November 18, 2019
By Michael Stanzel and Natalee Sembrick
Sustainable design in construction is motivating designers and developers to re-evaluate the materials, methods, and metrics used in creating greener communities. Sustainable structures must balance the environmental footprint, service life, social aspects, and economic factors. As modern society shifts to a circular, carbon-neutral built environment, concrete continues to deliver ‘best-in-class’ performance as a building material.
One of the most important challenges facing the world today is meeting the habitation and food needs of a growing world population, while mitigating climate change. With the construction, operation, and decommissioning of structures and infrastructure accounting for approximately 40 per cent of all man-made greenhouse gas (GHG) emissions, it is self-evident construction practices need to change and buildings must become low-carbon and more resilient to the changing climate.
Over the past few years, there has been considerable discussion about concrete construction and its impact on global warming. Concrete is the most widely used manufactured material in the world. Each year, more than 20 billion tonnes of concrete are produced globally. The environmental impact of concrete is due primarily to its widespread utilization as a building material. Concrete is, in the authors’ opinions, a relatively low-impact material, not only due to its durability, long service life, and recyclability, but also because of increasing investments in sustainable manufacturing technologies. It is locally available, and a good material for most applications, such as building foundations and structural or architectural elements, dams and bridges, schools and hospitals, pipe and water treatment facilities, residential homes, curb and sidewalk, pavement, etc.
Importance of life-cycle assessment
To build greener communities, a growing number of designers are relying on life-cycle assessments (LCAs) to measure the environmental impacts of construction projects at all stages, from raw material extraction and processing, transportation, and installation to use in service, and, if necessary, disposal.
“Emission Omissions: Carbon accounting gaps in the built environment,” a landmark study by the International Institute for Sustainable Development (IISD) argues LCA is the best approach to measure carbon emissions in buildings, but that more data, transparency, and robust standards are needed (Figure 1). All sources of carbon must be considered to ensure a big picture approach in prioritizing material efficiency, energy efficiency, and long service life in designs.
The study identified the omissions of emissions related to concrete, wood, and steel building products, and identified forestry products as the material with the greatest carbon accounting uncertainties, with up to 72 per cent of carbon emissions unaccounted for in current LCAs (Figure 2). When these emissions are included, concrete’s carbon footprint could be up to six per cent less intensive than that of wood products, giving designers and policy-makers reason for pause when making decisions about building materials.
P. Purnell, a professor at the University of Leeds, United Kingdom, argues many of the analyses on the embodied carbon of structures are simplistic and do not take into account the utility of each material and the structural purpose of the element. Purnell recommends defining a ‘functional unit’ that allows comparisons of like with like. As an example, a column designed to resist a particular compressive load at a specific height should evaluate the mass of carbon dioxide (CO2) per unit load capacity per unit height. For many structural member dimensions, reinforced concrete provides a competitive carbon footprint compared to other materials.
Studies from the Massachusetts Institute of Technology (MIT) demonstrate the passive energy efficiency of concrete’s thermal mass combined with smart design and the long service life of these structures results in the lowest annual operating costs and global warming potential (GWP). In some cases, particularly with insulated concrete forms (ICFs), the total life cycle GWP can be as much as eight per cent lower than alternative designs and materials and could potentially be less than 14 per cent with the increased use of low-carbon cementitious products.
A sustainable solution
Continued global growth and the trend toward urbanization means building material solutions must be abundant, affordable, and easily adapted to meet future needs. Concrete is a sustainable solution striking a strong balance between the social, environmental, and economic aspects of the communities’ infrastructure needs (Figure 3).
Concrete offers good compressive strength for the cost and weight of materials, where other structural characteristics can be enhanced through reinforcing materials. While the compressive strength of conventional concrete is typically less than 50 MPa (7 ksi), high performance concrete mix designs used in high-rise construction can have compressive strengths of up to around 100 MPa (14 ksi), and ultra-high performance concrete (UHPC) can have compressive strengths of more than 180 MPa (26 ksi). By carrying loads more efficiently, the quantity of materials and overall costs for a structure can be reduced. Similarly, lower strength and lower density concrete made with lightweight aggregates produced from recycled materials can help reduce the dead load of buildings.
