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.
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.
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.
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.
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.
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:
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.
This early 19th century building is being repointed with lime mortar. Photo: Travis McDonald.
|
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.
Caulking was inappropriately used here in place of mortar on the top of the wall. As a result, it has not been durable. Photo: NPS files.
|
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. 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.
Here, a hammer and chisel are being correctly used to prepare a joint for repointing. Photo: John P. Speweik.
|
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.
Repointing is both expensive and time consuming due to the extent of handwork
and special materials required. It is preferable to repoint only those areas
that require work rather than an entire wall, as is often specified. But, if
25 to 50 per cent or more of a wall needs to be repointed, repointing the
entire wall may be more cost effective than spot repointing.
When repairing this stone wall, the mason matched the raised profile of the original tuckpointing. Photo: NPS files.
|
Total repointing
may also be more sensible when access is difficult, requiring the erection of
expensive scaffolding (unless the majority of the mortar is sound and unlikely
to require replacement in the foreseeable future). Each project requires
judgement based on a variety of factors. Recognizing this at the outset will
help to prevent many jobs from becoming prohibitively expensive.
In scheduling, seasonal aspects need to be considered first. Generally
speaking, wall temperatures between 40 and 95 degrees F (8 and 38 degrees C)
will prevent freezing or excessive evaporation of the water in the mortar.
Ideally, repointing should be done in shade, away from strong sunlight in
order to slow the drying process, especially during hot weather. If
necessary, shade can be provided for large-scale projects with appropriate
modifications to scaffolding.
The relationship of repointing to other work proposed on the building must
also be recognized. For example, if paint removal or cleaning is anticipated,
and if the mortar joints are basically sound and need only selective
repointing, it is generally better to postpone repointing until after
completion of these activities. However, if the mortar has eroded badly,
allowing moisture to penetrate deeply into the wall, repointing should be
accomplished before cleaning. Related work, such as structural or roof
repairs, should be scheduled so that they do not interfere with repointing and
so that all work can take maximum advantage of erected scaffolding.
Building managers also must recognize the difficulties that a repointing
project can create.
A mechanical grinder improperly used to cut out the horizontal joint and incompatible repointing have seriously damaged the 19th century brick. Photo: NPS files.
|
The process is time consuming, and scaffolding may need
to remain in place for an extended period of time. The joint preparation
process can be quite noisy and can generate large quantities of dust which
must be controlled, especially at air intakes to protect human health, and
also where it might damage operating machinery. Entrances may be blocked from
time to time making access difficult for both building tenants and visitors.
Clearly, building managers will need to coordinate the repointing work with
other events at the site.
The ideal way to select a contractor is to ask knowledgeable owners of
recently repointed historic buildings for recommendations. Qualified
contractors then can provide lists of other repointing projects for
inspection. More commonly, however, the contractor for a repointing project
is selected through a competitive bidding process over which the client or
consultant has only limited control. In this situation it is important to
ensure that the specifications stipulate that masons must have a minimum of
five years' experience with repointing historic masonry buildings to be
eligible to bid on the project. Contracts are awarded to the lowest
responsible bidder, and bidders who have performed poorly on other projects
usually can be eliminated from consideration on this basis, even if they have
the lowest prices.
The contract documents should call for unit prices as well as a base bid.
Unit pricing forces the contractor to determine in advance what the cost
addition or reduction will be for work which varies from the scope of the base
bid. If, for example, the contractor has fifty linear feet less of stone
repointing than indicated on the contract documents but thirty linear feet
more of brick repointing, it will be easy to determine the final price for the
work. Note that each type of work--brick repointing, stone repointing, or
similar items--will have its own unit price. The unit price also should
reflect quantities; one linear foot of pointing in five different spots will
be more expensive than five contiguous linear feet.
Test Panels. These panels are prepared by the contractor using the
same techniques that will be used on the remainder of the project. Several
panel locations--preferably not on the front or other highly visible location
of the building--may be necessary to include all types of masonry, joint
styles, mortar colors, and other problems likely to be encountered on the job.
Unskilled repointing has negatively impacted the character of this late-19th century building. Photo: NPS files.
|
If cleaning tests, for example, are also to be undertaken, they should be
carried out in the same location. Usually a 3 foot by 3 foot area is
sufficient for brickwork, while a somewhat larger area may be required for
stonework. These panels establish an acceptable standard of work and serve as
a benchmark for evaluating and accepting subsequent work on the building.
