Q I am working on a residence that has raked mortar joints. Because of a change in the design, the first area was built with concave joints. Many voids in the head joints were exposed when the mortar in this area was ground out so that the wall could be tuckpointed with a raked joint to match the rest of the house. I have heard that not filling the head joints can lead to moisture problems.
What is the preferred way to correct the problem of incomplete head joints in the remainder of the house?
A Although poorly filled head joints are not desirable, this condition alone may not require repair. However, the combination of rake joints and poorly filled head joints can be a problem.
Water penetration in masonry walls is controlled by many factors. The first of these is the compaction of the tooled face of the joints. The joints that are well tooled with a concave jointer when the mortar is thumbprint hard are reasonably resistant to water penetration. If the amount of water that penetrates the wall is minimal, the presence of voids behind the tooled face is less significant.
Raked mortar joints, however, are not compacted and allow greater water penetration. A greater penetration rate in combination with multiple voids can allow excessive water to pass into and through the masonry, which can lead to efflorescence or freeze-thaw deterioration.
Raked joints, in addition to not being compacted, are often aggressively cleaned because of mortar smears on the exposed bed surfaces of the recessed joints. To remove these smears, they are often repeatedly cleaned with aggressive acid solutions, which can result in severely etched joints. Besides increasing water penetration, mortar joints etched with hydrochloric acid have elevated chloride ions. A greater chloride ion content in the mortar results in accelerated corrosion of embedded steel.
Because the mortar joints in this project were ground in order to create the appearance of raked joints, any compacted portions were removed and likely exposed more voids than would be discovered by acid cleaning. This type of cleaning also may have been used in combination with grinding to clean up the brick surfaces and to intentionally etch the surface of the joints to improve the appearance of the ground surface.
Before determining the repair, I recommend removing samples of the mortar and sending them to a laboratory to test for elevated chloride ion content. If the quantity of acid-soluble chlorides in the mortar exceeds 0.2% by weight of cement (0.06 by weight of mortar for Type N portland cement – lime mortar), and the walls are anchored to the backup with steel wall ties, the veneer may need to be removed and replaced.
If the chloride levels are not high, I recommend tuckpointing the joints. Some additional grinding likely will be needed to increase the depth to the joints to ¾ in. The joints should be filled in three ¼-in. deep layers, following the recommendations of the Brick Industry Association's Technical Note 7F.
In the first layer, all visible voids in the mortar joints should be filled. After this layer has stiffened to thumbprint hard, the next one should be installed, which typically takes only a few minutes.
By allowing each layer of mortar to stiffen to thumbprint hard before installing the next, the resulting back pressure allows each layer to be compacted. This process results in better compacted joints than would be provided by installing the joint in one layer. The final layer should be tooled with a concave jointer.
Making concrete corners
Q When making corners in a concrete masonry veneer wall, how do you maintain the running bond pattern at the corner if you are using 4-in. concrete masonry units?
The exposed face is only a nominal 4 in. rather than 8 in.; therefore, it does not work well with the typical running bond pattern.
A There are two basic approaches to form corners in 4-in. concrete masonry. The approach selected depends on the desired appearance and the availability of special units and the amount of time available to order special units.
Special corner units are available called “Return (or L) Corner Blocks.” These units have a 16-in. side and an 8-in. side, as shown in Illustration A. When they are used, the running bond pattern can be maintained.
If “L” corner blocks are not possible due to time constraints or availability in the area, corners can be created by cutting standard square end corner concrete masonry units to a length of 12 in., as shown in Illustration B.