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.