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Historic Preservation - Technical Procedures

Spectitle:

Granite: Characteristics, Uses And Problems

Procedure code:

0446501S

Source:

Outdoor Sculpture Manual - Center For Public Buildings

Division:

Masonry

Section:

Granite

Last Modified:

06/13/2012

Details:

Granite: Characteristics, Uses And Problems



GRANITE:  CHARACTERISTICS, USES AND PROBLEMS


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


INTRODUCTION

Granite is one of the most durable stones used in artistic and
architectural applications, including outdoor sculpture.  Granite
is defined by the American Society for Testing and Materials (ASTM)
as a "visibly granular, igneous rock generally ranging in color
from pink to light or dark grey, and consisting mostly of quartz
and feldspars, accompanied by one or more dark minerals".  The
definition goes on to point out that "some dark granular igneous
rocks, though not properly granite, are included in the
definition."  Some dark colored igneous rocks which are actually
basalt, gabbro, dionite, diabase and anorthosite are quarried and
sold as "black granite."  These stones contain little or no quartz
or alkali feldspars, but, for all practical purposes, they are used
interchangeably with true granites.

In addition to the quartz and feldspars, granite may also contain
other minerals such as mica, horneblend and occasionally pyroxene.
Compared to  calcareous sandstones, marble and limestone, granite
is not an acid soluble stone and is much more resistant to the
effects of acidic solutions, rainwater or cleansing agents.  In
general, igneous building stones, such as granite, have a more
inert composition; show much lower rates of deterioration; have
lower water absorption, and are harder than marbles, limestones and
sandstones.


TYPICAL USES

Granite, like other building stones, is used for a variety of
structural and decorative purposes.  Typical exterior uses for
granite include:

    1.   sculpture

    2.   sculpture bases

    3.   structural and veneer building stone

    4.   architectural trim

    5.   paving and curbstones, and

    6.   grave markers.

Some applications, especially the simpler or more limited ones, can
be monolithic, however most uses will require the joining of
smaller pieces through various mechanical methods.  Joining methods
and techniques must be identified and evaluated as an integral part
of the evaluation of the system of construction, because of the
integral role of the joint in maintaining the soundness of the
system.  


PROBLEMS AND DETERIORATION

Problems may be classified into two broad categories:  1) Natural
or inherent problems based on the characteristics of the material
and the conditions of the exposure, and 2) Vandalism and human-
induced problems.  


NATURAL OR INHERENT GRANITE PROBLEMS

BLISTERING:

A swelling on the surface followed by a rupturing of a thin,
uniform skin.  Although most common on sandstone, this problem may
occur with granite.  It is typically caused by de-icing salts
and/or ground water, therefore it is usually localized near ground
level.

This condition may stabilize and remain constant, however it
frequently precedes additional problems such as spalling or
exfoliation.  There is currently no established treatment other than
rectifying the conditions that cause the blisters, spalling, or delamination.  When this symptom is observed, it should be noted in assessment or inspection reports and reported to the
Regional Historic Preservation Officer (RHPO).  

CHIPPING:

The separation of small pieces or larger fragments from a masonry
unit, frequently at the corners, edges or mortar joints is called
"chipping."  These fractures are generally caused by the impact of
deterioration and repairs, especially the use of too hard a
pointing mortar, or by accident or vandalism.

For specific guidance on repairing chips in granite, see 04465-10-
R.

CRACKING:

Visual symptoms of cracking include appearance of narrow fissures
ranging from less than 1/16 to 1/2 inch or more wide in the stone.
It results from a variety of causes, for example, 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 also be an important early indication of
structural problems.  Cracks can be a point of entry of water into
the interior of the stone, promoting salt migration.  Repairs
include patching and replacement.

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

DETACHMENT:

This is not a failure within 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 structural 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 will prevent leakage and
penetration of water into the structural system.

For specific guidance on repairing detached granite, see 04465-11-R
and 04465-24-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 originate from mortar,
improper cleaning agents, rising damp, de-icing salts, chemical
landscaping treatments or air pollution.

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

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 RHPO.

For specific guidance on removing efflorescence from granite, 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
granite as well as any exposed material.  This is a less serious
problem with granite than with other stones, however inspections
should include examination  for any apparent loss of detail and
edge sharpness which could be due to erosion.

Erosion may have less serious implications on rock-faced or other
base applications, but more serious impact on granite sculpture and
ornament with finer 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 process.

FLAKING:

This is an early stage of more serious problems such as peeling,
exfoliation,  delamination or spalling and is 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.  Applications of water-repellent coatings may result in
flaking by trapping moisture beneath the surface.

The problem can also occur due to sub-florescence, so that if
flaking occurs, the area should be inspected closely to determine
if salt crystallization is occurring in the flaked areas.  The
symptom will be a thin coating of a whitish deposit where the sub-
surface is exposed.  Observations should be made as soon as
possible or inspection be directed to where flakes have not yet
separated, because evidence of sub-florescence may be washed away
after the subsurface is exposed.

For specific guidance on repairing exfoliated granite, see 04465-
15-R.

PEELING:

Peeling is flaking away of the surface from the substrate in strips
or layers.  It can result from the improper application of masonry
coatings which result in failure of the coating and/or stone
surface.

Encrustation of the surface caused by chemical reactions with
environmental elements may also peel or flake along the plane of
interface with stone.

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
the level 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.  Continued rising damp 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 layers or small
pieces of stone due to sub-florescence; freeze-thaw; improper
repointing with  too hard a mortar mix or portland cement, or
structural overloading of the stone.

Spalling is less common with granite than with softer sedimentary
stones.  Granite is hard enough to resist internal forces which
would cause spalling in other natural stones or fabricated masonry.

For specific guidance on repairing spalling granite, see 04400-03-
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 source of the salts can be de-icing salts; chemical cleaners or
landscaping products; mortar and/or air pollution.  The salts get
into the stone dissolved in rainwater or groundwater via natural
absorption, rising damp or poor joints.  The build-up of salts and
their subsequent crystallization can create substantial pressures
with the masonry, eventually causing pieces to break off along the
planes of deposition.  Efflorescence at the surface is an important
early indication that sub-florescence is a possible hazard.
Techniques for mitigating the problem include poulticing, removal
of identified salt sources, elimination of moisture in the stone
and damp-proofing.

STAINING:

A variety of stains may appear on stone, each having different
characteristics and requirements for removal.  Staining can be
caused from such sources as:

    1.   bird droppings

    2.   corroded iron or steel connectors within the masonry

    3.   salt crystallization (efflorescence)

    4.   run-off from bronze or metal sculpture/ornament

    5.   accretion of particulates (dirt, soot,etc.), and

    6.   graffiti.

Identification of the type of staining is necessary prior to
planning for the removal of stains.  For specific guidance on stain
removal, see the following:

    1.   Bird droppings, see 04510-02-R.

    2.   Dirt/soot, see 04400-01-P, 04400-02-P, 04400-03-P and
         04465-03-R

    3.   Copper/bronze stains, see 04465-02-R

    4.   Graffiti, see 04455-12-R and 04455-13-R

    5.   Rust stains from corrosion, see 04465-01-R

                         END OF SECTION
 


basalt,gabbro,dionite,diabase,anorthosite, blisters,chips,cracks,detachment,efflorescence,erosion,flaking,peeling,spalling,staining