Historic Preservation - Technical Procedures
- Resetting Limestone
- Procedure code:
- National Capitol Region Specifications
- Last Modified:
- Resetting Limestone
- Last Modified:
A. This procedure includes guidance on removing displaced
limestone panels, repairing the substrate and re-
installing the panels.
B. 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
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).
A. American Society for Testing and Materials (ASTM)
1.03 QUALITY ASSURANCE
A. Certifications: Prior to delivery, submit certificates
attesting compliance with applicable specifications for
grades, types and classes.
B. Joint Raking: Prior to raking out all areas, cut back
joints at location selected by the Contracting Officer
using the methods specified. Raking will continue at no
additional cost to the Government, until an acceptable
sample is achieved. This area will serve as standard for
joint raking for the entire job. It will be marked and
left unpointed until all other pointing is completed.
Point when directed by the Contracting Officer.
C. Sample Stone Pointing and Repair: Repoint joints, re-
attach stone fragments and patch stone using materials
and methods specified at a location selected by the
Contracting Officer. The samples accepted by the
Contracting Officer will serve as standard for the entire
job. They will be marked and left undisturbed.
1.03 PROJECT/SITE CONDITIONS
A. Environmental Requirements: No stonework will be
performed when the air temperature is 40 F or below
during and for 48 hours subsequent to the work.
A. Stone: To match existing in grade, color and finish.
B. Lime: Should conform to ASTM C207, Type S, high
plasticity, Hydrated Lime for Masonry Purposes.
1. Lime which meets this standard will "work" well,
resists drying during curing, and is sufficiently
strong for the purpose of repointing.
2. Lime expands as it hydrates, making high lime
mortars more resistant to crack formation.
C. Cement: Should conform to ASTM C150, Type I, White. It
should not have more than 0.60% alkali nor more than
0.15% water soluble alkali. Use gray portland cement
ONLY if a dark mortar is to be matched.
1. Cement meeting this standard should increase the
workability of the mortar, accelerate the setting
time and slightly increase the strength of the
2. The low alkali content will prevent efflorescence.
D. Sand: Free of impurities and conforming to ASTM C144.
1. Sand color, size, and texture should match the
original as closely as possible. Provide a sample
of the sand for comparison to the original, and
have it approved by the RHPO before beginning
2. When possible, use bar sand or beach sand rather
than crushed sand for the repointing mortar.
NOTE: BAR SAND OR BEACH SAND SHOULD BE WASHED TO
REMOVE THE SALTS BEFORE USING.
a. Crushed sand has sharp edges, which makes it
more "sticky" and difficult to work into the
b. Bar sand, on the other hand, has rounded edges
and flows easily during the mortar
c. The working characteristics of mortar made
with crushed sand may be improved by adding a
slight amount of portland cement. The amount
of cement should be determined by
experimentation, but should not exceed 20% of
the total lime/cement binder. 20% OR LESS OF
CEMENT HAS MINIMAL EFFECT ON THE HARDNESS OF
THE MORTAR. CEMENT CONTENT ABOVE 20% WILL
MAKE THE MORTAR TOO HARD.
E. Clean, potable water: If the water must be transported
or stored in a container, the container must not impart
any chemicals to the water.
F. Stone dust finely ground from the same stone as that to
G. Additives: NO antifreeze compounds or other admixture
shall be used.
NOTE: DO NOT USE ANTI-FREEZE COMPOUNDS. THESE COMPOUNDS
ARE DESIGNED FOR USE WITH CEMENT MORTARS, AND THEIR
EFFECTIVENESS WITH HIGH LIME MORTARS IS QUESTIONABLE.
FURTHERMORE, THE COMPOUNDS CONTAIN SALTS WHICH CAN LEAD
TO SERIOUS PROBLEMS IN THE MASONRY AT A LATER TIME.
NOTE: AIR ENTRAINING AGENTS ARE NOT RECOMMENDED. THESE
AGENTS ARE DESIGNED FOR USE WITH CEMENT RATHER THAN LIME,
AND THEY RESULT IN DECREASED BONDING OF THE MORTAR AND
THE MASONRY. AIR ENTRAINING IS NOT NECESSARY WITH HIGH
LIME MORTARS BECAUSE OF THE NATURAL ABILITY OF THESE
MORTARS TO FLEX WITH TEMPERATURE CHANGES.
H. Epoxy grout
I. Reinforcement/Anchors: Stainless steel type 302/304 3/8"
in diameter threaded rods, length as required.
A. Natural fiber or nylon bristle brushes
B. Wire bristle brush for cleaning anchors
C. Mixing Vessels
D. Joint tools
G. Hawk (plywood or steel mortar board)
J. Mesh screen
K. Putty knife
L. Wooden mortar boxes
A. Some factors to consider when mixing lime mortar include
durability, color and texture, and workability.
