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Patching Weathered, Exfoliated, Or Blistering Sandstone
COMPOSITE PATCHING WEATHERED, EXFOLIATED, OR BLISTERING SANDSTONE
PART 1---GENERAL
1.01 SUMMARY
A. This procedure includes guidance on composite patching of
sandstone. Composite patching is required when portions
of the stone surface are lost and must be replaced. For
retaining sandstone that is delaminating internally, see
04470-02-R "Repairing Sandstone by Through Surface
Repair".
B. Composite patching is the process whereby cement and sand
mixtures are applied as a series of stucco-like coats to
reconstruct missing stone surfaces. Three types of stone
deterioration that warrant composite patching include
weathering, exfoliation, and blistering.
C. See 01100-07-S for general project guidelines to be
reviewed along with this procedure. These guidelines
cover the following sections:
1. Safety Precautions
2. Historic Structures Precautions
3. Submittals
4. Quality Assurance
5. Delivery, Storage and Handling
6. Project/Site Conditions
7. Sequencing and Scheduling
8. General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performing
this procedure and should be followed, when applicable,
along with recommendations from the Regional Historic
Preservation Officer (RHPO).
D. For general information on the characteristics, uses and
problems associated with sandstone, see 04470-01-S.
1.02 REFERENCES
A. American Society for Testing and Materials (ASTM)
www.astm.org
1.03 SUBMITTALS
A. Samples: Routine testing of materials, of proposed
mortar mix, and of final work for compliance with this
procedure will be carried out by the RHPO or his\her
appointed representative.
1. Select sand and aggregate resembling a crushed
sample of the stone to be matched; Blend different
sands and pigments with crushed stone as necessary.
2. Keep track of the amounts added to each sample; The
color of the dry mix is a good indication of the
final repair color.
3. Add water and an acrylic latex admixture to make a
paste sample large enough to fill a 3 inch pie tin.
4. Cure the sample in a pie tin or similar container
for at least 48 hours.
5. Treat half of each sample with appropriate surface
finishing.
6. Compare samples to actual stone, and make new
samples as necessary to achieve a color match.
B. Mock-up: Apply a test patch to a small area.
1. Check to see that the composite patch matches the
stone in color, texture and surface treatment.
2. See that the patch adheres well to the adjacent
stone and does not shrink, crack or fall away.
3. See that the composite patch does not cause
deterioration of the old stone by differing too
greatly in hardness, moisture transmission, or
thermal expansion and contraction.
PART 2---PRODUCTS
2.01 MANUFACTURERS
A. Thoro System Products
www.thorosystems.com
2.02 MATERIALS
A. Cement: Portland cement ASTM C 150, Type II, white.
NOTE: DO NOT use gray cement; It is more difficult to
color and work, shrinks more in curing, and may cause
staining.
B. Lime: ASTM C 207, Type S, high plasticity: Increases
cohesion during mixing, slows down the rate of cure, and
moderates the qualities which could cause an excessively
strong and moisture-resistant cement repair to fail and
damage old stone.
C. Sand:
1. Local natural sand, graded or masonry mortar
conforming to ASTM C 144.
2. Sand color, size, and texture should match the
original as closely as possible to provide the
proper visual characteristics without other
additives. A sample of the sand is necessary for
comparison to the original, and should be approved
by the RHPO before beginning repointing work.
3. The color of the sand shall be the primary factor
used to make mortars which match existing adjacent
fabrics.
D. Crushed Sandstone:
1. Best repairs contain actual sandstone; Use stone
removed from the area to be repaired, or other old
stone with the same qualities.
2. Grind it fine enough to pass through a 16-mesh
screen, and wash thoroughly.
E. Dry Pigments:
1. Use when available crushed stone is not sufficient
to give a color match.
2. Use stable fade-proof mineral oxide pigments either
natural- or synthetic-fade.
NOTE: DO NOT exceed recommended manufacturer's suggested
maximum amounts; Too much pigment reduces strength and
gives unstable color. Maximum pigment/cement ratio to be
1/10 (verify with manufacturer).
F. Clean, potable water
G. Additives:
1. ACRYL-60 (Thoro System Products), or approved
equal: Use only latex admixtures that are labeled
nonreemulsifiable like ACRYL-60; Do not use bonding
agents that may break down in the presence of
moisture.
CAUTION: ADMIXTURE ABOVE RECOMMENDED AMOUNTS GIVES
A GLOSSY, ARTIFICIAL LOOK, AND CAUSES A GREENISH
TINT.
H. Hydrochloric Acid:
NOTE: Chemical products are sometimes sold under a
common name. This usually means that the substance is
not as pure as the same chemical sold under its chemical
name. The grade of purity of common name substances,
however, is usually adequate for stain removal work, and
these products should be purchased when available, as
they tend to be less expensive. Common names are
indicated below by an asterisk (*).
