Historic Preservation - Technical Procedures
- Mortar Patching Limestone And Marble Steps
- Procedure code:
- Developed For Hspg (Nps - Sero)
- Last Modified:
- Mortar Patching Limestone And Marble Steps
- Last Modified:
MORTAR PATCHING LIMESTONE AND MARBLE STEPS
SOME DEGREE OF AGING, WEATHERING AND COSMETIC DEFECT IS NATURAL
AND ACCEPTABLE AND CONTRIBUTES TO THE BUILDING'S CHARACTER.
NATURAL WEAR AND WEATHERING SUCH AS DEPRESSIONS IN THE STONE MAY
NOT BE SUFFICIENT CAUSE FOR REPAIR UNLESS LARGE ENOUGH TO BECOME A TRIPPING HAZARD.
A. This procedure includes guidance on mortar patching
limestone and marble steps, corners or nosings that are
broken up to 1/2 the length of the step, or where stone
deterioration presents a safety hazard.
B. Stone deterioration on steps is particularly common at
corners and nosings. Frost crystallization in the stone
and rusting metal rails are typical causes of some
deteriorated stone steps.
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
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 limestone, see 04460-01-S; for
marble, see 04455-01-S.
A. American Society for Testing and Materials (ASTM)
1.03 QUALITY ASSURANCE
A. Field samples:
1. Prepare several mortar mix tests (base and finish
coats): Start with 1 part white portland cement: 1
part lime: 3 parts stone dust (for the base coat,
use fine, sharp sand in place of stone dust); Mix
several variations (a cup or two in size) and let
them cure outside for 2 weeks to 3 months or more.
2. Test the samples for hardness by scraping with a
masonry chisel; Compare their resistance to that of
3. Vary the 3 component parts again to closely
approximate the color; Reduce the white portland
cement by 1/4 part and replace it with grey
portland cement if necessary; An additional part of
stone dust may also vary the color.
4. Vary the texture of the mortar either by changing
the grain size of the stone dust or by lightly
brushing the cured surface with a dilute solution
of muriatic acid.
A. Abatron, Inc.
B. Sika Corporation
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 (*).
A. Muriatic acid - generally available in 18 degree and 20
degree Baume solutions: (use a 5% solution)
1. A strong corrosive irritating acid.
2. Other chemical or common names include Chlorhydric
acid; Hydrochloric acid; Hydrogen chloride; Marine
acid*; Spirit of salt*; Spirit of sea salt*.
3. Potential Hazards: TOXIC, CORROSIVE TO FLESH;
CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS,
4. Available from chemical supply house, drugstore or
pharmaceutical supply distributor, or hardware
B. Limestone or marble dust
C. Portland cement ASTM C 150, Type II, white
D. Hydrated lime, ASTM C 207, Type S: CAUTION: AVOID SKIN
CONTACT WITH LIME.
E. Clean, sharp sand
CAUTION: ALWAYS USE RESPIRATORS AND RUBBER GLOVES WHEN
WORKING WITH EPOXIES. FOLLOW MANUFACTURER'S
RECOMMENDATIONS FOR APPLICATION AND DISPOSAL PROCEDURES.
1. As a mortar-to-stone bonding agent - specifically
formulated for bonding new mortar to old mortar or
existing stone, such as "Sika Hi-Mod" (Sika
Corporation), or approved equal.
2. For anchoring reinforcing rods - a gel-consistency
epoxy (contractor's building-supply houses,
plumbing suppliers, some large hardware stores),
(Abatron, Inc.), (Sika Corporation), or approved
G. 1/4 inch diameter stainless steel rods - at lengths to
fit 3/8" - 1/2" deep into the stone and yet not project
beyond the surface of the stone (commercial or
contractor's hardware stores)
H. Heavy-gauge stainless steel wire (1/4" diameter or less
for additional support between main reinforcing rods)
I. Clean, potable water
A. Masonry chisels: Varying sizes up to 1 inch
B. 3-pound hammer
C. Small pointing and caulking trowels
D. Wooden forms
E. Electric drill and masonry bits
F. Garden hose, spray bottle, or air compressor
G. Rubber gloves, respirator, and protective clothing
H. Slicker (straight edge)
I. Profile gauge
A. Mortar Mix: (Must match existing stone in durability,
color and texture)
1 part white portland cement
1 part lime
3 parts stone dust (marble or limestone)
B. Mortar-to-stone Bonding Agent Mix:
1 part portland cement
1/2 part lime
3 parts sand
C. Finish Coat Mix:
1 part white portland cement
1 part lime
3 parts (uniformly fine and sharp) sand
A. Common damage to stone steps includes spalling, erosion
A. Make the Wooden Forms:
1. Use a profile gauge and transfer the step profile
to a block of wood.
2. Cut out the wood profile.
B. Prepare the Damaged Surface:
1. Using a hammer and masonry chisel, cut back all
broken stone faces to a sound surface (a minimum of
1/4" deep); Undercut the stone slightly (about 30
degrees) to receive the mortar. DO NOT FEATHER THE
2. Remove any loose dirt and debris on the surface
with a stiff bristle brush.