1. Durability: Repointing and resetting mortar should
be softer than the masonry units and the original
mortar to reduce stresses at the edge of the
masonry and, in the case of lime mortar, to reduce
shrinkage which can cause cracks in the mortar.
a. If the new mortar is harder than the masonry
or the original mortar, it can cause serious
stresses within the wall during thermal
expansion and contraction, which can lead to
deterioration of the masonry units rather than
b. If the mortar is softer, any deterioration
which occurs will take place in the mortar,
which is easier to replace than the units
2. The repointing and resetting mortar should allow
the passage of water, both liquid and vapor. If
the mortar does not allow water to pass freely
through it, the water can become trapped inside the
wall, freeze and cause serious deterioration to the
3. Color and texture: The repointing mortar should
match the original mortar in color, texture and
a. Obtaining an accurate color match is best
achieved by selecting an appropriate sand.
1) Use sand which is similar to the original
in color and gradation. Sand from more
than one source may be required.
2) For repointing of natural stones, use
finely ground stone "dust" in the mortar
to match the joints as closely as
possible to the stone.
b. If the original mortar was tinted, or if it is
impossible to obtain a color match through the
use of sand, it may be necessary to use a
special mortar pigment.
CAUTION: PIGMENTS MAY REACT WITH OTHER
INGREDIENTS IN THE MORTAR TO FORM
EFFLORESCENCE. THEY MAY ALSO WEATHER AT A
DIFFERENT RATE THAN NATURAL COLORING AND CAUSE
A COLOR VARIATION IN THE MORTAR.
NOTE: IF PIGMENTS MUST BE USED, PURE MINERAL
OXIDES SHOULD BE USED BECAUSE THEY DO NOT FADE
OR LEACH OUT OF THE MORTAR. AMOUNT OF PIGMENT
SHOULD NOT EXCEED 2% OF THE MORTAR MIX BY
c. Many mortars used before the twentieth century
have small lumps of incompletely burned or
ground lime, or other impurities. To match
the original appearance of the masonry, these
impurities must be included in the new
repointing mortar. Use identical materials,
such as ground oyster shells (obtained at feed
stores) or lumps of lime, to duplicate
4. Workability: The workability or plasticity of the
mortar is a direct result of the selection of
B. Mortar Mix:
1. Have the existing mortar completely analyzed to
insure that the repointing mortar will not be less
permeable/harder than the masonry units or the
original mortar. IT IS BETTER TO HAVE MORTAR THAT
IS MORE PERMEABLE THAN LESS.
2. Measure all ingredients by cubic volume using a
pre-established uniform measure, such as a small
bucket, rather than a less uniform measure such as
3. For historic masonry set in lime mortar, use the
following mortar mix:
1 part portland cement
3 parts lime
8-12 parts sand (To match existing mortar as
closely as possible.)
NOTE: The exact mix required will relate to the
grain size and sharpness of the sand and will vary
depending on the supply.
For historic masonry set in standard mortar, use
the following mortar mix (ASTM C270 Type "0") as a
1 part portland cement
2 parts lime or lime putty
6 to 9 parts sand and stone dust (To match existing
mortar as closely as possible.)
For Limestone (ASTM C270 Type "N"):
1 part portland cement
1 parts lime
4-6 parts aggregate
Enough water to form a workable consistency
4. Mix a final "job-size" batch once the correct sand
color, cement content, etc. have been determined
through small tests to ensure the on-site mixing
conditions will result in the same final product.
3.01 ERECTION, INSTALLATION, APPLICATION
A. Carefully remove shifted and loose stone panels. Clean
of dirt, mortar, and loose debris. Retain for re-use.
Re-build support masonry and lay new bedding material.
B. Clean any exposed metal anchors of all corrosion by
scraping and brushing with stiff wire brushes. Replace
any unsound anchors as necessary with new stainless steel
anchors of same approximate size and shape. Bed new
anchors in epoxy grout.
C. Re-set limestone, maximum tolerances from plumb and level
of new work not to exceed variation from plumb and level
of adjacent existing work.
D. Wet masonry and lay in full bed of mortar. See 04100-03-
S for mortar mixing procedures. Construct uniform
joints. Shove vertical joints tight. Adjust stone units
to final position while mortar is soft and plastic. Set
stone with joints tooled back one inch. Point remaining
depth as the rest of the stone is pointed. For pointing
procedures, see 04520-02-R.
E. Keep mortar and stone damp (80-90% RH) for 72 hours or
until mortar is cured.
A. Immediately after repairing, patching, pointing and re-
setting the stone, remove mortar, grout and adhesives
from the face of the masonry.
B. Use only tools and equipment which are clean and free of
hardened or partially hardened material.
C. After all work is complete, clean stone only with fiber
bristle brushes and water. Use no acids, detergents, or
other cleaning agents.
END OF SECTION