Archive for category Mortar
An Interview with Lauren McCroskey, U.S. Army Corps of Engineers – Revised UFGS Historic Masonry Specification
I had the unique privilege to interview one of the leading historic preservation experts, Lauren McCroskey, Program Manager, for the U.S. Army Corps of Engineers, at the Seattle District, on the recent revision to the UFGS for the Restoration and Cleaning of Masonry in Historic Structures.
USACE Official Announcement:
Technical Center of Expertise (TCX), Preservation of Historic Structures and Buildings Technology Update
As part of its mission to provide leadership in historic buildings technology, the TCX announces a major revision of its specification, “Restoration and Cleaning of Masonry in Historic Structures.” The spec now reflects state-of-the-industry guidance for the treatment of historic masonry and mortar, and surpasses existing preservation guidance provided by other federal agencies.
Property managers and cultural resource specialists are encouraged to use the spec in contract documents to ensure that masonry work is performed appropriately to prolong the life of historic buildings. See Unified Facilities Guide Specification 04 01 00.91
Speweik: What is the official title of the specification?
McCroskey: The title is the UNIFIED FACILITIES GUIDE SPECIFICATIONS
DIVISION 04 – MASONRY SECTION 04 01 00.91
RESTORATION AND CLEANING OF MASONRY IN HISTORIC STRUCTURES
Speweik: Who originally authored it?
McCroskey: The Corps was the preparing agent and performed the processing. The exact author(s) are unknown, though Corps staff would have prepared it.
Speweik: How long has it been in use/circulation?
McCroskey: It’s been available since 1991.
Speweik: Who is authorized to use it?
McCroskey: The Guide is to be used by the Military Departments (Army, Navy, Air Force, etc.), the Defense Agencies and the DoD Field Activities for planning, design, construction, sustainment, restoration and modernization of facilities, regardless of funding source. But anyone can use the guide to adapt to a particular masonry project.
Speweik: What government agency owns it?
McCroskey: The Architectural Discipline Working Group are the owners of the Section; Scott Wick is the Corps representative of that group.
Speweik: What is your position with the USACE and what specific responsibilities do you have regarding historic preservation?
McCroskey: I manage the Technical Center of Expertise for the Preservation of Historic Structures and Buildings, a program of nationwide service. The program provides technical assistance and preservation planning for Corps Districts, DoD, and other federal agencies to ensure facility and property managers apply the best practices to historic structures. We try to set the highest standards of preservation practice through quality project work, training, and by developing technical information.
Speweik: What prompted you to request an update to the Historic Masonry Division Section this past year?
McCroskey: For several years I’ve had an awareness that the Corps’ existing standards and guidance for the treatment of historic masonry has lagged behind newer developments and technological advancements for treating historic brick, stone, and mortar. I receive inquiries from Corps Districts and other agencies asking for specific guidance to address deteriorated stone or brick. Property managers rarely know how to approach these issues from a historic preservation perspective, and often have maintenance and field crews tackle masonry problems. While their intent is good, the methods, materials, and applications are often not appropriate for historic structures, and can lead to further harm and long-term costly repairs. That’s why it’s essential for us to be able to pass along the most appropriate, state-of-the-industry techniques and standards.
Speweik: What do you believe to be one of the most significant changes to the specification?
McCroskey: There are many improvements, but one of the most important is the depth of information, which is far more educational for the user than the old spec. There is much to be learned from this document. Another key improvement is that materials application is not just described, but preceded by a thoughtful examination of building and masonry conditions. There is extensive information about how to investigate existing conditions so that the best decisions can be made about materials, conditions, and methods.
Speweik: How do you see this change making a positive difference for the quality-level of Historic Preservation Projects in the United States?
McCroskey: I believe the TCX is obligated to provide the best guidance regarding the treatment of historic structures and buildings. By encouraging the use of this guide, the rehabilitation of historic masonry should be performed in a manner that is appropriate, efficient, and prolongs the life of historic materials.
Speweik: How do you envision the revised specification affecting the work you do at the USACE?
McCroskey: The spec will be the only guidance we provide to customers, or when advising others on the best standards for masonry. Since this spec now surpasses all other historic masonry guidance, we now consider this document the “gold standard.” Of course, there are sub categories of masonry, such as terra cotta and concrete, which may require other technical information. But where brick, stone, and mortar are concerned, this is our “go to” standard.
Speweik: Did you consider the possible additional costs to Historic Preservation Projects as a result of some of the changes? And, if so, do you believe the additional cost is a significant percentage of overall project costs?
