PRESERVATION OF HISTORIC BUILDINGS |
ABOUT HISTORIC BUILDINGS
Buildings are everywhere. In fact, of the more than
67,000 listings in the National Register of Historic
Places, nearly 72% are historic buildings--houses,
schools, city halls, social halls, churches,
libraries, courthouses, stores, barns, factories and
mills, hotels, armories, and train depots. Now, as in
the past, they continue to provide shelter and
support human activity.
Unfortunately, building materials (masonry, wood, and
architectural metals) are all subject to damage,
decay, and loss over time. With ongoing preservation,
however, the historic buildings that define our
neighborhoods, towns, and cities will survive for
future generations to use and enjoy.
Repointing Mortar Joints in Historic Masonry Building
Table of Content
Historical
Background
Identifying
the Problem Before Repointing
Finding
an Appropriate Mortar Match
Properties
of Mortar
Mortar Analysis
Components
of Mortar
Mortar
Type and Mix
Visually
Examining the Mortar and the Masonry Units
Masonry--brick, stone, terra-cotta, and concrete block--is found on
nearly every historic building. Structures with all-masonry exteriors
come to mind immediately, but most other buildings at least have
masonry foundations or chimneys. Although generally considered
"permanent," masonry is subject to deterioration, especially at the
mortar joints. Repointing, also known simply as "pointing"or--somewhat
inaccurately--"tuck pointing"*, is the process of removing deteriorated
mortar from the joints of a masonry wall and replacing it with new
mortar. Properly done, repointing restores the visual and physical
integrity of the masonry. Improperly done, repointing not only detracts
from the appearance of the building, but may also cause physical damage
to the masonry units themselves.
The purpose of this Brief is to provide general guidance on appropriate
materials and methods for repointing historic masonry buildings and
it
is intended to benefit building owners, architects, and contractors.
The Brief should serve as a guide to prepare specifications for
repointing historic masonry buildings. It should also help develop
sensitivity to the particular needs of historic masonry, and to assist
historic building owners in working cooperatively with architects,
architectural conservators and historic preservation consultants, and
contractors. Although specifically intended for historic buildings,
the
guidance is appropriate for other masonry buildings as well. This
publication updates Preservation Briefs 2: Repointing Mortar Joints
in
Historic Brick Buildings to include all types of historic unit masonry.
The scope of the earlier Brief has also been expanded to acknowledge
that the many buildings constructed in the first half of the 20th
century are now historic and eligible for listing in the National
Register of Historic Places, and that they may have been originally
constructed with portland cement mortar.
*Tuckpointing technically describes a primarily decorative application
of a raised mortar joint or lime putty joint on top of flush mortar
joints.
Mortar consisting primarily of lime and sand has been used as an
integral part of masonry structures for thousands of years. Up until
about the mid-19th century, lime or quicklime (sometimes called lump
lime) was delivered to construction sites, where it had to be slaked,
or combined with water. Mixing with water caused it to boil and
resulted in a wet lime putty that was left to mature in a pit or wooden
box for several weeks, up to a year. Traditional mortar was made from
lime putty, or slaked lime, combined with local sand, generally in
a
ratio of 1 part lime putty to 3 parts sand by volume. Often other
ingredients, such as crushed marine shells (another source of lime),
brick dust, clay, natural cements, pigments, and even animal hair were
also added to mortar, but the basic formulation for lime putty and
sand
mortar remained unchanged for centuries until the advent of portland
cement or its forerunner, Roman cement, a natural, hydraulic cement.
Portland cement was patented in Great Britain in 1824. It was named
after the stone from Portland in Dorset which it resembled when hard.
This is a fast-curing, hydraulic cement which hardens under water.
Portland cement was first manufactured in the United States in 1872,
although it was imported before this date. But it was not in common
use
throughout the country until the early 20th century. Up until the turn
of the century portland cement was considered primarily an additive,
or
"minor ingredient" to help accelerate mortar set time. By the 1930s,
however, most masons used a mix of equal parts portland cement and
lime
putty. Thus, the mortar found in masonry structures built between 1873
and 1930 can range from pure lime and sand mixes to a wide variety
of
lime, portland cement, and sand combinations.
In the 1930s more new mortar products intended to hasten and simplify
masons' work were introduced in the U.S. These included masonry cement,
a premixed, bagged mortar which is a combination of portland cement
and
ground limestone, and hydrated lime, machine-slaked lime that
eliminated the necessity of slaking quicklime into putty at the site.
Identifying the Problem Before Repointing
The decision to repoint is most often related to some obvious sign of
deterioration, such as disintegrating mortar, cracks in mortar joints,
loose bricks or stones, damp walls, or damaged plasterwork. It is,
however, erroneous to assume that repointing alone will solve
deficiencies that result from other problems. The root cause of the
deterioration--leaking roofs or gutters, differential settlement of
the
building, capillary action causing rising damp, or extreme weather
exposure--should always be dealt with prior to beginning work. Without
appropriate repairs to eliminate the source of the problem, mortar
deterioration will continue and any repointing will have been a waste
of time and money.
