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
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 significant up to 25% on heating and cooling costs.
Precast concrete's 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.