Masonry Magazine March 2003 Page. 48
Control JOINTS
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.
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.
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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
Table 1-
Empirical Crack Control Criteria for Above Grade Exposed Walls
Criteria:
use horizontal reinforcement at an
equivalent area of at least
0.025 in.//ft of height,
AND
place control joints at a spacing of
11/2 times the wall height, not to
exceed 25 ft
Examples:
horizontal joint reinforcement
(W1.7 or 9 gage) at 16 inches on center
* one #3 reinforcing bar at 48 in. oc.
* one #4 reinforcing bar at 96 in. o.c.
* one #5 reinforcing bar at 144 in. o.c
* control joints every 15 ft for a
10 ft high wall
* control joints every 221/2 ft for a
15 ft high wall
* control joints every 25 ft for walls
17 ft high and taller