Finding an Appropriate Mortar Match
Preliminary research is necessary to ensure that the proposed
repointing work is both physically and visually appropriate to the
building. Analysis of unweathered portions of the historic mortar to
which the new mortar will be matched can suggest appropriate mixes
for
the repointing mortar so that it will not damage the building because
it is excessively strong or vapor impermeable. Examination and analysis
of the masonry units--brick, stone or terra cotta--and the techniques
used in the original construction will assist in maintaining the
building's historic appearance. A simple, non- technical, evaluation
of
the masonry units and mortar can provide information concerning the
relative strength and permeability of each--critical factors in
selecting the repointing mortar--while a visual analysis of the
historic mortar can provide the information necessary for developing
the new mortar mix and application techniques.
Although not crucial to a successful repointing project, for projects
involving properties of special historic significance, a mortar
analysis by a qualified laboratory can be useful by providing
information on the original ingredients. However, there are limitations
with such an analysis, and replacement mortar specifications should
not
be based solely on laboratory analysis. Analysis requires
interpretation, and there are important factors which affect the
condition and performance of the mortar that cannot be established
through laboratory analysis. These may include: the original water
content, rate of curing, weather conditions during original
construction, the method of mixing and placing the mortar, and the
cleanliness and condition of the sand. The most useful information
that
can come out of laboratory analysis is the identification of sand by
gradation and color. This allows the color and the texture of the
mortar to be matched with some accuracy because sand is the largest
ingredient by volume.
In creating a repointing mortar that is compatible with the masonry
units, the objective is to achieve one that matches the historic mortar
as closely as possible, so that the new material can coexist with the
old in a sympathetic, supportive and, if necessary, sacrificial
capacity. The exact physical and chemical properties of the historic
mortar are not of major significance as long as the new mortar conforms
to the following criteria:
The new mortar must match the historic mortar in
color, texture
and tooling. (If a laboratory analysis is undertaken,
it may be
possible to match the binder components and their
proportions with
the historic mortar, if those materials are available.)
The sand must match the sand in the historic mortar.
(The color
and texture of the new mortar will usually fall
into place if the
sand is matched successfully.)
The new mortar must have greater vapor permeability
and be softer
(measured in compressive strength) than the masonry
units.
The new mortar must be as vapor permeable and as
soft or softer
(measured in compressive strength) than the historic
mortar.
(Softness or hardness is not necessarily an indication
of
permeability; old, hard lime mortars can still retain
high
permeability.)
Methods for analyzing mortars can be divided into two broad categories:
wet chemical and instrumental. Many laboratories that analyze historic
mortars use a simple wet-chemical method called acid digestion, whereby
a sample of the mortar is crushed and then mixed with a dilute acid.
The acid dissolves all the carbonate-containing minerals not only in
the binder, but also in the aggregate (such as oyster shells, coral
sands, or other carbonate-based materials), as well as any other
acid-soluble materials. The sand and fine-grained acid-insoluble
material is left behind. There are several variations on the simple
acid digestion test. One involves collecting the carbon dioxide gas
given off as the carbonate is digested by the acid; based on the gas
volume the carbnate content of the mortar can be accurately determined
(Jedrzejewska, 1960). Simple acid digestion methods are rapid,
inexpensive, and easy to perform, but the information they provide
about the original composition of a mortar is limited to the color
and
texture of the sand. The gas collection method provides more
information about the binder than a simple acid digestion test.
Instrumental analysis methods that have been used to evaluate mortars
include polarized light or thin-section microscopy, scanning electron
microscopy, atomic absorption spectroscopy, X-ray diffraction, and
differential thermal analysis. All instrumental methods require not
only expensive, specialized equipment, but also highly-trained
experienced analysts. However, instrumental methods can provide much
more information about a mortar. Thin-section microscopy is probably
the most commonly used instrumental method. Examination of thin slices
of a mortar in transmitted light is often used to supplement acid
digestion methods, particularly to look for carbonate-based aggregate.
For example, the new ASTM test method, ASTM C 1324-96 "Test Method
for
Examination and Analysis of Hardened Mortars" which was designed
specifically for the analysis of modern lime-cement and masonry cement
mortars, combines a complex series of wet chemical analyses with
thin-section microscopy.
The drawback of most mortar analysis methods is that mortar samples
of
known composition have not been analyzed in order to evaluate the
method. Historic mortars were not prepared to narrowly defined
specifications from materials of uniform quality; they contain a wide
array of locally derived materials combined at the discretion of the
mason. While a particular method might be able to accurately determine
the original proportions of a lime-cement-sand mortar prepared from
modern materials, the usefulness of that method for evaluating historic
mortars is questionable unless it has been tested against mortars
prepared from materials more commonly used in the past.
Mortars for repointing should be softer or more permeable than the
masonry units and no harder or more impermeable than the historic
mortar to prevent damage to the masonry units. It is a common error
to
assume that hardness or high strength is a measure of appropriateness,
particularly for lime-based historic mortars. Stresses within a wall
caused by expansion, contraction, moisture migration, or settlement
must be accommodated in some manner; in a masonry wall, these stresses
should be relieved by the mortar rather than by the masonry units.
