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Historic Preservation - Technical Procedures

Spectitle:

Repairing Spalled Terra Cotta

Procedure code:

0421406R

Source:

National Capitol Region Specifications - Agriculture Bldg

Division:

Masonry

Section:

Terra Cotta Unit Masonry

Last Modified:

11/13/2012

Details:

Repairing Spalled Terra Cotta



REPAIRING SPALLED TERRA COTTA


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on repairing small
         spalls in terra cotta using a cementitious patching
         mortar.  For alternative guidance in repairing spalls in
         terra cotta using field-prepared mortar, see 04214-01-R.

    B.   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 SUBMITTALS

    A.   Samples: Color-match patch samples fabricated on pieces
         of appropriate masonry from or on the building using the
         specified repair mortar as required.


PART 2---PRODUCTS

2.01 MANUFACTURERS

    A.  Jahn Restoration Mortars
         www.jahnmortars.com



    B.   Edison Coatings, Inc.
         www.edisoncoatings.com



2.02 MATERIALS

    A.   Terra Cotta Patch Mix:  "Jahn M100 Terra Cotta Repair
         Mortar" (Jahn Restoration Mortars, Cathedral Stone
         Products, Inc.), "Custom System 45" (Edison Chemical
         Systems, Inc.), or approved equal.  Prepare samples for
         approval. The RHPO will select the color most closely
         matching the existing original terra cotta.

         1.   Mortar mix should be formulated to match color and
              texture of existing masonry.

         2.   Mortar mix should not contain any synthetic
              additives.

         3.   Mortar mix should be vapor permeable, frost and
              salt resistant, shrink resistant, and be physically
              compatible with the substrate, including, but not
              limited to, porosity, tensile, and compressive
              +strength.

    B.   Water: Clean, free of oils, acids, alkalis and organic
         matter.

2.03 EQUIPMENT

    A.   Stiff, natural bristle brushes

    B.   Hammer

    C.   Chisel

    D.   Trowel

    E.   Putty knife

2.04 MIXES

    A.   For Jahn Restoration Mortar:   Follow manufacturer's printed
         instructions.

    B.   For Custom System 45:   Follow manufacturer's
         printed instructions.


PART 3---EXECUTION

3.01 PREPARATION

    A.   Protection:  

         1.   Protect persons, motor vehicles, surrounding
              surfaces of building whose masonry surfaces are
              being restored, building site, and surrounding
              buildings from injury resulting from masonry
              restoration work.

         2.   Erect temporary protection covers over pedestrian
              walkways and at points of entrance and exit for
              persons and vehicles which must remain in operation
              during course of masonry restoration work.

         3.   Contractor shall test those areaway drains, window
              well drains, etc., which will be used to assure
              that drains are functioning properly prior to
              performing masonry restoration operations in those
              areas. The Contractor shall report immediately to
              the Construction Engineer the location of drains
              which are found to be stopped up or blocked.

         4.   Prevent grout or mortar used in repointing and
              repair work from staining face of surrounding
              masonry and other surfaces. Remove immediately
              grout and mortar in contact with exposed masonry
              and other surfaces.

         5.   Protect sills, ledges, windows, and projections
              from repair material droppings.

3.02 ERECTION, INSTALLATION, APPLICATION

    NOTE:  See manufacturer's product specifications for
    additional requirements in making terra cotta patches.  

    A.   Remove loose mortar and masonry at areas to be patched.

    B.   Cut back 1/2" to sound terra cotta with chisel and
         hammer.  Score surface to receive patch with chisel to
         provide a mechanical key with patching material.

    B.   Thoroughly clean all loose terra cotta particles, mortar
         and other dust and debris from surfaces to be repaired by
         air and then with a soft brush and water.

    C.   Pre-moisten terra cotta with clean water and a stiff
         natural bristle brush to prevent patching mortar from
         drying out prematurely.  Avoid over wetting masonry,
         which inhibits adhesion.  Prepare test patches as
         required for approval by the Contracting Officer's
         Representative to determine the correct degree of
         moistening.

    D.   Mix the cementitious patching material according to
         manufacturer's instructions.

         1.   The mixture should be compatible in texture and
              porosity to the adjacent terra cotta.

         2.   Patching mortar should be shapable without using
              molds and as it is being applied should hold its
              shape right away.  

         3.   Do not mix more material than can be used within 30
              minutes, or as recommended by manufacturer.

    E.   Apply mortar mix with a trowel.  Apply patching mortar in
         layers no greater than 3". If a second layer is required,
         allow the mortar to cure before applying the second
         layer.  If a cement skin forms, remove 1/16" of the
         surface using a metal straight edge or scraper.

    F.   Build up layers so that patch is slightly higher than
         adjacent surfaces.  Do not feather edges.  

    G.   Allow to harden for length of time as recommended by
         manufacturer, then carefully cut back and tool to match
         adjacent surfaces.  Maintain mortar joints; do not
         install patch across joints.

    H.   Keep patches damp for 72 hours after installations.

3.03 ADJUSTING/CLEANING

    A.   After mortar has fully hardened, thoroughly clean exposed
         masonry surfaces of excess mortar and foreign matter
         using stiff nylon or bristle brushes and clean water,
         spray applied at low pressure.

    B.   Use of metal scrapers or brushes will not be permitted.

    C.   Use of acid or alkali cleaning agents will not be
         permitted.

                         END OF SECTION
 


repairing spalled terra cotta, terra cotta spalls