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Charactenzation of Bncks arid Their Resistance to Detenoration Mechanisms GILBERT C. ROBINSON Brick and mortar are building materials of excellent durability, but they are subject to deterioration processes that can reduce their effectiveness. The rate of deterioration is a function of composition, pore structure, manufacturing procedure, structural design, and cleaning procedure. Deterioration of masonry results from several mechanisms, including freezing and thawing, salt crys- tallization, chemical attack by water and other substances, moisture expan- sion, other internal expansive reactions, and mismatch in dimensional char- acteristics of wall components. The susceptibility of brick to each mechanism is determined by pore struc- ture and composition. The glassy and amorphous phases are key items of composition. Illustrations are presented of the significance of glass-phase com- position. The manufacturing procedure, structural design, and cleaning pro- cedure can exert additional influence on deterioration mechanisms and rates. The danger of waterproof coatings is presented. Archeologists study ancient cultures by examination of brick and other fired clay artifacts that have persisted hundreds and thousands of years. Brick has an excellent record as a durable, versatile, and attractive building material. This is attested to by such buildings as Monticello, the Id North Church in Boston, and the Wren building at the College of William and Mary. Nevertheless, bricks do change with age, and Gilbert C. Robinson is Professor and Head, Department of Ceramic Engineering, Clem- son University. 145

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46 CONSERVATION OF HISTORIC STONE BUILDINGS the ravages of time produce varying results. In many cases the aging produces minor changes such as small shifts in color or accumulation of surface dirt. More severe damage may occur in other instances, with crumbling and disintegration or gross cracking of masonry units. The extent of change is dete''~ined by the severity of environmental ex- posure, the structural design, and the properties of the bricks. The properties of brick are determined by manufacturing procedures, and these have changed over time. It is important to consider this influence when contemplating restoration procedures or the likelihood of success in restoration. MANUFACTURING METHODS Raw Materials Raw materials have changed as the brick industry has evolved. The earliest bricks were made from clay similar to that used in making earthenware utensils. Clay is a rock or an integral part of the earth's crust that is composed of clay minerals and accessory minerals such as quartz, feldspar, and calcite. The clay mineral portion is made up of particles essentially smaller than 2 ,um; the accessory minerals may range from 2 mm down to 2 ,um. This clay was the major raw material for brick made in the first 100 years of this country's history. {Later, there was a marked switch to shales and similar rocks. These consol- idated materials offered certain advantages in manufacture.) The use of firecIays was a later raw materials development. These are coal- measure clays that produce light, fired colors of ivory, yellow, and gray; Pennsylvania and Ohio were major producing areas. The most recent addition to the raw materials mix has been kaolin and sericite schist, used by brick manufacturers in the Southeast. Shaping Bricks were hand-molded during the early years of this country. Suf- ficient water was added to the clay to produce a soft plastic mix, and the material was thrown into rectangular molds. Later (179~1819) machine-assisted equipment was developed to simulate the hand- molding, and the equipment would drop, throw, or vibrate the wet clay into the mold cavity." This type of molding began to decline with the development of extrusion equipment, and today only a small per- centage of building bricks are made by this method.

