The American Natural Cement Conference, 2011
March 31 – April 1, 2011
New York City: American Museum of Natural History and Governors Island
Preserve as Found? Clarifying repair philosophy in the age of authenticity.
John A. Fidler, RIBA, IHBC, Intl Assoc AIA, FRICS, FSA, FIIC, FAPT
Simpson Gumpherz and Heger, Inc., Los Angeles
Inevitably in a generic document aimed at many situations, the Secretary of the Interior’s Standards for the Treatment of Historic Properties (National Park Service, Department of the Interior, Washington D.C., 1992 revised 2001), have a wide degree of overlap in the principles and guidelines expressed for preservation, rehabilitation, restoration and reconstruction of historic buildings and therefore provide room for choice and interpretation in the application of the four standards. Preservation Brief #2, Repointing Mortar Joints in Historic Masonry Buildings (National Park Service, Department of the Interior, Washington D.C., 1976, revised 1980 and 1998) focuses attention on design choices for replacement mortar but the latest edition does not fully account for all the current binder choices in the market place e.g., natural cement and natural hydraulic lime. The properties of historic, traditional, and new mortar types are not generally understood with consequent results that can vary considerably from initial expectations.
Confusion in the preservation community about specifications regarding the repair of decayed mortar joints is compounded when attempts are made to establish the values and significance of building materials, especially when issues of authenticity are raised (UNESCO World Heritage Committee, Nara, 1994). For example, is it necessary to repoint a building with the exact same materials that were originally used? These materials may no longer be available, their formulation may have changed, and they may not satisfy performance requirements for the building’s changed circumstances.
Decisions on the repair or restoration of historic buildings need also to take into account important technical, logistical, economic, and legal considerations that include functional changes to the building, weathering risks, material conservation strategies, craft, material and tool capabilities, contractual warranties. and value for money. The paper aims to articulate these issues and to provide clear guidance to aid decision-making.
ASTM C1713: The New ASTM Mortar Specification for Historic Masonry
John M. Wathne, PE, Chair, ASTM Task Group C12.03.03
President/ Structures North Consulting Engineers, Inc., Salem, MA
Historic Preservation in Masonry has evolved into an ever-growing industry serving an aging building stock with an ever widening range of issues. As a result, every historic masonry material ever used (other than clay) is available in a replicated, manufactured form with its own ASTM specification.
ASTM C270 was written to address mortars for new construction, and by default mortars that may be used in the repair of modern-era structures. A new specification has been needed to address mortars for the repair of historic masonry.
I will discuss some of the fundamental differences that make ASTM C1713 unique, such as:
- Covering all binder materials other than clay.
- Allowing a wider variation in binder to aggregate ratios.
- Different sample curing regimes and curing times for carbonating vs. hydrating.
- Inclusion of water vapor transmission, porosity, and maximum compressive strength as tested properties.
- Including a clearly delineated Proportion vs. Property specification, with different requirements in each.
I will also give examples of how to use ASTM C1713 in practice, both from a basic specifier’s perspective, and as a tool in evaluating and selecting the most appropriate formulation for a given structure.
If time permits, I will also discuss the Appendix (Commentary) to the Specification that we have just begun writing, and field questions and suggestions from the audience for ways that we might improve the Specification and items that that could be included in the Appendix.
Characterization and Degradation of Cementitious Materials
at Fort Sumter National Monument
Denis A. Brosnan, PhD
Bishop Chaired Professor of Materials Science & Engineering, Clemson University
Cementitious materials from Ft. Sumter National Monument, including masonry mortars, concretes, and coatings, have been characterized as to their chemical, physical, and mineralogical properties. The pre-1860 mortars and rubble concretes were found to be based on a binder phase of Rosendale cement and lime. Concrete from the 1890’s era construction of Battery Huger contained Portland cement and lime with granite aggregate and sand. These findings are in agreement with the historical records on procurements for the Fort construction phases.