A
mortar that is stronger in compressive strength than the masonry units
will not "give," thus causing stresses to be relieved through the
masonry units--resulting in permanent damage to the masonry, such as
cracking and spalling, that cannot be repaired easily. While stresses
can also break the bond between the mortar and the masonry units,
permitting water to penetrate the resulting hairline cracks, this is
easier to correct in the joint through repointing than if the break
occurs in the masonry units.
Permeability, or rate of vapor transmission, is also critical. High
lime mortars are more permeable than denser cement mortars.
Historically, mortar acted as a bedding material--not unlike an
expansion joint--rather than a "glue" for the masonry units, and
moisture was able to migrate through the mortar joints rather than
the
masonry units. When moisture evaporates from the masonry it deposits
any soluble salts either on the surface as efflorescence or below the
surface as subflorescence. While salts deposited on the surface of
masonry units are usually relatively harmless, salt crystallization
within a masonry unit creates pressure that can cause parts ofthe outer
surface to spall off or delaminate. If the mortar does not
permitmoisture or moisture vapor to migrate out of the wall and
evaporate, theresult will be damage to the masonry units.
Sand. Sand is the largest component of mortar and the material
that
gives mortar its distinctive color, texture and cohesiveness. Sand
must
be free of impurities, such as salts or clay. The three key
characteristics of sand are: particle shape, gradation and void ratios.
When viewed under a magnifying glass or low-power microscope, particles
of sand generally have either rounded edges, such as found in beach
and
river sand, or sharp, angular edges, found in crushed or manufactured
sand. For repointing mortar, rounded or natural sand is preferred for
two reasons. It is usually similar to the sand in the historic mortar
and provides a better visual match. It also has better working
qualities or plasticity and can thus be forced into the joint more
easily, forming a good contact with the remaining historic mortar and
the surface of the adjacent masonry units. Although manufactured sand
is frequently more readily available, it is usually possible to locate
a supply of rounded sand.
The gradation of the sand (particle size distribution) plays a very
important role in the durability and cohesive properties of a mortar.
Mortar must have a certain percentage of large to small particle sizes
in order to deliver the optimum performance. Acceptable guidelines
on
particle size distribution may be found in ASTM C 144 (American Society
for Testing and Materials). However, in actuality, since neither
historic nor modern sands are always in compliance with ASTM C 144,
matching the same particle appearance and gradation usually requires
sieving the sand.
A scoop of sand contains many small voids between the individual
grains. A mortar that performs well fills all these small voids with
binder (cement/lime combination or mix) in a balanced manner.
Well-graded sand generally has a 30 per cent void ratio by volume.
Thus, 30 per cent binder by volume generally should be used, unless
the
historic mortar had a different binder: aggregate ratio. This
represents the 1:3 binder to sand ratios often seen in mortar
specifications.
For repointing, sand generally should conform to ASTM C 144 to assure
proper gradation and freedom from impurities; some variation may be
necessary to match the original size and gradation. Sand color and
texture also should match the original as closely as possible to
provide the proper color match without other additives.
Lime. Mortar formulations prior to the late-19th century used
lime as
the primary binding material. Lime is derived from heating limestone
at
high temperatures which burns off the carbon dioxide, and turns the
limestone into quicklime. There are three types of limestone--calcium,
magnesium, and dolomitic--differentiated by the different levels of
magnesium carbonate they contain which impart specific qualities to
mortar. Historically, calcium lime was used for mortar rather than
the
dolomitic lime (calcium magnesium carbonate) most often used today.
But
it is also important to keep in mind the fact that the historic limes,
and other components of mortar, varied a great deal because they were
natural, as opposed to modern lime which is manufactured and,
therefore, standardized. Because some of the kinds of lime, as well
as
other components of mortar, that were used historically are no longer
readily available, even when a conscious effort is made to replicate
a
"historic" mix, this may not be achievable due to the differences
between modern and historic materials.
Lime, itself, when mixed with water into a paste is very plastic and
creamy. It will remain workable and soft indefinitely, if stored in
a
sealed container. Lime (calcium hydroxide) hardens by carbonation
absorbing carbon dioxide primarily from the air, converting itself
to
calcium carbonate. Once a lime and sand mortar is mixed and placed
in a
wall, it begins the process of carbonation. If lime mortar is left
to
dry too rapidly, carbonation of the mortar will be reduced, resulting
in poor adhesion and poor durability. In addition, lime mortar is
slightly water soluble and thus is able to re-seal any hairline cracks
that may develop during the life of the mortar. Lime mortar is soft,
porous, and changes little in volume during temperature fluctuations
thus making it a good choice for historic buildings. Because of these
qualities, high calcium lime mortar may be considered for many
repointing projects, not just those involving historic buildings.
For repointing, lime should conform to ASTM C 207, Type S, or Type SA,
Hydrated Lime for Masonry Purposes. This machine-slaked lime is
designed to assure high plasticity and water retention. The use of
quicklime which must be slaked and soaked by hand may have advantages
over hydrated lime in some restoration projects if time and money
allow.
