Want to know why Jahn Mortars last and don't fail? It's all about Compatibility & Expert Training.
Dennis Rude cuts and patches stone as he discusses the principles that guide him and the masonry restoration products developed by Cathedral Stone.
Failures in masonry restoration are the norm—but they don’t have to be. Let me expand on that: Failures in masonry restoration are the norm when you use mortars that include bonding agents/polymers, or you use untrained/inexperienced installers.
When the right mortar is applied correctly, over a properly prepared surface, the repair can, and will, last more than 30 years (the Empire State Building used our Jahn M70 mortar in the late 80's and they've had no failures to date). But in spite of that record, time and again we hear about failures happening within three, four, or five years. Most of these are on projects either spec'd with polymer-based mortars or on low-bid projects.
“To select an appropriate repair material, an engineer must be aware of two factors: the repair material’s compatibility with the existing concrete, and the repair material application.”
Let's first deal with polymer-modified mortars.
Back in the 70’s and 80’s, I’d see failures in my own work. I knew we were all good masons, who knew our craft as well as anyone. For the life of me I could not figure out why these failures were happening. I had to go all the way to Holland to find the answer, which can be summed up in one word: Compatibility.
Brownstone, granite, limestone, sandstone, concrete—each has specific properties and characteristics in terms of breathability, compressive strength, and tensile strength. Any pointing or patching mortar used must also have compatible breathability, expansion and contraction characteristics, and tensile strength.
The trend over the last fifty years has been to use mortars mixed with bonding agents. This is a mistake. For those of you who aren’t familiar, bonding agents are essentially glue. By adding them to mortar, you get adhesion to pretty much any surface without needing to know much about the surface and without needing surface preparation and proper masonry techniques--other than the ability to slather it on like cake frosting. Bonding agents are popular because they result in a mortar with a much higher expansion-contraction rate and a much greater bond strength. This sounds good in theory, but the same properties that make it stick easily are also responsible for the failures. Glue inhibits breathability, which causes a buildup of water crystals and salts. And since the bond strength is so high, deterioration in the substrate is accelerated, which results in a failure.
You're taking something that is naturally doing something—the way a specific stone interacts with the elements--and you're stopping it from happening. The high bond strength that these mortar admixtures promise is too high--it's ridiculously too high.
“When the right mortar is applied correctly, over a properly prepared surface, the repair can, and will, last more than 30 years.”
I didn’t know these things either, and I didn't know the consequences--until I made it my business to know. Now we've developed the Jahn line to be even more substrate-specific. We have mortars that mimic the characteristics of specific stones, which is the first step in making long-term repairs that are not going to cause deterioration of the substrate. The repair will move with the masonry as it expands and contracts. With compatible porosity, fluids will move through the mortar the same way it moves through the natural stone, protecting both the repair and the substrate from freeze-thaw cycles. This is why Jahn mortars last so long and have gained such a good reputation in the restoration and preservation fields.
Don’t take my word for it. Like I mentioned, you can visit the Empire State Building and you’ll see 30-year-old repairs that are as strong as they were on day one. At Reagan National Airport, we used Jahn on 14,000 repairs--with no failures in 17 years.
Don't get the idea that this is a rant against what is modern and new, it is not. It’s a warning against short cuts and bonding agents. They're not even good short cuts.
To restore something is to take it back to its original state with the highest degree of integrity. That requires science and craft, not glue. Jahn mortars have the science built in, and our certification programs teach masons the traditional skills of the craft.
It’s what our historic buildings deserve, and what every specifier should demand.
— Dennis Rude, Stonecutter and Founder of Cathedral Stone Products
