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Historic Preservation - Technical Procedures

Spectitle:

Mortar Patching Limestone And Marble Steps

Procedure code:

0440005R

Source:

Developed For Hspg (Nps - Sero)

Division:

Masonry

Section:

Stonework

Last Modified:

07/02/2012

Details:

Mortar Patching Limestone And Marble Steps



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.


PART 1---GENERAL

1.01 SUMMARY

    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

         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 limestone, see 04460-01-S; for
         marble, see 04455-01-S.

1.02 REFERENCES

    A.  American Society for Testing and Materials (ASTM)
         www.astm.org


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
              the stone.

         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.


PART 2---PRODUCTS

2.01 MANUFACTURERS

    A.  Abatron, Inc.
         www.abatron.com
         
    B.  Sika Corporation
         www.sika.com (usa.sika.com)
         
2.02 MATERIALS

    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,
              FLAMMABLE.

         4.   Available from chemical supply house, drugstore or
              pharmaceutical supply distributor, or hardware
              store.

    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

    F.   Epoxy:

         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
              equal.

    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

2.03 EQUIPMENT

    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

2.04 MIXES

    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


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Common damage to stone steps includes spalling, erosion
         and chips.

3.02 ERECTION/INSTALLATION/APPLICATION

    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
              EDGES.

         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
              BY SPILLAGE.

         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
              steel wire.

              a.   Insert into drilled holes perpendicular to the
                   main rods.

              b.   For additional reinforcing, stretch wire
                   between the rods and bond them together with
                   epoxy.

         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
              plastic.

         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
         cured.

         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.

3.04 ADJUSTING/CLEANING

    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
 


mortar patching, limestone, marble, limestone steps, marble steps