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Bricks and Resistance to Deterioration Mechanisms 147 Extrusion equipment was developed about 100 years ago.i In this equipment an auger propels plastic clay through a die opening that forms the cross-sectional outline of the brick. Such machinery permits shaping brick from clay of much stiffer consistency, and recently de- veloped, high-horsepower machines produce an extruded column of high strength. It is possible to stack the bricks one on top of the other for travel through the dryer and the kiln, rather than drying them on pallets as required by the molding process. There has also been limited production of pressed brick. In this process, the clay is pressed into the mold cavity. The clay may be of a plastic consistency or may contain less water than required to develop plasticity. Firing The firing process is the key step in manufacturing brick. During firing, the porosity of the product is reduced, and a bond is developed between the particles by partial fusion and/or sintering of amorphous constit- uents. Firing is responsible for the development of strength in the brick. It is also responsible for color and resistance to disintegration by rain- water, freezing and thawing, or other disruptive forces. Thus the quality and extent of firing determine the major characteristics of the beck. Firing equipment was crude in the early history of this country. Bricks were normally stacked together in an appropriate configuration in a field. The dry bricks formed the walls and firing eyes of -the kiln as well as its load. Wood was used to fire these field kilns, resulting in large variations in temperature from one section of the kiln to another and a corresponding variation in the properties of the bricks produced. Various types of kilns evolved later: Permanent walls of fired brick were constructed for field kilns; periodic kilns were developed; and the round, down-draft kid became popular. This unit could produce much more uniform bricks than were obtained from field kilns. The tunnel kiln started a rapid ascendancy about 1940 and by the 1950s had largely replaced periodic kilns. The tunnel kiln produced excep- tionally uniform properties and allowed much shorter firing cycles perhaps 30 hours, compared with 5 days in the periodic kiln. Coloration During the early period of production, the fired color of brick was determined by the clay raw matenal. A variety of colors were obtained

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48 CONSERVATION OF HISTORIC STONE BUILDINGS by varying firing temperature and kiln atmosphere "flashing). Later, and particularly during the past 20 years, there has been a growing production of brick colored by coatings of different composition than the body of the beck. The coatings range from a sprinkling of natural or colored sand through mineral sTurnes to impervious glazes. INFLUENCE OF MANUFACTURING PROCEDURES ON PROPERTIES OF BRICK Shaping Method Bncks prepared by the molding process show higher porosity and larger pores than do extruded brick (Fi~re 1). Larger pores seem to enhance durability. The acceptable porosity is higher for molded brick than for extruded brick as a result of this difference in pore structure. Anna Method A large proportion of the charge in field kilns would turn out to be soft bricks of high porosity. The softer Units could be scratched with a steel knife blade and would exhibit orange or salmon colors. The 40 30 ; - cn o o 20 CL 10 SOFT MO L DED / / - - - . 'FAILED PAT 10 BRICK N.C. PAVING BRICK N. C. PAVING BRICK X X X FA I L E D PAV I N G B R I C K ......................... / ~ I I I 5 ~ 1 0.6 0.4 0.2 0.1 0.06 0.04 0.02 PORE SIZE, MICRONS FIGURE 1 The pore-size distribution of a soft molded brick of good durability compared to two extruded bricks that failed in service and two durable paving bricks.

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Bricks and Resistance to Deterioration Mechanisms 149 bricks in the higher-temperature zone of the kiln would be harder than steel, would have a good ring when struck together, and would be a darker brick-red or even brown or black. The variation in properties posed no problem at the time; the softer units were used in interior walls and the harder units for facing. Any reconstruction that exposes these interior bricks of former periods to the outdoor environment is poor practice; they probably lack the properties needed to survive in freezing and thawing environments. The extent of firing can be estimated by examining selected prop- erties of beck. Strength and hardness increase, absorption decreases, and color changes with increasing temperature or time at temperature. Properties from the surface of a brick to its center become more un~- fo~m with increasing time at temperature. An oxygen-deficient kiln atmosphere produces equivalent properties at Tower temperature than does a kiln with an oxidizing atmosphere. The pattern of properties at various firing temperatures is distinctive for each raw matenal. Table 1 shows patterns for selected matenals. AGING MECHANISMS Several processes operate over time to alter the properties of structures made of brick. The extent of the alteration depends on the ability of TABLE 1 Fired Properties and Durability of South Carolina Kaolin and North Carolina Shale Brick Fired to Selected Maturing Temperatures Firing Absorption (% Initial Apparent Durability. Temp. - Saturation Rate of Density (Cycles to Sample OF) Room Boiling Coefficient Absorption (g/cc~ Failure) SC kaolin 1900 15.1 16.2 0.93 41 4 2000 12.9 14.2 0.90 36 2.84 5 2010 11.3 12.8 0.88 31 2.50 5 2050 7.9 10.0 0.79 14 2.26 10 NC shale 1900 14.2 16.0 0.89 59 2.73 5 1950 11.0 13.0 0.85 45 2.71 5 2000 9.2 11.4 0.81 46 2.68 10 2050 7.6 9.8 0.77 32 2.66 10 2150 4.3 6.6 0.65 11 2.58 + 10 Durability was evaluated by a salt (sodium sulfate! crystallization testy The number of cycles to cause failure are listed. The + 10 means it withstood 10 cycles without failure.

