Due to the inherent sustainable qualities of precast concrete, the Precast/Prestressed Concrete Institute (PCI) believes that precast producers have a unique opportunity and obligation to participate in the sustainability movement by supporting green building practices and by continually improving their plant practices to reduce their environmental impact.
As the voice of the precast concrete structures industry, PCI has established a sustainability committee to provide leadership through education, sharing of green building practices using precast concrete technology, and best practices in the plants. PCI oversees a certification program that ensures quality and standardizes practices for precast plants and erectors, and has launched a sustainable plants program. A key initiative of PCI and its sustainability committee is the development of similar guidelines for sustainable practices and plant operations.
Precast concrete contributes to green building practices in significant ways. The low water-cement ratios possible with precast concrete (in the range of 0.36 to 0.38) mean it can be extremely durable. The thermal mass of concrete allows shifting of peak heating and cooling loads in a structure to help reduce mechanical-system requirements. Because precast concrete is factory-made, there is little waste created in the plant (most plants employ exact-batching technologies) and it reduces construction waste and debris on site, reducing construction indoor air quality concerns. The load-carrying capacities, optimized cross sections, and long spans possible with precast concrete members help eliminate redundant members, and concrete readily accommodates recycled content.
The primary ingredients of concrete—sand, gravel, and cement—are mineral based. When mixed with water, the cement chemically reacts to create a crystalline matrix with a high compressive strength. This matrix binds the sand and gravel together, creating concrete. Unlike other construction materials that can rust, rot, or otherwise degrade when in the presence of moisture, concrete can actually get stronger if there are unhydrated cement particles available to react with the water.
Precast concrete is different because it is made in a factory by highly experienced personnel who apply stringent quality-control measures. In the factory environment, precast producers are able to achieve consistency in temperature and moisture and low water-cement ratios that are not possible in field-fabricated concrete. Precast concrete can easily attain strengths of 5000 psi to 7000 psi or more, with densities that minimize permeability.
The thermal mass of precast concrete absorbs and releases heat slowly, shifting air conditioning and heating loads to allow smaller, more efficient heating, ventilating, and air conditioning (HVAC) systems. Insulation is often used in architectural panels and sandwich wall panels to increase thermal efficiency, with continuous insulation (ci) in walls being possible. The resulting savings are significant—up to 25% on heating and cooling costs.
Precast concrete's fresh and in-place performance can improve when several common industrial by-products are added. Fly ash, slag, and silica fume, which would otherwise go to landfills, can be incorporated into concrete as supplementary materials. These by-products can also reduce the amount of cement that is used in concrete. Reinforcement is typically made from recycled steel. (Steel is one of the most recycled building materials, and can be reused again and again.) Insulation and connections within the precast concrete also contain recycled content.
Precast concrete members are unique in that they are individually engineered products that can be disassembled. Designers can easily plan future additions to buildings, because the precast concrete components can be rearranged. Once removed, precast concrete members may be reused in other applications.
Precast concrete is also friendly to downcycling, in which building materials are broken down, because it comes apart with a minimum amount of energy and retains its original qualities. An example of downcycling would be the use of crushed precast concrete as aggregate in new concrete or as base materials for roads, sidewalks, or concrete slabs.
While the terms are sometimes used interchangeably, concrete and cement are not the same. Concrete is a building material, a composite of aggregates including sand and gravel, plus cement, water, and other materials. Cement is a key ingredient of concrete, typically making up 10% to 12% of the volume.
Cement does what its name implies-it cements the aggregates and other ingredients together. A fine powder that is usually gray or white in color, cement is hydraulic, meaning it chemically reacts with water. As the concrete components are mixed, cement helps turn the mixture into a flowable, formable emulsion, finally binding the components, as the concrete cures, into the rock-like substance used for everything from simple sidewalks to sophisticated skyscrapers.
