- Procedure code:
- National Capitol Region Specifications
- Last Modified:
- This procedure includes guidance on removing displaced limestone panels, repairing the substrate and re- installing the panels.
- 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).
- American Society for Testing and Materials (ASTM) www.astm.org
1.03 QUALITY ASSURANCE
- Certifications: Prior to delivery, submit certificates attesting compliance with applicable specifications for grades, types and classes.
- Joint Raking: Prior to raking out all areas, cut back joints at location selected by the Contracting Officer using the methods specified. Raking will continue at no additional cost to the Government, until an acceptable sample is achieved. This area will serve as standard for joint raking for the entire job. It will be marked and left unpointed until all other pointing is completed. Point when directed by the Contracting Officer.
- Sample Stone Pointing and Repair: Repoint joints, re- attach stone fragments and patch stone using materials and methods specified at a location selected by the Contracting Officer. The samples accepted by the Contracting Officer will serve as standard for the entire job. They will be marked and left undisturbed.
1.03 PROJECT/SITE CONDITIONS
- Environmental Requirements: No stonework will be performed when the air temperature is 40 F or below during and for 48 hours subsequent to the work.
- Stone: To match existing in grade, color and finish.
- Lime: Should conform to ASTM C207, Type S, high plasticity, Hydrated Lime for Masonry Purposes.
- Lime which meets this standard will "work" well, resists drying during curing, and is sufficiently strong for the purpose of repointing.
- Lime expands as it hydrates, making high lime mortars more resistant to crack formation.
- Cement: Should conform to ASTM C150, Type I, White. It should not have more than 0.60% alkali nor more than 0.15% water soluble alkali. Use gray portland cement ONLY if a dark mortar is to be matched.
- Cement meeting this standard should increase the workability of the mortar, accelerate the setting time and slightly increase the strength of the mortar.
- The low alkali content will prevent efflorescence.
- Sand: Free of impurities and conforming to ASTM C144.
- Sand color, size, and texture should match the original as closely as possible. Provide a sample of the sand for comparison to the original, and have it approved by the RHPO before beginning repointing work.
- When possible, use bar sand or beach sand rather than crushed sand for the repointing mortar.
NOTE: BAR SAND OR BEACH SAND SHOULD BE WASHED TO REMOVE THE SALTS BEFORE USING.
- Crushed sand has sharp edges, which makes it more "sticky" and difficult to work into the joints.
- Bar sand, on the other hand, has rounded edges and flows easily during the mortar application.
- The working characteristics of mortar made with crushed sand may be improved by adding a slight amount of portland cement. The amount of cement should be determined by experimentation, but should not exceed 20% of the total lime/cement binder. 20% OR LESS OF CEMENT HAS MINIMAL EFFECT ON THE HARDNESS OF THE MORTAR. CEMENT CONTENT ABOVE 20% WILL MAKE THE MORTAR TOO HARD.
- Clean, potable water: If the water must be transported or stored in a container, the container must not impart any chemicals to the water.
- Stone dust finely ground from the same stone as that to be repointed.
- Additives: NO antifreeze compounds or other admixture shall be used.
NOTE: DO NOT USE ANTI-FREEZE COMPOUNDS. THESE COMPOUNDS ARE DESIGNED FOR USE WITH CEMENT MORTARS, AND THEIR EFFECTIVENESS WITH HIGH LIME MORTARS IS QUESTIONABLE. FURTHERMORE, THE COMPOUNDS CONTAIN SALTS WHICH CAN LEAD TO SERIOUS PROBLEMS IN THE MASONRY AT A LATER TIME.
NOTE: AIR ENTRAINING AGENTS ARE NOT RECOMMENDED. THESE AGENTS ARE DESIGNED FOR USE WITH CEMENT RATHER THAN LIME, AND THEY RESULT IN DECREASED BONDING OF THE MORTAR AND THE MASONRY. AIR ENTRAINING IS NOT NECESSARY WITH HIGH LIME MORTARS BECAUSE OF THE NATURAL ABILITY OF THESE MORTARS TO FLEX WITH TEMPERATURE CHANGES.
- Epoxy grout
- Reinforcement/Anchors: Stainless steel type 302/304 3/8" in diameter threaded rods, length as required.
- Natural fiber or nylon bristle brushes
- Wire bristle brush for cleaning anchors
- Mixing Vessels
- Joint tools
- Hawk (plywood or steel mortar board)
- Mesh screen
- Putty knife
- Wooden mortar boxes
- Some factors to consider when mixing lime mortar include durability, color and texture, and workability.
