Masonry Magazine March 2003 Page. 50
Control JOINTS
of 0.00025 in/in is suggested. Similarly, a value of 0.000004 in/in/"F 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.
Table 2-
Engineered Crack Control Criteria for Above Grade Exposed Walls.
| | Crack Control | Coefficient, in/in. |
| :---------------------------------------------------------------- | :------------ | :------------------ |
| max length between control joints | 25 feet | 20 feet |
| max length/height ratio for wall panels | 21/2 | 2 |
| min horizontal reinforcement ratio (steel area/net cross-sectional area of masonry) | | 0.0007 |
| | 0.0010 | 0.0015 |
* Control joint spacing may be adjusted based on local experience.
* CCCs less than 0.0010 may be available, and table values may be adjusted accordingly.
* When masonry units are wet, consider using the higher CCC value.
* Walls less than half the maximum length shown are not required to have horizontal reinforcement.
Table 3-
Maximum Horizontal Reinforcement Spacing for Walls Without Control Joints in High Seismic Areas*
| Wall thickness, inches | Maximum horizontal reinforcement spacing for reinforcing bar sizes: |
| :--------------------- | :----------------------------------------------------------------- |
| | #6 | #5 | #4 |
| 6 | 48 in | 40 in | 32 in |
| 8 | 48 in | 32 in | 16 in |
| 10 | 48 in | 24 in | 16 in |
| 12 | 48 in | 24 in | 8 in |
* for partially grouted walls, closer spacings apply to fully grouted walls
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48 Masonry
March 2003
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