December 1, 2011
By Sherry Sullivan, MASc, P.Eng., LEED AP
From towering skylines and massive dams to modern bridges and centuries-old temples, concrete structures are the basis for much of civilization’s infrastructure and its physical development. Concrete is used worldwide, more than any other manufactured product—twice as much of it is used throughout the world than all other building materials combined. (See Bjørn Lomborg’s The Skeptical Environmentalist: Measuring the Real State of the World [Cambridge, 2001]). Each year, approximately four tonnes are used for every one of the nearly seven billion people on Earth. (This information comes from the 2009 U.S. Geological Survey). The world record for the largest concrete pour in a single project is the Three Gorges Dam in China’s Hubei province—the amount used is estimated at 16 million m3 (565 million cf) over 17 years. (“Concrete Pouring of Three Gorges Project Sets World Record,” an online article posted in January of 2001, comes from People’s Daily).
Still, as broadly as concrete is used, and as essential as it is to the built environment, people generally know very little about it. The material has evolved in many ways over the years; it is quite literally not your grandfather’s concrete.
Ingredients and environmental impact
In essence, concrete is a composite construction material containing aggregates and paste. The former makes up about 60 to 75 per cent of the total volume, while the latter comprises 25 to 40 per cent. In turn, the paste itself is made up of approximately:
The quality of the concrete depends on the paste and aggregate, and the bond between them. In properly made concrete, every particle of aggregate—and all the spaces between—is completely coated or filled with paste. (See Steven Kosmatka et al’s eighth Canadian edition of Design and Control of Concrete Mixtures [Portland Cement Association, 2011]).
Admixtures are those ingredients in concrete other than portland cement, water, and aggregates that are added to the mixture immediately before or during mixing. Chemical admixtures are materials in the form of powder or fluids added to the concrete to give it certain characteristics unobtainable with plain concrete mixes. In normal use, admixture dosages are less than five per cent by mass of cement and are added to the concrete at the time of batching/mixing.
The common types of admixtures and a general description are outlined in this section. (For more, visit the U.S. Federal Highway Administration (FHWA) site, www.fhwa.dot.gov/infrastructure/materialsgrp/admixture.html).
These admixtures introduce and stabilize microscopic air bubbles in the concrete, which improves concrete’s durability to freeze-thaw cycles. They also improve workability; segregation and bleeding are reduced or eliminated.
Water-reducing admixtures reduce the quantity of mixing water required. They help:
An increase in strength is generally obtained with water-reducing admixtures as the water-cement ratio is reduced.
Plasticizers are a type of high-range water-reducing admixture used with concrete with a low-to-normal slump and water-cementing materials ratio to make high-slump flowing concrete.
These ingredients are used to accelerate the concrete’s rate of hydration and strength development at an early age.
These admixtures delay the concrete’s rate of setting. They are useful in extending the setting time, but they are often also used in attempts to decrease slump loss and extend workability.
Hydration-control admixtures consist of a two-part chemical system:
Inhibitors are used in concrete for parking structures, marine structures, and bridges where chloride salts are present. These admixtures chemically arrest the corrosion reaction.
These admixtures are used where cracks and curling must be minimized for durability or esthetic reasons.
These components can be used to change the colour of concrete for esthetic reasons.
Other materials include alkali-silica reactivity (ASR)-inhibitors and admixtures targetting areas such as:
New cement for reducing emissions
Understanding concrete composition is critical because of longstanding and emerging environmental issues. Climate change is a much-discussed global problem; it can be caused by both natural processes and human activities, in particular those altering the atmosphere’s chemical composition. The buildup of greenhouse gases (GHGs) is the primary cause for concern about climate change now and into the immediate future. (For more on the topic, see Environment Canada’s site. Visit www.climatechange.gc.ca).
In recent decades, the Canadian cement industry has made significant progress in reducing GHG emissions through improvements in process and efficiency. Looking forward, further improvements are limited because carbon dioxide (CO2) production is inherent to the basic process of calcinating limestone, which is at the core of cement manufacturing.
With the demand for concrete worldwide estimated to double within the next 30 years, (For more, visit www.ecosmartconcrete.com). there is an increased need for the cement and concrete industry to develop sustainable solutions. Thus, the industry is continuously striving to find innovative solutions that reduce GHG emissions and air pollution. The recent introduction of Contempra—new carbon-efficient cement that reduces CO2 emissions by 10 per cent—and the increased use of supplementary cementitious materials (SCMs) are two key initiatives toward that end.
