Posts Tagged historic preservation
Unlike new masonry construction, restoration masonry requires matching to existing surfaces. Whether it’s a brick, stone, or mortar, samples must be submitted and often tested to determine the original material components. So what is the best way to specify a match? Well, first let’s talk about the way it is generally done now in the restoration business. An architect writes a specification that includes the details of matching the masonry; for instance, “match the brick in color, texture, size, and physical characteristics to that of the original historic brick”….nothing wrong with that, right?
Well consider the brick match needing to be located (research, calling around to suppliers, submittal of samples) after the contract has been awarded and the construction schedule is starting. The pressure to find a good and acceptable brick match is now the responsibility of the contractor who is thinking about mobilizing the site, balancing manpower to get the project done on time, and the overall responsibilities for the entire project. Question: Is this the best time and the right person to be carrying out the important responsibility of finding a successful brick match?
The same goes for the stone or mortar match as well. Question: Is placing these decisions on the back of the contractor at the start of the project in the best interest of the project? Under this pressure mistakes can be made and searching for the best most appropriate match compromises are often made (“that’s the best we can get, or, they don’t make that any longer”). So what might be a better strategy? A relatively new movement is occurring in the architectural design world in the restoration business.
Architects are working with building owners directly and sometimes with consultants to assist them in matching historic masonry materials – prior to bid….during the design development stage of the project (often 1-year in advance of the bidding process). The brick, stone and mortar testing work are accomplished and often times pre-approved in a pre-construction test panel installed by a local mason contractor or preservation consultant. This strategy helps to eliminate delays in the construction phase of the project and it gives more time, without the pressure, to find the best available match on the market.
So the next time you are considering specifying replacement masonry materials on a historic restoration project consider this new approach to an age old problem. It takes a little more planning on your part, and yes, the owners need to pay up front for some pre-construction test panels, installed into the actual masonry for evaluation. But in the end, the surprises related to change orders are often minimized and the team approach to getting the project done on-time and under budget becomes a reality-not just a dream. And, its money the owners will be spending anyway with the contractor after the bid award….. something to consider.
Fixing the faces of the stone using stone repair mortar has become a huge industry in the United States in the past 20 years. Manufacturer’s of these products all have their reasons why theirs is the best and most compatible to the original stone substrates. Each company guards its proprietary mixes with closely kept secrets as to what and why theirs is the best on the market. Some companies claim acrylics in the formulas, where others claim natural minerals – whatever that means. Most offer custom color matching, some texture matching. Either way, the products are here to stay and will be an option on most restoration projects large and small.
When is the best time to consider using stone repair mortar? Let’s first discuss the term – “repair mortar” not to be confused with “repointing” or “rebuilding mortar” that is used between stone and defined as mortar joints. A better term might be “substitute stone repair material” because that’s what it is – a substitute for the real thing. By definition a substitute stone repair material is a material that is used to patch damage or deteriorated surface masonry units’ insitu. When a stone is deteriorated beyond its original surface texture or carved feature it’s time to consider stone replacement or substitute stone repair patching.
Each condition and situation is unique to each building or monument and care must be taken in making/specifying this important decision. Cost of replacement may be problematic and even unrealistic. The size of the repair area may be so small in comparison to that of the entire stone that the substitute stone repair material may be justified. But the decision should also be compared to the old fashion Dutchman repair approach as an additional option. It seems that the repair material products are so easy to use that many situations that could merit a stone Dutchman repair simply get the “patch”.
On a recent project at the University of Wisconsin we were able to redress the deteriorated stone surfaces instead of applying a substitute stone repair material. The deterioration patterns were such that the surfaces had spalled and chipped away over the years leaving textures and stone massing voids from the profile of the walls. In many cases we were able to remove the damaged stone units redress them on a sand table, duplicate the original tools marks with hammer and stone chisels, and re-install the stone back to its original historic plumb lines of the wall. This was the best preservation practice approach for this project and one that limited the use of the substitute stone repair materials to the building. We did use a small amount of the material but only when all other stone treatment options were exhausted, which meant most substitute stone repair patches were on spots smaller than 1-inch in size.
