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Historic Preservation - Technical Procedures

Spectitle:

Limestone: Characteristics, Uses And Problems

Procedure code:

0446001S

Source:

Outdoor Sculpture Manual - Center For Public Buildings

Division:

Masonry

Section:

Limestone

Last Modified:

10/26/2012

Details:

Limestone: Characteristics, Uses And Problems



LIMESTONE:  CHARACTERISTICS, USES AND PROBLEMS


This procedure includes general information on the characteristics
and common uses of limestone and identifies typical problems
associated with the material.  See also 04400-01-S for guidance on
inspecting stone masonry failures.


INTRODUCTION

Limestone is a sedimentary rock composed principally of calcium
carbonate (calcite) or the double carbonate of calcium and
magnesium (dolomite).  It is commonly composed of tiny fossils,
shell fragments and other fossilized debris.  These fossils are
frequently visible to the unaided eye on close examination of the
stone surface, however this is not always the case.  Some varieties
of limestone have an extremely fine grain.  

Limestone is usually gray, but it may also be white, yellow or
brown.  It is a soft rock and is easily scratched.  It will
effervesce readily in any common acid.

Limestones may vary greatly in texture and porosity from coquina,
which is a matrix of whole or pieces of sea shells loosely cemented by calcite, to
oolitic limestones and microcrystalline limestones whose structures
are so fine that they can be seen only under magnification.

Limestone deposits can undergo metamorphism during major geological events resulting in a recrystallizing as marble.

Oolitic limestone consists of substantial amounts of "oolites" or
"ooliths."  Oolites are small spherical or sub-spherical grains of
concentric calcite.

The actual classification of limestones and marbles can be very
confusing to the non-geologists.  The same stone can be marketed
one time as a limestone and, at another time and place, sold as
marble.  The subtleties which sometimes differentiate between
grades and types of stones are frequently beyond the concern and
expertise of maintenance workers, building managers and historical
architects with responsibility for maintenance of the resources.
While this is understandable, it does not lessen or eliminate the
need to accurately identify the materials which must be treated and
maintained.  Failure to accurately identify a material to be
treated can result in the failure to consider important technical
details which subsequently results in irreversible damage to the
resource(s).

In an effort to improve accuracy in identifying the general
categories of limestones at a 'macro' level, the following section
contains descriptions of the most common types of limestone,
however this information is no substitute for training and
experience to correctly identify and catalog stone types.  The
following definitions are from the American Society for Testing and
Materials (ASTM) document, "Standard Definition of Terms Relating
to Natural Building Stones."

    Calcarenite:  Calcarenite is composed of sand-sized grains of
    calcite, usually in the form of tiny fossils, shell fragments
    and fossil debris.  Some calcarenites contain oolites and if
    the oolites are present in sufficient quantity, the stone is
    called oolite limestone.  Oolite limestone is a sub-category
    of calcarenite.

-    Coquina:  Coquina consists of raw, unaltered shell fragments,
    often quite large, loosely cemented by calcite.  It is
    generally very coarse and porous, frequently consisting of
    oyster and sea shells and fragments.

-    Dolomite:  Dolomite is a sedimentary carbonate rock composed
    of calcium and magnesium carbonate.  Also called "magnesium
    limestone", it contains from 5 to 40% magnesium carbonate.

-    Microcrystalline limestone:  This is a limestone structure of
    crystals too small to be seen without magnification.

-    Oolitic limestone:  Oolitic limestone is a calcite cemented
    calcareous stone composed of shell fragments, practically non-
    crystalline in character.  Generally without cleavage, and
    extremely uniform in composition and texture, oolitic
    limestone adjusts to temperature changes.

-    Travertine:  A calcium carbonate, usually light in color,
    travertine can be extremely porous or cellular.  It is usually
    deposited from solids in groundwater.

Limestone coloration is generally a consistent pure white to off-
white.  Many varieties do not take a polish well, so that the
surface is typically a matte finish, no-gloss surface.  Limestones,
like marble and other calcareous stones, are referred to as acid
sensitive.  Calcareous stones are readily dissolved in acid,
therefore acidic products should not be used on limestones and
marbles.


