Forts - Brick Construction
http://www.nps.gov/goga/history/seaforts/chap9&10/brick.htm
Brick Construction: General
Masonry work includes historic brickwork from the post-Civil War period and the materials and operations associated with its treatment. The brick used in the fortifications is a dense, reddish-brown, common brick laid in a running bond pattern with regularly spaced header joints. Laid in multiple wythe walls and utilizing arches and vaulting to span openings, the brickwork is set in a Portland cement-sand mortar. Little or no lime was used in the mortar.
Causes of Deterioration:
Rising damp from subsurface moisture sources.
Windblown moisture in the form of rain.
Condensation due to lack of ventilation.
Moisture infiltration through deteriorated moisture joints.
Moisture accumulation from the encroachment of vegetation.
Moisture from inadequate surface drainage.
Improper maintenance.
Improper coatings that trap moisture.
Failure of waterproofing, roofing, or protective coatings.
Identification:
Brickwork deterioration can be readily identified by visual inspection. Identifying characteristics include:
Mildew, staining, or efflorescence.
Soft, loose, or crumbly mortar.
Spalling.
Cracking.
Deflection.
Loose bricks.
Inspection and Testing:
Determining the causes and extent of deterioration of historic brick work requires careful field investigation, analysis, and laboratory testing. Review of drawings and other documents can enhance the identification of deterioration. Inspection and testing procedures include:
Field Inspection: Locate and record the extent of brickwork deterioration.
Document Review: Refer to original drawings and engineering reports and compare to field data. Check historic records and photographs.
Field Testing: If required, institute a field testing program including moisture meter readings and sampling of bricks and mortar for laboratory analysis.
Laboratory Analysis: Laboratory analysis includes:
Brick compression tests
Mortar composition analysis (by volume)
Moisture absorption of brick
Review of field inspection, field testing, and laboratory analysis should give a comprehensive view of the causes of deterioration. Based on the results, a plan for corrective treatment can be developed and tailored to meet treatment objectives.
Brick Construction: Identifying the Problem
Brickwork deterioration can be the result of individual causes or a number of related causes acting in concert.
Spalling:
Spalling is a loss of surface material due to moisture infiltration. It occurs when moisture inside the brick expands and contracts due to thermal action and when moisture evaporates at the surface of the brick. In climates where freeze-thaw cycles are frequent and extreme, spalling is more pronounced. Expansion and contraction, and evaporation, are mechanical actions that exert force and stress inside the brick. The presence of spalling may indicate that a mortar either too dense or too high in cement content has been used in joints. Dense and brittle mortars obstruct the migration of moisture from the interior of the brick to the point where evaporation occurs. Mortar joints act as sacrificial wicks allowing inevitable deterioration to occur at a location and in a material that is easily repaired. The critical relationship between bricks and mortar relies on the fundamental rule that the mortar should never be harder or more dense than the brick.
Cracking:
Cracking may occur along mortar joints or through bricks. Cracking can be caused by structural movement due to expansive soils, by tree roots too close to a building, by inherent defects in the original construction, by imbedded materials, or by the use of rigid mortars that do not allow normal expansion and contraction. Cracks that follow mortar joints can be addressed through normal treatment procedures while cracks through bricks indicate more severe structural problems. Cracks most often occur at masonry openings.
Cross section of brick wall showing header course and cement plaster parging on earthwork side of brick wall.
Efflorescence:
Efflorescence indicates that soluble salts are present within a brick and are migrating to the surface of the brick. An indication of chemical reactions within the brick, efflorescence can signal moisture-related deterioration.
Mortar Deterioration:
Loss of mortar, while an expected masonry condition, can contribute to further deterioration by allowing continued moisture infiltration. Loss of mortar can be caused by normal leaching-out of lime and through cracking of rigid, high-cement mortars.
Structural:
Structural deterioration is caused by excessive loading, differential load distribution, soil instability, and inadequate foundation support.
Brick Construction: Treatment Overview
Eliminate the Retention of Moisture:
Clean drains and clear drainage paths. Clear encroaching vegetation and slope grades to drain away from the structure. Install additional drains and repair existing drains.