Lime putty. Lime putty is slaked lime that has a putty or paste-like
consistency. It should conform to ASTM C 5. Mortar can be mixed using
lime putty according to ASTM C 270 property or proportion
specification.
Portland cement. More recent, 20th-century mortar has used portland
cement as a primary binding material. A straight portland cement and
sand mortar is extremely hard, resists the movement of water, shrinks
upon setting, and undergoes relatively large thermal movements. When
mixed with water, portland cement forms a harsh, stiff paste that is
quite unworkable, becoming hard very quickly. (Unlike lime, portland
cement will harden regardless of weather conditions and does not
require wetting and drying cycles.) Some portland cement assists the
workability and plasticity of the mortar without adversely affecting
the finished project; it also provides early strength to the mortar
and
speeds setting. Thus, it may be appropriate to add some portland cement
to an essentially lime-based mortar even when repointing relatively
soft 18th or 19th century brick under some circumstances when a
slightly harder mortar is required. The more portland cement that is
added to a mortar formulation the harder it becomes--and the faster
the
initial set.
For repointing, portland cement should conform to ASTM C 150. White,
non- staining portland cement may provide a better color match for
some
historic mortars than the more commonly available grey portland cement.
But, it should not be assumed, however, that white portland cement
is
always appropriate for all historic buildings, since the original
mortar may have been mixed with grey cement. The cement should not
have
more than 0.60 per cent alkali to help avoid efflorescence.
Masonry cement. Masonry cement is a preblended mortar mix commonly
found at hardware and home repair stores. It is designed to produce
mortars with a compressive strength of 750 psi or higher when mixed
with sand and water at the job site. It may contain hydrated lime,
but
it always contains a large amount of portland cement, as well as ground
limestone and other workability agents, including air-entraining
agents. Because masonry cements are not required to contain hydrated
lime, and generally do not contain lime, they produce high strength
mortars that can damage historic masonry. For this reason, they
generally are not recommended for use on historic masonry buildings.
Lime mortar (pre-blended). Hydrated lime mortars, and pre-blended
lime
putty mortars with or without a matched sand are commercially
available. Custom mortars are also available with color. In most
instances, pre-blended lime mortars containing sand may not provide
an
exact match; however, if the project calls for total repointing, a
pre-blended lime mortar may be worth considering as long as the mortar
is compatible in strength with the masonry. If the project involves
only selected, "spot" repointing, then it may be better to carry out
a
mortar analysis which can provide a custom pre-blended lime mortar
with
a matching sand. In either case, if a preblended lime mortar is to
be
used, it should contain Type S or SA hydrated lime conforming to ASTM
C
207.
Water. Water should be potable--clean and free from acids, alkalis,
or
other dissolved organic materials.
Other Components
Historic components. In addition to the color of the sand, the
texture
of the mortar is of critical importance in duplicating historic mortar.
Most mortars dating from the mid-19th century on--with some
exceptions--have a fairly homogeneous texture and color. Some earlier
mortars are not as uniformly textured and may contain lumps of
partially burned lime or "dirty lime", shell (which often provided
a
source of lime, particularly in coastal areas), natural cements, pieces
of clay, lampblack or other pigments, or even animal hair. The visual
characteristics of these mortars can be duplicated through the use
of
similar materials in the repointing mortar.
Replicating such unique or individual mortars will require writing new
specifications for each project. If possible, suggested sources for
special materials should be included. For example, crushed oyster
shells can be obtained in a variety of sizes from poultry supply
dealers.
Pigments. Some historic mortars, particularly in the late 19th
century,
were tinted to match or contrast with the brick or stone (Fig. 6).
Red
pigments, sometimes in the form of brick dust, as well as brown, and
black pigments were commonly used. Modern pigments are available which
can be added to the mortar at the job site, but they should not exceed
10 per cent by weight of the portland cement in the mix, and carbon
black should be limited to 2 per cent. Only synthetic mineral oxides,
which are alkali-proof and sun- fast, should be used to prevent
bleaching and fading.
Modern components. Admixtures are used to create specific
characteristics in mortar, and whether they should be used will depend
upon the individual project. Air entraining agents, for example, help
the mortar to resist freeze-thaw damage in northern climates.
Accelerators are used to reduce mortar freezing prior to setting while
retarders help to extend the mortar life in hot climates. Selection
of
admixtures should be made by the architect or architectural conservator
as part of the specifications, not something routinely added by the
masons.
Generally, modern chemical additives are unnecessary and may, in fact,
have detrimental effects in historic masonry projects. The use of
antifreeze compounds is not recommended. They are not very effective
with high lime mortars and may introduce salts, which may cause
efflorescence later. A better practice is to warm the sand and water,
and to protect the completed work from freezing. No definitive study
has determined whether air-entraining additives should be used to
resist frost action and enhance plasticity, but in areas of extreme
exposure requiring high-strength mortars with lower permeability,
air-entrainment of 10-16 percent may be desirable (see formula for
"severe weather exposure" in Mortar Type and Mix). Bonding agents are
not a substitute for proper joint preparation, and they should
generally be avoided. If the joint is properly prepared, there will
be
a good bond between the new mortar and the adjacent surfaces. In
addition, a bonding agent is difficult to remove if smeared on a
masonry surface.
