete limits use to low-load areas, and it must be laid properly to reduce susceptibility to freeze-thaw damage and sediment buildup.[56]
Urban heat[edit]
Both concrete and asphalt are the primary contributors to what is known as the urban heat island effect.[25] According to the United Nations Department of Economic and Social Affairs 55% of the world’s population reside in urban areas and 68% of the world’s population is projected to be urban by 2050; also, "the world is projected to add 230 billion m2 (2.5 trillion ft2) of buildings by 2060, or an area equal to the entire current global building stock. This is the equivalent of adding an entire New York City to the planet every 34 days for the next 40 years".[58] As a result, paved surfaces represent a major concern because of the additional energy consumption and air pollution they cause.[59]
The potential of energy saving within an area is also high. With lower temperatures, the demand for air conditioning theoretically decreases, saving energy. However, research into the interaction between reflective pavements and buildings has found that, unless the nearby buildings are fitted with reflective glass, solar radiation reflected off pavements can increase building temperatures, increasing air conditioning demands.[60]
Moreover, heat transfer from pavements, which cover about one-third of a typical U.S. city,[3] can also influence local temperatures and air quality. Hot surfaces warm the city air through convection, so using materials that absorb less solar energy, such as high-albedo pavements, can reduce the flow of heat into the urban environment and moderate the UHIE.[61] Albedos range from about 0.05 to about 0.35 for currently used pavement material surfaces. Over a typical life service, pavement materials that begin with high albedo tend to lose reflectance, while those with low initial albedo may gain reflectance[62]
The Design Trust for Public Space found that by slightly raising the albedo value in New York City, beneficial effects such as energy savings could be achieved.,[63] by replacement of black asphalt with light-colored concrete. However, in winter this may be a disadvantage as ice will form more easily and remain longer on light colored surfaces as they will be colder due to less energy absorbed from the reduced amount of sunlight in winter.[64]
Another aspect to consider is thermal comfort effect, as well as the need for more mitigation strategies, which don’t threat the health and wellbeing of pedestrians particularly during heat waves.[65] A study that appeared in Building and Environment in 2019 performed experiments to project the impact of heat waves and high albedo materials interactions in the northern Italian city of Milan. By calculating the "Mediterranean Outdoor Comfort Index" (MOCI) in presence of a heat wave, where high albedo materials was used in all surfaces. The study identified a deterioration of the microclimate where high amounts of high albedo materials were located. The use of the high albedo materials was found to "lead to the establishment of multiple inter-reflections and a consequent increase in micrometeorological variables such as average radiant temperatures and air temperatures. To be more detailed, these changes lead to an increase in the MOCI that in the afternoon hours can even reach 0.45 units".[66]
Overall urban configurations should remain of concern when making decisions as people are exposed to weather and thermal confort conditions. The use of high albedo materials within an urban environment can be of positive effect with proper combination of other technologies and strategies such as: vegetation, reflective materials, etc. Urban heat mitigation measures could minimize impacts on microclimate as well as human and wildlife habitats.[67]