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150 CONSERVATION OF HISTORIC STONE BUILDINGS the brick to withstand weather, chemical attack, and the effects of improper structural design or execution. Water Penetration and Bond with Mortar The ability to resist water penetration is a major determinant of the stain resistance and durability of brick structures. Dry walls will not effloresce, break up from freezing and thawing, or be subject to chem- ical attack. The two primary sources of water penetration are inadequate flash- ing and roofing and a poor bond between mortar and brick. The perme- ability of the mortar or brick represents a minor but possible source of water penetration. It is essential to correct the sources of water penetration if other restorative procedures are to succeed. The performance of a masonry wall depends on the properties of the mortar as well as on the properties of the brick and the compatibility of these two constituents. The properties of mortar important to bond development are water retentivity, workability, and tensile strength. Interfacial tension determines bond strength developed by the inter- action of brick and mortar. However, these properties are oversha- dowed by the practice of the mason and the weather, the predominant determinants of bond development. Mortars have changed with the period of history. In some early structures they were made of clay and sand, with perhaps a surfacing of lime mortar. Sometimes natural substances associated with the clay Isuch as gelatinous silica, alkali silicates, and marI) provide substantial resistance to weathering. More often the clay bond is susceptible to disintegration by water attack {slaking!. Erosion or removal of the high- lime surface exposes the interior clay mortar to disruptive water attack. Lime sand mortars have been used since early times and once were the predominant bonding medium. Later, Portland cement was added to lime mortars; today, prepared proprietary mixtures called masonry cements are commonplace. This trend in composition has produced mortars of higher compressive strength and more rapid strength de- velopment, but lower water retentivity and workability. As a conse- quence, the development of a good bond between brick and mortar has become more difficult and snore demanding on the properties of the brick. The use of a modem mortar with an old brick may be poor practice. Present day mortars lack the flexibility in structure and self- healing characteristics of early lime mortars. Another recent trend has been the use of air-entraining agents to improve the workability of

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Bricks and Resistance to Deterioration Mechanisms 151 mortar. These agents increase the porosity and permeability of mortar and reduce its compressive strength. The properties of brick sigruficant to bond development are the surface pore structure, the initial rate of adsorption, and the tensile strength. Mechanical gripping is the major bonding mechanism be- tween brick and mortar. The mortar paste enters the exposed surface pores and sets. The surface pores should be large enough to allow penetration and should provide undercuts, such as a spherical opening with a narrow neck, to assist adhesion. Cut surfaces on extruded brick can produce shaggy overhang projections that assist adhesion. Die- sTickened surfaces are less conducive to bond development. Other bonding mechanisms exist. Glassy brick with no surface pores will develop a bond strength of 10 psi. The same brick produced to give open surface pores will develop a bond strength of 100 pSi.3 The initial rate of absorption (IRA) of brick is dete~ined by the standard ASTM test C67,4 which indicates the speed with which the brick withdraws water from the mortar. This characteristic has a pro- nounced influence on bond strength. The capilIarity of the brick should be sufficient to pull mortar paste into the pores of the brick but not high enough to dewater the mortar before it penetrates the pores. The {RA iS determined by immersing one face of a dry brick to a depth of 3.18 mm in water for 1 minute. The water absorbed is expressed as grams per brick (per 194 cm2 of immersed areal. It has been foment that good; bond strength will develop with TRAS of between 10 and 40 g.5 Figure 2 shows the relationship between bond strength and IRA. Mortars with higher water retentivity show good bond strength at higher values of IRA, while Tower water retentivity depresses the relationship. Prewetting a brick to reduce its capillary suction may lead to a bond strength characteristic of the Tower IRA thus induced. Laying of frozen or saturated bricks will result in the poor or non- existent bond characteristic of brick with an MA of zero. Weather conditions that promote cIrying may interfere with bonding and hy- dration of the mortar's constituents. Freezing weather may disrupt the mortar as well as interfere with bond development. Exposure to Freezing and Thawing Conditions The saturation of brick or other building units with water followed by freezing can produce disruptive forces that will destroy the units. This is a consequence of the 9 percent volume expansion of water as it changes to ice and the hydraulic pressures generated ahead of the ice.

