Precast's proximity, energy efficiency, recyclability and minimal waste are keys to meeting environmental standards.
More and more owners are deciding to provide higher levels of sustainable construction in their buildings. Much of this attention is spurred by the Leadership in Energy & Environmental Design (LEED) standards created by the US Green Building Council (USGBC) and the Canadian Green Building Council (CaGBC), as well as by the growing attention to climate change and the desire to lower the consumption of energy and materials. To reach these goals, designers are often turning to precast concrete components that provide a number of "green" advantages.
Precast concrete components can help owners to reach as many as 21 of the 26 points needed to achieve LEED certification.
Precast Concrete Sandwich Wall Panels
Insulated sandwich wall panels, constructed with rigid insulation placed between two wythes of concrete, add energy efficiency to a precast wall panel's natural benefit of high thermal mass.
Precast concrete insulated sandwich wall panels can be finished on both sides, allowing the interior face to serve as the finished interior wall without additional finishes or furring out.
Precast sandwich wall panels can help achieve LEED certification in a variety of ways, including their ability to be recycled, being locally manufactured, having high thermal mass and integral insulation. These attributes help reduce the expended energy needed to manufacture, transport and erect precast concrete panels, key LEED requirements.
Minimum Energy Performance
New buildings must reduce the design energy consumption to comply with Natural Resources Canada’s Commercial Building Incentive Program (CHIP) requirements for a 25% reduction relative to the consumption of the reference building designed to the Model National Energy Code for Buildings 1997 (MNECB), including supplemental CHIP requirements, or: New buildings must reduce the design energy cost by 18% relative to the reference building designed to ASHRA/IESNA 90.1-1999 (without amendments).
Precast sandwich wall panels can achieve high R values and lower HVAC demands. In addition, large precast concrete panels have fewer, better sealed joints, reducing uncontrolled air infiltration. These attributes can help a project earn many of the maximum 10 LEED credits in the Optimize Energy Performance category.
Thermal Mass Not Appreciated
One of precast concrete's key benefits comes from its thermal mass, a property that helps concrete to store heat and moderate daily temperature swings. “Despite vast empirical evidence, modern understanding about thermal mass has taken some time to evolve," says a report from the Environmental Council of Concrete Organizations (ECCO). Few studies focused on the benefits provided by thermal mass prior to the oil crisis in the early 1970s. Then, prescriptive relief was addressed with readily available corrective measures, focusing on minimum R values, but R values underestimate energy saving thermal-mass characteristics.
Recent studies by the U.S. Department of Energy (DOE), have demonstrated the true benefit of thermal mass. The DOE report indicates that mass in exterior walls reduces annual energy costs in buildings. The U.S. Department of Housing & Urban Development (HUD) and the National Institute of Standards & Technology (NIST) also have done studies. Thermal mass helps shift peak loads from mid-afternoon in the summer to after 5 PM reducing energy consumption.
ECCO reports that "Modeling and testing have proven that the combination of insulation with thermal mass forms a superior wall system exhibiting the benefits of both." The most benefit comes from placing the insulation inside the thermal mass, as with precast concrete insulated sandwich wall panels. Adding insulation to the interior face of the wall, the other commonly used approach, isolates the wall from direct contact with the interior, reducing the benefits of the wall's thermal mass. Regardless of placement in the wall, thermal mass reduces loads and shifts peak loads in most climates.
The ECCO report says, "The guiding principle for all thermal-mass standards has been performance, whether individual components or the overall building envelope. These standards have successfully translated the behavior of thermal mass into understandable and easy-to-use terms. The result is that thermal mass has become a feasible element of building design." The benefits of thermal mass will become widely recognized by designers in the future.
Building Reuse (Materials Credit #1)
A one-point credit is available for maintaining 75% of the existing shell of a building. An additional point is offered for maintaining 100% of the shell, consisting of the skin and frame but not the windows, interior walls and other components. Concrete's durability gives it a strong advantage here. Total-precast concrete structural systems and architectural precast concrete panels can provide long service life.