Eliminate or Minimize Rising Damp:
Install damp course or mechanical barriers that resist hydrostatic pressure. The barrier may be in the form of thru-wall flashing, chemical injection, or surface-applied moistureproofing. Install the barrier above grade. Thru-wall barriers are best installed in mortar joints by raking out the mortar and inserting flashing material in small sections that do not exceed the width of the wall. Flashing sections may be shingled, or lapped as the work proceeds horizontally along the joint. Injection of chemical consolidant involves the saturation of a portion of the masonry with a material that will render the masonry impermeable. Such a procedure depends on the porosity of the masonry and requires extensive testing and coordination. Surface applied waterproofing involves digging out around the base of a wall and installing a vertical barrier from the base of the wall to a point above grade. The vertical barrier is in the form of a membrane material or trowel-applied asphaltic material. In some cases vertical barriers enhance hydrostatic pressure by creating a wick. Vertical barriers must be used in conjunction with other treatment methods that relieve the hydrostatic pressure before it can rise in the wall.
Brick spalling caused by moisture and thermal expansion and contraction and the use of high-cement mortars.
Consider Cracked Brick:
Cracked brick should be replaced only if the cracking goes all the way through the brick and is a part of a larger, more extensive cracking pattern. Replacement will depend on the availability of matching brick. Small hairline cracks that do not extend through the brick should be left alone. Cracks wider than one-sixteenth inch can be repaired with tinted grout that matches the color of the brick.
Brick Spalling is Almost Impossible to Repair:
In some cases, individual bricks may be removed from the wall, cleaned, and reinstalled with the damaged face to the inside of the wall. Where spalling is severe, and reversing the bricks is not possible, remedial efforts may be taken to save the remaining fabric of the material. Remedial measures include application of a water-repellent coating or the application of paint that matches the original brick color. Application of protective coatings is not generally recommended but if used, the coating should be breathable and should not significantly alter the brick finish. If salvaged or replica bricks can be obtained, severely damaged brick may be replaced.
Brick Construction: Mortar and Repointing
Mortar joints deterioration is the most common brick masonry problem. The repointing of deteriorated mortar joints requires the careful removal of deteriorated, inappropriate, or loose mortar; cleaning the joints; and installation of new mortar.
Identification and Inspection:
Original mortar should be tested to determine its original constituent materials by volume. The constituents will include cementitious materials and aggregate. The cementitious materials are composed of Portland cement and/or lime. The aggregate is usually sand. The usual and accepted ration of cementitious materials to aggregate is 1:3. That is: one part cementitious materials to three parts aggregate (by volume). While most nineteenth century mortars before 1880 were lime-sand mortars, the mortar used for fortification construction around San Francisco Bay for brick masonry was high in Portland cement content. The extensive use of Portland cement mortar was successful because of the relatively constant climatic conditions of humidity and temperature, and the rarity of freeze-thaw cycles. The hardness of the brick was also a factor allowing the use of a harder mortar. The character-defining features of the mortar depend on the color of the binders, the aggregate, and the joint treatment or finish. Mortar joints from the post-Civil War period were found to be flush to slightly concave joints about three-eighths to one-half inch wide.
Typical brick joint showing repointing technique where deteriorated mortar is raked out to a depth equal to about twice the joint width and replacement mortar is built up in layers.
Treatment:
Rake out loose mortar from joints using handtools, such as chisels, and remove dust and small debris with a brush of compressed air. Avoid damage to adjacent brick.
Mortar for repointing brickwork should be mixed in the following proportions, subject to adjustments based on laboratory analysis:
White Portland Cement:
ASTM C 207, Type S one part
White Hydrated Masons Lime:
ASTM C 150, Types I or III one part
Screened Local Beach Sand:
ASTM C 144 six parts
The mortar should be mixed in a paddle mixer or by hand with clean potable water. Based on laboratory analysis, the proportion of lime to cement may vary but the cementitious to aggregate ratio of 1:3 by volume must be maintained.
Finish joints to match original construction profile; concave or flush. After pointing, mortar may be tooled, brushed, or wiped (with burlap) when mortar has set to "thumbnail" hardness.
