Allowing for Movement of Masonry Materials
Building materials may experience dimensional changes and movement due to environmental conditions, such as temperature and moisture, or movement of adjacent building elements. If this movement is restrained, cracking may result. By accounting for movement in the wall design, cracking can be controlled. Movement joints are used to control and minimize cracking. There are two types of movement joints typically used in masonry construction; control joints and expansion joints.
Control joints are placed in concrete masonry walls to limit cracks due to shrinkage. Control joints are unbonded vertical separations built into a concrete masonry wall to reduce restraint and permit longitudinal movement. They are located where cracking is likely to occur due to excessive tensile stress. An expansion joint is typically used in brick masonry walls to provide means for expansion and contraction movements produced by temperature changes, loadings or other forces. Expansion joints allow for both expansion and contraction and may be vertical or horizontal.
Causes of Movement
Overall, the amount of movement due to temperature change in a cast stonewall is relatively small. For example, a wall constructed during 70°F (21°C) weather and subjected to a minimum temperature of 0°F (-18°C) results in a shortening of about 0.38 in. (9.7 mm) in a 100 foot (30.48 m) long wall using the 0.0000045in./in./°F (0.0000081 mm/mm/°C) coefficient.
Although mortar is also a cementitious product and does experience drying shrinkage, unit shrinkage has been shown to be the predominate indicator of the overall wall shrinkage principally due to the fact that it represents the largest portion of the wall. Therefore, the shrinkage properties of the unit alone are typically used to establish design criteria for crack control.
Maximum Linear Shrinkage Requirements for Cast Stone is specified in ASTM C 1363 Section 5.7 Linear Shrinkage shall not exceed 0.065% when tested in accordance ASTM C 426.
Cast Stone Units
Visible cracks exceeding 0.005 in. (1/200 in.) are regarded as 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 members can reduce the potential of 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 should be observed in most applications. However, in many cases shorter lengths may be advised. For example, bearing conditions, high wind loads, large lengths of banding 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 Cast Stone elements. Increases in temperature can cause Cast Stone pieces 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 8 ft. or more in length in any direction may experience up to 1/8 in. or more in expansion or contraction due to combined thermal and moisture movements.
Generally, the recommendations for concrete masonry seem appropriate for Cast Stone. Hairline cracks along mortar bed joints are not unusual in concrete masonry as both the mortar and the masonry units experience shrinkage, nor do they affect the integrity of the wall if properly designed. They can be minimized by keeping lengths of Cast Stone units to within the limits dictated by principles of masonry construction. Large cracks can be avoided by incorporating control joints and other recommended details. Cast Stone units that are to be wetted before installation must be wetted to achieve proper bond with the mortar and avoid cracking.
For Cast Stone units with linear dry shrinkage values up to 0.065% ±,