All concrete products and many natural stones, under varying conditions of moisture and temperature, are frequently subject to crazing. A manufacturer careful in proportioning of designs and watchful of compaction techniques and curing methods will minimize the likelihood of crazing as a result of manufacturing causes.

Crazing has been a subject of concern for producers of concrete products for as many years as concrete has been in existence. The appearance of small cracks on the surface, especially when filled with dirt, can be alarming since most people will assume that the product has failed, thinking that the fissures are running through the entire cross section.

Crazing can be caused by any factor which causes surface tension in excess of interior tension. Manufacturing causes include inadequate or improper curing, a surface film richer in cement and fines than the body of the concrete and plastic shrinkage cracking. Crazing can also be caused by design and installation factors which cause unusually high amounts of vapor transmission, excessive wetting and drying or inadequate ventilation behind the Cast Stone. There is some evidence that atmospheric carbonization can cause crazing.

Common installation problems which can cause or enhance crazing include the use of through-wall flashing without adequate drainage or masonry bond, lack of sufficient weep holes, use of Cast Stone without ventilated wythe, use of Cast Stone below grade or at planter type areas without proper moisture barrier, failure of joint materials which allow water entry, the use of hard mortar joints where sealant joints should be used and lack of sufficient allowance for movement via control joints.

Since crazing is only on the surface, the visual attributes can usually be removed by washing the affected areas with a mild acid solution. Severe cases of crazing may require application of a siloxane sealer, following etching, to penetrate the cracks and to keep dirt from settling into the surface.

Manufacturers of Cast Stone who experience crazing should review their mix designs, as well as compaction and curing techniques with the Institute and pay particular attention to the design and installation details, which can cause crazing, during the shop drawing submission process. Design professionals should ensure that the wall section details provide adequate ventilation and drainage behind Cast Stone and above flashing. Sealant joints should be used in accordance with CSI specifications and wherever thermal movement is likely.

Architectural cast stone is a product which has been used for many decades in all types of climatic conditions. In order to reasonably assure the user that the cast stone being supplied by a particular producer is durable in freeze / thaw conditions, the cast stone manufacturer has two options. The first, and most common method, is to show the purchaser similar architectural cast stone products made from the same materials by the manufacturer which have been in service for many years. The second option is to subject samples of architectural cast stone to laboratory testing.

Recent research has shown that architectural cast stone, as well as other dry cast concrete products, can be evaluated for durability when subjected to a modified version of ASTM C 666, Procedure A - Test Method for Resistance of Concrete to Rapid Freezing and Thawing. This technical bulletin outlines the modifications to ASTM C 666, Procedure A which are necessary to properly judge cast stone durability performance.

Test cast stone using ASTM C 666, Procedure A, but evaluate the product based on cumulative percent weight loss and not its relative dynamic modulus of elasticity or durability factor as is described in ASTM 666. Research has shown that certain cast stone may have a high durability factor, but its outer surface may deteriorate badly. Therefore, cumulative percent weight loss is more representative of the aesthetic performance of cast stone.

After the cast stone is 14 days of age or older, wet saw three 3" x 4" x 16" (76 mm x 102 mm x 406 mm) beams from a single sample of cast stone to represent three specimens for a single test. One surface of each beam is to be from the exposed formed face of the sample, and the remaining sides shall be cut from the sample with saws. The allowable size tolerance of the specimens shall be ± 1/8 inch (3.2 mm).

Do not oven dry the beam specimens until all freeze / thaw cycles are completed.

Submerge each beam specimen in lime-saturated water at 73.4 ± 3° F (23 ± 1.6°C) at least 48 hours prior to beginning freeze / thaw cycling. Subject each beam to freezing and thawing as described in Method C 666, Procedure A. Inspect each specimen every 30 to 36 cycles and collect all spalled material caused by freeze / thaw cycling from each specimen individually to monitor weight loss during testing.

For each specimen, oven dry and weigh the spalled material until loss in mass is not more than 0.2% in two hours of drying. Record the data individually and cumulatively for each specimen throughout the test until 300 cycles are completed, or 10% of the specimen's estimated mass has been lost due to spalling, whichever occurs first.

Specimens for this test shall then be oven dried at a temperature of 212 to 230° F (100 to 110° C) until the loss in mass is not more than 0.1% in 24 hours of drying. They shall be removed from the oven and allowed to cool at room temperature for approximately 30 minutes before measuring final dry weight. The initial dry weight of each specimen is considered to be the final dry weight of the specimen plus the total dry weight of spalled material collected from the beam throughout the test.

Calculate the cumulative percent weight loss for each beam specimen as follows:

CPWL (Beam) % - [S/(S+B)] x 100 where:
CPWL (Beam) = Cumulative Percent Weight Loss,
S = Total Dry Weight of Spalled Material, and
B = Oven Dried Beam Weight at the end of the test.
Calculate the Cumulative Percent Weight Loss, CPWL, for the sample. The CPWL of the sample is the average CPWL (Beam) of the three specimens.
The CPWL shall be less than 5% after 300 freeze / thaw cycles.

Seismic Considerations

Seismic and wind load are the principal live loads which are considered in designing masonry walls. Seismic loads may be a major, minor or inconsiderable factor in wall design, depending on the geographical location of the structure. Seismic zones, ranging in severity of effects to be considered, from a low of 0 to a high of 4, are delineated on maps in all major building codes. All buildings in seismic zones 1 through 4 must be designed in accordance with the appropriate code provisions.

Cast Stone elements are usually set in place in or on the structure as trim or veneer and as such, they do not normally add to or detract from the seismic resistance of the structure. The two critical design considerations with Cast Stone use in seismic areas are:

1. Use an anchoring system which will ensure that the Cast Stone remains an integral part of the wall. (Refer to pages C21, C25, C27, C35, & C41 of the CSI Technical Manual.)

2. Use an anchor and jointing design which allows a maximum of wracking of the structure without stressing the Cast Stone units. (Refer to page C21 of the CSI Technical Manual.) With all stone to unit masonry joints caulked with elastomeric sealant over backer rod.

When Cast Stone is used as a structural lintels or as a complete ashlar veneer system anchored to the structure, design should be in accordance with local building codes, ACI 318 and ACI 533. (Refer to pages C37, C39, & C41 of the CSI Technical Manual.)

References on masonry design for seismic forces include the National Building Codes and:

1. Reinforced Masonry Engineering Handbook by James E. Amrheim, 4th edition 1988, published by Masonry Institute of America (ISBN 0-94-116-05-07) pages 31, 42, 44, 46, 61 and 188.

2. Masonry Structures - Behavior and Design by Drysdale, Hamid and Baker, 1994 published by Prentice Hall (ISBN 0-13-562-26) pages 76, 677, 688 and 728.

3. Reinforced Masonry Design by Scheider and Dickey, 1987 published by Prentice Hall (ISBN 0-13-771776-1 04) page 459.

4. National Concrete Masonry Association Tek Notes No. 109, 1979 and No. 109A, 1989.