Precast concrete is made in a factory, where a dedicated batch plant produces a specifically 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, pre-stressing 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 precasters yard for finishing and storage prior to shipping to the jobsite.
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.
While the terms are sometimes used interchangeably, concrete and cement are not the same. Concrete is a building material, a composite of aggregates including sand and gravel, plus cement, water, and other materials. Cement is a key ingredient of concrete, typically making up 10% to 12% of the volume
Cement does what its name implies-it cements the aggregates and other ingredients together. A fine powder that is usually gray or white in color, cement is hydraulic, meaning it chemically reacts with water. As the concrete components are mixed, cement helps turn the mixture into a flow able, formable emulsion, finally binding the components, as the concrete cures, into the rock-like substance used for everything from simple sidewalks to sophisticated skyscrapers.
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.
The primary ingredients of concrete -sand, gravel, and cement- are mineral based. When mixed with water, the cement chemically reacts to create a crystalline matrix with a high compressive strength. This matrix binds the sand and gravel together, creating concrete. Unlike other construction materials that can rust, rot, or otherwise degrade when in the presence of moisture, concrete can actually get stronger if there are unhydrated cement particles available to react with the water.
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.
The thermal mass of precast concrete absorbs and releases heat slowly, shifting air conditioning and heating loads to allow smaller, more efficient heating, ventilating, and air conditioning (HVAC) systems. Insulation is often used in architectural panels and sandwich wall panels to increase thermal efficiency, with continuous insulation (ci) in walls being possible. The resulting savings are significantup to 25% on heating and cooling costs.
Precast concretes fresh and in-place performance can improve when several common industrial byproducts are added. Fly ash, slag, and silica fume, which would otherwise go to landfills, can be incorporated into concrete as supplementary materials. These by-products can also reduce the amount of cement that is used in concrete.
Reinforcement is typically made from recycled steel. (Steel is one of the most recycled building materials, and can be reused again and again.) Insulation and connections within the precast concrete also contain recycled content.
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.
2. Reliable supply
3. Made in purpose
4. Built factories and not weather affected
5. High level performance in thermal comfort
7. Acoustic separation
8. Resistance to fire
9. Resistant to massive water flow
10. Inherent strength and structural capacity able to meet engineering design standards for housing ranging from individual cottages to multi-stores apartments
11. Highly flexible in form, shape and available finishes
12. Ability to incorporate services such as electrical and plumbing in precast elements
13. High structural efficiency
14. Low wastage rates on site
15. Minimal waste, as most waste in the factory is recycled
16. Safer sites from less clutter
17. Ability to incorporate waste materials
18. High thermal mass, providing energy cost saving benefits
19. Simply designed for deconstruction, reuse or recycling