Masonry Magazine January 1966 Page. 39
NCMA TEK 3
THE NATIONAL CONCRETE MASONRY ASSOCIATION
An Information series from National Concrete Masonry Association
Control of wall movement with Q BLOCK, masonry
Excessive stresses and cracking of walls, floors, roofs and other sections can be the result of differential or restrained movement(s) of building elements.
When such cracking occurs in concrete masonry walls it generally is due to tensile stresses which develop when wall movements accompanying temperature and moisture change are restrained by other elements of the building, or when concrete masonry places restraint on the movements of adjoining elements.
There are three methods of controlling cracking in masonry structures:
1. Product specifications which limit moisture movement.
2. Steel reinforcement which increases crack resistance.
3. Control joints and other devices which accommodate movement.
With proper design, one or a combination of these three controls will eliminate cracking in concrete masonry construction.
Thermal coefficients depend largely upon material type. While temperature movement is of concern to design, thermal coefficients for building materials usually are not included in product specifications. This omission from specifications is reasonable when considering the major role of temperature change (generally an uncontrollable factor) in thermal movements.
Unlike thermal movement, moisture expansion and contraction is controlled by two properties of the masonry unit: (1) Potential shrinkage when dried from a saturated condition and (2) actual moisture loss or regain.
Shrinkage potential is determined by laboratory test. It is defined as the linear shrinkage, expressed as percent of length, of a concrete block during drying from a saturated condition to equilibrium weight and length with air at 122° F and 17 percent relative humidity. The test is difficult and expensive, and not suitable for product specifications. Fortunately, shrinkage potential of concrete masonry from a given source remains fairly constant. It depends almost entirely on raw materials and manufacturing techniques, and is unaffected by day-to-day variations.
For these reasons shrinkage limits are not included in product specifications, Moisture movement is controlled in specifications by limiting "moisture loss".
Moisture loss depends upon: (1) The initial moisture content of concrete masonry. (2) The temperature and relative humidity of drying atmosphere. Research has given us the general relationship between drying condition, moisture loss, and resultant shrinkage of concrete masonry. This relationship, together with field study, made it possible to establish specification limits on moisture content, limits which vary with:
1. Shrinkage potential of the masonry units, and
2. Expected drying conditions, expressed in terms of relative humidity.
These limits on moisture are incorporated in "Q BLOCK Specifications", the product standard of the industry's quality control program. Moisture limits for Type "I" Q BLOCK ensure minimum moisture movement. MOISTURE CONTROLLED TYPE "I" Q BLOCK CONCRETE MASONRY UNITS ARE RECOMMENDED FOR THE CONTROL OF WALL CRACKING.
product specifications
Changes in moisture contents and temperature cause expansion and contraction of masonry. In both instances, the magnitude of volume change depends upon two factors, one an intrinsic property of the masonry unit and the other a changing condition of temperature fluctuation and moisture loss.
tensile resistance
Horizontal steel reinforcement increases the resistance of masonry to cracking. Reinforcement may be incorporated as bond beams or as horizontal joint reinforcement. Its value in crack control is well established.
These recommendations apply to plain or non-reinforced concrete masonry in which reinforcement is employed primarily for crack-control-as opposed to reinforced concrete masonry in which steel is used to increase resistance to externally-applied forces. Reinforced concrete masonry normally contains sufficient steel that wall cracking is not a problem.