- Durability: Repointing and resetting mortar should be softer than the masonry units and the original mortar to reduce stresses at the edge of the masonry and, in the case of lime mortar, to reduce shrinkage which can cause cracks in the mortar.
- If the new mortar is harder than the masonry or the original mortar, it can cause serious stresses within the wall during thermal expansion and contraction, which can lead to deterioration of the masonry units rather than the mortar.
- If the mortar is softer, any deterioration which occurs will take place in the mortar, which is easier to replace than the units themselves.
- The repointing and resetting mortar should allow the passage of water, both liquid and vapor. If the mortar does not allow water to pass freely through it, the water can become trapped inside the wall, freeze and cause serious deterioration to the masonry.
- Color and texture: The repointing mortar should match the original mortar in color, texture and physical characteristics.
- Obtaining an accurate color match is best achieved by selecting an appropriate sand.
- Use sand which is similar to the original in color and gradation. Sand from more than one source may be required.
- For repointing of natural stones, use finely ground stone "dust" in the mortar to match the joints as closely as possible to the stone.
- If the original mortar was tinted, or if it is impossible to obtain a color match through the use of sand, it may be necessary to use a special mortar pigment. CAUTION: PIGMENTS MAY REACT WITH OTHER INGREDIENTS IN THE MORTAR TO FORM EFFLORESCENCE. THEY MAY ALSO WEATHER AT A DIFFERENT RATE THAN NATURAL COLORING AND CAUSE A COLOR VARIATION IN THE MORTAR.NOTE: IF PIGMENTS MUST BE USED, PURE MINERAL OXIDES SHOULD BE USED BECAUSE THEY DO NOT FADE OR LEACH OUT OF THE MORTAR. AMOUNT OF PIGMENT SHOULD NOT EXCEED 2% OF THE MORTAR MIX BY WEIGHT.
- Many mortars used before the twentieth century have small lumps of incompletely burned or ground lime, or other impurities. To match the original appearance of the masonry, these impurities must be included in the new repointing mortar. Use identical materials such as ground oyster shells (obtained at feed stores) or lumps of lime, to duplicate original lumps.
- Workability: The workability or plasticity of the mortar is a direct result of the selection of materials.
- Mortar Mix:
- Have the existing mortar completely analyzed to insure that the repointing mortar will not be less permeable/harder than the masonry units or the original mortar. IT IS BETTER TO HAVE MORTAR THAT IS MORE PERMEABLE THAN LESS.
- Measure all ingredients by cubic volume using a pre-established uniform measure, such as a small bucket, rather than a less uniform measure such as a shovel.
- For historic masonry set in lime mortar, use the following mortar mix:
1 part portland cement
3 parts lime
8-12 parts sand (To match existing mortar as closely as possible.) NOTE: The exact mix required will relate to the grain size and sharpness of the sand and will vary depending on the supply.
For historic masonry set in standard mortar, use the following mortar mix (ASTM C270 Type "0") as a starting point:
1 part portland cement
2 parts lime or lime putty
6 to 9 parts sand and stone dust (To match existing mortar as closely as possible.)
For Limestone (ASTM C270 Type "N"):
1 part portland cement
1 parts lime
4-6 parts aggregate
Enough water to form a workable consistency
- Mix a final "job-size" batch once the correct sand color, cement content, etc. have been determined through small tests to ensure the on-site mixing conditions will result in the same final product.
3.01 ERECTION, INSTALLATION, APPLICATION
- Carefully remove shifted and loose stone panels. Clean of dirt, mortar, and loose debris. Retain for re-use. Re-build support masonry and lay new bedding material.
- Clean any exposed metal anchors of all corrosion by scraping and brushing with stiff wire brushes. Replace any unsound anchors as necessary with new stainless steel anchors of same approximate size and shape. Bed new anchors in epoxy grout.
- Re-set limestone, maximum tolerances from plumb and level of new work not to exceed variation from plumb and level of adjacent existing work.
- Wet masonry and lay in full bed of mortar. See 04100-03- S for mortar mixing procedures. Construct uniform joints. Shove vertical joints tight. Adjust stone units to final position while mortar is soft and plastic. Set stone with joints tooled back one inch. Point remaining depth as the rest of the stone is pointed. For pointing procedures, see 04520-02-R.
- Keep mortar and stone damp (80-90% RH) for 72 hours or until mortar is cured.
- Immediately after repairing, patching, pointing and re- setting the stone, remove mortar, grout and adhesives from the face of the masonry.
- Use only tools and equipment which are clean and free of hardened or partially hardened material.
- After all work is complete, clean stone only with fiber bristle brushes and water. Use no acids, detergents, or other cleaning agents.