Portland cement is a typical ingredient of concrete, and the most widely used type of cement. It was invented in the early nineteenth century and named after the fine building stones it resembled that were quarried in Portland, England. The innovation of portland cement marked a milestone in the construction industry, as it created a far stronger bond than the plain crushed limestone of the day. Today it remains the best-performing and most economical binder used in concrete.
SCMs are used in concrete as cement replacement and/or to modify the properties of the fresh or hardened mixture. The ingredients are typically the by-products of other industrial processes, including fly ash, which is left over from coal burning power plants, and slag, which is produced during the production of steel. Other examples include silica fume and calcined clays.
As industrial by-products, some SCMs may not be part of an ideal future. As sustainable development extends to other industries, less and less of these materials may be available to be recycled into concrete. In the meantime, SCMs offer an opportunity to improve concrete performance with a recycled material that would otherwise have to be disposed of in landfills.
SCMs work with cement to bind the aggregates and other concrete ingredients, and can improve concrete's fresh properties as well as its strength and durability. Light-colored SCMs, such as white silica fume or metakaolin, are used in architectural-face concretes. It is important to note that although metakaolin is lighter in color and can be substituted for portland cement, it is not an industrial by-product and it requires energy to manufacture. In fact, replacement of portland cement with this material may not reduce the environmental impact of the concrete.
Certain SCMs, such as fly ash, may alter the color of the concrete or delay set times, which may be offset by chemical accelerating admixtures. SCMs work through either hydraulic or pozzolanic reactions.
These terms describe how concrete sets and then hardens. Hydraulic reactions occur when a reactive ingredient is mixed with water. Cement is hydraulic, and so are Class C fly ash and certain types of ground-granulated blast-furnace slags. Pozzolanic reactions occur in the presence of calcium hydroxide (Ca(OH)2), which is a by-product of the hydration of cement. Class F fly ash, silica fume, calcined clays, and most slags are pozzolanic.
Both hydraulic and pozzolanic reactions increase the strength and durability of finished concrete, and alter the fresh properties of concrete.
Precast concrete is made in a factory, where a dedicated batch plant produces a specially designed concrete for precast concrete products such as structural beams, columns, and double tees; architectural cladding; and wall systems. Aggregates usually come from nearby quarries, and cement and other ingredients are often supplied by local precast producers.
The mixed concrete is placed into a form around reinforcement and, often, prestressing strands that provide load-resisting camber to the finished precast concrete member. After the member is cured, the precast concrete product is stripped from the form and moved to the precast producer's yard for finishing and storage until it is ready to ship to the jobsite.
PCI Producer Members meet local and state ordinances and emissions requirements. Initiatives within the industry include:
Typical concrete contains approximately 10% to 12% cement by volume. The cement chemically reacts with water to bind together the aggregates and other ingredients of the concrete. According to the Department of Energy (DOE), cement production contributes between 1% and 2% of global carbon dioxide emissions through the burning of fossil fuels and process-related emissions.
The amount of cement used in precast concrete may be reduced by up to 60% through substitution by supplementary cementitious materials (SCMs). The amount of cement substitution possible is affected by the mixture design requirements, the products and processes of individual precast producers and plants, and the local availability of materials.
Since 1975, the cement industry has reduced CO2 emissions by 33%. Today, cement production accounts for less than 1.5% of U.S. carbon dioxide emissions, well below other sources such as electric generation plants for heating and cooling the homes and buildings we live in (33%) and transportation (27%). In 2000, the cement industry created a new way to measure CO2 emissions. Recently introduced guidelines allows for greater use of limestone as a raw material in cement, ultimately reducing CO2 by more than 2.5 million tons per year. By the year 2020, plans call for further reduction of CO2 emissions to 10% below the 1990 baseline through investments in equipment, improvements in formulations, and development of new applications for cements and concretes that improve energy efficiency and durability.
The United Nations Brundtland Commission Report (1987) defined sustainable development and urged the world to take note: “Sustainable development is that which meets the needs of the present without compromising the ability of future generations to meet their own needs.” A growing global population is straining the finite resources available on the planet. Sustainability seeks to balance the economic, social, and environmental impacts, recognizing that population growth will continue. Sustainable development brings this evaluation to the design and construction industries, which have significant potential to reduce the negative impact of human activities on the environment.
