Recently, here on our website, in past blog articles, we looked at several factors or forces of deterioration on historic masonry. Often, throughout the majority of Washington, DC, the historic buildings have been built with a low temperature fired brick and mortar that is relatively soft compared to modern mortar. You may know, if you’ve read a bit of the articles on our website before, historic mortar is softer and has a lower pounds per square inch (PSI) of compressive force resistance. It isn’t really substandard to modern mortar, it’s just different. That historic mortar though is compatible with historic bricks and when repointing or tuck pointing historic brick walls is important to understand the difference and use mortars that are both compatible on a structural level and also compatible in terms of matching physical characteristics.
Today we’re going to look at one of the primary causes of destruction and deterioration of historic brick and mortar: freeze-thaw conditions where there are instances or issues of overhydration in masonry. Over hydration is any amount of moisture that’s excessive, residual, and causes deterioration or exacerbates defects in masonry to lead to destruction and dilapidation. The picture below shows an example of an area of deterioration where a portion of a vertical sailor course brick has delaminated or detached from its multiple wythe substrate. Unlike a regular rectilinear brick, this particular brick is a specialty brick with an external soldier chamfer facing.
This delaminated separation is an example of an area that is susceptible to moisture entry and when water builds up in that area and then later freezes. A common occurrence during the winter months in our area here in Washington DC, it will lead to destruction or delamination of portions or parts of the brick masonry. Water expands to over 7 to 9% of its volumetric area when at freezing temperatures. This pressure can be excessive to the point that it can cause bricks to break apart.
In the next picture below, we show another example of a void in a typical masonry mortar joint, but this particular masonry mortar joint occurs at an area where there’s a transition from a cap or coping to a chamfer type brick that transitions to a wider base at the base of the masonry screenwall. This is a particular spot, like many, where they’re voids and pockets that become reservoirs at defects or deterioration that create openings, water can pull into areas like this then, in the winter season when water freezes, it can expand and crack the masonry.
If you look closely at this area, you can see that plant debris like parts of leaves and seeds from the trees above have collected in this area these type of materials also absorb moisture and after rains and precipitation will swell and hold water preventing it from draining freely. This type of retention of water adds to the risk of water being retained within the pocket of void which adds to the risk of potential damage from freezing water. As mentioned above, when water freezes within a pocket or void in masonry, it exerts extreme pressure on the masonry which can cause it to crack, deteriorate, dislodge and deteriorate further.
Truly horizontal ledges are generally a bit rare in masonry construction because almost all masonry surfaces should have a minimum of about 1/4 inch per linear foot of grade or fall from the area against the greater portion of construction to the outside this intentional grade or slope at the top of relatively horizontal areas allows water to drain freely and not collect or puddle or pond.
Here in the picture below, at a close up view of this same horizontal surface shown above, you can see there is a crack or thin void at the horizontal perpend joint of one of the bricks. This type of void is more problematic, from a perspective of freeze-thaw damage risk, in a horizontal position than in a vertical position. In fact, deterioration of this type, while still problematic in the vertical surface of a wall, is much less of a concern there than here in a horizontal surface.
Area of deterioration will allow water to enter and pool. Particularly during the winter months when temperatures hover around the freezing mark, the trapped freezing water can turn to ice and break the mortar and bricks.
The picture below shows a different brick and a horizontal position where the brick has been cracked and broken. It’s likely that some of the deterioration at this location is a direct cause of freezing water and precipitation.
Routine upkeep, maintenance, and repair, while still costly, can save a significant amount of cost in the long-term.
If you are looking for a masonry restoration contractor that takes historic preservation seriously, reach out to us. Our company can help.
You can reach us by telephone at (202) 796-7644 and you can reach us by email from the contact form on our website at https://duponttuckpointingmasonrydc.com/contact-us/.
