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Historic Preservation - Technical Procedures

Spectitle:

Patching Small Holes, Minor Chips And Spalls In Terra Cotta

Procedure code:

0421401R

Source:

US Custom House/Portland, Or - Gsa/Facilities Support Ctr

Division:

Masonry

Section:

Terra Cotta Unit Masonry

Last Modified:

11/13/2012

Details:

Patching Small Holes, Minor Chips And Spalls In Terra Cotta



PATCHING SMALL HOLES, MINOR CHIPS AND SPALLS IN TERRA COTTA


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on patching small holes
         (no greater than fist size), minor chips and spalls in
         terra cotta with mortar to match existing units.

    B.   Make patch repairs only where the area and depth of
         damage is large enough to make a patch which is at least
         1/2-inch thick; otherwise make surface finish repairs to
         protect from more weather damage and to maintain a
         uniform color and appearance.  For guidance on patching
         masonry cracks, see 04200-03-R; for guidance on
         reinforcing existing patches, see 04214-03-R.

    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).

1.02 REFERENCES

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

1.03 SUBMITTALS

    A.   Patching Mortar Sample:  Cured and dried sample of mortar
         proposed for use in masonry patching; provide with mix
         design and complete list of all materials and proportions
         with details on how mix proportions will be followed in
         the field; a sample area on the building may be used for
         this test.

1.04 PROJECT/SITE CONDITIONS

    A.   Environmental Requirements:  When outside air temperature
         is below 5 degrees C (40 degrees F), maintain mortar temperature between
         20 degrees C (70 degrees F) and 50 degrees C (120 degrees F) by heating water and/or
         aggregates.


PART 2---PRODUCTS

2.01 MANUFACTURERS

    A.  Thoro Consumer Products (BASF Construction Chemicals)
         www.thoroproducts.com



2.02 MATERIALS

    A.   Setting and Patching Mortar Materials:  Mortar should
         meet standard compressive strength and permeability
         requirements for terra cotta.  General ASTM Designation
         C 270-88a.

         1.   Portland Cement:  ASTM Designation C150-89, Type I
              or II.  Natural light gray or buff color.

         2.   Hydrated Lime:  ASTM Designation C207-79, Type S or
              equivalent lime putty.

         3.   Masons Sand:  The sand used is the primary giver of
              the cured mortar color.  Match  samples provided by
              RHPO from original mortar materials.

         4.   Mineral Colorants:  Only as necessary and approved
              to match terra cotta matrix or surface color.

         5.   Other admixtures:  No accelerators (Calcium
              Chloride) or additives may be used unless approved
              by the RHPO.

    B.   Patching Accessories:

         1.   Bonding Agents:  "ThoroBond" (Thoro Consumer
              Products), or approved equal.

         2.   Glaze Replacement Coating for Terra Cotta:
              Portland cement base, polymer modified, water vapor
              permeable, masonry coating; Thoroseal (Thoro Consumer
              Products) or approved equal.

              a.   Color to be selected by RHPO to match existing
                   terra cotta finish after curing.

         3.   Microcrystalline wax

         4.   Water-tolerant paint

    C.   Clean, potable water

2.03 EQUIPMENT

    A.   Masonry drill

    B.   Hacksaw blade

    C.   Chisels

    D.   Wooden mallet

    E.   Glass cutter

    F.   Wire brushes

    G.   Hand-held water bottle

    H.   Steel trowel


PART 3---EXECUTION

3.01 EXAMINATION

    A.   When inspecting for external damage, look for:

         1.   Missing units

         2.   Deteriorated or missing mortar

         3.   Large cracks running through multiple units

         4.   Material failures such as spalls, and parallel
              crazing

         5.   Presence or absence of water-shedding devices like
              flashing, gutters and downspouts

         6.   Bulges in the terra cotta, particularly at floor
              levels and at piers

         7.   Rust stains from failing anchors

         8.   Efflorescence from excessive moisture in the wall

    B.   When inspecting for internal damage:

         1.   Strike the surface of a unit with a wooden mallet.

         2.   Undamaged units give off a clear ringing sound,
              while broken units and those with internal damage
              give off a flat thud.

         3.   Consistent tonal quality among similar pieces is
              the key to locating damaged units.

3.02 PREPARATION

    A.   Surface Preparation:

         1.   Clean broken, spalled, and cracked terra cotta
              units by scraping, chipping, and wire brushing the
              damaged area to expose sound, clean, terra cotta
              surface.

         2.   Wash the cleaned surface with clean, clear water or
              blow compressed air to remove dusts.

         3.   Tool work areas to be patched to roughen for
              promoting bonding to patch material.

         4.   Undercut edges of area to be patched (roughly 30
              degrees).

3.03 ERECTION, INSTALLATION, APPLICATION

    A.   Using a glass cutter, cut a line through the fireskin or
         glaze around the damaged area at right angles to the
         edges of the block.

    B.   Using a masonry drill and a hacksaw blade, remove all
         damaged material.

    C.   Undercut at least 2 of the edges to provide a good key
         for the repair.

    D.   Mix patching mortar to a stiff consistency, to be
         workable, but not to run, sag, or be friable.

         1.   Consistently and accurately measure materials for
              each batch according to approved samples.

         2.   Mix for at least five minutes in a mechanical batch
              mixer or mortar box.

         3.   Mix trowel workable consistency for unit masonry
              setting and resetting.

         4.   Mix firm dry consistency for repointing.

         5.   Discard mortar not used within two hours after
              mixing; do not retemper at mixer.

    E.   Hand-spray the surface with water before placing the
         mortar.

    F.   Apply bonding agent to the clean terra cotta surface.

    G.   Completely fill patch area (no less than 1/2" thick, and
         no greater than fist size) by hand placing patching
         mortar, working and compressing it to remove air pockets
         and to effect a tight bond to surfaces.

         1.   Place patching mortar in maximum of 3/8-inch thick
              lifts, allowing an initial set, before proceeding
              with additional lifts, successively, until the
              whole patch is filled.  Hand-spray the mortar with
              water before each application.

         2.   Slightly over-fill the patch to allow for shrinkage
              and finishing.  

              NOTE:  Depth of lifts should be adjusted to type of
              mortar in order to allow for adequate curing.

    H.   Finish and dress patch surface to match adjacent
         surfaces, and begin curing.

         1.   For unglazed units, finish with 1 or 2 applications
              of microcrystalline wax.

         2.   For glazed surfaces, paint to match the glaze color
              and texture with water-tolerant paint.

         3.   For damaged areas that are smaller than 1/2 inch
              deep or for old patches where the color does not
              match the terra cotta, repair by applying one coat
              of colored masonry acrylic-cement finish to the
              surface to match terra cotta color.

              CAUTION:  WHEN REPLACING GLAZE, DO NOT COAT OR SEAL
              THE MASONRY JOINTS, AS THEY ARE THE ONLY AREAS
              WHICH ALLOW THE WALL TO RESPOND TO CLIMATIC
              CONDITIONS.

3.04 ADJUSTING/CLEANING

    A.   Wipe away all excess mortar as the work progresses.  

    B.   Dry brush at the end of each day's work.  

    C.   After mortar is thoroughly set and cured, clean new
         masonry surfaces, walls, sills, overhangs, etc., of all
         loose mortar and dirt and point up all nail holes,
         cracks, etc.

                         END OF SECTION
 


patching holes terra cotta, patching minor chips terra cotta, patching spalls terra cotta