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Maximum Length Considerations For Cast Stone: Design Tips – Technical Bulletin #53

Designers specifying Cast Stone often require guidance on the limitations of the possible sizes of Cast Stone elements. This Technical Bulletin addresses the many factors that must be considered in determining Cast Stone sizes including structural and physical properties of the material, manufacturing limitations, weight, transportation and handling, cost, and dimensional tolerances. By examining these factors, the designer in cooperation with Continental Cast Stone can develop details using sizes that are aesthetically pleasing, functional, and economical.

Structural and physical properties

The structural and physical properties of the Cast Stone as established by ASTM C 1364 are of prime importance in determining the likelihood of cracking and movement of a Cast Stone piece. While Cast Stone is a concrete product, appearance is much more important than for structural concrete or architectural precast concrete. In the case of Cast Stone, small hairline cracks are not aesthetically acceptable. The Cast Stone Institute states that visible cracks exceeding 0.005 in. (1/200 in.) are deficiencies in high quality Cast Stone installations. This is a much higher standard than is found in architectural concrete work. This has structural implications, as the structural stress limit of Cast Stone must be less than the modulus of rupture for the material to avoid any occurrence of cracking.

When considering non-structural pieces that do not carry any loads other than their own self-weight and transfer wind loads, limiting the length of the Cast Stone member can prevent cracking. A general rule of limiting the length of a Cast Stone trim element to no more than 15 times the least cross-sectional dimension is reasonable in most non-structural applications. However, in many cases shorter lengths may be advised. For example, bearing conditions, high wind loads, large quantities and unusual shapes are all factors that affect the structural stress and cracking potential, but vary from job to job.

Temperature and moisture changes can cause changes in the size of a Cast Stone element. Increases in temperature can cause Cast Stone piece to elongate. Decreases in temperature can have the opposite effect. Similarly, changes in the moisture content of the Cast Stone will affect its size. The magnitude of these physical properties depends in part upon the size of the member. The combined effects of thermal and moisture movements in Cast Stone elements and panels are often negligible. However, Cast Stone units 12 ft. or more in length in any direction may experience up to 3/16 in. or more in expansion or contraction due to combined thermal and moisture movements. Movements of this magnitude must be accommodated in the design and detailing of the Cast Stone. Expansion or control joints may be necessary. For this reason, Continental Cast Stone recommends limiting the maximum dimension of any Cast Stone piece to less than 8 ft. unless care is given to accommodate the possible expansion and contraction of the stone.

Manufacturing limitations

The manufacturing process may in some cases, limit the ultimate size of a Cast Stone piece. Continental Cast Stone recommends the following table as a guideline:

Minimum Thickness  Maximum Length
2-1/2″ 30″ 
3″ 45″ to 48″
4″ 60″ to 72″
5″ 75″ 
6″ 90″ 
weight limitations

Weight can also be a consideration during installation. Elements required to be hand-set should not exceed 250 pounds unless slings are used. Such limitations on size are common in restoration work where equipment access is limited. The weight of Cast Stone can be approximated as 144 to 150 pounds per cubic foot.

transportation and installation

Transportation and job-site installation issues may also limit sizes of Cast Stone pieces. Longer pieces typically require more care during transport than shorter ones of similar cross section. Similarly, thinner pieces require more care than stockier ones. Pallets used for shipping are typically 48″ wide and job-site forklifts typically are about 48″ long.

Cost

Repetition of Cast Stone pieces is the best way to achieve an economical design. In addition, limiting lengths of Cast Stone to those most commonly produced, 48″ – 60″ modules, minimizes the extra costs associated with special handling, custom forming, breakage, and transportation of longer pieces. Consult Continental Cast Stone for further information.

dimensional tolerances

Dimensional tolerances for Cast Stone are specified in ASTM Specification C 1364-97 for Architectural Cast Stone. Cross-sectional dimensions must not deviate from the approved dimensions by more than +/- 1/8 in. The length of a unit cannot deviate more than length/360 or +/- 1/8 in., whichever is greater, but not more than +/- 1/4 in. For units between 45 and 90 inches in length, the maximum tolerance is length dependent.

Cast Stone pieces must also conform to bowing or warping tolerances. The industry standard requirement is that bowing or warping cannot exceed length/360. This limit usually has no maximum tolerance, and this should be kept in mind when considering long pieces.

recommendations

While other sizes are possible, these recommendations provide general guidance on optimum design of Continental Cast Stone panels and elements.

1.  Specify panel and elements of at least 2-1/2 inches in thickness. Three inches or more is preferred to use standard stone anchors.
2.  Limit lengths to no more than 15 times the least cross-sectional dimension, unless specific project conditions are examined.
3.  Keep typical stone lengths to within 48″, with as much repetition as possible.
4.  Limit lengths to less than 8 ft unless provisions are made to accommodate possible thermal and moisture expansion and contraction.
5.  Recognize dimensional tolerances for longer pieces. 
6.  Weight of elements required to be hand-set should not exceed 250 pounds.

The information and suggestions contained in this Technical Bulletin are based on the available data and the experience of the staff of Continental Cast Stone. The information contained herein must be used in conjunction with good technical judgment and a basic understanding of the properties of architectural Cast Stone.  Final decisions on the use of the information contained in this Technical Bulletin are not within the purview of the Continental Cast Stone and must rest with the project architect, engineer and owner.


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