Masonry Magazine February 1977 Page. 14
Bonding
Bonding-Bed joints should be full face shell in width and should not be laid too far in advance of laying the concrete masonry unit. If the mortar is spread too far ahead of actual laying of the block it will begin to dry out, stiffen, and reduce bond with the block. When leaks occur through the bed joint, it usually is due to incomplete bond between the mortar bed and the bottom of the block. Incomplete bond is caused in most cases by partial drying of the mortar prior to bedding of the unit due to spreading of the mortar too far in advance. To assist a more complete bond, the trowel should be moved upward from the bottom of the bed joint when striking off the excess mortar.
Head joints should be buttered one unit at a time and that unit then put in place immediately. The unit should be positioned by shoving it firmly in place with enough force to cause an extruded head joint. Then in striking off the extruded mortar, the trowel should be moved in the direction of the unit previously laid, again to assist in the formation of a more intimate bond between mortar and unit.
The masonry unit should never be moved or relocated after it has been laid and the excess mortar struck off. Any slight movement will break the bond between the block and the mortar and lead to a leaky wall.
The time at which the mortar joints are tooled is important to wall weathertightness. The commonly accepted and quoted procedure is to wait until the mortar is "thumb print hard" and then tool the joints. From a practical viewpoint this is not always feasible. The mason laying overhand from the floor of a loadbearing multi-story concrete masonry structure has access to the joints in the outer wythe for a limited period of time. Consequently, the mortar joints must be tooled as soon as sufficient water has left the mortar to allow tooling without bringing excessive paste to the surface.
Finally with regard to bonding, the mason should make certain all joints are filled with mortar, even those joints in composite or cavity wall construction that are hidden from view in the finished wall. Unfortunately there have been a few cases where hidden head joints in high rise cavity wall construction were left unfilled and the omission discovered later when the building displayed bad leakage. The need to completely fill all hidden mortar joints is especially true in the case of the vertical collar joint in a structural composite wall. Here the filled collar joint not only serves as a barrier to water penetration, but also transfers horizontal shear across the joint from one wythe to the other thereby greatly increasing the wall's resistance to wind forces.
Flashing
Flashing-Flashing should be installed exactly as shown on the plans, without being cut or punctured. When plans call for flashing and joint reinforcement or ties at the same elevation, the designer should be consulted. Chances are the steel can be relocated one course above or below the flashing so as to avoid puncturing it and spoiling it as a water barrier.
Mortar droppings should not be allowed to accumulate: (1) on flashing, (2) in weep holes, or (3) in the cavity space of a cavity wall. Droppings not only plug weep holes, but also provide a bridge for water to travel across a cavity thereby encouraging a leaky wall.
Wall Coatings
Wall Coatings Good workmanship in the application of wall coatings is of paramount importance to weathertightness of concrete masonry construction. This is especially true with the clear water repellants where effectiveness depends upon solid content. Dilution of clear coatings prior to application is difficult or impossible to detect until it rains; then it becomes obvious.
flashing
weep holes
24" О.С.
CMU lintel
FIGURE 4. Flashing Detail
NATIONAL CONCRETE MASONRY ASSOCIATION
P.O. Box 135, McLean, Virginia 22101