Masonry Magazine December 2002 Page. 20
COVER STORY
Other stones
SEVERAL COMPANIES manufacture stone-like products that are used primarily as veneers on other substrate materials, such as concrete masonry units. These simulated stone products are manufactured to meet Construction Specifications Institute (CSI) Division 04730 requirements for simulated stone, cast stone is manufactured to meet Division 04720 requirements.
There are substantial differences between cast stone and simulated stone. Simulated stone is a lightweight product that is adhered to a structural wall. Therefore, simulated stone cannot be used to add to the load bearing capacity of a masonry wall. Cast stone, however, can be used to add to the load bearing capacity of a masonry wall and is usually integrated into the brickwork, becoming part of a composite wall system rather than being adhered to it. In addition, while simulated stone products are made light in weight so they will work easily on the exterior of other wall material, cast stone weighs approximately the same as natural cut limestone.
Cast stone has an ASTM Standard Specification, which is different than cultured or simulated stone products that usually refer to ASTM Test Methods. Test methods dictate how the materials are tested while the specification stipulates what the requirements of the test results must be, as well as the ingredients each product must contain. For example, cast stone is required to have a minimum compressive strength of 6,500 psi and maximum moisture absorption of six percent. It must pass a rigorous freeze-thaw test, the dimensions of each unit may not deviate by more than 1/8" from approved dimensions, and it is reinforced with billet steel reinforcing bars, unless otherwise specified, according to ASTM C 1364-Standard Specification for Architectural Cast Stone.
Estimating and ordering
THE ABILITY of the masonry contractor to estimate the number of pieces and shapes on bid day can be just as important as getting a price for the materials, since the degree of customization will have a significant impact on how many pieces can be set per day.
Improvements in the availability and economy of cast stone can almost universally be achieved when the number of special pieces is kept to a minimum. The indirect labor costs of layout, supervision and coordination with other trades that may be needed for a "full custom" job, where no field cutting is done can easily exceed the cost of working with typical modular lengths and simply cutting them to fit at the jobsite. This causes wide variances in the price of cast stone and places a premium on the product, which is not always necessary.
With that in mind, cast stone projects will generally fit into one of the following categories: standard, semi-custom or custom. As you will see, there are considerable cost differences among these categories.
Standard cast stone items are purchased according to a manufacturer's catalog, shop drawings, or inventory of molds. They may be ready-made items or made to order, the units are usually priced individually.
The mason contractor determines the quantities of each unit and the method of anchoring to the structure, if required. Cutting of units in the field is usually required, but this is often the most cost-effective way to work with pieces, such as bands, sills, coping and other types of trim.
Most types of cast stone are manufactured with dowel holes or slots that should be filled completely with mortar to receive non-corrosive anchors, usually two per stone. Stones which are three bricks (7-5/8") high or less can be set with brick ties in lieu of stone anchors.
Semi-custom cast stone is usually made to order either from custom or inventoried molds. The units may be priced either individually or as a lump sum contract for the project.
The cast stone manufacturer may want to prepare shop tickets for approval that show the quantities, cross section, and a schedule of lengths to be provided. The architect or general contractor should approve these tickets before production is allowed to begin. The cost of working this way is still considerably less than prefabricating all of the pieces for a custom fit.
Custom cast stone items are purchased strictly according to contract documents and according to shop drawings specially prepared to confirm the scope of the work. They are usually made to order from custom molds and the job is usually priced as a lump sum contract for the project, according to plans and specifications. Cutting of units in the field is usually not required with this type of installation.
The cast stone manufacturer prepares layout drawings for approval that show the details of stones, arrangement of joints, quantities, cross section, reinforcement, finished faces, anchoring methods, anchors, and the location of the units in the wall. The architect and general contractor should approve these drawings to confirm the cast stone manufacturer's interpretation.
The general contractor should also coordinate the interface of the cast stone setting with other trades.
Choosing the standard
WHEN THE ARCHITECT allows a standard or semi-custom design, which commits to a certain number of shapes and sizes, the approval process is significantly streamlined and delivery times are shortened. Standard shapes can usually be produced as soon as the color has been selected; semi-custom pieces can be made as soon as unit shapes are agreed upon. Full custom projects typically take six to eight weeks to begin delivering after the layout drawings have been approved and coordinated with the other trades.
The mason contractor will most likely prefer to receive a neat pallet of modular units that can be easily cut to fit the exact, as-built dimensions at the jobsite, rather than wait for special pieces to be fabricated and then sorting through them to find the piece that is needed.
For most installations, typical lengths will be the common unit, with 4'-0" modules (3'-11 5/8") as the most popular size.