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DESIGN TIPS—ALLOWANCE FOR MOVEMENT

 

The following are Design Tip recommendations for Allowances for Movement for Masonry and Cast Stone Material:

 

Reasons for Movement:

It is important to understand and consider the two most typical reasons for movement and dimensional changes most typically found with masonry & cast stone materials – Environmental (weather, temperature, moisture content) and Construction (building movement from adjacent building, surrounding masonry materials, foundation shrinkage and expansion, erosion, and shifting of soil).

 

Moisture and Masonry Materials:
Moisture plays a major component when it comes to the shrinkage and expansion of masonry materials. Materials will expand when moisture increases and shrink when the moisture decreases.

 

Freezing and Masonry Materials:
Generally, movement within a cast stone wall because of temperature changes is minimal.  Statistics (and reported within the Cast Stone Institute) demonstrate that a 100 feet long wall constructed during a 70°F temperature condition that is subjected to a freezing temperature condition of 0°F will lead to a shortening or shrinkage of approx .38 inch. (See Cast Stone Institute Bulletin #52 for further information.)

 

Warm Temperatures and Masonry Materials:
Areas that have extreme weather change, such as the Deep South (i.e. Texas with snow and ice in the winter months and 100°F plus days during the summer months), the hot desert areas, and Northern and mountainous snow areas, are prone to foundation movement because of the soil expansion and shrinkage throughout the seasons. Wall exposure and colors also impact the overall movement because of temperature changes. (Precautions after installation, such as watering the foundation during the summer months, can help control the shrinkage of soil under the foundation of a home thus minimizing the impact of the shift in the house that has been built using both Cast Stone, Clay Brick masonry as well as other construction materials.)

 

Shrinkage and Masonry Materials:
When concrete products age (including cast stone and mortar), they tend to lose their natural moisture content which results in shrinkage. Both mortar and masonry products, including cast stone material, are comprised of cement-based products (plus aggregates) and are susceptible to shrinkage. The amount of cement greatly influences the amount of shrinkage that should be expected since it is the cement that is bonding the components together. Therefore, it is very important to consider the overall unit shrinkage as it comprises the greater portion of the wall and preventive measures should be in place to compensate and control possible cracking.

 

Based upon its greater coverage of a wall, it is important to design the project from the start with the consideration that there will be movement from dry shrinkage. Maximum Linear Shrinkage Requirements of the ASTM C 1363, Section 5.7 indicates that Linear Shrinkage should not exceed 0.65% when tested in accordance to ASTM C 426.

 

Combined Effects of Thermal and Moisture Movement in Cast Stone:
Although temperature and moisture can impact Cast Stone pieces the overall combined effect of the thermal and moisture movement is often negligible. One should be aware that if they are working with a Cast Stone unit of 8 feet or more, statistics have shown that they may experience up to one–eighth inch or more in movement from expansion or contraction.

 

Types of Joints:
Control Joints are used in concrete masonry walls to limit cracking because of shrinkage. They are unbonded vertical separations built into the wall unit to reduce restraint and to permit longitudinal movement. They are typically used where tensile stress is likely to induce cracking.

 

Expansion Joints are commonly used in brick masonry walls where expansion and contraction movements, both vertically and horizontally, are more likely to occur from temperature changes, loading or other forces. This is important to keep in mind when butting cast stone material up to brick masonry material. (See below)

 

Recommendations when Cast Stone IS used with Clay Brick Products:
Although, this bulletin is designed to cover the movement affecting Cast Stone materials, note that clay brick units will experience irreversible expansion over time because of exposure to water and humid air. Clay Brick products will be the smallest when they are cooled after coming from the kiln. Throughout the first few weeks, they will experience the greatest increase in size resulting from moisture. They will continue to expand slowly over the next couple of years. When cast stone is used with clay materials, one must be aware of the movement that is likely to occur and take necessary measures and precautions to compensate for such.

 

When Cast Stone material is used as an accent piece within a clay brick wall unit, it is recommended that expansion joints for clay bricks be used.

 

When Cast Stone material, such as banding, is used in a continuous pattern, both the spacing for vertical expansion joints for clay brick and spacing for control joints for concrete masonry should be examined. Use the most strict requirement for that project portion consisting of the clay brick wall and the Cast Stone material.

 

The Brick Industry Association recommends placing a bond break between the clay brick and Cast Stone banding to accommodate differential movement that will occur. Additionally, flashing is often placed either directly above or below the banding pattern. It is not recommended that one uses the flashing in both the above and below portions of the banding unless proper mortar embedment of anchors in the veneer can be achieved.

 

Design Considerations for Cast Stone and Clay Brick Walls:
Cracking owed to movement of the masonry materials must be accounted for during early planning. ASTM C 1363 Standards provide for a dry shrinkage value up to 0.065% ±.  Therefore, there are a couple of design recommendations when following these guidelines:

DESIGN TIPS — ALLOWANCE FOR MOVEMENT (PDF)

 

The information within this and all our bulletins has been provided as a guideline and based upon statistical data and prior uses. We always suggest that you consult with your engineer, architect or contractor for the best design and use of cast stone for your project. Our design team is always available to answer any of your questions. We do not accept any liability from damages resulting from your interpretation of the data contained within.