About Cast Stone
This Guide gives general information on Cast Stone, its uses, cost, advantages and limitations. For more information on how Cast Stone measures up to other products, consult our Cast Stone comparison sheet.
Definition: Cast Stone is a highly refined architectural precast concrete building stone manufactured to simulate natural cut stone. One of the oldest known types of concrete, it is the most aesthetically refined form of concrete known today. Cast Stone is used as a masonry product to provide architectural trim, ornamentation or functional features on buildings and other structures. The earliest known use of Cast Stone was in the year 1138. The product was first used extensively in London in 1900 and in America around 1920. The Cast Stone Institute was incorporated in 1927. Since the early 1920s, Cast Stone has earned widespread acceptance in the architectural community as a suitable replacement for many masonry materials and for all types of natural cut building stones.
Manufacturing: Cast Stone is generally regarded as the most labor-intensive of all concrete product manufacturing processes. Of paramount importance to any Cast Stone manufacturer is the competence of his or her drafting department. In order to assure the success of the project, the detailer assigned to a Cast Stone job must have both knowledge and experience. He or she must have knowledge of architectural styles and designs and the individual must have experience in the manufacturing methods and the setting techniques employed by the masonry contractor.
Degrees of Customization: Since most other building products are either more “standardized” in nature (e.g. brick or window units) or less visible (e.g. structural steel) the Cast Stone usually becomes the custom element of the building facade which makes other materials fit together. The manufacturer details each piece with an aim toward simplification and standardization, each stone an architectural feat in itself. Cast Stone is available in various degrees of customization (Technical Bulletin #45).
The pattern or mold shop is the heart of any Cast Stone producer’s enterprise because of the extraordinary degree of craftsmanship required. The most successful producers have been in the business for many years and thus are able to cultivate such talent. Patterns for Cast Stone can be made from almost any material. The most common are wood, plaster, fiberglass and rubber. Other materials used are clay, gelatin, gypsum, styrofoam, plastic, concrete and, one of the earliest casting mediums, sand. Many advances have been made in rubber which have provided some very durable polyurethane and polysulfide which are suitable for not only casting final products, but, for obtaining impressions of existing and historical work as well. The journeyman patternmaker knows which material to select for a given application and exactly how to use it. Model maker, mould maker, artist, sculptor, plaster, master carpenter and leader of men; the modern Cast Stone plant’s pattern shop leader must be all.
Reading Rock Russell, LLC uses a “semi-dry, steam cured, earth moist concrete” which is neither “wet or dry”. The two most widely-used casting methods today are the “Vibrant Dry Tamp” (VDT) system and the “Wet Cast” method, neither of which precisely describe our manufacturing process. Each method requires a meticulously proportioned mix design consisting of carefully graded and washed natural gravel and sand combined with crushed graded stone such as granite, marble, quartz, or limestone meeting the latest requirements of ASTM C33. The mixtures are proportioned for maximum density and to produce the required “fine grained texture similar to natural stone with no bugholes permitted.” While Portland cement (ASTM C150) is usually used to achieve lighter colors and color consistency, blending of grey Portland cement and coloring pigments (ASTM C979) together with the white cement in order to achieve color is a fairly common practice. • Since a rich cement/aggregate ratio of 1:3 is normally used, a properly (warm-moist) cured Cast Stone unit will have a somewhat higher compressive strength (6,500 psi or higher, as tested by ASTM C1194), and a lower cold water absorption rate (6% or lower as tested by ASTM C1195) than natural limestone or normal concrete. The manufacturing method, due to its inherently lower water/cement ratio typically yields higher compressive strengths than low-slump concrete. The production system entails the vibratory ramming of earth moist, zero-slump concrete against rigid formwork until it is densely compacted and ready for immediate removal from the form. This process enables as many as 100 pieces to be cast from a single mold in an eight-hour day and is ideally suited to fast-track construction projects due to its production capability and low form work requirements. The process guarantees total absence of bugholes and a finish which is difficult to distinguish from natural stone. The limitation of the process is that it generally requires one flat, unexposed side to the design.
Sandblast or chemical retardation finishing methods (normally used in finishing of architectural precast concrete panels) are seldom used with Cast Stone because of the dulling of aggregates and the loss of fine detail which are not acceptable in quality Cast Stone work.
Cost per unit depends greatly on individual component design, component repetition, specification and bid documents. On average, however, Cast Stone costs less than quarried stone. There are several reasons. One is that it is a molded product and requires no further tooling after the initial pattern is made. Each piece of cut stone must be carved individually. Another reason is freight. Usually quarried stone must be hauled over long distances. Most of the limestone in this country is hauled from Indiana. Many stones come from overseas. Brownstone is now typically available from one main source, Germany.
