CAST STONE ARCHITECTURAL ELEMENTS

Properly manufactured architectural cast stone can be as durable --
or ever more durable -- than fabricated natural stone.

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As with other construction materials, quality levels can vary widely in architectural cast stone. The lowest quality level is made from a soupy garden-variety concrete that is poured into molds. Elements cast from soupy concrete have lower strength than dry-tamped material, are prone to "bug holes" from trapped air, and are more properly termed "precast concrete."

DRY-CAST FORMULATIONS

True cast stone is a portland-cement-based concrete product made using high-quality coarse and fine aggregates, such as limestone, marble, granite, or quartz. Other ingredients such as pigments, pozzolans, and chemical admixtures may also be added. Admixtures can include plasticizers, air-entraining agents, water-reducers, etc. Using a high ratio of fine aggregate creates a smooth, dense, consistent surface for cast building elements, with a very low level of water absorption -- which makes them very durable even when exposed to frequent freeze-thaw cycles.

High-quality cast stone is usually produced with a low water-to-cement ratio. This "dry" mix is packed into molds using vibration under pressure, or air-driven tampers. Elements produced in this way are often called VDT (vibrant-dry-tamped) cast stone.

WET-CAST FORMULATIONS

Cast-stone products made by the wet-casting method are required to meet the same specifications as VDT-cast products. Air-entraining agents, for example, can increase the durability of wet-cast concrete: having around 5% entrained air substantially increases the ability of wet-cast concrete to resist freeze-thaw cycles and meet ASTM standards. Other types of admixtures, such as water-reducers, super plasticizers, and self-compacting additives can also increase the strength and durability of wet-cast concrete so that it approaches the performance level of VDT-cast elements.

CONTROLLING CAST-STONE QUALITY

Both careful specifications and care in production are essential to creating strong, durable, good-looking cast-stone elements. Cast stone manufactured to high standards will have such characteristics as: (a) Hard, razor-sharp edges; (b) A fine-grained surface like natural stone; (c) No visible air holes. A cast-stone producer should conduct regular quality-control tests on the consistency of the raw materials (e.g. the aggregates), the manufacturing process, and the final product. The prudent client will take steps to make sure that the manufacturer performs adequate quality control testing and that the product meets ASTM standards. Alternately, or in addition, the client might also conduct its own random on-site testing and inspection at the plant to ensure that all specifications and quality standards are being met.

THERE'S MORE . . .

The accompanying list of cast-stone suppliers has been culled from recent issues of Traditional Building magazine. Each issue of TRADITIONAL BUILDING magazine contains 16 or more such reports covering products and services used in restoration, renovation, and historically inspired new construction. If you would like to subscribe, please visit our subscription page.

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