Q: Our company was asked to tuckpoint the top half of a 20-ft masonry wall. The mortar joints contained multiple cracks because of freeze-thaw deterioration. Water had been getting in at the top of the wall and freezing over the years because of some very poor coping/flashing details.
When getting ready to repoint, we noticed that the top of the wall appeared to have expanded about 3/8-in. relative to the roof. I have heard that brick expands over time; however, this amount seemed excessive. Also, the expansion appeared to be only occurring in areas with severe freeze-thaw damage.
What is going on?
A: Brick masonry does undergo permanent growth due to moisture expansion. However, as you pointed out, a 3/8-in. expansion would be excessive in a 20-ft wall. In fact, I would expect the expansion to be no more than 1/10 in. to 1/8 in. in this distance.
It is much more likely that the wall expanded due to the freezing of saturated mortar. When the mortar deteriorates as a result of freezing, multiple cracks develop, as shown in Fig. 1. As cracks form, small mortar fragments within them prevent the opening from closing completely when the masonry thaws. Since the masonry does not return to its original position, the presence of many small cracks results in a permanent expansion of the mortar in each of the joints.
The amount of expansion is essentially the sum of the width of all of the cracks in the mortar joints. I have seen badly deteriorated mortar joints with freeze-thaw cycles cause walls to raise roughly ½ in. in 10 ft, as shown in Fig. 2.
If the masonry has risen, it is likely that the deterioration is not just at the surface, but in fact extends through the thickness of the masonry. Tuck-pointing alone is not sufficient when this is the case.
If tuckpointing is performed only on the outer ¾ in., the remainder of the mortar joint is still unstable. As a result, the compressive stresses will be concentrated on the exterior face at the new tuckpointing mortar. Over time, the tuckpointed joint breaks free and falls from the wall, or can result in spalling of the brick face above or below the tuckpointed joints.
The best approach in these cases is to remove the masonry and rebuild it. Depending on the extent of the frost damaged mortar, it may be possible to point the wall to a depth of 2 in. or greater, which should provide adequate support for the masonry.
Coating concrete masonry
Q: We have a problem with a recently completed warehouse where all the cells in the single wythe concrete masonry walls are grouted solid. Wet spots appear in the masonry walls following heavy rains. It appears that water is able to leak through the wall at the webs between the grouted cells, which created a vertical line on the interior.
What can be done to correct this problem?
A: Single wythe concrete masonry walls are not particularly good at resisting wind-driven rains. Whereas the fully grouted cells are fairly resistant to water absorption and penetration, the webs and mortar joints between the grouted cells are not. If the wall does not contain an integral water repellant, the concrete masonry wicks moisture through it, resulting in even faster water transfer to the interior.
The only way to make this wall system resist rainwater penetration without rebuilding or over-cladding is to coat the concrete masonry to create a barrier on the exterior surface. There are many coating systems available. Most elastomeric systems have good elongation properties which allow them to bridge small cracks and voids without cracking. The application of a coating, however, changes the appearance of the wall.
A clear water repellant, such as a silane or siloxane, is used to minimize water penetration. These products, however, cannot bridge any significant gaps in the mortar joints and partial or full tuckpointing should be performed prior to applying the clear water repellant. Proper tuckpointing eliminates any large voids or separations between the mortar joints and concrete masonry. Clear water repellants have a relatively short life and need to be reapplied.