3. Clean the surface with a mixture of muriatic acid
and water; Using a 5% acid concentration, mix 1
part acid, 6 parts water.
CAUTION: ALWAYS WEAR RUBBER GLOVES AND SAFETY
GLASSES WHEN WORKING WITH ACID; ALWAYS POUR ACID
INTO WATER (NEVER THE REVERSE); KEEP A PAIL OF
WATER HANDY TO QUICKLY NEUTRALIZE ANY DAMAGE CAUSED
4. If the deteriorated area is very small, move
directly on to filling procedures (section E,
below); If the damage is not small, however,
installing reinforcing rods will be required.
C. Install Reinforcing Rods (if necessary):
1. Using a small electric drill with masonry bits,
drill holes at least 3/8 inch in diameter and 1/2
inch deep into the stone; Avoid drilling too near
the edge of the stone or with too large of a bit.
2. Clean dust out of holes with a small air compressor
or water from a garden hose.
3. Mix epoxy following manufacturer's instructions.
CAUTION: WHEN MIXING EPOXY, ALWAYS WEAR RUBBER
GLOVES, RESPIRATOR AND GENERAL PROTECTIVE CLOTHING.
NOTE: MIX ONLY AS MUCH AS CAN BE USED IN
APPROXIMATELY 20 MINUTES.
4. Place the epoxy in the drilled holes using a small
dowel rod or heavy wire.
5. Insert 1/4" stainless steel reinforcing rods
vertically into the epoxy-filled holes; Use a
straight edge to see that the rods do not extend
above the surface of the step or beyond the corner.
6. Place 1/4 inch stainless steel rods into the
drilled holes parallel to the top of the step.
7. If the broken area is large, supplement the
stainless steel rods with heavy-gauge stainless
a. Insert into drilled holes perpendicular to the
b. For additional reinforcing, stretch wire
between the rods and bond them together with
8. For wide areas of damage, drill small holes (1/4
inch or less) at 2 inch intervals along the stone
sub-surface to provide mechanical keying for the
bonding agent and mortar.
9. Allow the epoxy to cure for at least 24 hours.
D. Apply Bonding Agent to Aid in Bonding Mortar to Stone:
1. Mix an epoxy bonding agent -OR- mix 1 part portland
cement, 1/2 part lime, and 3 parts sand.
NOTE: MIX BONDING AGENT JUST PRIOR TO APPLICATION.
CAUTION: EPOXIES ARE TOXIC AND REQUIRE THE USE OF
SOLVENTS FOR CLEANING UP. THEY ALSO MAY CREATE A
WATER BARRIER BEHIND THE PATCH AND MAY CAUSE MORE
SERIOUS PROBLEMS, SUCH AS SPALLING.
2. Apply the bonding agent over entire sub-surface of
the stone using a small glue brush.
a. Apply immediately prior to applying the base
coat of mortar.
b. Avoid getting bonding agent on exterior
surface of step.
E. Prepare the Base Coat of Mortar: Mix 1 part white
portland cement, 1 part lime, and 3 parts stone dust; Use
the exact proportions determined from the mortar testing.
The wet mortar should be stiff for application.
F. Apply the Base Coat of Mortar: (for damaged areas more
than 1 inch (1") deep)
1. Apply the base coat to the damaged area leaving at
least 1/4 to 1/2 inch for the finish coat.
Note: Apply the base coat in the morning so that
it can set up and the finish coat can be applied in
the afternoon; If the base coat must sit overnight,
cover it lightly with damp cheesecloth and mist it
periodically with water, or cover the area with
2. Press the mortar into all crevices; Compact the
mortar and eliminate all air spaces. While the
mortar is still wet, score the surface of the base
coat to provide keys for the finish coat.
G. Apply the finish coat before the base coat is completely
1. Dampen the surface before applying the finish coat.
2. Fill all voids up to the nosing level.
3. Coat the wooden forms with liquid soap. Liquid
soap acts as a parting agent to keep the mortar
from sticking to the wood.
4. Fill the forms with mortar mix and tamp firmly into
place on the step; Do not leave any voids.
5. Secure the forms with clamps, props, or by fitting
them along the existing nosing.
6. Hand-tool all flat areas of risers and steps; Keep
the area damp and shaded to prevent shrinkage
cracks or rapid drying of the mortar.
7. Remove the wooden form after 2 or 3 hours; Loosen
the form and slide it off the new patch.
8. Using a 1/4 inch pointing tool, lightly smooth over
the mortar and reduce the high spots.
9. Keep the area shaded, covered with damp
cheesecloth, and misted for several days, or cover
the steps with plastic.
A. When the patch has cured, scrub off excess mortar with a
brush and water, or remove it with a solution of muriatic
acid and water.
B. If a weathered appearance is desired, run a light, dilute
acid wash over the patch.
C. Thoroughly rinse with clean, clear water in a few seconds
and allow to dry.
END OF SECTION