According to the U.S. Green Building Council (USGBC), buildings in the United States consume nearly 10% of the world’s energy, and over 30% of the total energy and more than 60% of the electricity in the United States. The U.S. Department of Energy reports that 51% of electricity comes primarily from the burning of coal, a fossil fuel that produces significant greenhouse gases during combustion.
With energy costs increasing, and concerns about environmental impact growing, the U.S. government is adopting green building programs. In addition, an increasing number of states are offering tax benefits for green public buildings, and large corporations are moving toward sustainable design for their facilities to reduce operations and maintenance costs.
The U.S. Office of the Federal Environmental Executive (OFEE) defines green buildings as those that: demonstrate the efficient use of energy, water, and materials limit impact on the outdoor environment provide a healthier indoor environment.
Studies show that green buildings offer improved air quality and more access to daylight in addition to energy and cost savings. The USGBC estimates that green buildings cost 8% to 9% less to operate, and have a 7.5% greater building value.
The USGBC cites an initial cost premium of anywhere from 0% to 2% for green buildings in the United States. As project teams become more experienced with building green, these costs should decrease. Generally, a 2% increase in construction costs will deliver a savings of 10 times the initial investment in operating costs for utilities (energy, water, and waste) in the first 20 years of the building’s life.
The financial payback of green building practice is measured in operating and maintenance savings over time offsetting initial costs of sustainable features. The payback varies from project to project, depending on the implemented sustainable features and other factors such as availability of materials and expertise of the design team. However, experienced design professionals maintain that green buildings do not have to cost more than non-green buildings.
Source: HOK Guidebook to Sustainable Design
Most project teams perform a comprehensive life-cycle cost assessment (LCC) prior to defining their sustainable goals for the project. The LCC predicts how long it will take to recoup additional first cost.
More and more local, regional, and national government agencies require sustainable building practices or LEED certification. The General Services Administration (GSA), U.S. Army, Department of State, Department of Energy (DOE), and Environmental Protection Agency (EPA) are adopting LEED or similar green-building standards. Twenty-five states including California, New York, Washington, and Oregon have adopted LEED, as have over 100 municipalities.
Gases that trap solar heat in the earth's atmosphere and contribute to global mean temperature are considered greenhouse gases. They take their name from the “greenhouse effect,” the term that compares the heat-trapping gases to the glass panes of a greenhouse for the way they warm the atmosphere. According to the National Energy Information Center, “Many gases exhibit these greenhouse properties. Some of them occur in nature (water vapor, carbon dioxide, methane, and nitrous oxide), while others are exclusively human-made (like gases used for aerosols).” Some greenhouse gases, such as methane and nitrous oxide, have more of an impact on global warming than carbon dioxide. In fact, methane (CH4) has 25 times the impact on global warming potential compared with carbon dioxide (CO2), and nitrous oxide (N2O) has 298 times the impact on global warming potential compared with carbon dioxide (CO2).
Studies show that human activities are rapidly increasing the amount of CO2 in the atmosphere, which in turn is increasing global mean temperature. The Intergovernmental Panel on Climate Change (IPCC) cites climate models predicting the effects of higher temperatures, including rising sea levels and greater frequency of heat waves and floods. These events could have severe impact on the biosphere, human health, political stability, and economic development.
A carbon footprint is the quantification of energy-related emissions from human activity expressed in units of carbon dioxide (CO2). It includes all the heat, light, power, refrigeration, and transportation emissions associated with the harvesting, manufacturing, use, and disposal of a particular material, product, or service.
Carbon footprints are most closely linked to the burning of fossil fuels. The USGBC estimates that in the next 25 years, CO2 emissions from buildings will grow faster than those from any other sector of the economy, with commercial building emissions predicted to increase 1.8% through 2030. New commercial buildings will add an estimated 12 million metric tons of CO2 per year.