Concrete is flexible, robust, and workable. Mix designs can be adapted to optimize performance and concrete can be placed in a variety of conditions by various methods while offering reliable and predictable performance. Due to its initial plastic state and workability, concrete is able to fill any shape or form to create elegant straight lines or complex geometries, offering a plenitude of architectural and structural options to the designer and owner.
The materials used for concrete construction are locally sourced, and readily available and mined from the earth’s crust, resulting in low-impact resource extraction and transportation. Aggregate, which comprises the largest proportion of material in concrete, requires little processing and is naturally occurring and locally available. Mineral extraction is tightly regulated, and sites can be restored to deliver a biodiversity gain to the surrounding environment.
Durability and resilience
Concrete is durable and able to provide a long service life in a variety of natural climates and environments, resisting weathering action, chemical attack, moisture, and abrasion while maintaining its engineering properties. Additionally, reinforced concrete offers resistance to natural disasters and a changing climate. Concrete is sometimes used in areas where it is exposed to substances that can cause deterioration. In such situations, high-durability mix designs using a low water-to-cementitious materials ratio and a high quantity of Supplementary cementitious materials (SCMs) can be enhanced by application of protective coatings or membranes (refer to the “Effects of Substances on Concrete and Guide to Protective Treatments” by Beatrix Kerkhoff). However, consideration must be given to the Living Building Challenge’s Red List.
Energy efficiency and healthy living environment
Concrete offers advantages to energy performance such as reduced heating and cooling costs through its excellent thermal mass, with a high heat capacity for storage (around 1000 J/kgoK) and moderate thermal conductivity (around 0.5 to 3 W/moK). Additionally, concrete can result in mitigation of the urban heat-island effect, and increased lighting efficiency due to an albedo reflectance of around 0.35 and solar reflective index greater than the threshold value of 29 required for hardscape in most green building standards and rating systems. It also offers improvements in air quality due to negligible levels of volatile organic compounds (VOCs) and elimination of uncontrolled through-wall infiltration, which can be enhanced through the use of photocatalytic cements that remove and decompose contaminants from the air. Concrete also offers effective sound attenuation, with concrete masonry units (CMUs) offering sound transmission classes (STCs) from 40 to over 60 depending on thickness, density, and design.
The use of concrete not only reduces maintenance and operational costs of buildings over their service life, with up to 10 per cent lower annual operating GWP than alternative designs, but also provides the option to repurpose, reuse, or recycle structures and the materials at the end-of-life, decommissioning stage.
Absorption of CO2
Just like trees, concrete naturally absorbs CO2 from the atmosphere. Science shows concrete absorbs the equivalent of up to 25 per cent of the emissions generated in creating it over its lifespan.
Innovations in the cement sector ranging from lower carbon cements and low-carbon fuels and materials from the waste stream to investment in carbon capture utilization and storage technologies are putting concrete on a path toward carbon neutrality. These effects could even transform the material from a significant emitter of carbon into a carbon sink.
Environmental impact of concrete
The primary environmental concerns regarding concrete are related to its CO2 footprint and the amount of energy required for manufacturing. These impacts are predominantly associated with concrete’s active ingredient, cement. Concrete is formed when cement is mixed with water, which binds the aggregate into a strong, cohesive structure. Cement production is energy- and CO2-intensive, but the product itself accounts for only 10 to 15 per cent of the volume of a concrete mix. The other ingredients in concrete consist of aggregates, taking up 60 to 75 per cent of the volume, and water, which accounts for around 15 per cent of the volume (Figure 4).
Cement is manufactured by heating a mixture of ground limestone and other minerals containing silica, alumina, and iron up to around 1450 C (2642 F) in a rotary kiln. At this temperature, the oxides of these minerals chemically transform into calcium silicate, calcium aluminate, and calcium aluminoferrite crystals. This intermediate product, called clinker, is then cooled and finely ground with gypsum (added for set-time control), limestone, and specialized grinding aids, which improve mill energy consumption and performance to produce cement (Figure 5). Those calcium silicates chemically react with the mixing water in concrete, through a process called hydration, to form an extended network of bonds. These bonds bring the aggregates together and give concrete its characteristic strength and durability.