Soluble salts were determined in mortar specimens by ion chromatography. All specimens contained levels of soluble salt that might be expected from sea water contact by direct impingement, sea spray, or immersion during hurricanes. Accompanying the ions expected in salt water were high levels of soluble calcium from mortars. X-ray diffraction of mortar specimens allowed identification of a number of alkali and alkaline earth salt reaction products between the mortar and environmental species.
There was direct microscopic evidence of selective dissolution of calcium carbonate from the mortars, a process well documented in technical literature. The result of this corrosion is a porous skeleton of material, with attack leading to an increase in porosity and generally an increase in the fraction of porosity less than one micron in diameter. A second effect noted was the salt scaling of clay bricks (cryptoflorescence), a wear process particularly evident in areas of modern repointing with Portland cement mortar. The primary source of the salt involved in cryptoflorescence was the calcium leached from repointing mortar by salt water contact. Scaling of the bricks occurred over successive drying periods for the Fort’s walls.
Because Rosendale cement was used extensively in the construction of Ft. Sumter, various instrumental techniques were utilized in analysis of modern Rosendale cement. Further,modern Rosendale cement was used in salt immersion experiments conducted to guide restoration decisions.
The short-term immersion experiments were conducted with candidate restoration mortar specimens that had been exposed to accelerated curing conditions. In these studies involving successive immersion and drying cycles, expansion was monitored along with calcium dissolution. Mortars containing substances to form pozzolans in-situ and specimens containing natural cement exhibited the best performance in these tests.
These studies have placed emphasis on the fact that calcium leaching by sea water infiltrated mortar is an essential factor to be known in restoration processes, as it affects the performance of the bricks as well as the mortar and the other cementitious materials. It is clear that direct sea water impact and rising damp is deleterious to masonry.
Modernist Vision and Inherent Vice: The Textile-Block System of Frank Lloyd Wright
Jeffrey M. Chusid
Associate Professor, Historic Preservation Planning Program, Cornell University
Late 19th and 20th-century modernism, as a design movement, has been differentiated from the periods that came before because of its particular challenges for preservationists. These are a consequence of experimentation in construction and materials choices, of attempts to reduce costs by cutting redundancy, and of the use of solutions that were rarely repeated, or were the products of now-defunct manufacturing processes. As a result, many modernist works contain “inherent vice,” flaws in design or construction that lead to serious problems later in the life of the structure.
Like many modernist architects, especially those working in the 1910s and 1920s, Frank Lloyd Wright experimented with various forms of concrete construction. In 1923-1925, Wright designed four concrete-block houses in Los Angeles, along with an unbuilt school. All but one of these was designed in what Wright would later call his “Textile-block” system, which consisted of 16″ square concrete tiles, 3 Â½” thick, interwoven with steel reinforcing. Wright would continue to use this system, and a closely related one he called the Usonian Automatic, for many projects over the rest of his life. In fact, the largest single collection of his buildings in any one place, Florida Southern College, is constructed using the textile-block system.
Wright’s goals for the system were quintessentially modernist:
- A mono-material system, in which a single material was structure, skin and ornament
- A low-skill, do-it-yourself system
- A system meant to be low cost, to increase housing for the middle-class
- Adaptable to virtually any site, climate, and scale of construction
However, many, if not most, of Wright’s block buildings are failing, despite the collective investment of many millions of dollars. This is due in large part to flaws in the textile-block system, both those resulting from his ideals (e.g., blocks made by hand by unskilled labor), and those inherent in the design (such as the mix specification and the exposed condition of the reinforcing). In fact, an in-depth analysis of the system provides a great case study of materials performance and conservation, and of the opportunities and challenges faced by modern architects and their experiments in concrete.
Based on the author’s involvement with the restoration of the Freeman and Ennis Houses in Los Angeles, and Florida Southern College in Lakeland, Florida, as well as surveys of other of Wright’s block projects, this paper will describe the design, manufacture, installation and history of use of the textile-block system to highlight the technical challenges being faced preserving this important body of work, and then to address the question of what constitutes integrity or authenticity in these works.
