Frequently Asked Questions (FAQs)
Q: Why is there demand for sustainable development?
A: According to the U.S. Green Building Council (USGBC) and the Canadian Green Building Council (CaGBC), buildings in North America consume nearly 10 percent of the world's energy, over 30 percnet of the total energy and more than 60 percent of the electricity. In fact, North American buildings use three times more energy than similar buildings in similar climates in Europe. Furthermore, North American construction projects generate up to 2.5 pounds of solid waste per square foot of completed floor space.
Q: What is a green building?
A: The North American regulatory agencies define green buildings as those that:
Studies show that green buildings offer improved air quality and more access to daylight in addition to energy and cost savings. The CaGBC estimates that green buildings cost 8 percent to 9 percent less to operate, and have a 7.5 percent greater building value.
Q: What is the difference between "green" and "sustainable"?
A: According to the CaGBC, "green" building practices can substantially reduce or eliminate negative environmental impacts and improve existing unsustainable design, construction and operational practices. Based on the Brundtland Commission's definition of sustainable development, green building practices constitute one effective approach to sustainable development.
Q: What is a "carbon footprint?"
A: 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.
Q: Why is carbon dioxide in the atmosphere a concern?
A: Trace amounts of carbon dioxide occur naturally and retain heat in the atmosphere, contributing to life on earth. However, CO2, along with methane, nitrous oxide, and fluorocarbons, are greenhouse gasses (GHGs) that are increasing as a by-product of human activity. As the concentration increases, it traps more and more heat that would otherwise be released into outer space in the natural cooling of the earth. This phenomenon has been termed the "greenhouse effect" because the increasing concentrations of GHGs trap infrared radiation in the same way as the panes of glass in a greenhouse.
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 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 that 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 the 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: Are precast concrete structures 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. See the reports by U.S. Department of Housing and Urban Development (HUD) and the National Institute of Standards and Technology (NIST)
Q: What are the recycling possibilities for precast concrete?
A: Concrete performance actually improves when several common industrial by-products 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 by-products can reduce reliance on cement as a binder. Even 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. 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 by-products like fly ash and consumes less energy than its competitors. According to the Canadian and U.S. 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 the 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 have 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 the built environment. Temperature increases have been measured as high as 80 degrees F. This additional heat causes air conditioning systems to work harder and consume more energy. The additional heat also contributes to 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: 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.