Listed as “portland-limestone cement” in Canadian Standards Association (CSA) A3001-08, Cementitious Materials Compendium, and CSA A23.1-09, Concrete Materials and Methods of Concrete Construction, Contempra is the branding used by members of the Cement Association of Canada (CAC). The CSA standards are referenced in the 2010 National Building Code of Canada (NBC), and this material is already widely available across the country.
Portland-limestone cement is manufactured by intergrinding a regular clinker with up to 15 per cent limestone—10 per cent more than in regular cement. As a result, using this material in concrete decreases CO2 emissions by 10 per cent when compared to conventional portland cement, while still delivering concrete’s well-known strength and durability. (See “Portland-Limestone Cement: A More Sustainable Material for the Construction Industry,” by Andy Vizer, P. Eng., LEED AP, in the May 2011 issue of Construction Canada. Visit www.constructioncanada.net and select “Archives.”)
This new carbon-efficient cement will contribute to more sustainable construction and cleaner air in Canada. Once it is adopted for all suitable concrete applications, the industry expects it could reduce Canada’s GHG emissions by up to 900,000 tonnes annually—the equivalent to having 172,000 fewer cars on the road or planting more than 23 million trees.
Since this new type of cement is optimized to provide performance equivalent to regular portland cement used domestically, no significant changes should be required to concrete mix designs when using it. While new to the Canadian market, the cement has an extensive proven track record; it has been used in Europe in various commercial and residential applications for more than a quarter-century.
The manufacturing process involves modifying the clinker and limestone proportions before the final grinding takes place. The limestone, being a softer material, is ground finer than the clinker; both of these ingredients are ground finer in Contempra than in regular portland cement. The particle size and distribution have a significant impact on the properties of the final concrete product.
The process of producing the proper size and distribution of particles is referred to as ‘optimizing’ the cement. The cement industry spent considerable resources in the last four years to optimize the properties of portland-limestone cement to be comparable to its regular counterpart. On average, the new clinker and limestone particles are smaller in size, producing a so-called particle packing effect, which will increase the concrete’s resulting strength.
Further, the limestone is subjected to three quality assurance tests before manufacturing to ensure the new cement provides suitable performance.
SCMs: what they are and how they help
Supplementary cementitious materials (SCMs) are substances interground with cement clinker to produce blended cement; they can also be directly added to a concrete mix as a complementary agent. They are waste products from other industries that would otherwise require disposal. In Canada, commonly used SCMs include fly ash, blast furnace slag, and silica fume.
Approximately 60 per cent of greenhouse gas emissions associated with cement production is irreducible process emissions that result from heating raw materials to form cement clinker. For each tonne of supplementary cementing materials used in place of pure portland cement, approximately one tonne of GHG emissions is avoided. This practice also has the added environmental advantages of:
Since 2004, the country’s cement industry has improved its clinker/cement factor—the proportion of pure cement—from 86 to 83 per cent. (For more on the topic, see Cement Association of Canada’s site. Visit www.cement.ca). In 2008, the Canadian average clinker-to-cement ratio was 0.83; based on 2006 data, the global average clinker-to-cement ratio was 0.78.
Currently, the use of blended cements (with SCMs) to produce concrete typically replaces 20 per cent of the energy-intensive clinker that would otherwise be required to produce a cubic metre of concrete. In 2008, this alone resulted in a 1.4 million-tonne reduction in CO2 emissions in Canada. SCMs not only help reduce GHG emissions, but they are also often added to concrete to make the concrete mixtures more economical, durable, stronger and easier to place.
It is important, of course, to take advantage of various ways to make concrete ‘greener,’ but it is also important to ensure standards and specifications are being followed, as these guidelines and regulations are based on sound scientific research. According to CSA A 3001, Contempra includes up to 15 per cent interground limestone and the industry standard concrete includes up to 30 per cent SCMs, in some regions in Canada. The Ready-mixed Concrete Associations (RMCAs) in each region can help identify what the industry standards are for a specific region.
Better understanding of what is in concrete can help design/construction professionals further grasp the importance of recent manufacturing breakthroughs and foster a sense of understanding how the material can contribute to Canada’s 2020 goal of reducing total greenhouse gas emissions by 17 per cent from 2005 levels. (For more information on this topic, see Canada’s Action on Climate Change. Visit the website at www.ecoaction.gc.ca/climatechange-changementsclimatiques/index-eng.cfm).
Sherry Sullivan, MASc, P.Eng., LEED AP, is the director of transportation and built environment for the Cement Association of Canada (CAC). She was previously a concrete engineer with the Ready-mixed Concrete Association of Ontario (RMCAO) and a technical services representative for St. Marys Cement, where she gained extensive experience in the cement and concrete industries. Sullivan can be contacted via e-mail at firstname.lastname@example.org.
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