Cast stone is created to simulate all types of natural cut stone and is a Portland cement-based architectural precast product manufactured using high-quality fine and coarse aggregate as its primary constituents.
The earliest known use dates to about the year 1138, and can still be seen at Carcassonne, France, a city that contains some of the finest remains of early architecture in Europe. Cast stone was first used extensively in London beginning around 1900, and has gained widespread acceptance in America since the 1920s. Fortunately, since that time many lessons have been learned and can be used to improve the mason contractor's utilization of cast stone.
The use of a high percentage of durable fine aggregate in cast stone creates a very smooth, consistent texture for the building elements being cast, resembling natural limestone, brownstone, sandstone, marble or granite. Applications that use cast stone can range from the simplest window sill to the most complicated classical architecture. Therefore, the number of profiles and sizes required for any given project can vary from a single shape shown on a sketch to an unlimited quantity of shapes, perhaps not so clearly shown in a set of architectural contract documents.
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. Longer lengths are available but generally should not exceed 15 times the minimum cross section thickness. Control joint spacing should be divided evenly across the size of the typical unit. Special corner and end units should be cast to suit the end-of-wall condition and intermediate units may be cut with a standard abrasive blade masonry saw.
In many specifications today, architects are requiring layout drawings that call for the location of every joint on the building facade to be precisely located, while other designers desire standard products and consider custom products to be cost prohibitive. Significant cost savings of 40 percent or more can be achieved by using typical lengths and standard products, so they should be used whenever possible.
To avoid the need for layout drawings, try to submit the following procedures to the approving parties:
- Follow the architect's jointing pattern as shown on contract drawings.
- Cut units to suit in-place wall dimensions.
- All window and door surrounds should consist of evenly sized units.
Window sills should be sized to fit the masonry opening or mullion spacing, with allowance for 3/8" joints. The height of all cast stone built into masonry walls should match the brick coursing.
Standard cast stone jobs are most easily tailored to designs that call for basic and popular items such as band courses and wall cap coping, pier caps, keystones, quoins and window sill units sized to replace brick. Semi-custom projects can include almost any application where the designer is willing to specify the dimensions of the stone units on the contract documents. Some of the largest manufacturers have online catalogs with hundreds of shapes, such as can be found in the Continental Cast Stone catalog, www.caststone.net/designer. These catalogs allow designers and contractors to choose profiles, stretchers, and corner or end units while keeping track of linear footage.
Layout drawings are needed for projects that have many different profiles running through changing wall sections, one-of a-kind installations such as entrances, porticos and signs, or for large sections that would be difficult to cut. Base courses of stone at changing grade elevations, radius walls and applications suspended from structural concrete or steel are other necessary applications for custom cast stone.
When the installation requires layout drawings, they should be sent to the jobsite before the first cast stone delivery arrives. When more than one delivery is involved, each delivery slip should be used to mark up the layout drawings to be sure that the pieces are onsite in the desired setting sequence before proceeding with the work. The stones should be placed around the site in convenient locations to minimize handling as much as possible.
On larger custom projects, the mason contractor should request that the delivery be segmented by the producer according to phase, elevation, floor or other location relative to the structure. The segment number should be marked on the stone in an easily identifiable manner, which should be decided during the submittal phase of the project before the stone is even made. This helps to keep the stones palletized in logical setting sequence and makes it easy to tell the final location of the stone.
Onsite personnel should be familiar with the applicable sections of the Cast Stone Institute's specifications and the project specification pertaining to delivery, storage, setting, patching, cleaning, pointing and sealing. Where the project specification does not include a particular issue, the industry standards should be followed. The Cast Stone Institute publishes several technical papers that are useful to stone setters including Technical Bulletin #37 Job Site Handling and Installation, which is available on the Internet at www.caststone.org.
Upon delivery, all cast stone should be checked for chips, cracks, stains or broken pieces. Any damage should be noted on the delivery slips and communicated to the manufacturer or the sales representative.
Storage of cast stone should be above the ground on non-staining planks or pallets. The storage site should be away from heavy construction traffic. Cast stone stored for an extended period of time should be kept on pallets or non-staining planking and covered with non-staining tarpaulins; be sure to allow for air circulation.
Cast stone should be handled in a way to minimize chipping. Care must be taken to not bump the stone into anything. Handle stones with the wide portion of the cross section in the vertical position to minimize breakage. After setting, columns, pilasters, entry jambs, window sills and all stone with projecting profiles should be protected during the remaining construction.
Prior to setting, insure climatic conditions are within thermal limitations of the mortar. Cold weather setting practices are basically the same as the International Masonry Industry All Weather Council (IMIAC) recommends for unit masonry. Mortar retarders and accelerators should be used according to the manufacturer's directions, but not with patching material. Set stone in full mortar joints and fill all dowel holes and anchor slots completely with mortar. Be sure uniform joint widths are within specifications tolerances.
Ensure that all specified flashing and damp proofing is installed. Flashing pierced by stone anchors must be sealed either by metal thimble, grommet, or approved sealant. Flashing locations are the responsibility of the architect and should not be assumed by the mason under any circumstances.
Concrete should never be poured against unprotected cast stone. Where poured in place concrete is placed against cast stone sills, be sure to use the appropriate material to separate them prior to pouring concrete.
Before setting, guarantee that the surfaces set in mortar are drenched with water. This will secure a good bond and help to prevent mortar shrinkage. Weep holes must be installed over windows, at relieving angles and at the bottom of walls. No mortar drippings should be allowed in the wythe between the back of the stone and the face of back-up structure.
Quite often, it is not the mason's responsibility to fill all of the joints between stones. All head joints at coping and sills, and joints at column covers, soffits, and, in general, all stone sections with projecting profiles, exposed top joints or rigid suspension connections to the supporting structure should have sealant joints applied by the caulking contractor. This contractor should prime the joints, insert a properly sized backup rod and gun in the sealant.
Chipped cast stone must be patched as the setting proceeds because waiting too long can cause color problems with the patch. A test patch should be approved before general patching is to begin.
Planter coping, fountain coping, swimming pool coping, treads, risers, stone pieces at grade, and pavers should be treated with a silane or siloxane water repellent after setting. This will minimize the likelihood of dirt and groundwater staining the surface of the stone, a frequent cause of staining, efflorescence and enhancement of crazing. Test the water repellent on a non-visible area to make sure it does not affect color or texture when dry.
Load bearing units should be reinforced as necessary. Remember, they may be designed to be handled only in the same orientation as they will be installed in the structure. Lintels and large panels must be kept vertical or they will be prone to cracking.
Regardless of the degree of care exercised during construction, a final wash down will be needed and, normally, whatever is specified to clean the brickwork will adequately clean the cast stone. A variety of commercial cleaners are available and most contain detergents combined with mild solutions of phosphoric and/or muriatic acids.
Extreme care should be taken when applying acidic cleaners to areas where joints are left open or where sealant is used as jointing material. The sealant manufacturer should be contacted to be sure of compatibility with cleaning materials. Acids left behind the stone on masonry wythe may cause corrosion problems later on.
Cast stone is a time-tested material of enduring architectural beauty that, when properly selected and installed, should result in a project with infinite durability which in concrete terms means a time period exceeding 100 years. With proper attention to the details of specifications, workmanship and design, the construction team has the ability and opportunity to build monumental structures with cast stone at reasonable cost for use and enjoyment by generations to follow.
Bill Russell is the Chairman of ASTM Committee C 27.2 on architectural and structural concrete products and President of Continental Cast Stone East, by Russell, Inc., Berlin, N.J. He is a past president of the Cast Stone Institute and served as its Technical Director for 14 years.
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