Want quick responses to our most asked precast questions? Our blog series, “FAQ Fridays,” is designed to answer your most popular questions, organized by topic and product category. In Part 2 of this series, we discuss manufacturing and concrete terminology.
Precast concrete is made in a quality-controlled environment where a dedicated batch plant produces concrete specially designed for precast products such as structural beams, columns, double tees, architectural cladding, and wall systems. Aggregates usually come from nearby quarries, and precast producers secure cement and other ingredients from other supplier partners.
The mixed concrete is placed into a form around reinforcements and 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 precast producer’s yard for finishing and storage until it is ready to ship to the job site.
To get a high-level overview of precast concrete, manufacturing process steps, and typical building applications, check out the Precast 101 eBook.
Forms for precast concrete manufacturing are made of various materials depending on product type and design requirements, including wood, steel, 3-D printed, plastics, rubber, concrete, and so on. With architectural precast, each precast form is entirely custom, so there are no premade forms, and all forms are made to order for each unique job. Surface textures, patterns, and shapes can be achieved by casting against various formliners, producing a dramatic aesthetic statement. These formliners can either be incorporated directly into the mold or attached to the surface, providing designers with more options and flexibility when designing a precast concrete building exterior.
Materials: Concrete & Cement
While the terms are sometimes used interchangeably, concrete and cement are not the same. Concrete is a building material and composite of aggregates, including sand, gravel, cement, water, and other materials. Cement is just one ingredient of concrete, but it’s a key ingredient, typically making up 10-12% of the volume.
Astonishingly, a small portion of precast concrete is cement. A fine powder that’s usually gray or white, cement is hydraulic, meaning it chemically reacts with water. As the concrete components mix, cement helps turn the mixture into a flowable, formable emulsion, binding the components as the concrete cures into a rock-like substance used for everything from simple sidewalks to sophisticated skyscrapers.
Is precast different from other types of concrete? Precast concrete is different, because it’s made in a factory by highly experienced personnel who apply stringent quality-control measures. In the factory environment, precast producers 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 most common type of cement, Portland cement, is a typical ingredient in concrete. It was invented in the early nineteenth century and named after the fine building stones it resembled that were quarried in Portland, England. The innovation of Portland cement marked a milestone in the construction industry, as it created a far stronger bond than that of the plain crushed limestone of the day. It remains the best-performing and most economical binder used in concrete.
The amount of cement used in precast concrete can be reduced by up to 60% through substitution by supplementary cementitious materials (SCMs), which are materials that can be partially substituted for cement in concrete mix. The ingredients are typically by-products of other industrial processes, including fly ash, which is left over from coal-burning power plants, and slag, which is produced during steel production. Other examples include silica fume and calcined clays. The amount of cement substitution possible is affected by the mixture design requirements, products and processes of individual precast producers and plants, and local material availability.
SCMs work with cement to bind the aggregates and other ingredients and improve concrete’s fresh properties and strength and durability. Light-colored SCMs, such as white silica fume or metakaolin, are used in architectural-face concretes. It’s important to note that although metakaolin is lighter in color and can be substituted for Portland cement, it’s not an industrial by-product, and it requires energy to manufacture. Certain SCMs, such as fly ash, may alter the concrete color or delay set times, which chemical accelerating admixtures can offset.
SCMs work through either hydraulic or pozzolanic reactions. Hydraulic reactions occur when a reactive ingredient is mixed with water. Cement is hydraulic and so are Class C fly ash and certain types of ground-granulated, blast-furnace slags. Pozzolanic reactions occur in the presence of calcium hydroxide (Ca(OH)2), which is a by-product of the hydration of cement. Class F fly ash, silica fume, calcined clays, and most slags are pozzolanic. Both hydraulic and pozzolanic reactions increase the strength and durability of finished concrete and alter the fresh properties of concrete.
Sustainability & Design Considerations
We’re often asked if precast concrete is a green building material. The low water-cement ratios possible with precast concrete (in the range of 0.36 to 0.38) allow for extreme durability. The thermal mass of concrete shifts peak heating and cooling loads in structures, reducing mechanical-system requirements. Because precast concrete is factory-made and most plants employ exact-batching technologies, there’s little waste created, which also reduces construction waste and debris on site and indoor air quality concerns. The load-carrying capacities, cross-section optimization, and long spans eliminate redundant members, and concrete readily accommodates recycled content.
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