A certain type of building stone and a special type of architectural precast concrete
In many specifications, architects specify precast concrete or architectural precast concrete, when what they really want is Cast Stone. Many builders have a temptation to furnish precast concrete where Cast Stone is clearly required.
There is a marked difference between Cast Stone, limestone, precast concrete, and architectural precast concrete in terms of product use, appearance, strength, durability and imperviousness to dirt. Click here for comparison chart.
Cast Stone is a highly refined architectural precast building stone made from fine and coarse aggregates such as limestone, marble, calcite, granite, quartz, natural sands, Portland cement and mineral oxide coloring pigments. The mixture is rammed into sturdy moulds to achieve a dense texture, similar to natural cut stone. It is available in any color and can look like limestone, brownstone, bluestone, granite, slate, travertine or marble. It can match terra-cotta or brick and makes a perfect substitute for brick shapes. Cast Stone’s appearance is improved by weather, year after year.
Cast Stone is ideally suited for use as architectural trim, facing or ornament. Since it can be reinforced, it has the aesthetic properties of natural cut stone combined with the structural advantages of concrete.
Precast Concrete is ordinarily made of sand, dark colored river gravel, dark colored pebbles or black trap rock. Very little attention is paid to gradation of aggregates. To the sand and pebbles is added, usually, gray cement, all of which is poured into a mould. The skin of cement on the surface is what gives the material its color and smooth appearance. After five years subject to the weather, the dark aggregates begin to show through the cement skin and the appearance is can be anything but pleasing.
Precast concrete products are best used where not exposed to view, or where the appearance of air voids and surface irregularities is not an important consideration.
Architectural Precast Concrete is not normally used as a masonry product and is usually used in the form of large curtain wall panel units or large spandrel members. An adaptation of precast concrete, it is commonly made from larger aggregates of various colors mixed with a wet slurry of sand and cement which is poured into a mould. The surface of it is usually treated by sandblasting or chemical means, to expose the aggregates, resulting in a pebbly-with-voids type of finish.
Architectural precast concrete is best suited for large crane set (vs. mason set) units and for projects where the appearance of a natural stone installation is not an important consideration.
Where exposed to view or weather, there is a definite advantage in using Cast Stone rather than precast concrete. It is a temptation for some manufacturers to supply precast concrete when Cast Stone is specified, because precast concrete is less costly to manufacture. Aggregates cost less and no finishing of the surfaces is required. Therefore, if Cast Stone and not precast concrete or architectural precast concrete is required, it is necessary to specify Cast Stone. Refer to Specifications Section on Cast Stone.