Mortars for repointing projects, especially those involving historic
buildings, typically are custom mixed in order to ensure the proper
physical and visual qualities. These materials can be combined in
varying proportions to create a mortar with the desired performance
and
durability. The actual specification of a particular mortar type should
take into consideration all of the factors affecting the life of the
building including: current site conditions, present condition of the
masonry, function of the new mortar, degree of weather exposure, and
skill of the mason. Thus, no two repointing projects are exactly the
same. Modern materials specified for use in repointing mortar should
conform to specifications of the American Society for Testing and
Materials (ASTM) or comparable federal specifications, and the
resulting mortar should conform to ASTM C 270, Mortar for Unit Masonry.
Specifying the proportions for the repointing mortar for a specific
job
is not as difficult as it might seem. Five mortar types, each with
a
corresponding recommended mix, have been established by ASTM to
distinguish high strength mortar from soft flexible mortars. The ASTM
designated them in decreasing order of approximate general strength
as
Type M (2,500 psi), Type S (1,800 psi), Type N (750 psi), Type O (350
psi) and Type K (75 psi). (The letters identifying the types are from
the words MASON WORK using every other letter.) Type K has the highest
lime content of the mixes that contain portland cement, although it
is
seldom used today, except for some historic preservation projects.
The
designation "L" in the accompanying chart identifies a straight lime
and sand mix. Specifying the appropriate ASTM mortar by proportion
of
ingredients, will ensure the desired physical properties. Unless
specified otherwise, measurements or proportions for mortar mixes are
always given in the following order: cement-lime-sand. Thus, a Type
K
mix, for example, would be referred to as 1-3-10, or 1 part cement
to 3
parts lime to 10 parts sand. Other requirements to create the desired
visual qualities should be included in the specifications.
The strength of a mortar can vary. If mixed with higher amounts of
portland cement, a harder mortar is obtained. The more lime that is
added, the softer and more plastic the mortar becomes, increasing its
workability. A mortar strong in compressive strength might be desirable
for a hard stone (such as granite) pier holding up a bridge deck,
whereas a softer, more permeable lime mortar would be preferable for
a
historic wall of soft brick. Masonry deterioration caused by salt
deposition results when the mortar is less permeable than the masonry
unit. A strong mortar is still more permeable than hard, dense stone.
However, in a wall constructed of soft bricks where the masonry unit
itself has a relatively high permeability or vapor transmission rate,
a
soft, high lime mortar is necessary to retain sufficient permeability.
Mortar Preparation. Mortar components should be measured and
mixed
carefully to assure the uniformity of visual and physical
characteristics. Dry ingredients are measured by volume and thoroughly
mixed before the addition of any water. Sand must be added in a damp,
loose condition to avoid over sanding. Repointing mortar is typically
pre-hydrated by adding water so it will just hold together, thus
allowing it to stand for a period of time before the final water is
added. Half the water should be added, followed by mixing for
approximately 5 minutes. The remaining water should then be added in
small portions until a mortar of the desired consistency is reached.
The total volume of water necessary may vary from batch to batch,
depending on weather conditions. It is important to keep the water
to a
minimum for two reasons: first, a drier mortar is cleaner to work with,
and it can be compacted tightly into the joints; second, with no excess
water to evaporate, the mortar cures without shrinkage cracks. Mortar
should be used within approximately 30 minutes of final mixing, and
"retempering," or adding more water, should not be permitted.
Using Lime Putty to Make Mortar. Mortar made with lime putty
and sand,
sometimes referred to as roughage or course stuff, should be measured
by volume, and may require slightly different proportions from those
used with hydrated lime. No additional water is usually needed to
achieve a workable consistency because enough water is already
contained in the putty. Sand is proportioned first, followed by the
lime putty, then mixed for five minutes or until all the sand is
thoroughly coated with the lime putty. But mixing, in the familiar
sense of turning over with a hoe, sometimes may not be sufficient if
the best possible performance is to be obtained from a lime putty
mortar. Although the old practice of chopping, beating and ramming
the
mortar has largely been forgotten, recent field work has confirmed
that
lime putty and sand rammed and beaten with a wooden mallet or ax
handle, interspersed by chopping with a hoe, can significantly improve
workability and performance. The intensity of this action increases
the
overall lime/sand contact and removes any surplus water by compacting
the other ingredients. It may also be advantageous for larger projects
to use a mortar pan mill for mixing. Mortar pan mills which have a
long
tradition in Europe produce a superior lime putty mortar not attainable
with today's modern paddle and drum type mixers.
For larger repointing projects the lime putty and sand can be mixed
together ahead of time and stored indefinitely, on or off site, which
eliminates the need for piles of sand on the job site. This mixture,
which resembles damp brown sugar, must be protected from the air in
sealed containers with a wet piece of burlap over the top or sealed
in
a large plastic bag to prevent evaporation and premature carbonation.
The lime putty and sand mixture can be recombined into a workable
plastic state months later with no additional water.
