Mortar Patching Limestone and Marble Steps
- Procedure code:
- Developed For Hspg (Nps - Sero)
- Last Modified:
PREFACE: 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.
- 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.
- Stone deterioration on steps is particularly common at corners and nosings. Frost crystallization in the stoneand rusting metal rails are typical causes of some deteriorated stone steps.
- See "General Project Requirements" for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performingthis procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO).
- For general information on the characteristics, uses and problems associated with limestone, see 04460-01-S; for marble, see 04455-01-S.
- American Society for Testing and Materials (ASTM)
1.03 QUALITY ASSURANCE
- Field samples:
- 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.
- Test the samples for hardness by scraping with a masonry chisel; Compare their resistance to that of the stone.
- 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.
- 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.
- Abatron, Inc.
- 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 (*).
- Muriatic acid - generally available in 18 degree and 20 degree Baume solutions: (use a 5% solution)
- A strong corrosive irritating acid.
- Other chemical or common names include Chlorhydric acid; Hydrochloric acid; Hydrogen chloride; Marine acid*; Spirit of salt*; Spirit of sea salt*.
- Potential Hazards: TOXIC, CAUSTIC TO FLESH; CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS, FLAMMABLE.
- Available from chemical supply house, drugstore or pharmaceutical supply distributor, or hardware store.
- Limestone or marble dust
- Portland cement ASTM C 150, Type II, white
- Hydrated lime, ASTM C 207, Type S: CAUTION: AVOID SKIN CONTACT WITH LIME.
- Clean, sharp sand
CAUTION: ALWAYS USE RESPIRATORS AND RUBBER GLOVES WHEN WORKING WITH EPOXIES. FOLLOW MANUFACTURER'S RECOMMENDATIONS FOR APPLICATION AND DISPOSAL PROCEDURES.
- 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.
- 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 equal.
- 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)
- Heavy-gauge stainless steel wire (1/4" diameter or less for additional support between main reinforcing rods)
- Clean, potable water
- Masonry chisels: Varying sizes up to 1 inch
- 3-pound hammer
- Small pointing and caulking trowels
- Wooden forms
- Electric drill and masonry bits
- Garden hose, spray bottle, or air compressor
- Rubber gloves, respirator, and protective clothing
- Slicker (straight edge)
- Profile gauge
- 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)
- Mortar-to-stone Bonding Agent Mix:
- 1 part portland cement
- 1/2 part lime
- 3 parts sand
- Finish Coat Mix:
- 1 part white portland cement
- 1 part lime
- 3 parts (uniformly fine and sharp) sand
- Common damage to stone steps includes spalling, erosion and chips.
- Make the Wooden Forms:
- Use a profile gauge and transfer the step profile to a block of wood.
- Cut out the wood profile.
- Prepare the Damaged Surface:
- 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 EDGES.
- Remove any loose dirt and debris on the surface with a stiff bristle brush.
- 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 BY SPILLAGE.
- 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.
- Install Reinforcing Rods (if necessary):
- 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.
- Clean dust out of holes with a small air compressor or water from a garden hose.
- 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.
- Place the epoxy in the drilled holes using a small dowel rod or heavy wire.
- 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.
- Place 1/4 inch stainless steel rods into the drilled holes parallel to the top of the step.
- If the broken area is large, supplement the stainless steel rods with heavy-gauge stainless steel wire.between the rods and bond them together withepoxy.
- 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.
- Allow the epoxy to cure for at least 24 hours.
- Apply Bonding Agent to Aid in Bonding Mortar to Stone:
- 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.
- Apply the bonding agent over entire sub-surface of the stone using a small glue brush.
- Apply immediately prior to applying the base coat of mortar.
- Avoid getting bonding agent on exterior surface of step.
- 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.
- Apply the Base Coat of Mortar: (for damaged areas more than 1 inch (1") deep)
- 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 plastic.
- 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.
- Apply the finish coat before the base coat is completely cured.
- Dampen the surface before applying the finish coat.
- Fill all voids up to the nosing level.
- Coat the wooden forms with liquid soap. Liquid soap acts as a parting agent to keep the mortar from sticking to the wood.
- Fill the forms with mortar mix and tamp firmly into place on the step; Do not leave any voids.
- Secure the forms with clamps, props, or by fitting them along the existing nosing.
- 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.
- Remove the wooden form after 2 or 3 hours; Loosen the form and slide it off the new patch.
- Using a 1/4 inch pointing tool, lightly smooth over the mortar and reduce the high spots.
- Keep the area shaded, covered with damp cheesecloth, and misted for several days, or cover the steps with plastic.
- 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.
- If a weathered appearance is desired, run a light, dilute acid wash over the patch.
- Thoroughly rinse with clean, clear water in a few secondsand allow to dry.