Brick Construction: Cleaning and Restoration
Masonry cleaning and restoration involves the removal of stains, mildew, dirt, grime, efflorescence, and paint from the brick surface. Masonry cleaning should be approached in a graduated manner. In arriving at an appropriate cleaning and restoration program, proceed from the least strong cleaning method to stronger methods. Use only enough chemicals and force to clean the material. It is preferable to retain existing imperfections than to permanently damage the structure by improper cleaning. Under no circumstances should brick masonry be abrasively cleaned or blasted.
Inspection and Testing:
Masonry cleaning and restoration should be accomplished only by experienced specialists implementing a comprehensive program. The cleaning and restoration program should be based on the approved results of field testing and sample panels. The cleaning and restoration program must be tailored to specific needs. Most cleaning can be accomplished with low pressure water blasting in association with scrubbing with a soft bristle brush. Isolated areas that retain staining or painted coatings such as graffiti after initial cleaning may require stronger measures.
Treatment:
Prior to the start of overall cleaning, clean a sample control panel for approval and reference. Demonstrate materials and methods to be used for cleaning the brick on the sample panel. The panel should be selected to include a range of cleaning and restoration requirements and should be of adequate size. Allow panel to stabilize for seven days before proceeding with other cleaning work. Longer observation may be appropriate.
Prepare a written program of procedures to be used including a description of the cleaning methods, working pressures, materials, equipment, and other information for each type of cleaning procedure. Comply with safety and environmental requirements.
Clean masonry surfaces only when the air temperature is between forty degrees Fahrenheit and eighty degrees Fahrenheit and will remain so for at least forty-eight hours after completion of the work.
Perform cleaning and restoration work in sequence with other masonry work. Clean masonry surfaces prior to repointing or other restoration work.
Proceed with cleaning in an orderly manner; work from the top to the bottom of each segment and from one end of a structure to the other. Clean in a uniform and consistent manner. Rinse off any residue by working upward from the bottom to the top of each treated area of each segment.
Apply water or cleaners in compliance with pressure, volume, and temperature requirements. Hold spray nozzle not less than six inches from the masonry surface and spray from side to side in overlapping bands to insure uniform coverage. Use low-pressure spray from 100psi to 300psi at three to six gallons per minute.
Pre-wet masonry to soften and loosen surface materials. Wash, scrub, and spray with low-pressure spray. Apply cleaner only in accordance to manufacturer’s written instructions. Rinse as required to remove all chemicals and residue. Repeat cleaning process if required.
Materials and Equipment:
Water: clean, potable, non-staining, and free of oils, acids, salts, and organic matter.
Brushes: fiber bristle only
Spray Equipment: low-pressure tank or chemical pump with a fan-shaped spray tip with an angle of not less than fifteen degrees.
Chemical Cleaning Solutions: dilute all cleaning solutions to produce mixes of a concentration not greater than that required to clean the masonry.
Note: Coordinate cleaning and restoration with other applicable sections in chapter 10.
Brick Construction: Graffiti Removal
Graffiti removal should be treated as a separate and distinct cleaning process. Graffiti removal will require specific treatment based on the type of paint used, the number of layers, the condition of the substrate, and the degree to which cleaning may permanently affect the historic materials involved. Where large areas have been painted with many coats of paint treatment may be different than small areas that have a single coat of paint. If graffiti removal, based on tests and sample panels, will permanently harm the historic materials a non-permanent, reversible sacrificial coating may be applied to enhance the visual effect.
Painting Out Graffiti:
Temporary solutions for problem graffiti areas include painting over the graffiti with two coats of any high quality latex paint of commercial grade. No special specifications are required.
Note: Refer to Finishes: General; Exterior Concrete Coatings; and Graffiti Removal, for details and coordination.
See also Martin E. Weaver, Removing Graffiti from Historic Masonry, Preservation Briefs, No.38, National Park Service, 1995, and, Anne E. Grimmer, Keeping It Clean: Removing Exterior Dirt, Paint, Stains and Graffiti from Historic Masonry Buildings, National Park Service, 1988.
Cavallo Battery. Preservation charette discussing treatments for graffiti cleaning or breathable, non-permanent coating.
Cavallo Battery. Graffiti on brickwork forming angle above vault.