If portland cement is specified in a lime putty and sand mortar--Type
O
(1:2:9) or Type K (1:3:11)--the portland cement should first be mixed
into a slurry paste before adding it to the lime putty and sand. Not
only will this ensure that the portland cement is evenly distributed
throughout the mixture, but if dry portland cement is added to wet
ingredients it tends to "ball up," jeopardizing dispersion. (Usually
water must be added to the lime putty and sand anyway once the portland
cement is introduced.) Any color pigments should be added at this stage
and mixed for a full five minutes. The mortar should be used within
30
minutes to 1½ hours and it should not be retempered. Once portland
cement has been added the mortar can no longer be stored.
Filling the Joint. Where existing mortar has been removed to
a depth of
greater than 1 inch, these deeper areas should be filled first,
compacting the new mortar in several layers. The back of the entire
joint should be filled successively by applying approximately 1/4 inch
of mortar, packing it well into the back corners. This application
may
extend along the wall for several feet. As soon as the mortar has
reached thumb-print hardness, another 1/4 inch layer of
mortar--approximately the same thickness--may be applied. Several
layers will be needed to fill the joint flush with the outer surface
of
the masonry. It is important to allow each layer time to harden before
the next layer is applied; most of the mortar shrinkage occurs during
the hardening process and layering thus minimizes overall shrinkage.
When the final layer of mortar is thumb-print hard, the joint should
be
tooled to match the historic joint. Proper timing of the tooling is
important for uniform color and appearance. If tooled when too soft,
the color will be lighter than expected, and hairline cracks may occur;
if tooled when too hard, there may be dark streaks called "tool
burning," and good closure of the mortar against the masonry units
will
not be achieved.
If the old bricks or stones have worn, rounded edges, it is best to
recess the final mortar slightly from the face of the masonry. This
treatment will help avoid a joint which is visually wider than the
actual joint; it also will avoid creation of a large, thin featheredge
which is easily damaged, thus admitting water. After tooling, excess
mortar can be removed from the edge of the joint by brushing with a
natural bristle or nylon brush. Metal bristle brushes should never
be
used on historic masonry.
Curing Conditions. The preliminary hardening of high-lime content
mortars--those mortars that contain more lime by volume than portland
cement, i.e., Type O (1:2:9), Type K (1:3:11), and straight lime/sand,
Type "L" (0:1:3)--takes place fairly rapidly as water in the mix is
lost to the porous surface of the masonry and through evaporation.
A
high lime mortar (especially Type "L") left to dry out too rapidly
can
result in chalking, poor adhesion, and poor durability. Periodic
wetting of the repointed area after the mortar joints are thumb-print
hard and have been finish tooled may significantly accelerate the
carbonation process. When feasible, misting using a hand sprayer with
a
fine nozzle can be simple to do for a day or two after repointing.
Local conditions will dictate the frequency of wetting, but initially
it may be as often as every hour and gradually reduced to every three
or four hours. Walls should be covered with burlap for the first three
days after repointing. (Plastic may be used, but it should be tented
out and not placed directly against the wall.) This helps keep the
walls damp and protects them from direct sunlight. Once carbonation
of
the lime has begun, it will continue for many years and the lime will
gain strength as it reverts back to calcium carbonate within the wall.
Aging the Mortar. Even with the best efforts at matching the
existing
mortar color, texture, and materials, there will usually be a visible
difference between the old and new work, partly because the new mortar
has been matched to the unweathered portions of the historic mortar.
Another reason for a slight mismatch may be that the sand is more
exposed in old mortar due to the slight erosion of the lime or cement.
Although spot repointing is generally preferable and some color
difference should be acceptable, if the difference between old and
new
mortar is too extreme, it may be advisable in some instances to repoint
an entire area of a wall, or an entire feature such as a bay, to
minimize the difference between the old and the new mortar. If the
mortars have been properly matched, usually the best way to deal with
surface color differences is to let the mortars age naturally. Other
treatments to overcome these differences, including cleaning the
non-repointed areas or staining the new mortar, should be carefully
tested prior to implementation.
Staining the new mortar to achieve a better color match is generally
not recommended, but it may be appropriate in some instances. Although
staining may provide an initial match, the old and new mortars may
weather at different rates, leading to visual differences after a few
seasons. In addition, the mixtures used to stain the mortar may be
harmful to the masonry; for example, they may introduce salts into
the
masonry which can lead to efflorescence.
Cleaning the Repointed Masonry. If repointing work is carefully
executed, there will be little need for cleaning other than to remove
the small amount of mortar from the edge of the joint following
tooling. This can be done with a stiff natural bristle or nylon brush
after the mortar has dried, but before it is initially set (1-2 hours).
Mortar that has hardened can usually be removed with a wooden paddle
or, if necessary, a chisel.
Further cleaning is best accomplished with plain water and natural
bristle or nylon brushes. If chemicals must be used, they should be
selected with extreme caution. Improper cleaning can lead to
deterioration of the masonry units, deterioration of the mortar, mortar
smear, and efflorescence. New mortar joints are especially susceptible
to damage because they do not become fully cured for several months.
