FREQUENT ASK QUESTION
SUSTAINABILITY AND PRECAST CONCRETE
Q: Is precast concrete a “green” building material?
A: Precast concrete contributes to green building practices in significant ways. Its inherent strength – 35 to 50 MPa – means precast concrete is extremely durable. Its mass can shift heating and cooling loads to help reduce mechanical system requirements. Because it’s factory-made, precast concrete reduces construction waste, both in the factory and on the job site, and does not add to construction indoor air quality (IAQ) concerns. The load capacity and long spans of precast members help eliminate redundant structures, and precast readily accommodates recycled content.
Q: What makes concrete so durable?
A: The primary ingredients of concrete, sand, gravel, and cement are mineral based. When mixed with water, the cement molecules chemically combine with the water to create a crystalline matrix of high compressive strength. This matrix binds the sand and gravel together, creating what is sometimes known as “liquid stone.” Unlike other construction materials, which rust, rot or otherwise degrade when in the presence of moisture, concrete actually gets stronger.
Q: Is precast concrete different from other types of concrete?
A: Precast concrete is different because it is made in a factory by highly experienced personnel who apply stringent quality control. In the factory environment, precasters are able to achieve consistency in temperature and moisture and low water to cementing material ratios that are not possible in field fabricated concrete. Precast strength easily achieves 35 to 50 MPa or more, whereas strength of field-cast concrete is far lower due to changing personnel, water added to mix during travel and curing conditions at the site. Much more permeable than precast concrete, field-cast concrete is also more susceptible to moisture and mechanical damage.
Q: Is precast concrete energy efficient?
A: Energy efficiency is part of the design of precast structures. The thermal mass of precast concrete can absorb and release heat slowly, shifting air conditioning and heating loads to allow smaller, more efficient heating, ventilating and air conditioning (“HVAC”) systems. Insulation can be incorporated in architectural exterior wall panels to increase thermal efficiency, and provide continuous insulation (“CI”) in walls. The savings can be significant – up to 25 percent on heating and cooling costs.
Q: What are the recycling possibilities for precast concrete manufacturing?
A: Concrete performance actually improves when several common industrial byproducts are added. High quality fly ash and slag that would otherwise go to landfills can be incorporated into concrete mixes as supplementary cementitious materials (SCMs). Use of these byproducts can reduce reliance on cement as a binder. Even the prestressed strand and reinforcing steel, almost always made from recycled materials, can be recycled and reused again and again. Insulation and connections within the precast also contain recycled content. Finally, when a precast concrete structure has served its purpose, it can be crushed and used as aggregate in new concrete or as base materials for roads, sidewalks and concrete slabs.
Q: Can precast concrete members be reused?
A: Precast concrete members are unique in that they are individually engineered products that can be disassembled. Designers can plan future additions to buildings, because the precast “kit of parts” can be rearranged and reinstalled. Once removed, precast members may be reused in other applications. Precast is friendly to downcycling, where building materials are broken down, because it comes apart with a minimum amount of energy and retains its original qualities. The process of downcycling does not contribute to the carbon footprint of precast to the same extent as other construction materials.
Q: How does concrete affect the environment compared to wood and steel?
A: Concrete is essentially inert; it does not rot, burn, off gas or rust, and provides durability that significantly outlasts many other building materials including wood and steel. The cement industry utilizes industrial byproducts like fly ash and consumes less energy than its competitors. According to the Canadian and US Departments of Energy, cement production accounts for 0.33 percent of energy consumption — lower production levels than steel production at 1.8 percent and wood production at 0.5 percent. In addition, it places less stress on our environment to acquire the raw materials for concrete than for steel or wood. Thus, concrete is an excellent choice for sustainable development.
Q: What is the urban heat island effect and how does concrete fit in?
A: Scientists observed that urban areas with more buildings and paving and less vegetation are typically warmer than surrounding rural areas. This is partially attributed to the dark surfaces of roofing and paving used to create our built environment. Temperature increases have been measured as high as 80F. This additional heat causes air conditioning systems to work harder and consume more energy, as much as 18 %. The additional heat also enhances the conditions for the creation of smog. Concrete’s natural light color can reduce urban heat islands. Light-colored concrete reflects more solar energy than dark-colored materials, such as parking lots, driveways, or sidewalks, thereby reducing the high temperatures.
Q: What is the difference between concrete and cement?
A: While the terms are sometimes used interchangeably, concrete and cement are not the same. Concrete is a building material, a composite of sand, aggregates and water plus other ingredients. Cement is a key ingredient of the concrete mix, typically comprising 10 to 12% of the volume of concrete.
Q: What is “Portland” cement?
A: Portland cement, also known as hydraulic cement, is a typical ingredient of concrete. It was invented in the early nineteenth century and named for the fine building stones that were quarried in Portland, England. The innovation marked a milestone in construction history, as Portland cement created a far stronger bond than the plain crushed limestone of the day. Portland cement remains the best-performing and most economical binder for concrete mixes worldwide.
