Posts Tagged historic masonry
Ever wonder how Type N mortar came to be? or Type M for that matter? Well the story goes something like this…In 1931 a group of non-mortar producers and representatives from the lime and cement industries got together and formed a committee to discuss the issue.
The issue was that mortar “types” needed to be established to distinguish high compressive strength mortars from soft flexible ones, so in 1944 the designations using A-1 (2,500 psi); A-2 (1,800 psi); B (750 psi); C (350 psi); & D (75 psi) were adopted, with minimum compressive strength requirements specified.
In the United States, “A-1” had become synonymous with “the best” or “top quality” and some committee members felt that the designation for the higher compressive strength cement mortar was misleading. The possibility did exist that an architect desired a flexible lime mortar type for a particular project, but he might mistakenly specify the A-1 type, thinking it was the best. In an effort to avoid confusion on the subject, the committee adopted a new mortar type designation in 1954.
The new designation letters were taken from the two words, MASON WORK utilizing every other letter. The compressive strength minimums for each mortar type are still recognized in the current ASTM mortar specification C270.
(2,500 psi) Type M replaced A-1
(1,800 psi) Type S replaced A-2
(750 psi) Type N replaced B
(350 psi) Type O replaced C
(75 psi) Type K replaced D
Most historic load-bearing masonry buildings have original mortars with low compressive strength, but yet are very durable (well carbonated lime mortar). We have plenty of architectural inventory around the world that supports this statement. High compressive strength in historic masonry mortar (Type O or higher) is not a direct reflection of durability and maximum life-cycle performance.
In fact, to give you some perspective, a certain material scientist/university professor studied historic mortar for his entire career. Traveling the world he collected samples from some of the oldest historic masonry structures. Very seldom did he ever run across a historic mortar with compressive strength of over 300 psi.
As you climb the scale from Type K upward, you are adding more and more portland cement by volume. As a result, the mortar becomes less permeable, less breathable, and more inflexible in exchange for the increased compression. Historic masonry on the other hand needs mortars to accommodate building movement (flexibility), exchange moisture readily from the face of the wall (breathability), and most of all have excellent bond strength-all natural properties of lime mortar (“Type L” introduced in 1998, ref. NPS Preservation Brief 2).
Preservation Brief 2, “Repointing Mortar Joints in Historic Masonry Buildings” http://www.cr.nps.gov/hps/tps/briefs/brief02.htm
The History of Masonry Mortar in America 1720-1995 http://www.lulu.com/product/paperback/the-history-of-masonry-mortar-in-america-1720-1995/11271764
Let me save you some time and trouble if you are considering specifying lime putty (ASTM C1489-01) for your next historic masonry restoration project. Forget about the standard way of mixing mortar with a gas-powered paddle mixer or drum type machine used in new masonry construction. These machines require the mortar ingredients to have a high rate of flow by adding enough water into the mixer to keep everything moving and mixing thoroughly. Not so with lime putty. This material is generally 50 percent water and 50 percent solid (looks like thick cream cheese) and requires a mixer that provides pressure or a kneading action to evenly incorporate the sand particles into the material.
Mixing lime putty and sand together works well when mixed by hand with a mortar hoe and shovel as you can place pressure into the mix by pressing down during the process. Ramming rods made from wood with handles also work well to beat the mortar into submission forcing the sand particles into the lime putty.
What is interesting about mixing lime putty mortar, if you have never had the pleasure to do so, is that it requires no additional water once properly mixed. There is enough water in the lime putty to create a good workable mixture that can be used for repointing. For years we have used a vertical shaft mixer that whips the material into form from the outside-in once all the ingredients are in the shaft mixer. So whatever you decide to do on that next historic masonry restoration project, if it involves lime putty, be ready for some good-old-fashion hand mixing or get ready for some buckshot of lime putty balls coated with sand!
The next time you come across a historic masonry building take a close look at the surface of the mortar joints. Yes, I know they often get over-looked in competition with the brick or stone, but trust me on this one. The first thing you should notice is the sand. The sand is the largest part of the mortar by volume and is the material that gives the joint its color, texture and cohesiveness. The next thing you should notice is white specs or small chunks of carbonated lime putty. If this evidence is identified you’ve got yourself a truly historic lime putty mortar. No need to hire a fancy consultant or pay for an expensive mortar test, you can with confidence declare your finding.