American Museum of Natural History
Kyle Normandin, WJE Associates
The Seventy-seventh Street wings of the museum were designed by Cady, Berg & See and completed in the last decade of the 19th century. The pink granite facades are graced with a sweeping granite clad stairway, punctuated by magnificent cherry wood windows and capped with brilliant red slate roofs.
Appropriate restoration of these facades required research and methodical investigations including survey, testing, analysis and trial repairs. Laser scanning was used to develop CAD base drawings for use by the design team. Hands-on masonry surveys and probes were conducted at a majority of the facade surfaces. Access to the facade was gained through the use of boom trucks and via rope.
Masonry repairs included traditional dutchman patches, mortar repairs, and miscellaneous stabilization operations. Matching the multicolored pink and red granite required more than a half dozen stone sources and individual matching for each of the nearly 400 dutchman repairs. Granite was cleaned through the use of a gentle micro-abrasive system without harsh chemicals that had damaged granite surfaces in the past.
At the plaza, a new raised central planter and seat wall modeled on the facade was constructed with complimentary granite. Plantings are in the form of a parterre bordered by low boxwood hedges. The fountain placed at the center references historical renderings depicting a much larger fountain originally proposed for this location.
New facade lighting consists of low wattage lights at grade near the facades that provide a glow to the lower portions of the facade that fades as the eye travels up the facade. At the two corner towers, the illuminated conical stone roofs punctuate the extent of the facades.
Rosendale Pointing Mortar:
Full repointing was recommended due to the deteriorated nature of the mortar. Laboratory analysis indicated seven types of mortar including natural cement hydrated-calcitic lime mortars suggestive of Rosendale natural cement. Several samples revealed a deep wine color representing the original finish pointing mortar. According to historic reference Cummings in American Cements, “the American Museum of Natural History was built using ‘American Rock Cement’ from Rosendale, New York, also known as ‘natural cement'”.
Authentic Rosendale cement had recently been brought back to market by Edison Coatings but was not initially specified as WJE laboratory testing suggested it lacked workability and could be difficult for masons to work with. Through a succession of trial samples that paralleled the development of ASTM C10 Standard Specification for Natural Cement, Edison was able to reformulate the Rosendale mortars for full-scale trials at the museum facade. The trials were successful and the job proceeded accordingly.
Key Project Facts:
- Dates of Construction 1891-1897
- Dates of Restoration 2007-2009
- Facade Area 100,000 Square Feet
- Restoration Cost $30 Million
- Central Park Cement Sites Tour
Central Park Police Precinct
Kate Ottavino, A. Ottavino Corp.
The Central Park Police Precinct is a national and New York City landmark and has been home to the New York Police Department since 1936.Â The stationhouse was originally designed as stables in the 1870s by Jacob Wrey Mould, who served as Central Park’s chief architect and also created Bethesda Fountain’s terrace, Belvedere Castle and the Sheepfold, better known until just recently as Tavern on the Green.
The $50 million restoration project was initiated to restore the building itself, accommodate a new larger program from the police, and allow for a modernized police precinct stationhouse. Restoration mortar replicated the original 1870’s natural cement mortar.
Bethesda Terrace and Arcade Tour
Matt Reiley, Central Park Conservancy
Bethesda Terrace and Arcade is the architectural centerpiece at the heart of Central Park in New York City. The magnificent split-level terrace, which overlooks the park’s lake and world-renowned Bethesda Fountain, has seven arches and a columned passageway with walls on either side forming an arcade adorned with trompe l’oeil paintings. The large ceiling, part of the arcade’s original design, was first installed in 1867. It contained 16,000 handcrafted Minton ceramic tiles.
In addition to the intricate restoration of the tile ceiling, the Conservancy undertook repointing of the sandstone columns and structure using a natural cement mortar, matching the original materials.
Lecture and Hands-on Demonstration 1
Rosendale and French Lime Mortars.
Proportioning Natural Cement Mortars
By Chad Lausberg, Presenter: Michael Edison
Many variables affect the compressive strength of a natural cement mortar. Understanding these variables and learning from past mix design mistakes are beneficial when designing mixes for natural cement repointing and rebuilding mortars.