Joint Preparation. Old mortar should be removed to a minimum depth of
2 to 2-1/2 times the width of the joint to ensure an adequate bond and to
prevent mortar "popouts." For most brick joints, this will require
removal of the mortar to a depth of approximately ½ to 1 inch; for stone
masonry with wide joints, mortar may need to be removed to a depth of several
inches. Any loose or disintegrated mortar beyond this minimum depth also
should be removed.
Although some damage may be inevitable, careful joint preparation can help
limit damage to masonry units. The traditional manner of removing old mortar
is through the use of hand chisels and mash hammers. Though
labor-intensive, in most instances this method poses the least threat for
damage to historic masonry units and produces the best final product.
The most common method of removing mortar, however, is through the use of
power saws or grinders. The use of power tools by unskilled masons can be
disastrous for historic masonry, particularly soft brick. Using power saws on
walls with thin joints, such as most brick walls, almost always will result in
damage to the masonry units by breaking the edges and by overcutting on the
head, or vertical joints.
However, small pneumatically-powered chisels generally can be used safely and
effectively to remove mortar on historic buildings as long as the masons
maintain appropriate control over the equipment. Under certain circumstances,
thin diamond-bladed grinders may be used to cut out horizontal joints
only on hard portland cement mortar common to most early-20th century masonry
buildings. Usually, automatic tools most successfully remove old
mortar without damaging the masonry units when they are used in combination
with hand tools in preparation for repointing. Where horizontal joints are
uniform and fairly wide, it may be possible to use a power masonry saw to
assist the removal of mortar, such as by cutting along the middle of the
joint; final mortar removal from the sides of the joints still should be done
with a hand chisel and hammer. Caulking cutters with diamond blades can
sometimes be used successfully to cut out joints without damaging the masonry.
Caulking cutters are slow; they do not rotate, but vibrate at very high
speeds, thus minimizing the possibility of damage to masonry units. Although mechanical tools may be safely used in
limited circumstances to cut out horizontal joints in preparation for
repointing, they should never be used on vertical joints because of the
danger of slipping and cutting into the brick above or below the vertical
joint. Using power tools to remove mortar
without damaging the surrounding masonry units also necessitates highly
skilled masons experienced in working on historic masonry buildings.
Contractors should demonstrate proficiency with power tools before their use
is approved.
Using any of these power tools may also be more acceptable on hard stone, such
as quartzite or granite, than on terra cotta with its glass-like glaze, or on
soft brick or stone. The test panel should determine the acceptability of
power tools. If power tools are to be permitted, the contractor should
establish a quality control program to account for worker fatigue and similar
variables.
Mortar should be removed cleanly from the masonry units, leaving square
corners at the back of the cut. Before filling, the joints should be rinsed
with a jet of water to remove all loose particles and dust. At the time of
filling, the joints should be damp, but with no standing water present. For
masonry walls--limestone, sandstone and common brick--that are extremely
absorbent, it is recommended that a continual mist of water be applied for a
few hours before repointing begins.
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.
This 18th century pediment and surrounding wall exhibit distinctively different mortar joints. Photo: NPS files.
|
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: Assessing Cleaning and Water-Repellent Treatments for Historic Masonry Buildings, Robert C. Mack, FAIA, and Anne Grimmer, Washington, D.C.: Technical Preservation Services, National Park Service, U.S. Department of the Interior, 2000; 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.
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.
Mortar
Types (Measured
by volume) |
Designation |
Cement |
Hydrated
Lime or Lime Putty
|
Sand |
M |
1 |
1/4 |
3 - 3 3/4 |
S |
1 |
1/2 |
4 -
4 1/2 |
N |
1 |
1 |
5 -
6 |
O |
1 |
2 |
8 -
9 |
K |
1 |
3 |
10
- 12 |
"L" |
0 |
1 |
2 1/4 - 3 |
Suggested
Mortar Types for Different Exposures |
|
Exposure
|
Masonry
Material |
Sheltered |
Moderate |
Severe |
Very
durable:
granite, hard-cored brick, etc. |
O |
N |
S |
Moderately
durable: limestone, durable stone, molded brick |
K |
O |
N |
Minimally
durable: soft hand-made brick |
"L" |
K |
O |
For the Owner/Administrator. The owner or administrator of a historic building should remember that repointing is likely to be a lengthy and expensive process. First, there must be adequate time for evaluation of the building and investigation into the cause of problems. Then, there will be time needed for preparation of the contract documents. The work itself is precise, time-consuming and noisy, and scaffolding may cover the face of the building for some time. Therefore, the owner must carefully plan the work to avoid problems. Schedules for both repointing and other activities will thus require careful coordination to avoid unanticipated conflicts. The owner must avoid the tendency to rush the work or cut corners if the historic building is to retain its visual integrity and the job is to be durable.