McCroskey: When good preservation practices are used, the life and performance of historic materials is extended. When improper practices are applied, greater costs can be incurred when the wrong treatment or method causes damage that requires repair. Taking short cuts by using commercial products that are not suited to historic stone or brick, or using techniques that are not consistent with historic methods can cost more long-term, and rarely satisfy the Secretary of the Interior’s Standards for Rehabilitation and treatment of historic structures that all federal agencies must follow.
Speweik: How do architects, owners, consultants and contractors find out more about this important specification document?
McCroskey: The guide is now available on-line at the TCX web page: http://www.nws.usace.army.mil/BusinessWithUs/HistoricPreservation.aspx
For additional information or clarification regarding the spec’s application, your readers may contact me at:
Technical Center of Expertise
Preservation of Historic Structures & Buildings
U.S. Army Corps of Engineers
Our team last week installed a repointing test panel on a project in St. Louis. Being the first week of December in the Midwest we knew we were taking a chance with the weather. Question: Are we going to have enough time for the mortar to cure before frost set in? We prepared for winter protection and had the wall and working area at 75F during the installation of the hydraulic lime mortar and left the temperature at that level for five hours after installation.
If you have not seen a frozen surface of a newly installed mortar joint well now you have! As it turns out the weather was below freezing for the critical 4 days after our installation and thus the surface of the mortar froze. I have only received images at this point on the condition so my involvement is somewhat limited as to the severity of the problem. The solution to fix frost bitten mortar is to reinstall the material if the damage is justifiably deep and out of the specification requirement for the joint profile finish. We do have an open stipple finish profile which may come to our rescue, but we must be certain the bond between the mortar and stone has not been jeopardized in any way.
As I sit and write this blog I realize I am writing about style and approach to a specific task that many of you have been doing for many years. So I will start with this disclaimer – I will share my experiences with you on many successful repointing projects that involved lime putty mortar and most required no washing after wards except an occasional vinegar quick rinse to remove the white film from red brick units. I have made mistakes along the way and worked over the years to attempt to figure out what is the best practice approach to this task of mortar replacement called repointing.
Repointing, unlike tuckpointing, requires the full preparation of the joints to a depth of 2 to 2-1/2 times the width as discussed in yesterdays post. The American version of tuckpointing is only a skim coating of cement-based mortar over the top of existing mortar joints without the removal process (Chicago). The British version of tuckpointing is where a mortar joint is actually made to the same color as the units and a grapevine line is established in the center of the joint and tucked with a different color and mixture of mortar – true tuckpointing. This true tuckpointing is remarkably difficult to learn but in the end it makes brickwork appear straighter and rubble stonework look like it was laid up in ashlar units.
Mixing lime putty mortar is straight forward. You mix the putty into the sand placing the sand in the wheelbarrow first. Create a hole in the sand for the putty and place the putty into the hole and start to twist and knead the material together. What is interesting is that you do not need additional water during the mixing process; there is enough water in the lime putty to give you a good brown-sugar consistency for repointing. If you are laying brick you can add a small amount of water to create a spreadable mortar. A good repointing mortar should not be able to be spread with a trowel, if it is, than you have too much water in the mixture. If this occurs all is not lost, simply set the material aside on a dry sheet of plywood and allow the excess water to run off (be absorbed into the wood) until you get the desired consistency. This may take several days.
Pre-soak the wall with water. Be sure to get the water between the units and back into the existing original mortar joints. I have had projects where the water pre-soaking process never made it back into the joints – the sprayer was held too far away from the wall surface and while the unit faces received water the joints remained dry – especially the tighter joints. To ensure that you are getting the water back into the joints run the sprayers right on the walls and into the joints filling them with water. Allow the water to absorb into the wall materials and become Saturated Surface Dry (SSD) – no standing water to the touch, no drips, no glistening or shining if viewed from an angle – usually takes 8 to 10 minutes depending on the rate of absorption. You are now ready to start placing the mortar material.
It is most important that your repointing tools fit between the masonry units to enable you to compress the mortar back to the original material. If your repointing tools are too wide you will stain the face of the masonry as you attempt to get the material back into the joint and you will not get the necessary compression required for a good job. You may need to alter your tools by using a bench grinder to create thinner repointing tools. Most big box stores only carry 3/8 inch size even though ¼ inch sizes are available. Work from a hawk. If you are right-handed, work from right to left, on a slight angle, leading with the tip of the repointing tool. Overfill the mortar joints past the surface of the masonry units at least ¼ inch. Allow to dry and become thumbprint hard. Scrap away the excess mortar using a margin trowel and follow the contours of the masonry edges if required. Match the mortar joint profile of the original work. Stipple brush finish the joints by beating the faces with a churn brush – opening up the surfaces to expose the aggregate and create a texture that will encourage prompt evaporation of water and rapid carbonation as the mortar cures.