Specific durability can be hard to quantify, because it depends on so many variables, including weather, maintenance and type of finishes. Compared to precast concrete, many other building materials simply don't last as long without significantly more maintenance. High-quality precast concrete is an ideal choice to achieve durability and extend the life of a building. As concrete is made of natural materials, it reduces the use of chemical-based materials.
Total-precast concrete systems can offer long interior spans via double tee and hollow core floors and roofs. Buildings are easier to remodel or reconfigure as occupant needs change. This ensures the structure can remain in place longer with only minor adjustments needed.
Construction Waste Management (Materials Credit #2)
This two-point credit is available for reducing construction, demolition and land-clearing waste that ends up in landfills. At least 50% of these materials must be kept out of landfills. The second credit is offered if 75% is diverted. This credit can be used in conjunction with the Building Reuse credit to achieve as many as four points if existing materials, such as precast concrete wall panels, are reused in the project. In that case, the materials preserved can be applied to this credit as well as to Materials Credit #1.
Concrete's inorganic composition makes it ideal for recycling. Concrete is frequently crushed and reused as aggregate for road bases or construction fill. As with the Building Reuse credit, these options are available only in the future after the building has been constructed and is being reconsidered for other uses. Concrete’s impact on these points for future use can play a role in specifications made today.
|Steel||Iron Ore Mining||2.25|
|Wood||Boreal Timber Harvesting
Coastal Timber Harvesting
Source: Environmental Council of Concrete Organizations (ECCO), 2004
ECCOs indexing of the impact of various materials against concrete showed that iron-ore mining, for example, has 2.25 times the impact on the environment as concrete-aggregate extraction.
Precast concrete offers other waste-saving benefits; less material is required to produce precast components because thinner sections, precise mix designs and tighter tolerances can be achieved. Less concrete is wasted because the quantities of constituent materials are tightly controlled in a precast plant.
The waste materials also are more likely to be recycled because concrete production takes place in one location under controlled conditions. Grey water can be recycled into future mixes. Between 5 and 20% of the aggregate in a mix can be recycled concrete aggregates. Sand used in finishing can be reused. Wood and steel forms and other materials used in casting can also be reused many times before recycling.
There also is less dust and waste at the construction site because precast components are delivered when needed. There is no debris from formwork, rebar and concrete pouring. Fewer trucks and less time are needed because precast concrete is made offsite. This is particularly beneficial in urban areas where minimizing traffic disruptions and noise pollution are critical.
A properly designed precast concrete system will result in smaller structural members, longer spans and less material used on-site. This translates directly into economic and environmental savings. Using less material means using fewer natural resources and less energy during manufacturing and transportation.
Recycled Content (Materials Credit #4)
This two-point credit is achieved for using materials with post-consumer recycled or post-industrial recycled content. Precast concrete components can contribute to this requirement because supplementary cementitious materials can replace a portion of cement in a mix. The use of recycled materials is growing, and will continue to grow as more designers learn about this option and its benefits.
Cement comprises 7 to 15% of the total volume of precast concrete components. Cement is a manufactured product that uses energy during its production. The Canadian cement industry has reduced energy usage per ton of cement by 35% since 1972. Water comprises 14 to 21% of the precast concrete volume, aggregates accounts for the rest.
Supplementary Cementing Materials (SCM) can significantly reduce the embodied energy of precast concrete products by substituting waste materials for relatively high energy consuming hydraulic cement. SCMs are mostly by-products of other industrial processes. Their judicious use in concrete production is desirable both for environmental and energy conservation as well as for the technical benefits they can provide. SCMs are added to concrete as part of the total cementitious system, either as an addition or partial replacement of hydraulic cement.
When properly used, the SCMs can enhance the following properties of concrete:
- Generally improve the workability and finishing of fresh concrete
- Reduce bleeding and segregation of fresh concrete
- Lower the heat of hydration beneficial in mass pours
- Improve the pumpability of fresh concrete
- Generally improve the long term strength gain
- Reduce permeability and absorption (especially silica fume)
- Reduce alkali-aggregate reactivity
The effect of replacing cement with supplementary cementitious materials on the embodied energy of concrete is appreciable. For example, a 1% replacement of cement with fly ash results in an approximately 0.7% reduction in energy consumption per unit of concrete.