Brick Construction: General
Masonry work includes historic brickwork from the post-Civil War period and the materials and operations associated with its treatment. The brick used in the fortifications is a dense, reddish-brown, common brick laid in a running bond pattern with regularly spaced header joints. Laid in multiple wythe walls and utilizing arches and vaulting to span openings, the brickwork is set in a Portland cement-sand mortar. Little or no lime was used in the mortar.
Causes of Deterioration:
Rising damp from subsurface moisture sources.
Windblown moisture in the form of rain.
Condensation due to lack of ventilation.
Moisture infiltration through deteriorated moisture joints.
Moisture accumulation from the encroachment of vegetation.
Moisture from inadequate surface drainage.
Improper maintenance.
Improper coatings that trap moisture.
Failure of waterproofing, roofing, or protective coatings.
Identification:
Brickwork deterioration can be readily identified by visual inspection. Identifying characteristics include:
Mildew, staining, or efflorescence.
Soft, loose, or crumbly mortar.
Spalling.
Cracking.
Deflection.
Loose bricks.
Inspection and Testing:
Determining the causes and extent of deterioration of historic brick work requires careful field investigation, analysis, and laboratory testing. Review of drawings and other documents can enhance the identification of deterioration. Inspection and testing procedures include:
Field Inspection: Locate and record the extent of brickwork deterioration.
Document Review: Refer to original drawings and engineering reports and compare to field data. Check historic records and photographs.
Field Testing: If required, institute a field testing program including moisture meter readings and sampling of bricks and mortar for laboratory analysis.
Laboratory Analysis: Laboratory analysis includes:
Brick compression tests
Mortar composition analysis (by volume)
Moisture absorption of brick
Review of field inspection, field testing, and laboratory analysis should give a comprehensive view of the causes of deterioration. Based on the results, a plan for corrective treatment can be developed and tailored to meet treatment objectives.
Brick Construction: Identifying the Problem
Brickwork deterioration can be the result of individual causes or a number of related causes acting in concert.
Spalling:
Spalling is a loss of surface material due to moisture infiltration. It occurs when moisture inside the brick expands and contracts due to thermal action and when moisture evaporates at the surface of the brick. In climates where freeze-thaw cycles are frequent and extreme, spalling is more pronounced. Expansion and contraction, and evaporation, are mechanical actions that exert force and stress inside the brick. The presence of spalling may indicate that a mortar either too dense or too high in cement content has been used in joints. Dense and brittle mortars obstruct the migration of moisture from the interior of the brick to the point where evaporation occurs. Mortar joints act as sacrificial wicks allowing inevitable deterioration to occur at a location and in a material that is easily repaired. The critical relationship between bricks and mortar relies on the fundamental rule that the mortar should never be harder or more dense than the brick.
Cracking:
Cracking may occur along mortar joints or through bricks. Cracking can be caused by structural movement due to expansive soils, by tree roots too close to a building, by inherent defects in the original construction, by imbedded materials, or by the use of rigid mortars that do not allow normal expansion and contraction. Cracks that follow mortar joints can be addressed through normal treatment procedures while cracks through bricks indicate more severe structural problems. Cracks most often occur at masonry openings.
Cross section of brick wall showing header course and cement plaster parging on earthwork side of brick wall.
Efflorescence:
Efflorescence indicates that soluble salts are present within a brick and are migrating to the surface of the brick. An indication of chemical reactions within the brick, efflorescence can signal moisture-related deterioration.
Mortar Deterioration:
Loss of mortar, while an expected masonry condition, can contribute to further deterioration by allowing continued moisture infiltration. Loss of mortar can be caused by normal leaching-out of lime and through cracking of rigid, high-cement mortars.
Structural:
Structural deterioration is caused by excessive loading, differential load distribution, soil instability, and inadequate foundation support.
Brick Construction: Treatment Overview
Eliminate the Retention of Moisture:
Clean drains and clear drainage paths. Clear encroaching vegetation and slope grades to drain away from the structure. Install additional drains and repair existing drains.