Protect the mortar for the first 24 hours after placement from wind driven rain and direct sunlight. Keep the material moist – spraying the entire wall with water three times per day for the first three days after application. The material carbonates as it goes through wetting and drying cycles – a minimum of at least nine (9) cycles. Do not allow the mortar to dry out too quickly. Repointing into hot or warm masonry units (south elevation) during summer months is not recommended. The masonry units simply will draw out the water from the mortar to quickly as the temperature rises reducing the chances for proper curing and carbonation to take place.
Repointing tools are available at: http://ushg.macusa.net/heritagedetail1.1.php?Current_Name=Restoration%20Pointing%20Iron
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 the market increasingly becomes aware of the use of building lime for historic masonry restoration there will always be challenges in making sure everyone understands the decisions they are making, why, and most importantly, the materials they are working with. Take lime for instance, everyone seems to believe that going back to the old mixes of yesteryear is a better choice than that found in Isle 14 at the local Home Depot when it comes to mortar selection for historic masonry structures. But just knowing about a subject and really understanding a subject are two entirely different things. The product of lime is pretty basic. You have lime putty, purchased wet (Philadelphia cream cheese consistency) in a bucket or barrel, and you have dry hydrate lime purchased in a 50lbs. (fifty pound) bags (fluffy and very light weight).
Common cement/lime mortar mix formulations in the restoration industry center around 1:1:6; 1:2:9; and 1:3:12 (Type N, O, and K respectively- ASTM C270-10, proportion specification). The second numeral reflects the amount of lime to be added to the formulation to create the desired mortar and thus the characteristics of that mortar. Generally, a mortar with more lime will tend to have better workability, higher flexural bond and more autogenious healing properties than a mortar with less. If its compressive strength your after than 1:1:6 is your answer, if you are looking for the flexibility to accommodate for future movement than you will likely be happy with a 1:2:9 or 1:3:12 formula. And then of course there is the historic straight 1:2.5 lime-sand mortar almost always made with lime putty and not dry hydrate lime, let me explain one of the reasons why.
Lime, like portland cement, is measured as a dry powder when mixing individual ingredients at the job site. Small batches of mortar are mixed from opened bags using a coffee can or some other used drinking cup (seven-eleven big gulp cup works good) up to a five gallon bucket depending of the project needs. But here’s the real scoop – Dry hydrate lime experiences a significant volumetric loss when converted to a wet paste during mixing. Let me say that again, Dry hydrate lime experiences a significant volumetric loss when converted to a wet paste during mixing. Volume changes that occur when dry hydrated lime is converted to a wet paste can cause sizable errors in proportioning mortar formulations; the most likely error is over-sanding.
A given amount of hydrated lime occupies far more volume as a dry powder than it does after mixing with water. Thus, when lime is measured as a dry powder, less is actually put into the mixture than is used if the lime is measured as putty. When wetted, dry hydrate lime will typically contract, on average, to 75% of the original dry volume. Using a nominal 1:2:9 mixture (Type O) cement/lime/sand, the variation caused by wet verses dry measure of the lime results in a 1:1.5:9 mixture. This ratio exceeds the allowable sand content in ASTM C270 of 2.5 to 3 times the binder, and is actually 3.6 times the cement plus lime; thus an unintended over-sanded mixture results. To avoid this problem an additional amount of dry hydrate lime (25%) must be added to all formulations during the proportioning stage, or just use lime putty. Note: Portland cement does not experience this volumetric loss when converted to a wet paste during mixing.
Phillips, Morgan, A Source of Confusion about Mortar Formulas, APT Bulletin 1993 http://www.jstor.org/pss/1504465
I often get asked this time of year, “How late in the construction season can we work with lime-sand mortars?” Well the quick and fast answer is 45 days before the first hard frost. Which means you should have your projects wrapping up by the end of October at the latest just to be safe. The reason for this safe period (as suggested by lime mortar manufacturers) is because of the way lime-sand mortars initially cure, by carbonation – absorbing CO2 back into the material through wetting and drying cycles. Most specifications call for a minimum of nine (9) wetting and drying cycles of misting the walls down with generous amounts of water after installation and allowing them to dry out naturally – drawing in the CO2 from the evaporation of the water. Obviously, this wet-curing process can become problematic during freezing temperatures.