Silica fume is a waste product recovered from the reduction of high-purity quartz with coal in electric furnaces in the production of silicon and ferrosilicon alloys. Silica fume improves the quality, strength and durability of concrete by making the concrete much less permeable and more resistant to corrosion of the steel reinforcement.
Fly ash is a pozzolan waste product collected from coal-fired power plants. Fly ash contains some heavy metal (normally more than silica fume), so the heavy metal content of the concrete will increase. Fly ash refines the pore structure of the concrete, making it more resistant to chloride penetration. Not all fly ash is suitable for use in concrete. Fly ash must conform to the requirements of CSA A3000 and must be specified in accordance with Tables 7 and 8 in CSA A23.1.
Although fly ash offers environmental advantages, it also improves the performance and quality of concrete. Fly ash affects the plastic properties of concrete by improving workability, reducing water demand, reducing segregation and bleeding, and lowering heat of hydration. Fly ash increases strength, reduces permeability, reduces corrosion of reinforcing steel, increases sulphate resistance, and reduces alkali-aggregate reaction. Fly ash reaches its maximum strength more slowly than concrete made with only hydraulic cement.
Blast Furnace Slag
Production of blast furnace slag consumes about 1/3 of the energy required to produce cement. Substitution of slag at levels exceeding 35% for the hydraulic cement in precast concrete is considered a high volume SCM application, and its suitability for intended use must be prequalified. The addition of slag cement usually results in reduced need for water, faster setting time, improved pumpability and finishability, higher 28-day strength, lower permeability, resistance to sulfate attack and alkali-silica reactivity (ASR), and lighter color. One life cycle inventory study investigated fly ash substitution of 20 percent for a 21 MPa ready mixed concrete mixture. In this example, slag cement provided one and a half to more than twice the savings in emissions, energy and extracted material when compared with fly ash. Not all slag is suitable for use in concrete. Blast-furnace slag must conform to the requirements of CSA A3000 and must be specified in accordance with Tables 7 and 8 in CSA A23.1.
Too Much Fly Ash?
Some designers try to push high volume fly ash concretes too far without understanding the chemistry and ramifications involved. There are several decades of research supporting the performance benefits of reasonable quantities of fly ash. Even before sustainability influenced North American markets, the concrete industry used about one million tons of fly ash annually to achieve better durability. The optimum amount of supplementary materials varies by type of cement and type of supplement. The reactivity of fly ash with other materials can vary by coal seam and the electrical-generating plant from which it was obtained.
Normally 10 to 20% of the cement can be replaced with fly ash to reduce the environmental burden of the concrete. Substitution of fly ash at levels exceeding 25% is considered to be a high volume SCM application. Care must be taken to perform appropriate testing to ensure desired performance. The use of fly ash can increase setting times. This may be an economic factor in precast concrete manufacturing if casting cannot be maintained on a daily cycle.
The best approach is to talk to CPCI members in advance and avoid prescribing an amount of fly ash beforehand. Ask the producer to use the maximum amount that is effective and to supply a performance standard for the application, not just a percentage. The goal of the long-term performance of a building will optimize the use of fly ash, not maximize it.
Local/Regional Materials (Materials Credit #5)
A one-point credit is offered when at least 20 percent of building materials are manufactured within an 800 km radius of the site. An additional point is offered when half of the regionally manufactured products are extracted or recovered within 800 km.
Precast concrete meets both of these requirements in virtually all cases. Most precast plants are within 300 km of a project, and the raw materials used to produce the precast concrete components (cement, aggregate, reinforcement) are usually obtained from sources within 300 km of the precast plant. This advantage leads many designers to replace granite, stone and other imported products with precast concrete wall panels.
Innovation in Design (Innovation and Design Process)
Up to four points can be awarded for innovative green design strategies that do not fit into existing LEED categories. An additional point is available for using LEED-certified professionals on the design team.
- Certified (26-32 points)
- Silver (33-38 points)
- Gold (39-51 points)
- Platinum (52-70 points)