Chemical cleaners, particularly acids, should never be used on dry
masonry. The masonry should always be completely soaked once with water
before chemicals are applied. After cleaning, the walls should be
flushed again with plain water to remove all traces of the chemicals.
Several precautions should be taken if a freshly repointed masonry wall
is to be cleaned. First, the mortar should be fully hardened before
cleaning. Thirty days is usually sufficient, depending on weather and
exposure; as mentioned previously, the mortar will continue to cure
even after it has hardened. Test panels should be prepared to evaluate
the effects of different cleaning methods. Generally, on newly
repointed masonry walls, only very low pressure (100 psi) water washing
supplemented by stiff natural bristle or nylon brushes should be used,
except on glazed or polished surfaces, where only soft cloths should
be
used.**
New construction "bloom" or efflorescence occasionally appears within
the first few months of repointing and usually disappears through the
normal process of weathering. If the efflorescence is not removed by
natural processes, the safest way to remove it is by dry brushing with
stiff natural or nylon bristle brushes followed by wet brushing.
Hydrochloric (muriatic) acid, is generally ineffective, and it should
not be used to remove efflorescence. It may liberate additional salts,
which, in turn, can lead to more efflorescence.
Surface Grouting is sometimes suggested as an alternative to
repointing
brick buildings, in particular. This process involves the application
of a thin coat of cement-based grout to the mortar joints and the
mortar/brick interface. To be effective, the grout must extend slightly
onto the face of the masonry units, thus widening the joint visually.
The change in the joint appearance can alter the historic character
of
the structure to an unacceptable degree. In addition, although masking
of the bricks is intended to keep the grout off the remainder of the
face of the bricks, some level of residue, called "veiling," will
inevitably remain. Surface grouting cannot substitute for the more
extensive work of repointing, and it is not a recommended treatment
for
historic masonry.
**Additional information on masonry cleaning is presented in
Preservation Briefs 1: The Cleaning and Waterproof Coating of Masonry
Buildings, Robert C. Mack, AIA, Washington, D.C.: Technical
Preservation Services, National Park Service, U.S. Department of the
Interior, 1975; and Keeping it Clean: Removing Exterior Dirt, Paint,
Stains & Graffiti from Historic Masonry Buildings, Anne E. Grimmer,
Washington, D.C.: Technical Preservation Services, National Park
Service, U.S. Department of the Interior, 1988.
Visually Examining the Mortar and the Masonry units.
A simple in-situ comparison will help determine the hardness and
condition of the mortar and the masonry units. Begin by scraping the
mortar with a screwdriver, and gradually tapping harder with a cold
chisel and mason's hammer. Masonry units can be tested in the same
way
beginning, even more gently, by scraping with a fingernail. This
relative analysis which is derived from the 10-point hardness scale
used to describe minerals, provides a good starting point for selection
of an appropriate mortar. It is described more fully in "The Russack
System for Brick & Mortar Description" referenced in Selected Reading
at the end of this Brief.
Mortar samples should be chosen carefully, and picked from a variety
of
locations on the building to find unweathered mortar, if possible.
Portions of the building may have been repointed in the past while
other areas may be subject to conditions causing unusual deterioration.
There may be several colors of mortar dating from different
construction periods or sand used from different sources during the
initial construction. Any of these situations can give false readings
to the visual or physical characteristics required for the new mortar.
Variations should be noted which may require developing more than one
mix.
1) Remove with a chisel and hammer three or four unweathered samples
of
the mortar to be matched from several locations on the building. (Set
the largest sample aside--this will be used later for comparison with
the repointing mortar). Removing a full representation of samples will
allow selection of a "mean" or average mortar sample.
2) Mash the remaining samples with a wooden mallet, or hammer if
necessary, until they are separated into their constituent parts. There
should be a good handful of the material.
3) Examine the powdered portion--the lime and/or cement matrix of the
mortar. Most particularly, note the color. There is a tendency to think
of historic mortars as having white binders, but grey portland cement
was available by the last quarter of the 19th century, and traditional
limes were also sometimes grey. Thus, in some instances, the natural
color of the historic binder may be grey, rather than white. The mortar
may also have been tinted to create a colored mortar, and this color
should be identified at this point.
4) Carefully blow away the powdery material (the lime and/or cement
matrix which bound the mortar together).
5) With a low power (10 power) magnifying glass, examine the remaining
sand and other materials such as lumps of lime or shell.
6) Note and record the wide range of color as well as the varying sizes
of the individual grains of sand, impurities, or other materials.
Other Factors to Consider
Color. Regardless of the color of the binder or colored additives, the
sand is the primary material that gives mortar its color. A surprising
variety of colors of sand may be found in a single sample of historic
mortar, and the different sizes of the grains of sand or other
materials, such as incompletely ground lime or cement, play an
important role in the texture of the repointing mortar. Therefore,
when
specifying sand for repointing mortar, it may be necessary to obtain
sand from several sources and to combine or screen them in order to
approximate the range of sand colors and grain sizes in the historic
mortar sample.
