Posts Tagged stone
After a careful evaluation and clear understanding of why the mortar joints have deteriorated (or not, in the case of removing hard portland cement mortars) it’s time to repoint the wall. First, is the question of how deep to cut the old mortar out from the joints in preparation to receive the new replacement mortar. The Preservation Brief 2 “Repointing Mortar Joints in Historic Masonry Buildings 1998 published by the U.S. Department of the Interior’s National Park Service , Heritage Preservation Services gives us a good place to start.
Old mortar should be removed to a minimum depth of 2 to 2-1/2 times the width of the joint to ensure an adequate bond and to prevent mortar “popouts”. For most brick joints, this will require removal of the mortar to a depth of approximately 1/2 to 1 inch; for stone masonry with wide joints, mortar may need to be removed to a depth of several inches. Any loose or disintegrated mortar beyond this minimum depth also should be removed”(page 9).
I like that the Brief advises on a range of mortar depths in correlation with the width of the joints. It makes sense to approach the removal in this way. Often I see contractors bidding repointing projects calling for the depth of the removal at ¾ inch. For most mortar joints that are the thickness of your little finger, about 3/8 inch, this is not deep enough. It does not cost the contractor any more money to remove another 1/4 inch of material during preparation and it makes for a better job.
Luckily for the projects requiring the removal of hard portland cement mortars from old historic lime mortar walls, the contractors of years past, did not follow this quality protocol of 2 to 2-1/2 times, otherwise the portland cement mortars would be much more difficult to remove. Instead, we most often find these projects only skim coated with the harder material. It is important as a mason contractor to know what you are getting into prior to bidding a project that has been pointed in portland cement. Questions you should be asking are; How deep is the non-original portland pointing?,… How hard is the mortar?,… and how difficult will it be to remove it without causing damage to the surrounding masonry units? Sometimes the only way to really know for sure about the answers to these questions is to commission a test panel prior to bid.
A four-step approach to removing mortar joints in historic masonry buildings has been the industry method and best practice approach now for the past 15 years or so. First, the use of thin diamond-blade (turbo-type) grinders has been successful in cutting down the center of the horizontal (bed) joints for the removal of hard portland cement mortars. Second, followed by hand hammer and chisels or pneumatic chisels, to remove the excess mortar from the top and bottom of the masonry units. The vertical (head) joints are removed by chisel and hammer once the bed joints are removed. The third step is to use a caulking cutter with a diamond sickle type blade to clean the top and bottom of the masonry units and create a square cut back to the original lime mortar.
In the fourth and final step, mortar debris from the process should be removed by compressed air or a vacuum system. We do not use large amounts of water to flush out the debris during the cooler fall or spring months as it takes a long time for the walls to dry out especially the north facing elevations that do not benefit from the direct sunlight. We have unintentionally had efflorescence become an issue on some projects – waking up salts that lie deep within the masonry wall system due to excess water flushing.
As promised from yesterdays post we were going to take a look at how water enters a building. We know water can enter a building in many ways, through masonry walls, roofs, windows, and saturated soil surrounding the foundation just to name a few. There is also interior sources of water, such a condensation from cooking, cleaning, showering – generally occupying the home. However, this is not the whole story when it comes to historic masonry walls. What we don’t talk about much is the “embodied water,” the water that remains in the walls at all times. Old historic masonry walls are moist in the center, due to the porosity of the masonry units, the lime mortars, and the thickness of the wall – it’s just hard for them to dry out completely. They really never do completely dry out, especially if they are foundation walls. We must consider this condition and account for moisture already present within the wall, because if we don’t, we could unintentionally trap moisture even when the outside of the wall feels normal and dry.
