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Historic Preservation - Technical Procedures

Spectitle:

Surface Repair Of Limestone By Consolidation And Use Of Lime Mortar

Procedure code:

0446002R

Source:

Developed For Hspg (Nps - Sero)

Division:

Masonry

Section:

Limestone

Last Modified:

02/24/2012

Details:

Surface Repair Of Limestone By Consolidation And Use Of Lime Mortar



SURFACE REPAIR OF LIMESTONE BY CONSO= LIDATION AND USE OF LIME MORTAR


EXECUTION OF THIS PROCEDURE REQUIRES A HIGH DEGREE OF EXPERTISE.
CONSIDERABLE SKILL IS REQUIRED IN COLOR MATCHING AND IN MATCHING
MORTAR REPAIRS.


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on repairing damaged
         limestone by flooding the surface with limewater for
         several days followed by surface repair with lime mortar.

    B.   This procedure describes three stages in what is known as
         Baker's Lime Method.  

         1.   Developed by Robert Baker in the 1950's, Baker's
              Lime Method is a 4-stage process of limestone
              cleaning and repair which includes cleaning with a
              hot lime poultice, repair, consolidation and
              preservation.  

         2.   This procedure describes the repair, consolidation
              and preservation stages.  The cleaning stage of the
              process is described in 04460-01-R.

    C.   Safety Precautions:  Check manufacturer's literature for
         precautions and effects of products and procedures on
         adjacent building materials, components, and especially
         vegetation. Take appropriate protective measures.

    D.   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 Precaution= s

         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).

    E.   For general information on the characteristics, uses and
         problems associated with limestone, see 04460-01-S.


PART 2---PRODUCTS

2.01 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 adequa= te
    for stain removal work, and these products should be purchas= ed
    when available, as they tend to be less expensive.  Com= mon
    names are indicated below by an asterisk (*).

    A.   Calcium Hydroxide:

         1.   Other chemical or common names include Calcium
              hydrate*; Hydrated lime*; Lime hydrate*; Slaked
              lime*.

         2.   Potential Hazards:  SKIN IRRITANT, AVOID INHALATION
              OF THE DRY POWDER.

         3.   Available from chemical supply house, construction
              materials yard, construct= ion specialties
              distributor, garden and lawn supply center, or
              hardware store.

    B.   Casein

    C.   Formalin:

         1.   Other chemical or common names include
              Formaldehyde; Formic aldehyde; Methanal; Methyl
              aldehyde; Oxomethane; Oxymethylene.

         2.   Potential Hazards:  TOXIC AND MODERATELY FLAMMABLE;
              SKIN IRRITANT; MAY DISCOL= OR THE SKIN, MAKING IT
              DARKER DUE TO THE TANNING ACTION OF THE CHEMICAL.

         3.   Available from chemical supply house, dairy supply
              distributor, drugstore or pharmaceutical supply
              distributor, photographic supply distributor (not
              camera shop), or printer's supply distributor.

    D.   Lime mortar - "High Calcium", Non-hydrau= lic Type (see
         mixes in Section 2.03 below)

    E.   HTI Powder (white refractory brick powder - "= High
         Temporary Insulation") - may be used as a pozzolanic
         additive to make a weak hydraulic mortar= .

    F.   Aggregate - size, grade and color to be determined by
         RHPO (see mixes in Section 2.03 below)
    G.   Cotton-wool packs

    H.   Polyethylene sheets

    I.   Clean potable water

2.02 EQUIPMENT

    A.   Hand spray bottle

    B.   Mechanical spray equipment

    C.   Dental picks

    D.   Small, soft bristle brushes (short-haired)

    E.   Trowels and spatulas

    F.   Hand pump

    G.   Sponges

    H.   Rubber gloves

2.03 MIXES

    NOTE:  PROPORTIONS MAY VARY BASED ON SIZE, GRADE, COLOR AND
    FUNCTION OF AGGREGATE

    A.   Repair mortar:  Mix 1 part lime with 2 parts aggregate.
         Aggregate often includes LESS THAN 10% HTI powder
         additive.

    B.   Adhesive mortar (spalls):  Mix 1 part lime with 1 part
         aggregate.  Aggregate often includes a 10% HTI powder
         additive.

    C.   Grouting mortar (cracks):  Mix 1 part lime with 1-1/2
         parts aggregate.  Aggregate often includes a 10% HTI
         powder additive.

    D.   Shelter coating:  Mix 1 part lime with 2-3 parts
         aggregate.  Aggregate often includes LESS THAN 10% HTI
         powder additive.