Pointing Style. Close examination of the historic masonry wall and the
techniques used in the original construction will assist in maintaining
the visual qualities of the building. Pointing styles and the methods
of producing them should be examined. It is important to look at both
the horizontal and the vertical joints to determine the order in which
they were tooled and whether they were the same style. Some late-19th
and early-20th century buildings, for example, have horizontal joints
that were raked back while the vertical joints were finished flush
and
stained to match the bricks, thus creating the illusion of horizontal
bands. Pointing styles may also differ from one facade to another;
front walls often received greater attention to mortar detailing than
side and rear walls. Tuckpointing is not true repointing but the
application of a raised joint or lime putty joint on top of flush
mortar joints. Penciling is a purely decorative, painted surface
treatment over a mortar joint, often in a contrasting color.
Masonry Units.The masonry units should also be examined so that any
replacement units will match the historic masonry. Within a wall there
may be a wide range of colors, textures, and sizes, particularly with
hand-made brick or rough-cut, locally-quarried stone. Replacement units
should blend in with the full range of masonry units rather than a
single brick or stone.
Matching Color and Texture of the Repointing Mortar
New mortar should match the unweathered interior portions of the
historic mortar. The simplest way to check the match is to make a small
sample of the proposed mix and allow it to cure at a temperature of
approximately 70 degrees F for about a week, or it can be baked in
an
oven to speed up the curing; this sample is then broken open and the
surface is compared with the surface of the largest "saved" sample
of
historic mortar.
If a proper color match cannot be achieved through the use of natural
sand or colored aggregates like crushed marble or brick dust, it may
be
necessary to use a modern mortar pigment.
During the early stages of the project, it should be determined how
closely the new mortar should match the historic mortar. Will "quite
close" be sufficient, or is "exactly" expected? The specifications
should state this clearly so that the contractor has a reasonable idea
how much time and expense will be required to develop an acceptable
match.
The same judgment will be necessary in matching replacement terra
cotta, stone or brick. If there is a known source for replacements,
this should be included in the specifications. If a source cannot be
determined prior to the bidding process, the specifications should
include an estimated price for the replacement materials with the final
price based on the actual cost to the contractor.
Conclusion
A good repointing job is meant to last, at least 30 years, and
preferably 50- 100 years. Shortcuts and poor craftsmanship result not
only in diminishing the historic character of a building, but also
in a
job that looks bad, and will require future repointing sooner than
if
the work had been done correctly. The mortar joint in a historic
masonry building has often been called a wall's "first line of
defense." Good repointing practices guarantee the long life of the
mortar joint, the wall, and the historic structure. Although careful
maintenance will help preserve the freshly repointed mortar joints,
it
is important to remember that mortar joints are intended to be
sacrificial and will probably require repointing some time in the
future. Nevertheless, if the historic mortar joints proved durable
for
many years, then careful repointing should have an equally long life,
ultimately contributing to the preservation of the entire building.
Selected Readinng
"Lime's Role in Mortar." Aberdeen's Magazine of Masonry Construction.
Vol. 9, No. 8 (August 1996), pp. 364-368.
Phillips, Morgan W. "Brief Notes on the Subjects of Analyzing Paints
and Mortars and the Recording of Moulding Profiles: The Trouble with
Paint and Mortar Analysis." Bulletin of the Association for
Preservation Technology. Vol. 10, No. 2 (1978), pp. 77-89.
Preparation and Use of Lime Mortars: An Introduction to the Principles
of Using Lime Mortars. Scottish Lime Centre for Historic Scotland.
Edinburgh: Historic Scotland, 1995.
Schierhorn, Carolyn. "Ensuring Mortar Color Consistency." Aberdeen's
Magazine of Masonry Construction. Vol. 9, No. 1 (January 1996), pp.
33-35.
"Should Air-Entrained Mortars Be Used?" Aberdeen's Magazine of Masonry
Construction. Vol. 7, No. 9 (September 1994), pp. 419-422.
Sickels-Taves, Lauren B. "Creep, Shrinkage, and Mortars in Historic
Preservation." Journal of Testing and Evaluation, JTEVA. Vol. 23, No.
6
( November 1995), pp. 447-452.
Speweik, John P. The History of Masonry Mortar in America, 1720-1995.
Arlington, VA: National Lime Association, 1995.
Speweik, John P. "Repointing Right: Why Using Modern Mortar Can Damage
a Historic House." Old-House Journal. Vol. XXV, No. 4 (July-August
1997), pp. 46-51.
Technical Notes on Brick Construction. Brick Institute of America,
Reston, VA.
"Moisture Resistance of Brick Masonry: Maintenance."
7F. February
1986.
"Mortars for Brick Masonry." 8 Revised II. November 1989.
"Standard Specification for Portland Cement-Lime
Mortar for Brick
Masonry." 8A Revised. September 1988.
"Mortar for Brick Masonry-Selection and Controls."
8B Reissued.
September 1988. (July/August 1976).
"Guide Specifications for Brick Masonry, Part V Mortar
and Grout."
11E Revised. September 1991.
"Bonds and Patterns in Brickwork." 30 Reissued. September
1998