Many designers use concrete masonry based on masonry's rich history, unique design flexibility, and proven durability to ensure a beautiful and sound structure for generations. Aspects such as the structural design, aesthetics and energy efficiency are all important considerations influencing the ultimate success of the project. An additional consideration that should not be overlooked is the control of shrinkage cracking, which can detract from the visual continuity of the design and could potentially lead to water penetration problems.
Careful structural design and detailing will prevent the majority of wall cracks those due to excessive deflection, structural overload and settlement, for example. Shrinkage cracks, however, are not structural they will not weaken the building but they can be unsightly and can lead to water penetration in exterior walls. Because they are strictly an aesthetic concern, efforts to control shrinkage cracking are typically applied to exposed above grade walls. Attention to the following guidelines will help keep shrinkage cracks to a minimum.
Causes and Prevention of Shrinkage Cracks in Concrete Masonry
Shrinkage cracks occur to alleviate internal wall stresses. Over time, concrete materials have a natural tendency to shrink and to move in response to changing temperature and moisture conditions. These movements tend to decrease the length of the wall. The degree of movement is small, typically less than an inch in a one-hundred-foot long wall, vertical cracks form at regular spacings to accommodate this movement if the ends of the wall are restrained from moving. The cracked wall then acts as a series of separate panels, each with the ability to move slightly without impacting the next panel.
To prevent shrinkage cracks, stresses within the wall must be reduced through the use of control joints. Conversely, horizontal reinforcement can be used to distribute the stresses, which results in more, but smaller, cracks held tightly together.
Control joints are planned vertical wall separations. They basically divide a wall into separate panels, similar to what happens naturally after shrinkage cracks occur.
There are several common methods to incorporate control joints (see Figure 1), and all of them share certain characteristics. First and foremost, the control joint provides a vertical bond break within the wall, which allows longitudinal movement. This is typically accomplished by eliminating mortar between adjacent units or using building paper to break the bond if units are grouted (detail D). Next, if the wall is subject to lateral loads, the joint should allow some out-of-plane load transfer under wind or other lateral loads. This can occur by using a preformed joint filler (detail A), filling the space between "flanged" units with grout and providing a bond breaker to allow longitudinal movement (detail D), by using smooth dowels with a bond breaker of grease or plastic sleeve across the joint (detail E), or by using special control block units (detail F). In addition, for exterior walls, the joints must be weather-tight. Using backer rod and sealant in the mortar joint spaces accomplishes this.
Nonstructural reinforcement, such as horizontal joint reinforcement used to control shrinkage cracking, should be discontinued at the control joint. Any reinforcement crossing the joint effectively restrains the wall and may negate the benefit of the joint. Structural horizontal reinforcement, such as bond beam reinforcement at floor and roof diaphragms, however, may be required to be continuous across the control joint to maintain the structural integrity of the wall.
Control Joint Location
For walls that will include control joints, they should be placed where stress concentrations (and, therefore, cracks) are expected, such as:
For walls without openings or other stress concentration points, control joint spacing has traditionally relied on specifying low moisture content units and on using various combinations of horizontal reinforcement. Prior to the 2000 edition of ASTM C 90 Standard Specifications for Loadbearing Concrete Masonry Units, low moisture content was specified by requiring a Type I moisture controlled unit. The intent was to provide designers an assurance of units with lower moisture content to minimize potential shrinkage cracking. However, there are several limitations to relying on moisture content alone since there are other factors that influence shrinkage which are not accounted for by specifying a Type I unit, as will be discussed later. Additionally, Type I units were not always inventoried by concrete masonry manufacturers. Most importantly, Type I units needed to be kept protected until placed in the wall, which was proven to be difficult on some projects. Because of the above problems associated with the Type I specification, ASTM removed the designations of Type I, Moisture-Controlled Units and Type II, Nonmoisture Controlled Units from the standard.
- at changes in wall height or thickness (including at pilasters);
- at movement joints in adjacent foundations, floors, and roofs;
- near door and window openings if adequate vertical and horizontal reinforcement is not provided around the opening (at one side of an opening less than six feet wide, and at both sides of openings wider than six feet); and
- near corners and intersecting walls (locate control joint within half the typical control joint spacing for the wall).