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52 c.4 80 ,_ 60 Z _ I`J O Cat oh ~ ~ 40 Z o ~ 't 20 > o CONSERVATION OF HISTORIC STONE BUILDINGS ~~ ~1 1 1 1 1 1 1 1 1 ft: I I I : :~I 4 40 60 80 100 120 0 20 BRICK "SUCTION'' 9/30 IN2/MINUTE FIGURE 2 Bond strength versus BRA for mortar of different water-retaining capacity.5 Any structural configuration that entraps water in a void or impedes the flow of water in advance of a moving ice front will cause damage. The failure normally appears as a crumbling or disintegration or a delamination of the unit j see Figure 31. Most brick can withstand freezing and thawing without damage, but some units with inadequate properties will fail. Certain properties are keys to this type of darnage. One is porosity. The porosity of brick ordinarily is evaluated by determining the weight percentage of water it absorbs. Water adsorptions in excess of 12 percent begin to suggest deterioration by freeze-thaw mechanisms. The greater the absorption, the greater the likelihood of damage; usually, adsorptions above 15 percent are unacceptable for extruded brick.6 Pressed and molded brick will give acceptable performance with higher adsorptions, perhaps 14 to 17 percent. Porosity, as indicated by absorption, is not the sole determinant of durability. Pore structure also is significant. Early attempts to evaluate pore structure used saturation coefficient as a measure of the propor- tions of large pores and small pores. Present efforts are directed toward determining pore-size distribution, but the saturation coefficient re- mains a useful if imperfect index of the durability of brick.6 7

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Bricks and Resistance to Deterioration Mechanisms 153 FIGURE 3 The appearance of brick disintegrated (upperJ and cracked in lamination planes {lower) by exposure to freezing and thawing.

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154 - a' C1 - o o Z o ~ . cn < ~ < 't t ~r ! ~t t l~.tat t I t ~ o _ 1 1 _ O O O .0 O O O O O O O O 0 U) ~ C\l ~t ~ ID at ~t ~t et ~ et ~ ~ ~t ~t ~t't ~ ~ ~t t ~t et ~ at t t , t t t ~r ~ ! ~r~ ! ~t at ~t' t ~t ~r '.t m ~ ~t .$ t 't ct ~r ~t at ~S at ~t OCR for page 145
Bricks and Resistance to Deterioration Mechanisms 155 The saturation coefficient is the ratio of water absorbed during 24 hours of immersion in room-temperature water to that absorbed during 5 hours of immersion in boiling water. The room-temperature absorp- tion is supposed to indicate the amount of water a brick will pick up from rainfall, while the added absorption from boiling indicates the quantity of pores that can be penetrated only under abnormal pressure. These pores, or voids, should serve to relieve the hydraulic pressures in advance of a freezing front passing through the brick. Saturation coefficients below 0.75 give good assurance of durability. However, there are examples of brick with excellent durability having values above 0.85. In these instances, other factors of manufacturing procedure are predominant in determining durability. Extruded bricks exhibit a laminar structure. This structure may be made up of thin, flaky elements, or it may show a large, spiraling crack within the unit. Attempts have been made to relate lamination to lack of durability, but the relationship is questionable. A brick with un- favorable pore structure will delaminate under repeated cycles of freez- ing and thawing; however, the cause of failure is the inadequate pore structure and not the laminar structure. It is interesting to observe that materials that do not have an extruded laminar structure will still fad] by delamination. Soft molded brick and granite are two illustrations of this mode of failure. Strength seems logically related to freeze-thaw resistance. Never- theless, attempts to correlate compressive strengths with durability have been unsuccessful. Figure 4 shows freeze-thaw resistance versus strength for commercially manufactured units. It can be seen that there is no correlation. The lack of correlation is believed to result from faults in the method of measuring compressive strength and perhaps from the use of the wrong type of strength determination. There has been little investigation of the permeability of bricks. However, it has been suggested that the speed with which water pen- etrates a brick bears some relation to durability. Experiments have shown some relation between MA and durability (Table 1), and the MA test has proven useful for predicting durability. Other work is being conducted to obtain more information on the role of permeability in freeze-thaw resistance. The properties of brick are usually determined on a whole or a half brick, and the values reported are really averages for the entire brick. Sampling at different locations from the surface to the interior of the brick will show different properties, including different pore structures. These differences have become particularly significant with the fast- firing schedules of modem tunnel kilns. High production rates allow

