Patching Spalled Limestone
- Procedure code:
- National Capitol Region Specifications
- Last Modified:
Technical Procedures Disclaimer
Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
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:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- 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 www.jahnmortars.com
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 and water.
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 30 minutes.
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.