Redressing Stone Training: http://www.youtube.com/user/SpeweikPreservation#p/u/10/oQSt_9sDcTI
Companies that currently sell substitute stone repair materials are: Cathedral Stone Products, Edison Coatings, Transmineral, Pennsylvania Lime Works, U.S. Heritage Group, Conproco, Keim, and Virginia Lime Works
Best way I know of to match a historic mortar is to first identify whether you are dealing with a cement-based binder or a straight lime binder. You can do this by dissolving a sample of original historic mortar in a solution of hydrochloric acid and some water. Watch the reaction of the material as the solution makes its first contact. It’s best to place the material into the solution rather than the solution into the material for best results. If it is a lime binder the material will break down quickly and form foam at the top of the material, bubbling and hissing as the calcium breaks down. If the solution just sits there with no reaction, only a few bubbles – but know foam or hissing action you have a portland cement mortar.
Allow the materials to soak in this solution until all the binder materials are gone. You can check and see this visually by looking at what is left – the aggregate of the mortar. It should be clean and free of any particles of binder still attached. Lime binder mortar can dissolve quickly, sometimes in a matter of hours. Cement binders can take up to several days to dissolve down. Now comes the fun part – identifying the aggregate or sand in the mortar.
Drain off the solution through filter paper to collect the fines. Dry the material in an oven at around 200F on a hot tray. Weigh the material into an even gram amount then run the sand through the series of ASTM E11 sieves specified through ASTM C144 and calculate the percentage of grain particles on each sieve. Then create a sand gradation chart which depicts to sand particle size, shape and color to make for an easy way to match the material.
Chances are if you match the sand and binder materials in a historic mortar you should not need to assistance that oxide pigments can provide. However, these materials are used in the industry to assist masons in matching mortars regularly. We typically stick with a 1 part binder to 2.5 parts sand in our formulas as this was the standard in the industry for the exception of butterjoint brickwork which is often a mixture of one part binder to one part sand by volume.
The performance characteristics of a historic mortar; bond strength, flexibility, breathability, vapor permeability, and compressive strength (in that order of importance) will typically fall into place if the necessary time and testing of the original material is carried out. Most historic lime mortars are very durable and well carbonated and worth replicating. It is not best practice to trump a historic mortar formulation and go to the next higher mortar type, i.e., Type L to a Type O for example. It is best to fix the problem of why the historic mortar deteriorated in the first place – more than likely a water related issue.
Ever wonder why portland cement gets so hard? Why it is so high in compressive strength? This blog post is a continuation from an earlier one from last week titled “Hot Rocks” which most people seemed to enjoy. Early lime kilns could operate only up to around 1600F — just hot enough to activate the clay, turn the limestone (calcium carbonate) to quicklime (calcium oxide) and combine them both to form belite (dicalcium oxide) to make hydraulic lime.
Joseph Aspdin (1824) had achieved a slightly higher temperature of around 2200F forming some liquid phase and combining the belite with the remaining quicklime to create alite (tricalcium silicate), the base compound for portland cement. The vertical shaft kilns could burn slightly hotter, but temperatures were mainly kept below the maximum heat achievable to limit the liquid phase and prevent the danger of a kiln blockage, a situation created when the molten mass of materials cooled within the kiln. Kiln blockage was something kiln operators historically and understandably feared; crawling down into a shaft kiln and chopping away with a hammer and chisel at a molten mass of material was to be avoided at all costs.