PART 3---EXECUTION

3.01 ERECTION/INSTALLATION/APPLICATION

    A.   Preparing the Limewater:

         1.   Mix 0.14g of calcium hydroxide in 100ml of water at
              60=F8F and let stand until the water is clear.

         2.   Cover the surface of the limew= ater in its container
              with a float of polystyre= ne sheetrock pierced by a
              siphon tube with a filter.  NOTE:  IT IS IMPORTANT
              TO PROTECT THE LIMEWATER FROM AIR SO THAT IT DOES
              NOT CARBONATE AND BECOME INEFFECTIVE.

         3.   Draw off the limewater as needed and hand pump it
              into spray bottles; Make sure that the water is
              clear and not cloudy.

    B.   Consolidation by Limewater:

         1.   Carefully remove any cementiti= ous filling to spalls
              and cracks.

         2.   Cut out cavities to be consoli= dated where new
              spalls and splits have developed and where previous
              spalls and cracks were found.

         3.   Remove all loose dust and debris by flushing with
              clean water.

         4.   Flood the limestone surface with a biocide if
              necessary to provide a clean, sterile surface for
              the new mortar filling.
         5.   Treat the cleaned surfaces and open cavities with
              clear limewater to attempt to consolidate the more
              friable areas.  If water becomes cloudy, stop work
              and allow lime to settle to the bottom and water to
              again become clear.  = ;Proceed when water is clear.

         6.   Flood the limestone surface with the limewater for
              several days or as long as the surface will absorb
              (up to 40 applications may be necessary).

         7.   Remove any excess limewater on the surface with
              sponges; Squeeze them out in clean water.

    C.   Follow Consolidation with Surface Repairs Using Mortar:

         NOTE:  All mortar repairs are based on lime - NO PORTLAND
         CEMENT OF AN KIND IS USED.

         1.   Flush out cavities and cracks again with water from
              the hand sprays to avoid de-watering the repair;
              Make sure the surface remains damp.

         2.   Fill deep cavities with a slurry of repair mortar
              (see Section 2.03 A. above).

         3.   Follow this by inserting small pieces of limestone
              into the cavity to reduce the thickness which needs
              to be built up in fine repair mortar.

         4.   Brush a thin slurry of repair mortar containing
              "HTI" powder into the cavity or fracture to provide
              an additional key for the repair.

         5.   After the slurry has dried, wet the slurry again,
              knead the first repair mortar and push it into
              place with fingers, dental plugging tools or
              spatulas; Not more than 5-6 mm should be pressed in
              at one time.

         6.   Protect the treated area from direct sunlight or
              strong drafts to avoid rapid drying out.

         7.   Apply wet cotton-wool packs over the repair mortar
              to avoid rapid drying.

         8.   When the cavity is dry, wet it again and repeat
              steps 5-7 until the cavity has been filled
              completely.

         9.   Using a spatula, trim off excess mortar to the
              desired profile.

         10.  To achieve a texture matching the stone, take a dry
              sponge (hessian pads, stencil brushes or purpose-made
              plastic scrapers will also work) and press it
              against the repaired surface (be careful not to
              press too hard as it may absorb moisture from the
              repair).

    D.   To protect the limestone and slow down the weather= ing
         process, apply a shelter coating (or sacrificial
         coating):  This is similar in compo= sition to the repair
         mortar except the proportion of aggregate to lime is
         slightly higher and the sand and stone dusts of aggregate
         are more finely crushed.

         1.   Mix 1 part lime with 2-3 parts aggregate; Add water
              and mix to achieve a consistency of thin cream;
              continue to mix for anoth= er 20 to 30 minutes.

         2.   Add Casein and Formalin to the mixture.

         3.   Thoroughly wet the stone with water using hand
              spray bottles.  Wet the stone until it can no
              longer absorb any more water.

         4.   Apply the coating to the stone surface using a
              short-haired bristle brush.

         5.   Using a second short-haired (worn) bristle brush,
              work the coating into the texture of the stone.

         6.   Cover the treated area with polyethylene sheets and
              intermittently spray mist the area with water
              during the first few hours to avoid rapid drying of
              the coating.  NOTE:  RAPID DRYING MAY RESULT IN A
              POWDERY APPEARANCE.

         7.   During the drying stages, additives of finely
              ground stone dust may be dusted on to the surface
              to achieve subtle color variations in the stone
              appearance.

                         END OF SECTION