SPECIFICATIONS & HISTORY OF CAST STONE
WHAT IS CAST STONE?
DEFINING ARCHITECTURAL CAST STONE
Cast stone is defined as “a refined architectural concrete building unit manufactured to simulate natural cut stone, used in unit masonry applications”. Cast stone is a masonry product, used as an architectural feature, trim, ornament or facing for buildings or other structures.
Cast stone can be made from white and/or grey cements, manufactured or natural sands, carefully selected crushed stone or well graded natural gravels and mineral coloring pigments to achieve the desired color and appearance while maintaining durable physical properties which exceed most natural cut building stones. Cast stone is an excellent replacement for natural cut limestone, brownstone, sandstone, bluestone, granite, slate, coral rock, travertine and other natural building stones.
Today, cast stone is a Portland cement-based architectural precast concrete product manufactured using high quality fine and coarse aggregate as its primary constituents. The use of a high percentage of fine aggregate creates a very smooth, consistent texture for the building elements being cast, resembling natural cut stone. Other ingredients such as chemical admixtures, pozzolans, and pigments also may be added.
Historical Overview & Early Developments
Cast stone has been a prime building material for hundreds of years. The earliest known use of Cast stone dates about to the year 1138 and was seen at Carcassonne, France, the city which contains the finest remains of medieval fortification in Europe. Cast stone was first used extensively in London in the year 1900 and gained widespread acceptance in America in 1920.
One of the earliest developments in the industry was Coade stone, a fired ceramic, but most artificial stone consists of fine cement concrete placed to set in wooden or iron moulds. It was cheaper and more uniform than natural stone, and widely used. In engineering projects, it had the advantage that transporting the bulk materials and casting them near the place of use was cheaper than transporting very large pieces of stone.
A well-known variety was Victoria stone, which is composed of finely crushed Mount Sorrel (Leicestershire) granite and Portland cement, carefully mixed by machinery in the proportions of three to one, and filled into moulds of the required shape. When the blocks are set hard the moulds are loosened and the blocks placed in a solution of silicate of soda for about two weeks for the purpose of indurating and hardening them. Many manufacturers turn out a material that is practically non-porous and is able effectually to resist the corroding influence of sea air or the impure atmosphere of large towns.
PRODUCTION METHODS & ASTM STANDARDS
Vibrant-Dry-Tamped (VDT) & Wet Cast
Cast stone frequently is produced with a low water-to-cement ratio mixture with a “dry” (or “earth moist”) consistency. The mixture is consolidated into a mould using an air-driven tamping device or vibration under pressure. For slumpable consistency mixtures, internal or external vibration is applied to the production mould.
Admixtures & Durability
Integral waterproofing formulas improve strength by as much as 20% while reducing absorption by 40%. Commercial test results show VDT freeze-thaw weight loss at just 0.23% after 300 cycles. Air entraining agents (4% to 6%) allow wet cast concrete to withstand numerous freeze-thaw cycles without failing.
ASTM C 1364 Standard
Today, the Industry Standard for physical properties and raw materials constituents is ASTM C 1364, the Standard Specification for Architectural Cast Stone. The ASTM consensus process ensures that interested individuals representing industry, academia, consumers, and governments all have an equal vote in determining standard content.