On average in Canada, 1 tonne of cement can results in approximately 0.78 tonnes of CO2. Approximately 1/3 of this is from the energy and heat requirements for manufacture, and 2/3 is from the calcination of calcium carbonate into calcium oxide and CO2.
How is the industry addressing sustainability?
The CO2 impact of concrete construction is due to the sheer volume required to keep up with global needs.
While the cement and concrete industry has long been committed to providing responsible and sustainable high-performance options, there has been a stronger focus on enhancing concrete’s inherent sustainability in recent years.
Cement producers have made significant strides in operational efficiency and heat recovery, plant modernization, and recycling of industrial byproducts as raw material sources. Over the past 20 years, the Canadian cement industry has reduced the energy consumption required for manufacturing by 20 per cent and GHG emissions by almost 15 per cent.
On top of this, producers are also offering lower carbon products. Portland-limestone cements are specially formulated to provide performance equivalent to traditional Portland varieties, but with a portion of the limestone diverted past the pyro-processing stages and incorporated directly into the product. This avoids around 10 per cent of the CO2 emissions from calcination and combustion.
SCMs, such as ground slag, are recycled materials that react with relatively inert byproducts of the hydration reaction (mainly calcium hydroxide) to form compounds that densify the cementitious matrix and enhance later age strength and permeability. SCMs are added to concrete as part of the total cementitious system, and judicious use is desirable not only for the technical advantages, but also environmental benefits. In Canada, it is common to see 25 per cent replacement or more of the cement in concrete with these materials—this corresponds to a comparable reduction in manufacturing energy consumption and GHG emissions, as captured in environmental product declarations (EPDs). Due to these benefits, the industry is trying to increase SCM utilization and investigate new sources of potential materials.
The substitution of traditional fossil fuels with lower carbon alternatives derived from non-recyclable waste, including single-use plastics and waste biomass, has the potential to reduce the industry’s carbon emissions by 20 to 30 per cent across Canada.
Finally, breakthrough carbon capture, utilization, and storage (CCUS) technologies could yield, when scaled, carbon-neutral or even carbon-negative cement and concrete.
Admixtures are small quantities of various chemicals or nanomaterials added to concrete to improve both performance and efficiency. They can provide air entrainment, control setting characteristics, and improve the workability and constructability of fresh concrete. For hardened concrete, they can increase compressive strengths, reduce shrinkage, and help lower permeability. Admixture producers are evolving their technology to enhance concrete’s durability and longevity, while reducing the need for higher quantities of environmentally intensive and costly materials within a particular mix.
Concrete producers develop and optimize mix designs to balance the performance requirements of a project while minimizing the environmental impact, resource use, and waste. Several concrete associations in Canada and internationally have developed eco-certification programs for their members. The industry has also been diligent with developing independently verified regional EPDs for various mix designs to communicate transparent and comparable information about the life-cycle impact of products.
The academic research community is at the leading edge of testing and developing new materials and innovative technologies. This involves working with the standards community to identify requirements for material properties and use, developing appropriate test methods to measure performance, and ensuring the fitness of these products. This includes CCUS-related innovations, such as replacing virgin aggregate with alternatives manufactured from captured carbon as well as various methods for injecting carbon directly into the concrete mix as it sets.
Sustainable concrete meets the performance requirements of the owner, designer, contractors, and the material supplier while minimizing energy consumption, GHG emissions, virgin material extraction, and waste generation. It will also take advantage of local and recycled materials and have high durability and a long service life. Specifications for concrete should not restrict the concrete mixtures being supplied to the jobsite from being more sustainable.
During a project, several parties will be involved in the production and construction process. The custody of the concrete will change hands several times, with each party having the ability to affect the material’s final performance. Problems can arise when conflicts exist between the specification and performance requirements. Fundamentally, these issues arise due to unfamiliarity with the most current and appropriate standards and lack of knowledge regarding local materials, communication, and planning.
The two main options for specifying concrete are prescriptive and performance-based specifications.
With prescriptive specifications, the owner stipulates the material types, sources, quantities, air content, slump, and construction processes, while the contractor plans construction methods around those parameters. The concrete supplier then verifies the concrete complies with those criteria. The specifier assumes responsibility for concrete performance.