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156 CONSERVATION OF HISTORIC STONE BUILDINGS insufficient time for heat to penetrate to the interior of the unit, which can cause property differentials. The property differentials wiD be greater for a solid unit than for a cored unit because the heat must penetrate more material. Salt Crystallization Saturation of a building brick with salt solution can produce failure similar to that resulting from freezing and thawing. The salt will crys- tallize within the pores of the unit and with repeated wetting and temperature cycling may produce an expansive force that wfl] disin- tegrate the unit. This phenomenon can be observed in structures sub- jected to saltwater spray. Figure 5 shows solar screen tiles that have been disintegrated by this mechanism in Miami Beach. It is interesting that the mortar withstood the assault better than the bricks. Evaluation of the bricks showed them to have been underfired. They had an absorption of 18 percent and a saturation coefficient of 0.95 and were soft and easily scratched. They contained 14 percent dissolved salt. This type of brick, however, might be satisfactory for back-up or in- terior locations or even exposed locations in nonfreezing environments that are free of soluble salts. There are other sources of salts within masonry structures. Brick- work around flower beds or gardens may collect soluble salts from FIGURE 5 Salt disin- tegration of solar screen tile in Miami Beach, Florida. ,.~2 Am; ~ ~ ~ ~ ~ .0 ~ _ -_ ,,, FW -. : ~~.~ -

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Bncks and Resistance to Detenoration Mechanisms 157 fertilizers. The use of calcium chloride to prevent freezing of mortars will introduce a large quantity of soluble salts that may concentrate in one part of the masonry structure. Cleaning agents dissolve cement and other substances to produce soluble salts; injudicious use of these agents can contribute to increased salts within the masonry structure. Chemical Attack Properly fired bricks have unusual resistance to attack by chemical agents. They can form containers for hydrochloric acid and are subject to disruptive attack only by hydrofluoric acid. However, bricks that are inadequately fired are subject to attack. Even water wiB react slightly with the glassy constituents of a highly porous brick. Such a reaction will produce some reduction in the strength of the unit and contribute to moisture expansion. This process is usually not continuous; instead, the attack proceeds to a certain limit and then stops without further disintegration. Acidic solutions will accelerate such an attack. Thus, the formation of sulfuric acid solutions from atmospheric constituents will increase the dissolution of the brick approximately 10-fold. Even in this cir- cumstance, however, the maximum dissolution of the brick is usually about 1 percent. The resulting salts are more damaging to the appear- ance of the building than to its strength. These salts are a source of staining and efflorescence on the masonry structure. The sulfur triox- ide required for this attack can come from pollutants in the atmo- sphere, but can also come from the masonry itself. The production of bricks in a high-sulfur atmosphere may yield residual sulfates that will become acidic in the presence of water. The cement constituents of mortars may be relatively high in sulfates end alkalies, and solutions of these constituents may accelerate the attack on the brick. Moisture Expansion Most porous materials will exhibit some moisture expansion. The moisture expansion of brick is usually slight, about 0.04 percent. In a few instances, underfiring may give higher expansions (perhaps-O.1 percent). Particles of lime or gypsum within the clay raw material can cause still higher moisture expansions, at times sufficient to disinte- grate masonry units or crack and destroy walls. Also, reactions between brick and alkali solutions from the cement or other sources may result in large expansions.

