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.
- This procedure includes guidance on patching scaling concrete with a mortar patch.
- Scaling of concrete masonry is the local flaking or peeling away of the near surface portion of concrete or mortar. It is often caused by:
- Overtroweling: Weakens the bond of the surface concrete to the concrete below by bringing an excess of fines to the surface.
- Insufficient concrete strength: Often a result of cool temperatures that do not allow the concrete to cure properly and gain sufficient strength to withstand freeze/thaw cycles and the application of de-icing chemicals.
- The use of concrete with low air contents in areas exposed to severe winter weather.
- Heavy application of salts and/or de-icing chemicals.
- The use of high slump concrete.
- Improper curing methods or lack of curing.
- Insufficient protection of fresh concrete during cold temperatures.
- See "General Project Guidelines" 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).
- Scaling: The local flaking or peeling away of the near surface portion of concrete or mortar.
- Peeling: Thin flakes of mortar are broken away from the concrete surface.
- Light scaling: Loss of surface mortar without exposure of coarse aggregate.
- Medium scaling: Loss of surface mortar without exposure of coarse aggregate.
- Severe scaling: Loss of surface mortar 5 to 10 mm in depth with some loss of mortar surrounding aggregate particles 10 to 20 mm in depth so that aggregate is clearly exposed and stands out from the concrete.
- Very severe scaling: Loss of coarse aggregate particles as well as surface mortar surrounding aggregate, generally greater than 20 mm in depth.
- Spalling: a more extreme form of scaling.
- Check to see that all gutters, downspouts and other water run-off systems are in good repair and clear of debris.
- Correct conditions of "rising damp", splash-back, andfoundation wetness.
- Remove water-entrapping vegetation from on or near the concrete walls.
- Do not use salt to melt snow anywhere near concrete walls.
- Thoro System Products
- Mortar Patching Material: Composition to be determined by Regional Historical Preservation Officer (RHPO)
- Bonding Agent or Cement paste such as "Acryl" (Thoro System Products), or as specified by RHPO
- Rebars and/or stainless steel pins as required
- Epoxy as required
- Clean, potable water
- Hammer and chisel
- Mortar board
- Masonry drill
3.01 ERECTION,INSTALLATION, APPLICATION
- Cut back damaged area to stable material. Roughen the surface with a hammer and chisel.
CAUTION: AVOID GRINDING THE MATERIAL IN THIS PROCESS, BECAUSE IT LEAVES THE SURFACE TOO SMOOTH TO ACHIEVE A PROPER BOND.
- Remove all rust from any exposed rebar.
- Cut out any rebar which is severely corroded and splice Paint freshly cleaned rebar with an epoxy coating to prevent further rusting.
- If the patch is unusually large, drill holes into soundsubstrate and insert stainless steel pins anchored with epoxy.
- Remove all dust and debris by water blasting, air blasting, or with a broom or vacuum.
- Square off the perimeter of the area to be patched sothat a feathered edge will not be required.
- About 1 hour before making repair, moisten surface of area to be patched.
- To insure a good bond between patch and substrate, brush substrate surface with either a cement wash or a bonding agent such as Acryl.
- Apply patching material, with a trowel, in 3/4" layers, compacting thoroughly after each layer.
- Work the finished surface carefully making sure to match texture and appearance of surrounding surfaces.
NOTE: PATCHING MIX SHOULD MATCH AS CLOSELY AS POSSIBLE, THE TEXTURE AND COLOR OF THE ORIGINAL.