The Brick Industry Association, March 1992, Technical Note 1 states, “Mortar which freezes is not as weather-resistant or as watertight as a mortar that has not been frozen. Furthermore, significant reductions in compressive and bond strength may occur. Mortar having a water content over 6 to 8 percent of the total volume will experience disruptive expansive forces if frozen due to the increase in volume of water when it is converted to ice. Thus, the bond between the unit and the mortar may be damaged or destroyed.”
But what if your schedule backs you up against old man winter and you have no choice but to work into November? Don’t lose heart. We carried out some testing 8 years ago on a project in Chicago [Lime Mortars, Two Recent Case Studies, Ed A. Gerns and Joshua Freedland] to find out how late in the season you could repoint a building using lime-sand mortars. Trials were conducted at various times in the fall and early winter at approximately six week intervals. The last installation occurred 48 hours before the first frost on November 23, 2003. Observations during installation, following initial curing, and periodically through the winter and following year were noted. The high-calcium lime putty and sand mortar showed no signs of shrinkage cracks, the bond between the mortar and brick units was well-adhered, and no erosion or cyclic freeze-thaw damage was observed. We were working with butter joint brickwork of 1/8 inch mortar joints.
To supplement the insitu testing, limited concurrent laboratory testing was conducted to evaluate the depth of carbonation and the impact of freezing temperatures had on the depth and rate of carbonation of the lime. Two inch mortar cubes were made from the same mortar formulation (1:2.5) – no additional water was added. The mortar cubes (eight sets) each went under freezing temperatures once for a four-hour duration at various times after initial mixing. The exposure to freezing temperatures (10F) was established at 24 hrs; 48hrs; 72hrs; 96hrs; 1 week and 2 weeks. The mortar cubes were then broken at various times and the depth of carbonation was measured using a phenolthalein solution as an indicator.
After 2 weeks, the depth of carbonation suggested that mortar cubes that were exposed to a freezing temperature for a limited duration during the first week had less depth of carbonation than the cubes that did not experience any freezing temperatures. After three weeks, however, this difference was no longer observable. All mortar cubes seemed to equalize after three weeks. Interesting. The success of this trial may be a result of the forgiving nature of lime-sand mortars, the low water content of the repointing mortar, and the narrow joint width of this particular project.
This paper was presented at the 2005 International Building Lime Symposium in Orlando, Florida. Proceedings are available on CD (ISBN 0-9767621-0-2). The CD includes 39 papers by authors from 10 countries. Also, included are several important historical documents related to building lime–some as old as 1920.
The conference proceedings are available for $25 at: http://www.lime.org/documents/publications/free_downloads/summary-ibls-2005.pdf
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.
Testing masonry materials for durability and performance has been going on for some time. It is important to study and test materials prior to construction of an actual wall to prevent wasted material, time and labor. Some of the earliest recorded tests performed on masonry mortar ingredients were carried out by the U.S. Army Corps of Engineers in the construction of fortifications during the early part of the 19th century.
The durability of masonry buildings relied heavily upon the past performance of the actual structures and the master mason’s experience with the individual materials. Much of the heritage knowledge of making good mortar was passed down from generation to generation through the trades. Testing mortar ingredients historically involved the masons working with the architects in a team approach for common understanding. However, signs of change began to appear as early as the 1890s.
Uriah Cummings writes in his book, “American Cements,” which was first published in 1898. “With their former teaching and experience on the one hand, and the testing machine on the other, the question was not long in doubt. The machine was victorious, and henceforth all judgment founded on experience was laid aside and they became blind believers in the tensile strain tests. What matter though they were continually befogged by the frequent, unreasonable, and capricious pranks of the machine, they had found a god, and were determined to worship it. And so it came to be established as a fixed belief among engineers and architects that the best cement was the one which tested the highest, and the manufacturer had no alternative but to strive to make his product test as high as possible.”
Seems from the tone of Mr. Cummings writing that he knew the industry was going in the wrong direction toward high compression. Is it high compression that destroys historic masonry? Well indirectly it does. Most mortars that are very high in compressive strength are very low in vapor permeability. The ability a mortar has to capture and release water easily through evaporation. What tends to happen in a historic masonry wall is moisture infiltrates by various ways; rising damp, poor roof/parapet/flashing details; driving rain; capillary action through cracks among other ways. The water needs to escape from inside the walls through the mortar joints ideally keeping everything dry from the inside out. Hard, high strength mortar prevents water from escaping thus trapping it inside the wall potentially causing damage to the masonry units of brick and stone as well as terra cotta over the course of time. It’s always better to insist on a lower compressive strength lime mortar that readily breathes with the masonry allowing quick evaporation of water, and in addition, provides the natural flexibility needed for traditional load-bearing masonry walls to perform at their best.
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