Masonry Magazine September 1975 Page. 16
Prisms should be built under a mason contractor's supervision using the mason, materials, and workmanship used or to be used in the structure. In building the prisms, the consistency of the mortar and grout, the thickness and tooling of joints, and the moisture content of the units at time of laying should be the same as is used in the structure, insofar as possible.
All block in the prisms should be laid with the thicker end of the face shell up, as this provides a larger mortar-bedding area. Full mortar bedding (complete coverage of webs and face shells) is always used in solid masonry construction. By definition, masonry units that are only 75 percent solid are classified as solids. Hollow units with two plain ends (corner block) or prisms to be solidly filled with grout should have face shell bedding with the two end webs also buttered. Other hollow units should have only face shell bedding, unless the wall is laid up with full mortar bedding.
The prisms should be grouted only when the walls are to have all cores of the hollow units or the cavity space in two wythe construction filled with grout. Do not include any reinforcement in the specimen except for the metal ties necessary for bonding multi-wythe walls.
When prisms are to be grouted the mortar joints should cure 24 hours before pouring of the grout. Place the grout into the cavity in one layer to within 1 1/2 in. of the top of the prism. Rod the grout 25 times with a 1 x 2 in. wooden puddling stick to eliminate air bubbles. A mechanical vibrator should be used if wall grout is to be vibrated. Wait about 30 minutes to allow the masonry to absorb some of the excess water from the grout, then fill the cavity to the top with grout. Again rod or vibrate the grout 25 times and allow the stick or vibrator to penetrate about 2 to 3 in. into the underlying layer to reconsolidate the previous layer. Then level the top of the prism with a trowel. The strength of the grout and its cohesion to the block can affect the prism test results so care must be exercised in grouting the prisms.
Where prisms are made during construction, the masons should build them at the job site in a place where they will not be disturbed for 48 hours and where they will be subjected to air conditions similar to that in the masonry structure. Record the maximum and minimum temperatures as well as weather conditions during the field storage. After 48 hours, place a 3/4 in. plywood board, the same size as the prisms, at the top and bottom of the prisms and tightly wrap them with two wire ties or steel straps to keep the joints from being disturbed. The prisms should then be further protected or insulated against jarring so as not to break the bond between the units. The prisms should be transported to the laboratory under the direction of the testing laboratory personnel for additional curing.
Anchorage
Engineered design of load bearing masonry construction utilizes the diaphragm action of walls and floors to provide stability to the building. For this reason, it is very important that the inspector check to make sure that connections between intersecting walls and between walls and floors are carried out in accordance with the plans.
Chases
Electrical and mechanical trades must not be permitted to cut chases or recesses into the wall at their convenience. Unless designed for, this would create a plane of weakness. Chases should not be allowed unless shown on the plans; the location, spacing and depth of chases should be checked.
NATIONAL CONCRETE MASONRY ASSOCIATION
P.O. Box 9185, Rosslyn Station, Arlington, Virginia 22209