For the Architect/Consultant. Because the primary role of the consultant is to ensure the life of the building, a knowledge of historic construction techniques and the special problems found in older buildings is essential. The consultant must assist the owner in planning for logistical problems relating to research and construction. It is the consultant's responsibility to determine the cause of the mortar deterioration and ensure that it is corrected before the masonry is repointed. The consultant must also be prepared to spend more time in project inspections than is customary in modern construction.
For the Masons. Successful repointing depends on the masons themselves. Experienced masons understand the special requirements for work on historic buildings and the added time and expense they require. The entire masonry crew must be willing and able to perform the work in conformance with the specifications, even when the specifications may not be in conformance with standard practice. At the same time, the masons should not hesitate to question the specifications if it appears that the work specified would damage the building.
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.
Ashurst, John & Nicola. Practical Building Conservation. Vol. 3: Mortars, Plasters and Renders. New York: Halsted Press, a Division of John Wiley & Sons, Inc., 1988.
Cliver, E. Blaine. "Tests for the Analysis of Mortar Samples." Bulletin of the Association for Preservation Technology. Vol. 6, No. 1 (1974), pp. 68-73.
Coney, William B., AIA. Masonry Repointing of Twentieth-Century Buildings. Illinois Preservation Series. Number 10. Springfield, IL: Division of Preservation Services, Illinois Historic Preservation Agency, 1989.
Davidson, J.I. "Masonry Mortar." Canadian Building Digest. CBD 163. Ottawa, ONT: Division of Building Research, National Research Council of Canada, 1974.
Ferro, Maximillian L., AIA, RIBA. "The Russack System for Brick and Mortar Description: A Field Method for Assessing Masonry Hardness." Technology and Conservation. Vol. 5, No. 2 (Summer 1980), pp. 32-35.
Hooker, Kenneth A. "Field Notes on Repointing." Aberdeen's Magazine of Masonry
Construction. Vol. 4, No. 8 (August 1991), pp. 326-328.
Jedrzejewska, H. "Old Mortars in Poland: A New Method of Investigation." Studies in Conservation. Vol. 5, No. 4 (1960), pp. 132-138.
"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 1988.
Useful Addresses
Brick Institute of America
11490 Commerce Park Drive
Reston, VA 22091
National Lime Association
200 N. Glebe Road, Suite 800
Arlington, VA 22203
Portland Cement Association
5420 Old Orchard Road
Skokie, IL 60077
Acknowledgments
Robert C. Mack, FAIA, is a principal in the firm of MacDonald & Mack, Architects, Ltd., an architectural firm that specializes in historic buildings in Minneapolis, Minnesota. John P. Speweik, CSI, Toledo, Ohio, is a 5th-generation stonemason, and principal in U.S. Heritage Group, Inc., Chicago, Illinois, which does custom historic mortar matching. Anne Grimmer, Senior Architectural Historian, Heritage Preservation Services Program, National Park Service, was responsible for developing and coordinating the revision of this Preservation Brief, incorporating professional comments, and the technical editing.
The authors and the editor wish to thank the following for the professional and technical review they provided: Mark Macpherson and Ron Peterson, Masonry Restoration Contractors, Macpherson-Towne Company, Minneapolis, MN; Lorraine Schnabel, Architectural Conservator, John Milner Associates, Inc., Philadelphia, PA; Lauren B. Sickels-Taves, Ph.D., Architectural Conservator, Biohistory International, Huntington Woods, MI; and the following National Park Service professional staff, including: E. Blaine Cliver, Chief, Historic American Buildings Survey/Historic American Engineering Record; Douglas C. Hicks, Deputy Superintendent, Historic Preservation Training Center, Frederick, MD; Chris McGuigan, Supervisory Exhibits Specialist, Historic Preservation Training Center, Frederick, MD; Charles E. Fisher, Sharon C. Park, FAIA, John Sandor, Technical Preservation Services Branch, Heritage Preservation Services, and Kay D. Weeks, Heritage Preservation Services.
The original version of this brief, Repointing Mortar Joints in Historic Brick Buildings, was written by Robert C. Mack in 1976, and was revised and updated in 1980 by Robert C. Mack, de Teel Patterson Tiller, and James S. Askins.
Washington, D.C. October, 1998
Home page logo: Soft mortar for repointing. Photo: John P. Speweik.