Figure 8-14.-Installing a closure block.

important. First, tip it slightly toward you so that you can see the edge of the course below. Then place the lower edge of the new block directly on the edges of the block below (figure 8-13). Make all position adjustments while the mortar is soft and plastic. Any adjustments you make after the mortar stiffens will break the mortar bond and allow water to penetrate. Level each block and align it to the mason's line by tapping it lightly with a trowel handle.

Fifth and last, before installing the closure block, butter both edges of the opening and all four vertical edges of the closure block with mortar. Then, lower the closure block carefully into place (figure 8-14). If

Figure 8-15.-Cutting off excess mortar from the joints.

any mortar falls out, leaving an open joint, remove the block and repeat the procedure.

To assure a good bond, do not spread mortar too far ahead when actually laying blocks. If you do, the mortar will stiffen and lose its plasticity. The recommended width of mortar joints for concrete masonry units is 3/8 inch. When properly made, these joints produce a weathertight, neat, and durable concrete masonry wall. As you lay each block, cut off excess mortar from the joints using a trowel (figure 8-15) and throw it back on the mortar board to rework into the fresh mortar. Do not, however, rework any mortar dropped on the scaffold or floor.

Weathertight joints and the neat appearance of concrete masonry walls depend on proper striking (tooling). After laying a section of the wall, tool the mortar joint when the mortar becomes "thumb print" hard. Tooling compacts the mortar and forces it tightly against the masonry on each side of the joint. Use either concave or V-shaped tooling on all joints (figure 8-16). Tool horizontal joints (figure 8-17, view 1) with a long jointer first, followed by tooling the vertical joints (view 2). Trim off mortar burrs from the tooling flush with the wall face using a trowel, soft bristle brush, or by rubbing with a burlap bag.

Figure 8-16.-Tooled mortar joints for weathertight exterior walls.