Introduction to Cast Stone

Cast Stone is a highly refined architectural precast concrete building “stone”, similar in appearance and manufactured to simulate the color, texture and appearance of natural cut stone. It is the most aesthetically refined form of concrete known. Cast stone is used as a masonry product to provide architectural trim, ornamentation or functional features on buildings and other structures in various forms such caps, sills, bands, medallions, veneer panels and in many landscaping and monument applications with architectural finish. The earliest known use of Cast Stone was in the year 1138. The product was first used extensively in London beginning in 1900 and in America around 1920. The Cast Stone Institute was incorporated in 1927.

Since the 1920’s, Cast Stone has earned widespread acceptance in the architectural community as a sustainable replacement for many masonry materials and for all types of natural cut building stone. The manufacture of Cast Stone is generally regarded as the most labor-intensive of all concrete products manufacturing processes. Of paramount importance to any Cast Stone manufacture is the competence of the design staff. It requires knowledge and experience with architectural styles and designs, as well as the manufacturing methods and installation requirements.

Since most other building products are standard (e.g. brick, windows, doors, etc.) or less visible (e.g. structural components), the Cast Stone is the custom element of the building façade which fits with the standard products to provide the architectural character or aesthetic appeal of the building or structure. The manufacture designs and details each piece with an aim toward simplification and standardization. Each stone becomes an architectural feature of the building.

The mold fabrication shop is the heart of any Cast Stone production facility. This is because of the extraordinary degree of craftsmanship required. American Artstone Company has the sustained reputation as the most successful producer in the industry and has many years experience including cultivated talent which has been passed down from generation to generation.

While the molds for Cast Stone can be made from many materials, the most common materials are wood, plaster, fiberglass and rubber. Other materials used are clay, gelatin, gypsum, Styrofoam, plastic, concrete and sand. Many advances have been made in rubber which have provided very durable polyurethanes and polysulfide’s which are suitable for not only casting products, but for obtaining impressions of existing and historical work as well.

Two widely used casting methods are the “Vibrant Dry Tamp” (VDT) system and the “Wet Cast” method. Each method requires a meticulously proportioned mix design consisting of carefully graded and washed natural gravel and sands combined with crushed graded stone such as granite, marble, quartz or limestone meeting the latest requirements of ASTM C 33. The mixtures are proportioned for maximum density and to produce the required fine grained texture similar to natural stone with no bug-holes permitted. White Portland cement (ASTM C 150) is usually used to achieve lighter colors and color consistency. Blending of grey Portland cement and coloring pigments (ASTM C 979) together with white cement in order to achieve color is common practice. Since the rich cement/aggregate ratio of 1:3 is normally used, a properly cured Cast Stone unit will have a high compressive strength (6,500 psi) and a lower cold water absorption
rate (6%) than natural limestone or normal concrete. The VDT method, due to its inherent lower water/cement ratio typically yields higher compressive strengths than low slump concrete.

The VDT production method 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 for fast-tracked construction projects due to its production capability and low formwork requirements. The VDT process guarantees total absence of bug-holes and a finish which is difficult to distinguish from natural stone. The limitation of this process is that it generally requires one flat, unexposed side to the design making “L” shapes and similar shapes more costly to produce.

The wet cast process for manufacturing Cast Stone is similar to the manufacturing process used for making architectural precast concrete, but produces a finish much more closely resembling natural stone. Mix designs usually require graded sieve coarse aggregates typically 1/2” or smaller and are composed of an abundance of fines which, combined with careful hand and vibratory concrete placement techniques, leaves little or no voids after finishing of exposed surfaces. The compression strength is approximately 6,500 psi with a lower cold water absorption factor making Cast Stone less permeable to water, weather and dirt.

The final process in the manufacture of Cast Stone is the removal of the cement “skin” from the outer surface. This exposes the fine aggregates which make up the matrix. The removal of the cement skin assures that the product will undergo minimal color and texture changes as a result of weathering. Muriatic (hydrochloric) acid etching is the most popular method of finishing Cast Stone, because of the brilliance of the etched aggregates and the ability of the resulting finished surface to remain clean. American Artstone also offers a honed or polished finish which provides a glossy finish similar to granite or marble. Sandblast or chemical retardation finishing methods (normally used in finishing 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 typically desired in quality Cast Stone work.

Cost depends greatly on requirements from specifications and bid documents. On an average, Cast Stone costs less than quarried stone. Since Cast Stone is a molded product, it requires no further tooling after the initial pattern is made. Each piece of natural stone must be carved individually. Another cost advantage of Cast Stone is lower freight cost since natural stone requires hauling long distances, quite possibly overseas. Cast Stone is a highly versatile, sustainable architectural precast building material which when compared natural stone Cast Stone provides a similar look, is stronger than, weathers better, has greater color consistency, can be reinforced, and cost less than natural cut stone.