Ten typical natural cement mortar mixes, representing a range of proportions as found in existing historic structures, were replicated. These mortar mixes were allowed to cure over a period of four months, and compressive strength was determined at different intervals. Test results were plotted to produce compressive strength curves, providing the specifier with greater insight into how natural cement mortars develop strength over time and how sand, lime, and pigment, added to natural cement, affect mortar compressive strength. The results provide useful guidance in proportioning natural cement mortar mixes to achieve particular target strengths.
The results of this research show that natural cement with the least amount of additives will develop the highest compressive strengths over time. Further, the analyses of several historic mortars indicate that natural cement-rich mortars were often used, and that common contemporary mortar proportions of between 2Â¼ and 3 parts aggregate to 1 part binder by volume should not be adopted as default proportions for current mix designs of natural cement repointing and rebuilding mortars.
Lecture and Hands-on Demonstration 2
Matching Visual Appearance and Surface Textures In the Repair
of Historic Concrete and Stucco
John E. Harry, Preservation Consultant
John E. Harry Restoration Services, Allentown, PA
The author will discuss techniques that can be employed in matching cement-based repairs to the surrounding, weathered surfaces of the original material. The importance of matching the components of the repair material to the original masonry composition will be stressed along with the role of petrographic analysis and the information that this can provide. Color is obviously an important part of matching appearance and is often problematic. Color of the finished patch depends on the components (sands, aggregates, cement, and pigmentation) and the techniques used to treat the surface of the patch and the methods to cure the material.
There are a number of ways of evaluating color including the use spectrophotometers and calibrated digital photography to assist in determining the color components of the surface and in assessing the accuracy of color matches. The exposure of sand and aggregate affects both color and texture of the surface. The texture of surfaces is an often-overlooked aspect of repair work. There are a number of simple and effective techniques for exposing aggregates and texturing surfaces. And, the conditions under which patches cure can permanently affect the color.
A prime requirement of most preservation work is to do repairs that are structurally sound and long-lasting. This requirement does not have to conflict with concerns about visual appearance. The author will describe ways to accomplish aesthetic matches while following accepted procedures for preparing surfaces and installing patching.
Case studies will be presented to show the application of the techniques that the author has used for matching colors and finishes. The projects will include: Randolph Hall and Porter’s Lodge at the College of Charleston, Charleston, South Carolina – natural cement stucco; Lakefront Airport Terminal, New Orleans, Louisiana – exposed-aggregate, decorative Portland-based stucco; Katy Trail railroad overpasses, Dallas, Texas – weathered concrete; and Merritt Parkway bridges and overpasses – multiple Portland cement concrete bridges with varying colors and weathering.
Attendees will be able to see and participate in the process of matching cementitious finishes.
Documentation of the use of Iron-Laden Mortars by the Army Corp of Engineers in New York Harbor and Analysis of the Iron-Laden Mortar at Castle Williams, Governor’s Island, NY
Dede Nash & Mark Kanonik
Einhorn Yaffee Prescott Architecture & Engineering PC, Albany, NY
Iron-laden mortar, used by the US Army Corp of Engineers in the construction of a defensive system for New York’s inner harbor, is evident in the original, 19th century, load-bearing masonry walls of Castle Williams, a circular fortification located at the west point of Governors Island.
The mortar used to lay the walls was described by the designer of the building as a cement of mortar and iron borings. While the presence of iron borings is easily identified in the existing mortar, the nature of the cement and other binders is more complex and difficult to identify. Methods of analysis will be discussed in this case study, including petrography, metallurgy, field evaluation, and historic records documentation. Through petrographic analysis of the remarkably cohesive, 200-year-old setting mortar found extant in the walls, the component binders will be identified in proportion to the iron borings utilized in the mortar mix.
Additional research will relate the historic, iron-laden mortar to modern metallic non-shrink grout to determine how the iron borings, as an expanding agent, was affected by the mineral inclusions in the mortar and ultimately affected the stone it was set into.