Under the performance option, the owner specifies the structural and durability requirements and other performance criteria. The contractor is responsible for procuring concrete and working with the material supplier who will establish mix proportions to meet the plastic and hardened requirements. With performance-based requirements, the concrete supplier assumes responsibility for material as delivered. The contractor is responsible for the concrete as placed. This option provides an advantage as the involved parties are free to use their expertise to innovate and ensure the most efficient, economical, and sustainable product is finally used.
There are a number of obstacles to achieving sustainable construction, but the two most significant and easily managed issues are:
It is important to ensure that sufficient time is allowed for project bidding so material suppliers and contractors can discuss requirements, evaluate options, develop and test mix designs, and conduct any necessary prequalification testing and optimization.
Prescribing mixture proportions or specific materials should be avoided as restrictions on material types or source often result in the utilization of unfamiliar products, greater overdesign (i.e. material inefficiency), potential material incompatibilities, and increased transportation distances.
Outlining the details of construction methods should also be avoided as this falls within the contractor’s realm of expertise and experience. Slump specifications, in particular, are best addressed by the designer, supplier, and contractor discussing construction requirements and working together.
It is also important to not insist on faster construction schedules than required unless there is a tangible and measurable benefit to the project. Sometimes, the more sustainable option may be using a concrete with slower strength development.
Restrictions on reasonable changes to concrete mixture proportions should also be avoided as slight adjustments throughout a project might be necessary to maintain performance as material and environmental conditions change over time.
Planning and communication are keys to success. It is important to discuss concrete performance requirements with the producer and contractor to allow them to optimize mix designs. It is advisable to ask them how they can contribute to the project’s sustainability as they can often see the whole picture related to material efficiency and can come up with solutions providing a win for all the involved parties.
Best practices onsite
Onsite testing should be carried out by a competent agency following proper testing procedures per Canadian Standards Association (CSA) A23.2, Test methods and standard practices for concrete. False negatives in test results lead to greater overdesign and material waste. It is important test results are shared with all parties in a timely fashion to allow control and optimization of concrete mixes for performance and sustainability.
Improper scheduling and estimates or insufficient labour and resources on the project can lead to delays onsite and excess waste generation, while undersized loads increase vehicular emissions. It is critical the site is adequately prepared, access and traffic plans are in place, and designated staging and washout areas are established to avoid delays and potential safety impacts or site damage.
Concrete must be properly protected and cured to reach its potential. Curing is necessary for the hydration reaction between the cement and water to continue. A lack of curing significantly reduces the durability and service life of concrete. Additionally, pouring during extreme weather conditions without proper precaution can result in delays and increased energy consumption to maintain favourable concrete conditions and risk of failure of an element.
It is important to ensure proper concreting practices are in place, per CSA A23.1, Concrete materials and methods of concrete construction, the design followed, reinforcement properly spaced, jointing is timely, and excessive water addition beyond the design parameters is avoided.
Pre-placement and routine update meetings, site-specific plans, risk assessments, and procedures for document control and inspection and verification practices must be established to ensure all parties contributing to a project are on the same page.
The final word
Sustainable design in construction is motivating designers and builders to re-evaluate the materials, methods, and metrics used in constructing greener communities. A focus on the material’s performance allows each of the parties involved to bring their knowledge, expertise, and innovation to the table. Concrete offers a robust, reliable, and sustainable solution in many applications as it consumes minimal materials and energy and is durable.
Michael Stanzel is the technical services manager for Lehigh Cement. Stanzel holds a bachelor’s degree in chemical engineering from Queen’s University, with more than 19 years of experience in both cement and concrete quality and operations. He is a member on the Canadian Standards Association (CSA) A3000, Cementitious Materials Compendium, and an associate member on CSA A23.1, Concrete materials and methods of concrete construction. Stanzel can be reached via e-mail at firstname.lastname@example.org.
Natalee Sembrick is a marketing specialist at Lehigh Hanson in Irving, TX. She holds a bachelor’s degree in marketing from the University of Arkansas, with a minor in supply chain management. Sembrick can be reached at email@example.com.
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