Masonry Magazine October 1996 Page. 12
Brick and Block Units
This Section restricts the wetting of concrete masonry units at the time of laying. Concrete block expand with moisture, therefore, laying wet units will aggravate cracking in masonry by shrinkage of the units during the curing process. Shrinkage of any Portland cement product is inevitable and care must be taken to minimize the effects of cracking.
On the other hand, clay brick expands over a long lifespan. Clay masonry units with high absorption rates rapidly dry the fresh mortar which impairs the bond between the mortar and clay unit. Wetting of brick units to accommodate this situation is desirable.
As a general rule of thumb, a rough, but effective, field test for a rate of absorption is to place a quarter on a brick and trace a circle with a pencil. Draw a circle around the pencil mark with a wax crayon. With a medicine dropper, quickly drop water within the circle until it is full, using care not to overfill the circle, until 20 drops have been placed. The brick will absorb water. Note the time for all of the 20 drops to be absorbed into the brick, beginning with the time the circle is first filled. If the time is less than 1 1/2 minutes, the brick should be pre-wetted. If more than 1 1/2 minutes, the brick do not need pre-wetting.
Portland Cement
Often overlooked is the type of cement allowed in various regions. Many small masonry contractors use plastic cement for masonry, which is not recognized by the Code. Masonry cement is popular, but it is restricted to Seismic Performance Categories A, B and C. Mortar cement is acceptable for all conditions, and the traditional Portland cement-lime mortar is acceptable in all regions for the proper type of mortar. There are limitations on the use of Type N mortar in Seismic Performance Categories D and E. The contractor should be aware of the acceptable cement materials for the project. Using plastic cement may 'get the job done' without using lime, but it is not recognized as an acceptable material for structural masonry. Masonry cement may be acceptable for a Seismic Performance Category C project on one side of the street, but unacceptable for a higher Seismic Performance Category D project on the other side of the street. Manufacturers are now blending mortar at the factory, and it can now be shipped to the jobsite in sack or bulk. Typically, these pre-blended mortars are Portland cement-lime based, with the sand added. The material is marked with the mortar type (M, S or N) and the contractor adds water to make the mortar workable. The availability of pre-blended mortar should go a long way in eliminating the use of plastic cement for masonry.
Construction Tolerances
Perhaps the hottest topic of industry standards for quality are the allowable dimension tolerances for masonry construction. There are at least two different methods for quality consideration of installed masonry.
The Commentary of ACI 530.1 states that the dimension tolerances given in the section are based on structural performance, not aesthetics. The Architect or Owner may use the dimensional tolerances for acceptance with regard to aesthetics even though the values are based on structural design. This is a practical approach since the actual dimension variations can be measured, and then be subjectively applied to acceptable tolerance parameters. Since most masonry is designed in 4 inch or 8 inch modules, and unit manufacturing processes are slightly imperfect, it is the work of the mason to constantly adjust the mortar joint thickness to maintain the desired vertical and horizontal bonds.
Bed joints are to be + 1/8 inch from the desired bed joint, that is, for concrete block masonry, with a typical mortar joint of 3/8 inch, the joint size may range from 1/4 inch to 1/2 inch (3/8" + 1/8 "). For a brick with an actual height of 2 1/4 inches, an 8 inch bond can be achieved with 53/128 inch mortar joints. One cannot measure, much less construct, such a precise dimension. A 3/8 inch joint will yield an actual bond of about 7.9 inches, and the balance should be left to the bricklayer to adjust. In this particular application, mortar joints between 1/4 inch and 1/2 inch are readily acceptable.
As with concrete, the permissible variation from level for bed joints is + 1/4 inch in 10 feet and + 1/2 inch maximum.
Head joints may vary from minus 1/4 inch from the specified dimension to plus 3/8 inch, which means head joints will vary from virtually nothing to as large as 3/4 inch. 3/8" +³/s" or½" Allowable tolerances for variation from plumb and laying true to line are + 1/4 inch in 10 feet, + 3/8 inch in 20 feet and + 1/2 inch maximum.