For historic masonry buildings, softer mortars are actually better. This sounds counterintuitive. Naturally a lot of people think that stronger is better. In most cases in building construction, that is true. However, when it comes to historic masonry, softer is actually better than stronger.
Modern mortar is very hard. Modern type M building mortar typically has compressive strength of over 2,500 pounds per square inch (PSI). It is essentially important for the mortar to have a compressive strength that is lower than the historic brick masonry. That means that if a historic brick has a compressive strength of only 1,200 to 2,000 PSI you need for the restoration mortar to have an even lower compressive strength.
In dry form, one mortar may look very similiar to the next, but may actually have significantly different structural properties.
Buildings have mico-movements, constant small movements occurring to and in the building. It sounds surprising, but buildings, even big heavy ones actually move around on a small scale. They don’t necessarily move a great distance, but when an incredibly heavy building moves a tiny bit, even just one one hundredth of an inch, that is a big deal. There is a lot of energy transferring and a lot if force, even in a tiny movement like that. (We talk more about passive building movement in a related article at this link.) Solid materials, such as building materials like masonry, glass, wood, steel, etc. actually expand a very very tiny bit when they warm up, in the sun for example. Wind, moisture, and earth movements also play as a factor in building movements.
When an extremely heavy building moves a very tiny bit, the softer bricks get pinched between hard mortar. This can lead to lots of problems such as the abbreviated following list:
- Brick spalling
- Propagation cracking
- and Crushing of bricks.
These concepts are part of the fundamentals of historic masonry restoration, tuckpointing, and brick repair.
The links in the list above will take you to other articles with more information on defects and failures of historic masonry. You can learn a lot more on our blog. Check it out. If you have questions about the historic masonry of your building in Washington DC, fill out the webform below and drop us a line. We will be in touch if we can help.