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OCR for page 145
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
OCR for page 146
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.
OCR for page 147
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
OCR for page 148
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.
OCR for page 149
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.
OCR for page 150
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
OCR for page 151
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.
OCR for page 152
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
OCR for page 153
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.
OCR for page 154
154
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OCR for page 155
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
OCR for page 156
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 -. : ~~.~
-
OCR for page 157
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.
OCR for page 158
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
OCR for page 159
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.
OCR for page 160
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
OCR for page 161
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-
OCR for page 162
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~.
Representative terms from entire chapter:
deterioration mechanisms