1. A strong corrosive irritating acid.
2. Other chemical or common names include Chlorhydric
acid; Hydrogen chloride; Muriatic acid* (generally
available in 18 degree and 20 degree Baume
solutions); Marine acid*; Spirit of salt*; Spirit
of sea salt*.
3. Potential Hazards: TOXIC, CORROSIVE TO FLESH;
CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS,
FLAMMABLE.
4. Available from chemical supply house, drugstore or
pharmaceutical supply distributor, or hardware
store.
2.03 EQUIPMENT
A. Trowels
B. Hawks
C. Stiff natural bristle brushes
D. Hammer and cold chisel
E. Wood screeds
2.04 MIXES
A. Slurry Coat:
1. 1 part white Portland cement
2. 2 parts Type S lime
3. 6 parts sand
4. Mix with water and ACRYL-60 in 3:1 ratio
B. Scratch Coat:
1. 1 part white Portland cement
2. 1 part Type S lime
3. 6 parts sand
4. Mix with water and ACRYL-60 in 5:1 ratio
C. Finish Coat:
1. 1 part white Portland cement
2. 1 part Type S lime
3. 2-3 parts sand
4. 3-4 parts crushed sandstone
5. Dry pigments (maximum 10% by weight)
6. Mix with water and ACRYL-60 (or equivalent) in 5:1
ratio
PART 3---EXECUTION
3.01 EXAMINATION
A. Deterioration of sandstone due to moisture is evident as
spalling, erosion, cracking, flaking and deteriorated
mortar joints.
B. Before proceeding with any type of repair, examine the
sandstone to determine the extent and the cause of the
damage. Compare undamaged stone with areas of suspected
decay. Use a magnifying glass if necessary. Look
closely at the following:
1. Color: What color is the stone? Is there
variation in color within individual stones? Is
there variation between stones?
2. Pattern: Are there swirls, bands, or veins of
color within the individual stones?
3. Texture: Is the stone surface rough or smooth? Is
it hard or crumbly? Is the texture uniform or
varied?
4. Surface Tooling: Is the face of the stone rough or
smooth? Are there any chiseled grooves? Are there
any decorative surface patterns? Are any parts
damaged or missing?
5. Sand Grains: Is the grain size large or small?
Are the grain shapes regular or irregular, uniform
or varied? Does the grain structure appear densely
or loosely packed together? Are there mica flakes
present in the stone (these will often appear to
glitter on the surface)?
6. Cementing Material: What color is the material
between the grains? Do the grains project from the
stone surface, giving the surface a rough texture?
7. Decay and Old Repairs: Is there evidence of
erosion, crumbling, spalling or other types of
deterioration? Is there evidence of previous
patching or repairs?
3.02 ERECTION/INSTALLATION/APPLICATION
A. Cut or chip out all loose stone with a hammer and cold
chisel to a minimum thickness of 1/2"; Undercut the stone
so the patch will lock firmly.
B. Drill holes approximately 1/2 inch deep by 1/4 inch in
diameter at varying angles about 2 inches apart along the newly
exposed surface.
C. Remove stone dust from the patch area with bristle
brushes and lightly spray the area with water.
D. Apply a thin slurry coat of approximately 1 part white
Portland cement, 2 parts lime and 6 parts sand and any
additives as required. Final mix will depend on field
testing of mix to get correct color and texture match.
E. Build the scratch coat layers up to within 3/16 inch of
the surface; Each layer should be no less than 3/4 inch
and no more than 3 inches thick. Do not feather the
edges.
F. Use a trowel to gouge many scratches into the surface of
each layer in order to provide keying; Allow 2-4 hours
for each coat to cure, but apply each layer while the
previous layer is still damp.
G. Use wood screeds set in adjacent mortar joints to prevent
repairs from extending continuously between separate
blocks of stone and remove when the mortar is partly set;
Repoint the joint after the patch has cured.
H. Trowel on a final coat of brownstone stucco.
I. Work a straight edge back and forth across the width of
the patch to strike it off flush.
J. Execute resurfacing carefully. Finish the surface repair
by one of the following:
1. Acid etching: After the surface has cured 48
hours, brush on Technical Grade hydrochloric acid,
diluted 1:5 with water; Rinse the surface
thoroughly with clean, clear water.
2. Rubbing stones: Coarse or fine grade (grits #60,
80, 100, 120); Use dry or with water to hone the
surface of well cured repairs.
3. Stipple with a damp sponge or dry-towel with a
wooden float.
4. Score partially cured repair with stone tools to
match original tool marks and patterns.
END OF SECTION
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