Load-bearing historic masonry walls (16 inches to 24 inches in thickness) are porous, capable of wicking large amounts of water great distances due to capillary suction. The smallest cracks and pores found in mortar, brick and stone can bring on water in a big way. The action of wicking is energized by the horsepower of smallness. The smaller the pores or cracks in the wall, the more powerful the draw of the wicking action. If you have given blood, you may recall the very small glass tube the nurse uses to take a droplet of blood from your fingertip with a seemingly invisible vacuum cleaner. The blood droplet instantly goes up the tube – effortlessly, capillary suction at work.
The horsepower of smallness regarding the capillary action of water should be cause for concern, because architects and contractors are often focused on repairing large cracks in buildings while leaving the small ones untreated. Don’t be deceived. The small cracks, even the hairline cracks, are where the suction power is. One way to slow down the power of capillary suction is to reduce the surface area of the material that comes in contact with the water source. For small hairline cracks, injecting dispersed hydrated lime (DHL) into the crack with a syringe will sometimes do the trick. Unlike epoxy or cement, DHL remains flexible after it cures and maintains good vapor transmission, allowing the wall to breathe while at the same time halting the water – pulling the plug on the vacuum of capillary suction.
Dispersed Hydrate Lime (DHL) is a product imported from Germany and has been used successfully on many historic masonry structures here in the United States for over 10 years. Information on the product can be obtained by contacting the U.S. Heritage Group based in Chicago. Other vender choices I’m sure are available, but this is the product we have specified and are most farmiliar with using.
SPC Training video on DHL injection: http://www.youtube.com/user/SpeweikPreservation?blend=4&ob=5#p/u/4/mlGm2XvEGF8
The new term on the streets used by industry consultants to describe the details of how a building takes on water and then (hopefully) sheds it is “water management.” The longevity of historic masonry walls relies heavily on how water is managed in and around them. I am personally not yet convinced we can control water. I can work to manage where it goes, and possibly how long it stays – by redirecting it, but in the end it goes where it wants, the easiest way. When you attempt to fight water it usually wins. The ways water impacts a building depends on how long it stays – which is directly correlated to its architectural design, geographic location, topography, soil, the water table, the type of brick, stone or mortar, and whether the building has recently been restored.
Sometimes the understanding of historic load-bearing masonry walls built with lime mortar materials is not established, or respected, prior to a restoration project being undertaken. While the joints may look like they are in need of repointing due to deterioration, it’s important to know why they deteriorated in the first place. The cause is most likely from water saturation – then freezing and thawing or extreme temperature variations. One of the challenges is understanding that a building can, and does, breathe though its mortar joints as well as its masonry units.
The shear thickness of most load-bearing masonry walls keep the water out. The original building materials made for quick evaporation of the water on the surface of the walls and kept the inside dry, but this breathability does takes its toll on old lime mortar joints and they need to be repointed in high moisture areas every 75 to 100 years or so. Problems start when an architect specifies a replacement mortar that is harder than the original (in an effort to make it last longer) than potentially traps moisture inside the wall system. The effort in the restoration repairs is totally focused on keeping water out from coming in through the exterior side of the wall. The problem is that old masonry walls contain a certain amount of moisture already and often do not perform very well with harder/stronger mortar joints surrounding them.
When the goal of the restoration project is to create a Watertight Envelope you’d better run the other way – fast. “Watertight Envelope” and “Historic Load-bearing Masonry” should not be used in the same sentence. Keeping water on the outside would seem to be an honorable goal for any restoration project, but observing the current condition of some masonry buildings restored in the past 10 years tells us a much different story, a troubling one. Basically, the buildings subjected to this watertight-envelope theory are not doing very well.
Where waterproofing and harder cement-based mortars are applied we find decay patterns that are surprising – in just a decade after application. Instead of the mortar surfaces wearing, there is a new pattern of brick and stone decay. Strong osmotic and hydrostatic pressures build up in brick and stone that are subjected to these hard, strong, and water resistant materials.
Tomorrow we will discuss how water enters a building.
