Sustainability and Precast Concrete FAQ
Precast concrete contributes to green building practices in significant ways. The low water-cement ratios possible with precast concrete -0.36 to 0.38- mean it can be extremely durable. The thermal mass of concrete allows shifting of 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 IAQ 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.
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, precasters 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.
PCI Producer Members meet local and state ordinances and emissions requirements. Initiatives within the industry include:
- Use of local materials in all mixtures; local aggregate resources
- Water reclamation and recycling
- Reducing cement requirements by lowering watercement ratios
- Admixtures such as hardening accelerators to eliminate applied heat in curing
- Use of self-consolidating concrete (SCC) for quicker placement, no vibration, and reduced surface defects
- Use of environmentally friendly thin brick in place of conventional brick in precast concrete systems
- Carbon-fiber reinforcement that allows lighter and larger concrete sections with less embedded energy and no corrosion
- Use of supplemental cementitious materials (SCMs) to reduce cement consumption; participation in Cool Climate Concrete
- Enclosed sandblasting facilities with 100% process-waste control
- Standardizing wood form parts for multiple reuse; recycling discarded forms into mulch or fuel
- Recycling all scrap steel and reinforcement
- Reducing and reusing product packaging received in facilities
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 concrete manufacturers 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 will allow 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.
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. 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.
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.