TYPICAL USES

Limestone is widely used in architectural applications for walls,
decorative trim and veneer.  It is less frequently used as a
sculptural material, because of its porosity and softness, however,
it is a common base material.  It may be found in both bearing
(structural) and veneer applications.


PROBLEMS AND DETERIORATION

Weathering may have a degrading effect on the appearance and
structural soundness of limestone.  Factors include rain, snow,
temperature, wind and atmospheric pollutants.  Generally these
factors act in combination with one another or with other agents of
deterioration.

Rainwater, especially in combination with atmospheric gases often resulting in acid rain can
result in dissolution of the limestone, causing higher levels of
salt movement within the stone structure.  Temperature can effect
rates of deterioration and (in larger stones) movement of the
pieces, as well as patterns of salt migration within the stone.
Most of the natural or inherent problems which can occur with
limestone require some degree of moisture to occur, however other
problems such as wind erosion and vandalism may occur
independently.


NATURAL OR INHERENT LIMESTONE PROBLEMS

WEATHERING:

Limestone subjected to exterior exposures deteriorates due to
weathering or the natural effects of wind, rain, and thermal
change.  Limestone is extremely durable. It does, however, absorb
water and, since it is a carbonate rock, it is highly reactive when
exposed to acids or even mildly acidic rain water, and it can
suffer substantial deterioration.  The most common effect of
weathering and erosion is loss of precise detail.

Little can be done to restore edge detailing short of re-carving
the stone which is usually infeasible.

EROSION:

Erosion can be the result of general weathering described above, or
it can be a more localized phenomenon based upon handling or
exposure.  Wind driven airborne abrasives may selectively wear away
detailing on certain elevations, based upon the direction of
prevailing winds.  One of the few effective ways to address this
problem is by landscaping where plantings and/or grade can deflect
the wind.  Such landscaping and/or grading may range from the
simple and inexpensive up to a major and expensive intervention.
It would have to be consistent with appropriate policy for the
management of cultural landscapes.  It may, however, be cost
effective when considering the extended life of the stone.

The symptoms of erosion can be as simple as the loss of edge
sharpness as described above, or it can be very localized, specific
wear due to contact with landscaping and mowing equipment.
Localized damage due to contact by mowing or other maintenance
equipment is preventable.  Where there is evidence of recurrent
physical damage, steps should be taken to protect the resource(s).

STAINING:

Discoloration of the limestone, whether general or localized, is
staining.  Staining, may be the result of exposure to a variety of
exterior substances or to internal occlusions in the stone or
structural elements.

Some of the most common types of staining and the causative agents
are:

1.   Oil/grease stains:  These stains are usually the result of
    vandalism or use.  A variety of organic or inorganic oils may
    be absorbed into the stone upon contact.  The depth of
    penetration will depend upon the viscosity of the oil/grease,
    temperature, stone porosity, finish and dryness.

    The appearance of grease/oil stains will usually consist of a
    darkening of the stone at the area of contact.  The edges of
    the staining will generally be diffused, especially after an
    extended period.  There are standard techniques for removing
    oil and grease stains.

    For specific guidance on removing oil/grease stains from
    limestone, see 04455-10-R and 04455-11-R.

2.   Dyes and inks:  The staining could be any color depending on
    the type and source of the dye.  This type of stain is likely
    to be extremely localized around the area of contact.  The
    liquid containing the coloration may be absorbed into the
    stone and during the normal process of evaporation, the
    coloring pigment is deposited in the stone.

    For specific guidance on removing ink and dye stains from
    limestone, see 04455-18-R.

3.   Organic stains:  Organic stains are caused by direct contact
    with decomposing organic matter, such as leaves, bird or
    animal droppings, flowers, tea or coffee. Regardless of the
    source these stains tend to be a slight reddish-brown in
    color.  They also frequently disappear after the source has
    been removed.  These stains may be left to weather and bleach
    or oxidize out after the removal of the organic source,
    however a residue  may still remain on the stone.

    For specific guidance on removing organic stains from
    limestone, see 04455-14-R.