Eliminate or Minimize Rising Damp:
Install damp course or mechanical barriers that resist hydrostatic pressure. The barrier may be in the form of thru-wall flashing, chemical injection, or surface-applied moistureproofing. Install the barrier above grade. Thru-wall barriers are best installed in mortar joints by raking out the mortar and inserting flashing material in small sections that do not exceed the width of the wall. Flashing sections may be shingled, or lapped as the work proceeds horizontally along the joint. Injection of chemical consolidant involves the saturation of a portion of the masonry with a material that will render the masonry impermeable. Such a procedure depends on the porosity of the masonry and requires extensive testing and coordination. Surface applied waterproofing involves digging out around the base of a wall and installing a vertical barrier from the base of the wall to a point above grade. The vertical barrier is in the form of a membrane material or trowel-applied asphaltic material. In some cases vertical barriers enhance hydrostatic pressure by creating a wick. Vertical barriers must be used in conjunction with other treatment methods that relieve the hydrostatic pressure before it can rise in the wall.
Brick spalling caused by moisture and thermal expansion and contraction and the use of high-cement mortars.
Consider Cracked Brick:
Cracked brick should be replaced only if the cracking goes all the way through the brick and is a part of a larger, more extensive cracking pattern. Replacement will depend on the availability of matching brick. Small hairline cracks that do not extend through the brick should be left alone. Cracks wider than one-sixteenth inch can be repaired with tinted grout that matches the color of the brick.
Brick Spalling is Almost Impossible to Repair:
In some cases, individual bricks may be removed from the wall, cleaned, and reinstalled with the damaged face to the inside of the wall. Where spalling is severe, and reversing the bricks is not possible, remedial efforts may be taken to save the remaining fabric of the material. Remedial measures include application of a water-repellent coating or the application of paint that matches the original brick color. Application of protective coatings is not generally recommended but if used, the coating should be breathable and should not significantly alter the brick finish. If salvaged or replica bricks can be obtained, severely damaged brick may be replaced.
Brick Construction: Mortar and Repointing
Mortar joints deterioration is the most common brick masonry problem. The repointing of deteriorated mortar joints requires the careful removal of deteriorated, inappropriate, or loose mortar; cleaning the joints; and installation of new mortar.
Identification and Inspection:
Original mortar should be tested to determine its original constituent materials by volume. The constituents will include cementitious materials and aggregate. The cementitious materials are composed of Portland cement and/or lime. The aggregate is usually sand. The usual and accepted ration of cementitious materials to aggregate is 1:3. That is: one part cementitious materials to three parts aggregate (by volume). While most nineteenth century mortars before 1880 were lime-sand mortars, the mortar used for fortification construction around San Francisco Bay for brick masonry was high in Portland cement content. The extensive use of Portland cement mortar was successful because of the relatively constant climatic conditions of humidity and temperature, and the rarity of freeze-thaw cycles. The hardness of the brick was also a factor allowing the use of a harder mortar. The character-defining features of the mortar depend on the color of the binders, the aggregate, and the joint treatment or finish. Mortar joints from the post-Civil War period were found to be flush to slightly concave joints about three-eighths to one-half inch wide.
Typical brick joint showing repointing technique where deteriorated mortar is raked out to a depth equal to about twice the joint width and replacement mortar is built up in layers.
Treatment:
Rake out loose mortar from joints using handtools, such as chisels, and remove dust and small debris with a brush of compressed air. Avoid damage to adjacent brick.
Mortar for repointing brickwork should be mixed in the following proportions, subject to adjustments based on laboratory analysis:
White Portland Cement:
ASTM C 207, Type S one part
White Hydrated Masons Lime:
ASTM C 150, Types I or III one part
Screened Local Beach Sand:
ASTM C 144 six parts
The mortar should be mixed in a paddle mixer or by hand with clean potable water. Based on laboratory analysis, the proportion of lime to cement may vary but the cementitious to aggregate ratio of 1:3 by volume must be maintained.
Finish joints to match original construction profile; concave or flush. After pointing, mortar may be tooled, brushed, or wiped (with burlap) when mortar has set to "thumbnail" hardness.
