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Moravian College • Preservation Master Plan because it exacerbates rising damp and related moisture problems. The desire for a clean building must be weighed against the possible risks to the masonry materials. Holes, vents, or borings in brick and stone masonry that can admit water should always be limited in size and number. Any opening should be cut through mortar joints with minimal harm to the adjacent masonry. Openings with protruding metal elements, such as pipes or conduit, should be treated with an appropriate sealant colored to match the existing material. Openings that have been abandoned with no protruding elements should be repaired by completely moving any embedded metal and installing a cementitious patch that matches the existing masonry surface in color, strength, texture, and finish. Masonry Crack Repair All cracks in masonry must first be properly diagnosed. Cracks caused by structural stresses must be investigated by a structural engineer who can recommend remedial repairs. As with moisture problems, the underlying structural problems must be addressed before performing repairs. Cracking from a one-time event, such as small-scale settlement, may require a cementitious mortar or grout repair. Cracking through masonry joints should be repaired by repointing the affected joints. Cracking through masonry units may require the installation of reinforcement and a cementitious patch or grout repair. Long, deep cracks in the masonry must be patched using a knife-grade patching compound to prevent further moisture penetration. The visual impact of such a repair should be minimized by using a colored mortar that is similar to the color of the masonry being patched. It is important to note that not all cracks in masonry require repair. Cracks may simply be a part of the natural weathering process for some stone masonry. Small, hairline cracks on vertical surfaces of stone masonry should not be repaired unless they are deep enough to allow water to infiltrate into the masonry wall. However, such cracking on horizontal wash surfaces should be patched with a knife-grade patching compound to prevent water infiltration. Corroding Lintel Repair The repair of corroding lintels is a costly and invasive procedure. The steel lintel is a substantial structural element, which causes major masonry disruption as the metal corrodes and expands. The greater the amount of corroding metal in the wall, the greater the disruption of the masonry units. The long-term solution to the problem of corroding lintels is to remove the piece of corroding metal in its entirety. First the window must be properly shored, using techniques that have been approved by a structural engineer. Then the overlying brick or stone units must be removed, a minimum of four courses of brick above the window and one foot to either side. The original bricks or stones should be removed as whole units, cleaned of mortar, and salvaged for use in the masonry repair. The corroding lintel can then be removed and replaced with a new one. The new lintel must be properly flashed, with the flashing cut into the masonry backup at the top, then run vertically against the masonry backup for a minimum of eight inches, and then flashed over the steel angle. If the flashing cannot be cut into the masonry backup, it must be attached using a termination bar and high-quality sealant. The salvaged brick or stone should be used to reconstruct the masonry facing over the window lintel. It is important that the masonry repair follow the brick or stone size, pattern, and row height in the surrounding wall. Replacement mortar must match the original in its thickness, color, texture, and finishing. Patches and Dutchmen Small pieces of masonry lost through spalling or delamination can be repaired with a cementitious patching compound that matches the color and hardness of the primary masonry. Proprietary patching compounds must only be installed by trained masons. Many manufacturers offer training courses and product certification for masons. Commercially-available patching compounds can be either Portland cement-based or natural hydraulic lime-based. It is important to choose a patching compound that is compatible with the compressive and flexural strengths and permeability characteristics of the masonry to be repaired. Damaged areas of masonry that are too large to patch may be repaired by installation of a masonry dutchman. In this procedure, the deteriorated portion of the masonry is cut away and a new piece John Milner Associates • October 2009 • Chapter 9 • Building Guidelines • 288
Moravian College • Preservation Master Plan of masonry (the dutchman) is installed to match the existing. Dutchman repair is a much more durable repair than a cementitious patch repair. A cementitious patch may need to be replaced after 10-15 years, while a properly-installed dutchman should last as long as the masonry itself. Dutchman repairs require skill to install correctly and should not be attempted by inexperienced personnel. Consolidation Consolidation of masonry should only be considered in situations where the masonry is friable (prone to crumbling) and exhibits surface disintegration. Consolidation works on a microscopic level to strengthen cohesion between grains. Consolidation is not appropriate for delamination, spalling, or large-scale cracking. Commercially-available consolidants are not appropriate for masonry containing a calcium carbonate binder, such as limestone. Consolidants have only proven effective on silicate-based masonry such as brick or sandstone. If the masonry type and deterioration warrants consolidation, the first step is to characterize the masonry through petrographic analysis and materials characterization. Testing is performed in a laboratory to investigate how the consolidant treatment affects the physical characteristics of the stone. It is also important to investigate how effectively the masonry takes up the consolidant treatment. Consolidants must not be used if the masonry is adversely affected or if the consolidant is not properly absorbed. If laboratory testing determines that the consolidant is well-absorbed by the masonry and does not significantly alter its physical characteristics, then the consolidant should be tested in field mockups. Consolidation is an irreversible treatment and should not be undertaken by unskilled personnel or without proper laboratory and field testing. Cleaning Careful project planning is essential when making decisions about cleaning historic masonry. The initial assessment should evaluate the historic material, the type of soiling, the reason for cleaning, and the cleaning method. Cleaning should be undertaken only where dirt or other material obscures significant architectural features, or is causing, or has the potential to cause, damage to masonry. Cleaning methods should be carefully selected to do the job without harming the historic material. It is also important to repoint deteriorated mortar joints prior to cleaning to ensure that water does not penetrate the wall during cleaning. Cleaning should never remove the patina that is evidence of a structure’s history and age. It should never be performed for the sole purpose of achieving a new appearance. Moreover, it is important to have realistic expectations: historic buildings will never have the same clean appearance as those that were recently built. It is better to under-clean than to over-clean because the latter inevitably causes damage. Cleaning treatments fall into three general categories: water-based, chemical, and mechanical methods. Water-based methods include pressurized water spray, heated water treatments, and mist-spray. Chemical methods involve the use of soaps, detergents, acidic and basic cleaners, and biocidal treatments in a variety of gels, liquids, pastes, and poultices. Mechanical cleaning methods include the use of tools, such as brushes, scrapers, and specialized rotating and laser-based cleaning equipment. It is possible to combine treatments for the best results, such as combining mild mechanical methods with low-pressurized water spray. The preservation approach is to always employ the gentlest cleaning method starting with low-pressure water and natural bristle brushes. Water pressure should be no stronger than 150-200 pounds per square inch (psi). High-pressure water spray can damage masonry surfaces and drive moisture into cracks and joints. Several cycles of mist-spray can be effective in removing some black gypsum crusts with minimal scrubbing. Any cleaning method using water should not occur when the temperature will fall below 50 degrees Fahrenheit for three days (72 hours) after cleaning. Chemical treatments should be approached with great caution because they can cause irreversible damage. They should never be applied by unskilled personnel. Understanding the physical properties of the masonry and type of soiling in question is a vital first step before proposing or testing any chemical cleaners. If chemicals must be used, test panels should be developed and carefully evaluated to avoid over cleaning. Chemical cleaners must be chosen by a knowledgeable professional who understands the type and condition of the masonry material to be cleaned. Only non-acidic neutral pH detergents should be used in conjunction with non-metallic brushes or scrapers; metal brushes John Milner Associates • October 2009 • Chapter 9 • Building Guidelines • 289
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