4.   Metallic stains:  Two major categories of metallic staining
    occur, they tend to be based on either iron or copper.  The
    source of the staining may be internal structural components
    or elements.  A major source is the water wash, or run-off,
    from adjacent metallic elements, especially bronze and copper.

    a.   Rust stains:  These stains are reddish-orange and are
         caused by the oxidation (rusting) of iron.  The source of
         iron staining is usually the structural or connecting
         components.  These components are usually hidden and
         protected; however, water penetration from bad joints or
         cracks can activate or accelerate rusting.  The
         discoloration may be within the stone or it may be a
         deposit of rust on the surface of the stone.  Surface
         deposits of rust may sometimes be removed by hand rubbing
         with a clean cloth.  The examination of the stain should
         include such rubbing to determine if it is only a surface
         deposit.

         For specific guidance on removing rust stains from
         limestone, see 04400-06-R.

    b.  Bronze and Copper stains:  Stains from water run-off from bronze can
         range in color from a light green to a dark brown.  The
         staining results from the dissolved copper salts (from
         copper or bronze) which wash onto the stone, then
         oxidize. The pattern of the staining is likely to be
         localized, streaked and in the path of the run-off from
         the metallic source.

         For specific guidance on removing bronze and copper stains from
         limestone, see 04400-07-R.

CRUMBLING:

This condition is indicative of a certain brittleness or tendency
of the stone to break up or dissolve.  It may be caused by an
inherent weakness in the limestone or gradual breakdown of the
binder, or it may be the result of external factors affecting the
strength and durability of the limestone.

This condition may be caused by the use of de-icing salts, or any
other source of salt migration, such as that which can occur when
rising damp is present.  There is currently little which can be
done to repair the damage once this condition has developed,
however the early detection of potential problems and elimination
of sources of salts is critical to arresting the process.  When
this condition is severe and obviously caused by the heavy or
inappropriate use of de-icing salts, it is sometimes called "Salt
Fretting".  Regular preservation maintenance may eliminate the
causes promoting crumbling, however, once the condition has
occurred, its correction or repair is beyond the level of a
maintenance procedure.  The Regional Historic Preservation Officer
(RHPO) should be contacted for assistance.

CHIPPING:

The separation of small pieces or larger fragments from a masonry
unit, frequently at the corners, edges or mortar joints is known as
chipping.  These fractures are generally caused by deterioration
and repointing, especially due to the use of too hard a mortar, or
by accident or vandalism.

Repairs include detachment repairs, patching and splicing.  Repair
of chipped stone requires a skilled mason and is not a maintenance
procedure.  If chipping is due to occasional impact from mowing or
other landscape maintenance, steps should be taken to prevent
future damage.  

For specific guidance on repairing chips in limestone, see 04455-
03-R.

CRACKING:

This condition is manifested by the appearance of narrow fissures
ranging from less than 1/16 to 1/2 inch wide or more in the stone.
It results from a variety of causes, such as structural overloading
due to settlement, the use of too hard a mortar mix or a flaw in
the material.  Minor cracking may be no problem, in and of itself,
but it can be an indication of structural problems and the cracks
can be a point of entry of water into the interior of the stone,
promoting salt migration.  Cracking, which allows water or salts to
enter the stone, increases the possibility of failure along the
limestone and may result in subsequent spalling.  Repairs include
patching and replacement.

For specific guidance on repairing cracks in limestone, see 04455-
03-R.

DETACHMENT:

This is not a failure of the material per se but a failure of the
construction system, i.e. the connectors and/or joints.  The
definition implies that the failed component survives intact and
may be re-installed using appropriate mechanical techniques.

The failure of anchors or metal connectors which lead to detachment
may be caused and/or accelerated by the penetration of water into
the structure behind the stone, causing rust and corrosion.
Adequate pointing and caulking can prevent leakage and penetration
of water into the system.

For specific guidance on resecurring detached limestone blocks, see
04460-07-R and 04460-13-R.