Due to removal of the unit type designations from ASTM C90, two methods of determining control joint spacings have been devised irrespective of unit type. The empirical method is the most commonly used method and is applicable to most conventional building types. This method is a "rule of thumb" approach based on a successful history of experience over a broad geographic area and was recently revised by the concrete masonry industry to reflect a larger database of in-field performance.
The alternative engineered approach is a new method, developed to provide an analytical approach, which can give designers a greater level of confidence in mitigating shrinkage cracks. The engineered method is generally used only when unusual conditions are encountered such as dark colored units in climates with large temperature swings.
Empirical Method for Crack Control
The empirical criteria for locating control joints were recently revised by the concrete masonry industry. Where previous criteria applied only to Type I (moisture-controlled) units, the current criteria apply to all concrete masonry units. The empirical approach uses a combination of horizontal reinforcement to keep unplanned cracks closed, and control joints to allow unrestrained movement, as detailed in Table 1.
Because this criterion is based on a very broad range of material types and geographic locations, there are situations where these requirements may seem too conservative. Local experience may justify increasing, or decreasing, the control joint spacings presented above. However, the concrete masonry industry recommends 25 feet as a maximum control joint spacing unless the engineered method is used.
Alternative Engineered Method for Crack Control
The newly-developed engineered crack control method is intended to provide more effective crack control than the empirical, but can also require additional design effort.
A Crack Control Coefficient (CCC) is used to determine combinations of horizontal reinforcement and spacing of control joints. The CCC is an estimate of expected wall movement due to three masonry unit characteristics: total linear drying shrinkage, measured in a laboratory; carbonation shrinkage (a chemical reaction between cement and carbon dioxide in the air) and the unit's coefficient of thermal expansion. Since there currently is no ASTM test to determine carbonation shrinkage, an average value of 0.00025 in/in is suggested. Similarly, a value of 0.000004 in/in/šF is recommended for the thermal coefficient by the Building Code Requirements for Masonry Structures (ACI 530-02/ASCE 5-02/TMS 402-02). These three values are summed to determine the Crack Control Coefficient. Higher CCC values indicate more shrinkage is expected, so tighter crack control measures should be employed. Table 2 summarizes these requirements. As for the empirical criteria, these requirements can be adjusted based on local experience.
The engineered method also recognizes the need for not cutting horizontal reinforcement in high seismic areas and the fact that walls with substantial horizontal reinforcement may not need control joints to effectively control shrinkage cracking in the masonry. For example, horizontal reinforcement spacings up to four feet have effectively controlled cracking when standard reinforcing bar sizes are used. Accordingly, walls with minimum horizontal reinforcement areas of 0.002 times the net cross-sectional area of the masonry are not required to have control joints. Theoretically, this will limit crack widths to a maximum of 0.02 inches an unobtrusive size that would limit water entry if surface water repellent treatments are used. Reinforcement requirements for these walls are presented in Table 3.
This is a brief overview of the industry empirical and the alternative engineered crack control methods. For more detailed information consult the following publications of the National Concrete Masonry Association: NCMA TEK 10-2B, Control Joints for Concrete Masonry Walls - Empirical Method and TEK 10-3 Control Joints for Concrete Masonry Walls - Alternative Engineered Method. These TEK may be viewed free online at sponsoring NCMA member web sites. A listing of these members and hot-links to their sites can be found at www.ncma.org.
Designers now have several options to minimize shrinkage cracking in concrete masonry walls. Horizontal reinforcement can be used with or without control joints to help ensure the beauty of the concrete masonry project remains unblemished.
Maribeth Bradfield, P.E., is a consulting engineer practicing in Arlington, Virginia. She can be reached by e-mail at mbradfield@ tidalwave.net.
This article, from Concrete Masonry Designs March 1999 edition, is reprinted with permission by the National Concrete Masonry Association (NCMA), Herndon, Va. The article has been slightly modified and updated from its original form by NCMA.
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