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158 . .. CONSERVATION OF HISTORIC STONE BUILDINGS :~AO2.~W ..~.. FIGURE 6 Surface spelling resulting from application of water-impermeable coating to a wet wall. Water-Impermeable Coatings Water-impermeable coatings can be a factor leading to damage of ma- sonry units. Water may enter the interior of the wall and dissolve soluble salts, which then migrate toward the barrier and crystallize behind it. The exertion of a disruptive force will pop the waterproofed face away from the masonry unit. Thus, it is particularly dangerous to apply a waterproof coating over an entire masonry wall. This practice may result in no damage, however, if the interior of the wall is dry and no water can enter the interior from other sources, such as the roof. If water does enter, salt crystallization wid cause severe damage in the wall See Figure 61. Structural Design A common source of damage to masonry walls' in those few instances where damage occurs, is inadequate design of the structure. Inadequate

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Bricks and Resistance to Deterioration Mechanisms \ 159 flashing at the roof line or other systems that allow water to penetrate the interior of the wall can be quite damaging. Any restoration pro- cedure should emphasize proper construction techniques to prevent leakage of water from the roof or other structures to the interior of walls. Another source of damage is inadequate expansion joints that permit unrestrained movement of the masonry as a result of temper- ature cycling. Mismatch of Matenals It should be recognized that steel, concrete, and brick masonry show different dimensional behavior with fluctuating temperatures and from shrinkage. These differences can lead to cracking of a masonry wall. Figure 7 shows brick masonry fitted tightly around a metal fixture for holding electric lights; the difference in dimensional behavior between the metal and the brick has caused cracking within the brickwork. Restoration procedures should avoid this type of mismatch and correct . . . any exlstmg mismate nest Cleaning Procedures Cleaning brick masonry walls generally can be accomplished with the use of water alone or in combination with chemical reagents or some- times by sandblasting. FIGURE 7 Cracking of masonry from mismatch in dimensional behavior of metal light box and brickwork.

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160 CONSERVATION OF HISTORIC STONE BUlEDINGS It should be recognized that chemical cleaning or sandblasting has the potential for causing serious damage to the masonry. Cleaning actions depend on solution or abrasion of the offending substance, and these actions are avaflable also for attack of the masonry. Furthermore, chemical cleaners produce soluble salts that may penetrate the ma- sonry and cause development of new discoloration or contribute to new deterioration. The injudicious selection and/or mung of various cleaning reagents can cause worse staining than that originally present. The extent of damage is influenced by the concentration and quantity of cleaning agent and the duration of the cleaning procedure. The susceptibility of the masonry to attack wfl] depend on its composition and manufacturing history. Thus, lime in mortar is more susceptible to solution than Portland cement, and underfired bricks are more read- fly attacked by abrasion and reaction than properly fired units. The many variables that influence cleaning make it important to follow recommended cleaning practices7 ~ and to pretest the selected procedure in a small inconspicious place on the masonry wall. Ob- servations should be made of the cleaning effectiveness and of any unacceptable discoloration or damage to the masonry. RESTORATION PROCEDURES ,. Restoration procedures should be selected after examination of the masonry structure. Visual examination may be sufficient to indicate staining problems: the mode of failure, if any, of the brick; sources of water penetration, such as cracks between mortar and brick; unfilled joints; wall cracks; and inadequate flashing or roofing. In other instances, sampling and testing of the wall may be required. The scratch-hardness test remains a simple but effective means of establishing the firing history and soundness of brick. Bricks or pieces of brick can be removed from a wall and their adsorptions and satu- ration coefficients deterrnine`d to provide additional clues to durability. Determining the possibility of increasing the structural load on a building is a complex problem. There is probably no sure way to de- termine the load-caITying potential of a structure short of loading it to destruction. Strength testing of small sections of a wall has limited value because of the large variation in properties of brick produced in earlier times. Table 2 shows the strengths of individual bricks removed from the capitol of Florida. The strength varies from good to non- existent. However, even if all the bricks have the highest strength, a wall might be weak because of gaps in the mortar joint in some lo- cations. The variabilities in properties and construction practices make

