Patching Small Holes, Minor Chips And Spalls In Terra Cotta
- Procedure code:
- US Custom House/Portland, Or - Gsa/Facilities Support Ctr
- Terra Cotta Unit Masonry
- Last Modified:
PATCHING SMALL HOLES, MINOR CHIPS AND SPALLS IN TERRA COTTA
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
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).
A. American Society for Testing and Materials (ASTM)
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
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
A. Thoro Consumer Products (BASF Construction Chemicals)
A. Setting and Patching Mortar Materials: Mortar should
meet standard compressive strength and permeability
requirements for terra cotta. General ASTM Designation
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
A. Masonry drill
B. Hacksaw blade
D. Wooden mallet
E. Glass cutter
F. Wire brushes
G. Hand-held water bottle
H. Steel trowel
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
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.
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
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
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
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
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
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
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,
END OF SECTION