As a young boy growing up in a family of stone masons mixing mortar was like brushing my teeth…I did it every day, at least during the summer months when school was out. Who would have thought that in the age of technology, speed and convenience that my great great grandfather’s 1846 mortar formulation would return. The trend seems to be one that is sweeping across Scandinavia, Europe and Canada as architects and heritage masons work together to preserve their country’s historic masonry properties built hundreds and often thousands of years ago. They call it the “Lime Revival” It’s been 30 years for Sweden, 20 years for England, 10 years for Canada….its America’s turn now.
The oldest archaeological sites in the world are, of course, masonry. As early as 2450 B.C., masons began using lime and sand for mortar. Lime is made from limestone (calcium carbonate) which has been heated to temperatures exceeding 1,650F where the heat drives off the carbon dioxide and water turning the limestone into quicklime (calcium oxide). Traditionally this quicklime (sometimes called lump lime or hot lime) was delivered fresh to construction sites or made on-site in a temporary kiln just for the job. The quicklime was mixed with damp sand and stacked up into piles for slaking into a hydrate powder (calcium hydroxide) and run through a screen or the quicklime was combined with water in the ground, formed into a putty (also calcium hydroxide), and mixed with the sand at a later time depending on the project needs. Either way, the mixtures were left to mature or rest for a time before use, due to the expansion of the lime particles during slaking.
The lime was generally mixed with local sand in a ratio of 1 part lime putty to 3 parts sand by volume. Other ingredients like crushed brick, clay, lamp black, and natural cement were sometimes found in smaller quantities before 1870; however, the basic lime putty/quicklime sand mortar formulation has remained unchanged for centuries.
Portland cement was first manufactured in America in 1871, but did not become truly widespread until the 20th century. As late as 1883 there were only three portland manufacturing plants in the U.S. Up until the turn of the last century portland cement was considered an additive, or “minor ingredient” to help accelerate mortar set time. By the 1930s, most masons were using equal parts of portland cement and lime putty or quicklime. Thus, masonry structures built between 1871 and 1930 might be pure lime and sand mixes or a wide range of lime and portland combinations.
What we do know about lime, and the reason for its come-back, is its incredible performance characteristics, and versatility as a time-tested building material – and not just as a masonry mortar either, but also as paint, (limewash/whitewash) exterior stucco/render, and interior plaster as well. Lime, when properly combined with clean, sharp, well graded sand can perform for many centuries in masonry applications. Lime has the ability to handle water without trapping it within a wall structure. It is breathable, flexible, obtains high bond strength to masonry units, it is truly sustainable (less energy is required to heat a ton of lime as compared to a ton of cement) and it has autogenious healing capabilities, often referred to as “self-healing” where hairline cracks do develop over time water combines with the lime again to re-knit the cracks back together. Limes durability comes through a process of what’s called carbonation. Carbonation is a process by which lime turns back to limestone by reabsorbing the CO2 back from the atmosphere though wetting and drying cycles. You can say that the material interacts with nature on a daily basis.
As portland cement became more widely used many lime sand mortars were being “covered-up” during repair projects. Exterior masonry buildings suffered badly from hard portland cement mortars (1940s until today) which did not accommodate for movement or stresses within the wall systems, and as a result, many historic brick and stone units got damaged by this un-sacrificial material. When cracks did occur, in the portland cement mortars, water would migrate into the wall cavities and not be able to escape or evaporate back out as they once had done with the lime sand combination mortars.
But times are changing. We are seeing signs of the “Lime Revival” hitting the shores of the United States. Mortar manufacturing companies are now offering lime mixes now for restoration and a few specialty companies offer traditional lime putty, quicklime and imported hydraulic lime for sale.
Lime mortar materials, that I am currently aware of, are available from the following U.S. companies listed in alphabetical order for your convenience. Be sure to ask questions about each of the company’s offerings, as they differ. Some still use portland cement in their lime mortars. It’s best to know what you need first – then go out and find a supplier that can meet that need.
Cathedral Stone Products
U.S. Heritage Group
Virginia Lime Works
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