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Bricks and Resistance to Deterioration Mechanisms TABLE 2 Strength and Absorption of Brick from a Candidate Building for Restoration Year of Construction Unrestrained Compressive Strength (psi) Absorption {%) 1902 1 168 19 1902 1900 22 1845 494 26 1845 0 161 it difficult to predict the maximum permissible loading for a structure; one must allow large safety factors and use judgment based on expe- . . nence in sue. ~ cases. To prevent further deterioration of a damaged structure, the cause of deterioration must be removed. For example, structural stress caused by mismatch of dimensional movements among different materials must be eliminated. Walls should be Nor before waterproofing agents are added. The use of coatings to strengthen units has been studied by nu- merous authors. Lenin and Charola suggest alkoxysilanes as the most promising coatings.~ Lal Gauri discusses the use of fluorocarbons and a series of polymer solvent mixtures with increasing concentration of polymer. Drisko discusses different waterproofing materials and gives recommended coatings for different situations. Cleaning procedures should be selected to suit the stains and ma- terials involved. Any procedure should be tested at a small, inconspic- uous place before it is applied to the entire structure. REFERENCES 1. P.E. Jeffers, The Building of America, Brick and Clay Record, 169~11:19-30 {1976~. 2. G.S. Robinson, An Accelerated Test Method for Predicting the Durability of Brick, MS thesis, Clemson University, Clemson, S.C. {1976~. 3. H.D. Martin, Adhesion Mechanisms in Masonry Mortars, MS thesis, Clemson University, Clemson, S.C. t1965~. 4. Standard Methods of Testing Brick and Structural Clay Tile, ASTM Designation C67-78, 1980 ASTM Annual Book of Standards, Part 16, pp. 45-54 {American Society for Testing and Materials: Philadelphia, Pa., 1980~. 5. L.A. Palmer and D.A. Parsons, A Study of the Properties of Mortars and Bricks and Their Relation to Bond, Nate. Burl Stand. [. Res. 12 {1965J. 6. G.C. Robinson, J.R. Holman, and J.F. Edwards, Relation Between Physical Prop-

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162 CONSERVATION OF HISTORIC STONE BUILDINGS erties and Durability of Commercially Marketed Brick, Am. Ceram. Soc. Bull. 56~12~: 1071-1076 {1977~. 7. B. Butterworth, Frost Resistance of Bricks and Tiles: A Review, I. Br. Ceram. Soc. 1~2~: 203-223 t1964~. 8. Cleaning Clay Products Masonry, Technical Notes on Brick and Tile Construc- tion, 20. Brick Institute of America, McLean, Va. {1964~. 9. Good Practice for Cleaning New Brickwork. Brick Association of North Carolina, Greensboro, N.C. {no date}. 10. S.Z. Lewin and A.E. Charola, "The Physical Chemistry of Deteriorated Brick and Its Impregnation Technique," paper presented;at the Congress for the Brick of Venice, October 22, 1979. 11. K. Lal Gauri, The Preservation of Stone, Scientific American, June 1978. 12. R.W. Drisko, An Introduction to Protective Coatings, Public Works, pp. 8(}83 (August 1979~.