Mortars that display lime inclusions were typically mixed using quicklime and sand mixed on the jobsite with a shovel or mixing hoe by hand and with a lot of hard work I might add. Often, the moisture would be added to the sand first then the quicklime added to the damp material. The quicklime would slake first into a hydrate of lime then into putty if more water was added to the mixture.
The batch of mortar would be tossed and turned until the masons yelled out “MUD!” then the material would find its way onto the laborers back then unloaded onto the boards. The mortar would be placed in the wall as construction proceeded. Mortar consistency might certainly vary from batch to batch with this serve as you go system in place. There might be a time when a laborer catches up with the demand for mortar and has more time to mix a particular batch-thus breaking up the lime inclusions into smaller pieces and even dissolving them altogether.
If it is your desire to match these inclusions you have a couple of options. Use a mortar mixture made from damp sand and quicklime (hot lime mortar mix- allow 24 hrs before use), or make lime inclusions from straight lime putty by allowing the material to air dry then running the harden pieces through a series of aggregate sieves to match the inclusion size you specify. Then simply add the inclusions to your lime putty mortar just before application taking care not to over mix. The inclusions in the image above were added to the masons hawk just before installation and were protected from the initial mixing of the lime putty and sand to keep them from breaking apart.
Whitewashing has been used for many years to cover and protect historic masonry, even before it was historic! A whitewashing application involves mixing lime putty with water in a ratio of 1:5 then vigorously stirring the material until the lime putty fully dissolves in the water. Colors can be added from earth pigments but most material was used white – thus the name. The lime (calcium hydroxide) sets slowly by absorbing carbon dioxide from the air. The chemical reaction that occurs produces crystals of calcite. These crystals are unusual because they have a double reflective index: light entering each crystal is reflected back in duplicate. This results in a wonderful surface glow that is characteristic of whitewashed surfaces and is not found in modern paint products or imitation coatings.
The application of whitewash acts more like an absorptive stain. It is not a coating so it will not peel-off. After it hardens whitewash remains vapor permeable and will not trap moisture in the wall. One of the attractive attributes is that it gradually wears off the surface of the wall over time leaving a very pleasant uneven aged look.
Many architects and designers seek this look but have had challenges because they have been using the wrong products, such as paint, to achieve the effect. If it is a traditional look you want to specify than its best to go back with the traditional material that will get you there.
The key; however, is in thinly applied coats. This facilitates the carbonation process of curing and prevents crazing and cracking. It is helpful to specify onsite application training – as most painting contractors will treat the product like paint and attempt to get the surfaces coated in paint-thickness applications. Most raw masonry surfaces require 3 to 5 applications of whitewash, then after that, just a single coat will do the trick to freshen things up later.
Additional reading: http://www.slideshare.net/speweikpreservation/speweik-limewash-returns-2000
I once overheard a colleague of mine describe the process of deteriorating mortar as “romantic decay”, I guess all the years of his travels throughout Europe and the Scandinavian countries he had seen his share of crumbing bricks and mortar and had become un-alarmed about the condition. Interestingly enough, it seems the opposite is true here in the United States. We tend to get all worked up about crumbling mortar, especially the condition when the material turns to sand in your hand when you rub on it vigorously between the brick.
But according to the Preservation Briefs 2, “Repointing Mortar Joints in Historic Masonry Buildings” describe a different view of a good working replacement mortar. In fact, the mortar could be called a “romantic decaying mud” depending on how you look at it.
Here is what Brief 2 has to say about repointing mortar. “In creating a repointing mortar that is compatible with the masonry units, the objective is to achieve one that matches the historic mortar as closely as possible, so that the new material can coexist with the old in a sympathetic, supportive and, if necessary sacrificial capacity.” We have mortar all over this country trying to sacrifice itself for the good of the masonry units by falling apart in historic masonry walls!
But unfortunately, we also have many engineers and architects, building owners and contractors doing their best to prevent the process from occurring. When the romantic decay is identified the sure tell remedy is usually a stronger mix design one that contains a large amount of portland cement to go back in with during the repairs. A new mortar without the romance and certainly no sacrifice.
As we continue our work on important historic masonry structures lets – let the mortar help us to identify the real problems – usually the water infiltration somewhere, somehow. Blame the water not the historic romantic mud!