Q: What does cement do to concrete?
A: Cement does what its name implies – it “cements” the sand, aggregates and other ingredients together. A fine powder usually gray or white in color, cement is “hydraulic,” meaning it is activated by water. As the concrete mix is agitated, cement helps to turn it into a flowable, formable emulsion, finally curing into the rock-like substance used for everything from simple sidewalks to sophisticated bridges and skyscrapers.
Q: What are Supplementary Cementitious Materials (SCMs)?
A: SCMs are typical concrete ingredients that are byproducts of other industrial processes. Examples include 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 byproducts, some SCMs may not be part of an ideal future. As sustainable development extends to other industries, less and less of these materials will 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 clog landfills.
Q: What do SCMs do to concrete?
A: SCMs work with cement to bind the aggregates and other concrete ingredients, and can improve concrete’s strength and durability. SCMs can fill the pores in concrete making the resulting mix more resistant to chlorides and sulphates. Light colored SCMs such as white silica fume or metakaolin are used for architectural face mixes. Certain SCMs such as fly ash may slow the time of set, which may be offset by chemical accelerating admixtures. SCMs work through either hydraulic or pozzolanic reactions.
Q: What are hydraulic and pozzolanic reactions?
A: These terms describe how concrete mixtures set and then harden. 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 slag. Pozzolanic reactions occur in the presence of calcium hydroxide (Ca (OH)2), which is a byproduct of the hydration of cement. Class F Fly Ash, silica fume, calcined clays, and most slags are pozzolanic. In both cases, these reactions increase the strength and durability of finished concrete.
Q: How is precast concrete made?
A: Precast concrete is made in a manufacturer’s plant or factory under controlled conditions. A dedicated batch plant produces a specially designed mix for structural and architectural products and systems. Sand and aggregates usually come from nearby quarries, while cement and other ingredients are usually supplied from local sources. The mixed concrete is poured around reinforcing and, often, prestressing strands that provide load-resisting force in the finished precast member. Factory production permits experienced workers to exercise a high degree of quality control, resulting in quality products containing high strength concrete of 35 to 50 MPa or more that is extremely durable and long lasting.
Q: How is cement made?
A: Limestone along with small amounts sand and clay are quarried, usually near the cement plant. These ingredients are ground and blended together. The materials are then heated in a giant rotary kiln at temperatures of 1870°C (3400°F). The high temperature causes the limestone to undergo a chemical reaction that changes its properties, imparting its characteristics as a binder in concrete. During the firing process, an intermediate product called clinker is formed. Once cooled, the clinker is ground with a small amount of gypsum, forming the fine gray-colored powder called hydraulic (Portland) cement.
Q: Isn’t cement manufacturing one of the most energy intensive manufacturing processes?
A: No. According to the US Department of Energy, cement production accounts for 0.33 (one-third of one percent) percent of energy consumption. The current level is low compared with other industries, such as petroleum refining at 6.5 percent, steel production at 1.8 percent, and wood production at 0.5 percent.
Q: What is being done about CO2 emissions during the cement manufacturing process?
A: The cement industry was among the first to address climate change, and has remained at the forefront of developing policies and improving the manufacturing process. Since 1975 the industry has reduced CO2 emissions by 33 percent. Today, the cement production accounts for less than 1.5 percent of 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%).
The UPF created a new way to measure CO2 emissions. Recently introduced guidelines will allow for greater use of limestone as a raw material of 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.
Q: Is precast manufacturing environmentally friendly?
A: When compared to jobsite operations, precast manufacturing is definitely more environmentally friendly. Less waste is generated, less material is used within comparable products, forms or molds have a longer service life, noise is reduced, and both quality and safety are improved.
At the manufacturing plant, UPF members will be monitoring their operating processes and implementing changes to ensure:
effective safety programs are in place to safeguard workers and assets
effective maintenance programs are in place to ensure equipment safety and efficiency
effluent water is adequately detained and neutralized for discharge to the receiving environment, and/or recycled for re-use
air quality is monitored and controlled: dust from cement silos, mixer operations and sandblasting, is minimized; unpaved road dust is addressed; and welding operations are adequately vented
solid waste is monitored and controlled: excess concrete is effectively used; culls are minimized; waste product is crushed into re-usable road-base material; steel is separated and recycled; wood forms and steel forms are recycled; paper use is minimized
energy use is monitored and controlled: heat curing is in a closed system; process heat is adequately controlled; flue gases are monitored and energy sources are properly tuned, with heating pipes and conduits insulated; hydro power factors and demand are monitored and adjustments made to minimize consumption of fuel and oil tanks are adequately contained to prevent any ground contamination from possible spillage
effective continuous improvement programs are in place to ensure that tomorrow’s performance will be even better than today’s