Brick Construction: Cleaning and Restoration
Masonry cleaning and restoration involves the removal of stains, mildew, dirt, grime, efflorescence, and paint from the brick surface. Masonry cleaning should be approached in a graduated manner. In arriving at an appropriate cleaning and restoration program, proceed from the least strong cleaning method to stronger methods. Use only enough chemicals and force to clean the material. It is preferable to retain existing imperfections than to permanently damage the structure by improper cleaning. Under no circumstances should brick masonry be abrasively cleaned or blasted.
Inspection and Testing:
Masonry cleaning and restoration should be accomplished only by experienced specialists implementing a comprehensive program. The cleaning and restoration program should be based on the approved results of field testing and sample panels. The cleaning and restoration program must be tailored to specific needs. Most cleaning can be accomplished with low pressure water blasting in association with scrubbing with a soft bristle brush. Isolated areas that retain staining or painted coatings such as graffiti after initial cleaning may require stronger measures.
Treatment:
Prior to the start of overall cleaning, clean a sample control panel for approval and reference. Demonstrate materials and methods to be used for cleaning the brick on the sample panel. The panel should be selected to include a range of cleaning and restoration requirements and should be of adequate size. Allow panel to stabilize for seven days before proceeding with other cleaning work. Longer observation may be appropriate.
Prepare a written program of procedures to be used including a description of the cleaning methods, working pressures, materials, equipment, and other information for each type of cleaning procedure. Comply with safety and environmental requirements.
Clean masonry surfaces only when the air temperature is between forty degrees Fahrenheit and eighty degrees Fahrenheit and will remain so for at least forty-eight hours after completion of the work.
Perform cleaning and restoration work in sequence with other masonry work. Clean masonry surfaces prior to repointing or other restoration work.
Proceed with cleaning in an orderly manner; work from the top to the bottom of each segment and from one end of a structure to the other. Clean in a uniform and consistent manner. Rinse off any residue by working upward from the bottom to the top of each treated area of each segment.
Apply water or cleaners in compliance with pressure, volume, and temperature requirements. Hold spray nozzle not less than six inches from the masonry surface and spray from side to side in overlapping bands to insure uniform coverage. Use low-pressure spray from 100psi to 300psi at three to six gallons per minute.
Pre-wet masonry to soften and loosen surface materials. Wash, scrub, and spray with low-pressure spray. Apply cleaner only in accordance to manufacturer’s written instructions. Rinse as required to remove all chemicals and residue. Repeat cleaning process if required.
Materials and Equipment:
Water: clean, potable, non-staining, and free of oils, acids, salts, and organic matter.
Brushes: fiber bristle only
Spray Equipment: low-pressure tank or chemical pump with a fan-shaped spray tip with an angle of not less than fifteen degrees.
Chemical Cleaning Solutions: dilute all cleaning solutions to produce mixes of a concentration not greater than that required to clean the masonry.
Note: Coordinate cleaning and restoration with other applicable sections in chapter 10.
Brick Construction: Graffiti Removal
Graffiti removal should be treated as a separate and distinct cleaning process. Graffiti removal will require specific treatment based on the type of paint used, the number of layers, the condition of the substrate, and the degree to which cleaning may permanently affect the historic materials involved. Where large areas have been painted with many coats of paint treatment may be different than small areas that have a single coat of paint. If graffiti removal, based on tests and sample panels, will permanently harm the historic materials a non-permanent, reversible sacrificial coating may be applied to enhance the visual effect.
Painting Out Graffiti:
Temporary solutions for problem graffiti areas include painting over the graffiti with two coats of any high quality latex paint of commercial grade. No special specifications are required.
Note: Refer to Finishes: General; Exterior Concrete Coatings; and Graffiti Removal, for details and coordination.
See also Martin E. Weaver, Removing Graffiti from Historic Masonry, Preservation Briefs, No.38, National Park Service, 1995, and, Anne E. Grimmer, Keeping It Clean: Removing Exterior Dirt, Paint, Stains and Graffiti from Historic Masonry Buildings, National Park Service, 1988.
Cavallo Battery. Preservation charette discussing treatments for graffiti cleaning or breathable, non-permanent coating.
Cavallo Battery. Graffiti on brickwork forming angle above vault.
posted by te oki at 11:58 PM
0 Comments:
Post a Comment
<< Home