boffintech -
I think what your are concerned with is the bond between the mortar and the joint reinforcement wires.
The cover for protection from the weather/earth is similar in concept to that required for coverage for steel in poured concrete.
Joint reinforcement is not structural, except in cases where codes allow it to be used to make a stack bond wall act like a running bond wall.
Frankly, crack control recommendations and requirements are not really scientifically established, but are based on historic performance of masonry where joint reinforcement was used. In some areas/applications it is not needed or required. The placement and mortaring process you described is entirely accurate. It is obvious that there is not 100% uniformity of the mortar/steel bond when masonry units are initially laid. Due to the block laying process (unit movement, vibrations, and the vertical loads of other units above), the bond is improved to the level reflected in he historical performance, which has dictated the current requirements.
Crack control is the subject that can be easy to get lost in the variables, theories and possible moisture and temperature conditions. The bottom line is that since few engineers can afford to take the time to to analyze the specific combination of conditions, it is more practical to set a standard for the worst conditions since it is convenient and not cost prohibitive.
Originally, the crack control criterial was set to match the shrinkage results from the old ASTM 426. More recently, a more practical approach was taken to simply the design and requirements. This encompasses ALL the factors that can affect the shrinkage and expansion of a wall.
Crack control/shrinkage of a wall is a function of many factors:
1. The potential curing shrinkage of the individual masonry units.
2. The temperature differential of the wall between as-laid conditions and in-service conditions.
3. The moisture conditions of the wall between as-laid conditions and in-service conditions.
4. The curing shrinkage of the mortar.
5. The carbonation shrinkage of the concrete units and the mortar.
6. The long term expansion of clay units'
7. The differential temperature/moisture reaction between the wyths of a wall composed of different materials (brick & block).
8. If you really want to confuse matters, add in the effect of grouting, bond beam filling and wall exposure to moisture and temperature during construction.
As you see, the factors provide a wide range of results depending on the conditions that a portion/wythe of the wall is subjected too.
On one extreme, you have a wall built with very dry low shrinkage units at a temperature of 80 to 100 degrees and never exposed to a temperature below 70 degrees (interior cavity wall where joint reinforcement is not really needed). The other extreme is the high shrinkage units laid when at the upper permitted moisture limts at a temperature of 80 to 100 degrees and exposed when fully dried to temperatures of -20 degrees (single wythe wall in a cold climate). Because of the crack problems observed, the most critical conditions set the standard.
In the end, the masonry engineering, code and standards organizations decided to look at the performance of conventionally built walls when setting the requirements for joint reinforcement, since an excessive of reinforcement amount posed no downside performance and guanteed performance at least equal to well performing structures.
P.S. - it took many, many years to get a proper standard since it also required modifying material standards also.
Dick