The rotary kiln, however, not only helped to overcome the kiln blockage challenge, but it also allowed the higher temperatures necessary to produce portland cement with a much higher compressive strength. Before the rotary kiln was invented (1889) most kilns were constructed vertically, loaded with alternating layers of limestone and fuel (wood and/or coal or both) until the materials reached the top. A fire was started at the base of the kiln and allowed to burn for a day or two depending on how large the kiln was. The entire kiln was cooled down another few days until the materials were discharged from the bottom. The process would start over again for the next batch, a very labor intensive process. The rotary kiln also allowed for continuous feeding of the kiln without the usual starting and stopping and could run 365 days per year without interruption in production.
So just how hard is portland cement? Well, in a paper presented at the American Lime Conference in Lynchburg, Virginia in March 2003, Paul Livesey of Castle Cement presented the following information about the answer to this question. When matching a “portland cement” mortar from the period between 1871 and 1920 then, we should not be confused by the terminology. Portland cement of 1871 was a different material from that of 1920 which, in turn, is totally different from portland cement today.
Portland cement products produced at the turn of the last century were fired at temperatures from 2300F to 2600F. In comparison manufacturers making portland cement in this century fire at temperatures ranging from 2800F to 3000F. The differences in mineralogy and the effect that burning at higher temperatures has on strength development are well known. For example, in 1871, portland cement tested in the 1800 psi range; today’s portland cement are in the range of 8,000 psi, an increase of 344 percent in compressive strength. Indeed, the portland cement of 1871 bears more relation to modern, higher-strength hydraulic lime (NHL 5, 1000 to 1500 psi) than it does to modern portland cement.
And when making decisions on the appropriate mortar match for historic buildings remember that the National Park Service as well as leading American experts agree that it is always better to err on the side of a lower strength mortar replacement in order to protect the historic masonry materials. Even adding a small amount of modern portland cement can have a significant affect of increasing strength when maybe you did not intend to in the first place.
It was David O. Saylor back in 1871 that produced the first portland cement in the United States in Lehigh Valley, Pennsylvania. Between 1871 and 1920, American portland cement production skyrocketed, due in part to the increasing demand for automobiles and the attendant need for roadways during this period. The years 1871 to 1920 also saw a major change from “traditional” manufacture of cement to a more technically aware, science-based industry.
As a consequence, it was inevitable that cement products that reflected both traditional and modern scientific production methods were on the market at the same time. Thus, anyone looking to match a historic portland cement mortar from the time period between 1871 and 1920 will benefit from considering the evolution of cement production technology during this period.
As popularity of portland cement grew over the period, so did its compressive strength, from 1800 psi in 1871 to 3000 psi by 1920, an astounding 110 percent increase. An examination of the limestone and clay used to produce portland cement, however, shows that they changed very little after 1871. What, then, had changed? Mainly it was the production process and the ability to fire the raw materials to consistently higher temperature.
The main technological breakthrough came with the invention of the rotary kiln, which was first used commercially in Lehigh Valley, Pennsylvania in 1889. Initial trials by Ransome and Stoke in England had lacked the necessary financial backing to succeed. In the end, it was left to Seaman and Hurry in the United States to make the final technical refinements that could produce sufficient temperatures and efficiency to unleash the massive portland cement industry we know today.
The traditional volume mix design of 1 part lime putty to 3 parts sand may be insidious to follow straight up without more details. First, mix designs historically used quicklime as the 1 part of lime mixed to the 3 parts of sand by volume. Quicklime when it is slaked with water will increase volumetrically 70-100 percent – or basically double its size. This fact would reduce the sand content closer to that of a 1 part lime to 1.5 parts sand – a much sticker richer mix design.
Secondly, sand must be measured in a damp loose condition according to ASTM C270 when mixing mortar. Dried sand will bulk up to 30 percent and grow volumetrically by the addition of a small amount of water. This can send your mortar mix designs at the construction site off the specified requirements.
I recommend everyone take a moment to read Gerard Lynch’s article on the subject it is well done. http://www.buildingconservation.com/articles/mythmix/mythmix.htm
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
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!