Historic Preservation - Technical Procedures
Patching Spalled Limestone
National Capitol Region Specifications
Patching Spalled Limestone
PATCHING SPALLED LIMESTONE
A. This procedure includes guidance on repairing spalls in
limestone using a cementitious patching material.
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
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. Sample Stone Repair: Reattach stone fragments and patch
stone at locations selected by the Contracting Officer's
Representative using methods specified. Provide one
sample each for granite, limestone and marble.
A. Jahn Restoration Techniques and Research
A. Cementitious patching material such as "M70 Stone
Restoration Mortar" (Jahn Restoration), or approved
equal; mixed and applied following manufacturer's guidance.
B. Clean, soft cloths
C. Clean, potable water
A. Stiff natural bristle brush
E. Putty knife
3.01 ERECTION, INSTALLATION, APPLICATION
A. Cut back to sound stone with chisel and hammer. Score
surface to receive patch with chisel to provide a
mechanical key with patching material. Undercutting if possible is recommended to hold patch more securely.
B. Thoroughly clean all stone dust and debris from areas
that are to be repaired by air and then with a soft brush
C. Pre-moisten stone with clean water and a stiff natural
bristle brush to prevent patching mortar from drying out
prematurely. Avoid over wetting stone, which inhibits
adhesion. Prepare test patches as required for approval
by the Contracting Officer's Representative to determine
the correct degree of moistening.
D. First mix the cementitious patching material well in a
dry state, then add water as per manufacturer's
instructions, depending on the porosity of the limestone;
between 150-200 cc of water to one kg of dry material.
Patching mortar should be shapable without using molds
and as it is being applied should hold its shape right
away. Do not mix more material than can be used within
E. Apply with trowel so that patch is slightly higher than
adjacent surfaces. Jahn Patching mortar can be put on
from 3 mm to any required thickness at once. Allow to
harden for 7 days, then carefully cut back and tool to
match adjacent surfaces.
END OF SECTION