EFFLORESCENCE:

The appearance of a whitish deposit locally or uniformly over the
surface may be efflorescence, the surface deposition of soluble
salts.  There are numerous sources for the soluble salts which
create the hazy appearance; salts can come from mortar, improper
cleaning agents, rising damp, de-icing salts, chemical landscaping
treatments and air pollution.

Efflorescence can be a salt residue resulting from improper
chemical cleaning, i.e. too strong a chemical cleaner or inadequate
rinsing.  It can also be an indication of water problems.  Salt
migration and/or sub-florescence and efflorescence should be
considered a symptom which should be investigated to identify the
source of the soluble salts and/or the source of moisture.
Corrective action should then be taken to eliminate the source of
the problem once it is identified.

Some efflorescence may occur naturally with new stones, mortar and
installation materials.  Normally, this efflorescence will be
removed by natural rain and weathering processes and/or by regular
washing.  The new or continued appearance of efflorescence is a
stronger indicator of problems like rising damp or inappropriate
cleaning methods, all of which should be referred to the Regional
Historic Preservation Officer (RHPO).

For specific guidance on removing efflorescence from limestone, see
04500-02-R.

EROSION:

Erosion is the wearing away of the material surface by the natural
action of wind, windblown particles and water.  It can occur with
limestone as well as any exposed materials. Inspections should
include examination  for any apparent loss of detail and edge
sharpness which could be due to erosion.

Erosion may be less of a problem on rock-faced or quarry-faced
marble, but may be a more serious problem on stone with more
precise detail.  Little can be done to correct this problem once it
occurs, other than to protect the surface from further exposure.
This may stop or at least retard the erosion process.

FLAKING:

This is an early stage of peeling, exfoliation, delamination or
spalling evidenced by the detachment of small flat thin pieces of
the outer layers of stone from a larger piece of stone.  Flaking is
usually caused by capillary moisture or freeze-thaw cycles which
occur within the masonry.  

The problem can also occur due to sub-florescence, so that if
flaking occurs, the area should be examined to determine if salt
crystallization is occurring in the flaked areas.

PEELING:

Peeling is the flaking away of the stone surface from the substrate
in strips or layers.  It may result from the improper application
of masonry coatings which result in failure of the coating and/or
stone surface.  It may also result from a defect in the stone, or
from weathering.

Encrustations of the surface caused by chemical reactions with
environmental elements may also peel or flake along the bedding
plane.

RISING DAMP:

Rising damp is the suction of ground water into the base of masonry
through capillary action.  Moisture is drawn up into the stone and
may rise and fall due to conditions of temperature; humidity; site
grading; absence or failure of damp courses, and/or treatments to
the masonry surfaces which affect evaporation.

During active wet periods, rising damp may be visible as a
darkening of the stone along the base at ground level.  Due to the
continuous changing of the moisture level due to varying exposure
conditions, staining or efflorescence may be visible at a range of
several feet up from the ground.  Continuation of the problem can
lead to more severe problems of flaking, peeling and/or spalling,
but the correction of the problem requires the elimination of the
source of water or the interruption of its path into the stone by
physical or chemical damp-proofing.

SPALLING:

Spalling is the separation and breaking away of pieces of stone due
to sub-florescence, freeze-thaw, improper repointing with  too hard
a mortar mix containing too much portland cement, or structural overloading of the
stone.

Spalling is less frequent with limestone than with sedimentary
stones which are also less hard.  Limestone is hard enough to
resist internal forces which would cause spalling in other natural
stones or fabricated masonry.

For specific guidance on repairing spalling limestone, see 04400-
03-R and 04455-03-R.

SUB-FLORESCENCE:

This is a potentially harmful internal accumulation of soluble
salts deposited under or just beneath the masonry surface as
moisture in the wall evaporates.

The build-up of salts and their crystallization can create
substantial pressures within the masonry, causing pieces to break
off along the planes of deposition.  Efflorescence at the surface
is an indication that sub-florescence is possible.  Techniques for
mitigating the problem include poulticing, removal of identified
salt sources, elimination of moisture in the stone and damp-
proofing.

                         END OF SECTION
 


limestone, oolitic limestone, calcarenite, coquina, dolomite, travertine, limestone staining, limestone crumbling, limestone efflorescence