Cast Stone - Physical Properties

Physical Properties

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.

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, or electric, tamping device or vibration under pressure, which is much like the formation of natural sedimentary rock. Products manufactured in this manner are referred to as vibrant-dry-tamped (VDT) cast stone. For cast stone mixtures produced with a slumpable consistency mixture, the concrete typically is consolidated using internal or external vibration applied to the production mould, or increasingly by the use of self-compacting additives.

Over the last decade, new types of admixtures have been developed for VDT concrete products. These new admixtures do not normally work with "wet cast" concrete. These new plasticizers are more efficient than using air-entraining agents to increase compaction in VDT concrete. Some plasticizers have chemical properties that react with the cement to increase ultimate strengths of semi-dry concrete. Another important type of admixture for VDT concrete is integral waterproofing formulas. Tests have shown that some of these integral waterproofing admixtures have improved strength by as much as 20% while reducing the absorption by 40%. The increased strength and reduced absorption results in improved freeze/thaw durability.

On recent commercial testing laboratory results the freeze-thaw for VDT concrete using integral waterproofing admixtures was 0.23% weight loss after 300 cycles. Air entraining agents have been the salvation of wet cast concrete as it pertains to its durability. Wet cast concrete with 4% to 6% entrained air normally can withstand numerous freeze-thaw cycles without failing. Other types of admixtures such as water-reducers, super plasticizers and the new self-compacting additives can add to improving the strength and durability of wet cast concrete products. Cast stone products manufactured by the wet cast method are required to meet the same standards as VDT cast products.

Today, the Industry Standard for physical properties and raw materials constituents is ASTM C 1364, the Standard Specification for Architectural Cast Stone. Membership in ASTM International (founded in 1898 as the American Chapter of the International Association for Testing and Materials and most recently known as the American Society for Testing and Materials) exceeds 30,000 technical experts from more than 100 countries who comprise a worldwide standards forum. The ASTM method of developing standards has been based on consensus of both users and producers of all kinds of materials. The ASTM process ensures that interested individuals and organizations representing industry, academia, consumers, and governments alike, all have an equal vote in determining a standard's content.

In the UK and Europe it is more normal to use the Standard “BS 1217 Cast stone - Specification” from the BSI Group, and the European Commission's “Construction Products Regulations” legislation states that by mid 2013, all cast stone sold in Europe must be certified to this standard.