Precast concrete is made in a factory, where a dedicated batch plant produces a specially designed concrete for precast 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 manufacturers.
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 precaster s yard for finishing and storage prior to shipping to the jobsite.
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 water cement 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 reuses; 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.