Cast Stone is a highly versatile architectural precast building material which looks like, is usually stronger than, weathers better, has greater color consistency, can be reinforced, and costs less than natural cut stone.
|Attributes||Cast Stone||Limestone||Precast Concrete|
|Constituents||Portland cement, fine crushed natural stone, and manufactured or natural sand. Iron oxides added for color.||Carbonated lime||Sand, cement, larger aggregates|
|Size of aggregate||3/8″ or less||none||3/8″ to 3/4″|
|Structural use||Can be reinforced, but has limited structural applications.||Cannot be reinforced, used primarily as non- structural veneer||Structural or non-structural|
|Typical uses||Architectural feature, trim, ornament, or facing: copings, sills, banding, door and window surrounds, bases||Architectural feature, trim, ornament, or facing: copings, sills, banding, door and window surrounds, bases||Architectural concrete structural pieces and non-structural panels, window units, etc.|
|Anchors supplied by manufacturer||No||No||Usually supplied by onsite erector.|
|Method of anchor attachment to stone||Anchor attaches to pre-formed anchor slots and dowel holes||Anchor attaches to cut anchor slots and dowel holes||Inserted in piece as it is poured at manufacturing facility or installed separately at the job site|
|Surface texture||Fine-grained to simulate natural stone, textures optional||Fine-grained, with directional grain lines||Usually exposed aggregate or other finishes not intended to simulate natural stone|
|Method of forming into shapes||Ramming moist zero-slump mix into rigid formwork until densely compacted and demoulded immediately||Mined, cut, and fabricated||Pouring wet concrete mix into form, vibrating, and removing forms the following day|
|Colors available||Almost unlimited||Limited to white, buff, gray||Almost unlimited|
|Compressive strength||6,500 psi minimum ASTM C1194||4,000 psi minimum ASTM C170||5,000 PSI minimum ASTM C39|
|Water absorption||6% maximum||7.5% maximum||7% maximum|
|Bug holes or air voids||No||No||Yes|
|Industry association||Cast Stone Institute
813 Chestnut Street, PO Box 68
Lebanon, PA 17042
Phone: (717) 272-3744
Fax: (717) 272-5147
|Indiana Limestone Institute of America, Inc., Stone City Bank Bldg,
Bedford, IN 47421
Phone: (812) 275-4426
|Architectural Precast Association
325 John Knox Rd, Suite L103,
Tallahassee, FL 32303
Phone: (850) 205-5637
Fax (850) 222-3019
|Keys to optimum economy||Piece repetition, average sizes 1.5 – 2.0 cu. ft., one flat unexposed surface||Pieces with simple profiles, short distance of jobsite from mines||ideal for larger structural pieces such as panels, precast lintels|
Cost: Keys to economical manufacturing and competitive pricing depends on the size of the individual piece, piece design, repetition of the individual profile, ornamentation, and the color desired.
Size: Rather than cost per lineal foot or cost per square foot, the wide range of sizes that can be made in Cast Stone makes it more convenient to view cost in terms of cubic feet per piece. The ideal unit cost (per cubic foot) is achieved when individual piece sizes are 1.5 to 2.0 cubic feet. The ideal length for straight pieces such as copings, sills, and banding are between 4′ to 6′.
Piece Design: By including one flat unexposed side in the design of the individual piece (see Value Engineering section), production time and curing time would be reduced to a minimum and costs would be kept low. Typical profiles with one flat unexposed side include base, water table, banding, sills, header, and coping. If more than one flat unexposed side is designed such as L-shaped or U-shaped pieces, costs increase due to the intricate mold required and the additional production time. Pieces such as balusters and balls that require molds that are two-sided, with minimal unexposed surface, add costs due to the longer time to construct molds and extreme care that must be used during the production process.
Profile Repetition: The more that an individual mold can be amortized over a production run, the lower the cost per piece, even if the length of the profile varies. Various piece lengths can made by using an adjustable filler or stop in the mold during the production process.
Profiles and Rustication: Cast stone can be molded to a variety of shapes and the architect is encouraged to exercise design freedom when drawing Cast Stone profiles. In contrast to natural cut stone, profiles with curves and bevels are encouraged because the additional cost to incorporate these shapes into molds is minimal compared to flat pieces with defined edges.
Ornamentation: It should be noted that highly intricate designs that require making a model, a “positive” needed to make the mold be sculpted from clay or other material, adds to the cost. This would be the case when reproducing shapes on historical buildings.
Color: The more chromatic the desired color, the more each piece costs. Cast stone colors that are standard and that have no additional costs are the colors that are indicated as “Standard Colors” in the colors section of this website.
|The advantages of Cast Stone include its ability to:|
|• Replicate natural stone at a fraction of the cost|
|• Offer precise colors and form a wide variety of shapes|
|• Consistently deliver quality because it is a manufactured product|
|• Can easily be cut to length in the field|
|• Simulate the look of natural stone|
|• Deliver a time-tested freeze-thaw durability (Technical Bulletin #40)|
|• Replace the stone on historical buildings.|
Since Cast Stone is a division 4 masonry material, sizes and connection methods should be within the scope of work to be performed by the masonry contractor. In addition to not designing structural pieces, the vibrant dry tamp method of Cast Stone, which involves ramming earth moist concrete against rigid formwork until ready for immediate removal from the mould, does not lend itself to large panels. The ideal panel size is approximately 2′ x 5′. Typical panel thickness is 3 5/8″. This size enables our plant to be most productive, and therefore, we can be most competitive on bids that contain this or similar sizes. The typical maximum size should be no larger than 3′ x 8′. Here you can create the content that will be used within the module.
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