Removing and Replacing Deteriorated Stone Masonry
- Procedure code:
- Developed For HSPG (NPS - Southeast Regional Office)
- Last Modified:
PREFACE: This procedure should be performed under the direction of an historical architect or engineer to determine the most efficient and least destructive manner for executing the work. Approval from the regional historic preservation officer is required.
- This procedure includes guidance on removing and replacing deteriorated stone masonry. It should be used in conjunction with procedure "Repointing Masonry Using Lime Mortar" and "Preparing Lime Mortars for Repointing Masonry".
- Damage to stone in the form of cracking, splitting and spalling often occurs in areas sheltered from direct rain-washing.
- 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).
- American Society for Testing and Materials
1.03 PROJECT/SITE CONDITIONS
- Environmental Requirements:
- Weather: Do not proceed with patching orreplacement under adverse weather conditions, or when temperatures are below or above manufacturer's recommended limitations for installation; Proceed with the work only when forecasted weather conditions are favorable for proper cure.
- Wet Weather: Do not apply or mix mortar on outside surfaces with standing water or outside during rain.
- Cold Weather, winter construction is not allowed without consent of Regional Architect; Winter construction (midwest region) is defined as any time between December 1 and March 1 and/or when surface temperature of masonry is below 40 degrees (F). or air temperature is predicted to be below 40 degrees (F). within 48 hours; Heat mortar materials to above 40 degrees (F). if necessary.
- Hot Weather: The surface temperature of the work, not the ambient temperature, should not be higher than 100 degrees(F).; Mortar mixing should be done only in the shade; Cover mortar in hot weather to reduce evaporation; Pointing work should be done in the shade; Work around the building during the day so that the fresh work will be shielded from direct sunlight to reduce evaporation rate.
- All work must be suspended during frosty weather unless a heated enclosure is provided. Work should not be done in full sun at temperatures above 27 degrees (C). unless shading of the walls is provided and the masonry wall temperature is kept below this point; Burlap sacking and water misting may be necessary to control evaporation; High temperatures can cause flash setting of cements and rapid evaporation of water in the mix, leading to lack of development of final strength by the cement.
- Special Precautions and Notes: Do not allow masonry to freeze until mortar is thoroughly dry and hardening almost complete (approx. three days time); The setting of lime mortar is very much slower than that of cement mortar because the curing requires the absorption of carbon dioxide to form hard lime carbonates; It is a very lengthy process, so do not expect it to become hard immediately, especially at the core of large masses of masonry.
- Repointing Tools: Available from good hardware stores, building material suppliers or mail-order catalogues.
- Goldblatt Tool Co.
- Marshalltown Trowel Co.
- Salvaged Stone: Approved, sound, crack free, clean stone without face chips larger than 1/2 inch, salvaged from removed face stone work.
- Replacement Stone: Upon approval of Regional Historic Preservation Officer, provide natural building stone of type (classification) and with color, and surface texture to match existing stone on building. Match existing stones in size and shape.
- Naturally colored binder such as (PFA) low sulphate pozzolanic fly ash
- HTI Powder (white refractory brick powder - "High Temporary Insulation") - may be used as a pozzolanic additive to make a weak hydraulic mortar.
- Mortar and Grout Materials:
- Portland Cement: ASTM C150, Type I.
- For stonework and other masonry indicated, provide non-staining white cement complying with staining requirement of ASTM C91 for not more than 0.03% water soluble alkali.
- Hydrated Lime: ASTM C207, Type S.
- Aggregate for Mortar: ASTM C144, unless otherwise indicated.
- Colored Mortar Aggregate: Natural or manufactured sand selected to produce mortar color indicated.
- Colored Mortar Pigment: Natural and synthetic iron oxides and chromium oxides, compounded for use in mortar mixes. Use only pigments with record of satisfactory performance in masonry mortars.
- Water: Clean, free of oils, acids, alkalis and organic matter.
- Aggregate for Grout: ASTM C404.
- Manufactured and raw materials accessory to the primary products.
- Supports, fasteners, installation accessories, and anchors: Stainless steel only for use with granite. Brass, bronze, or stainless steel for use with limestone and other masonry.
- Flashings or barriers: Sheet copper or as detailed.
- Trowels: range in length from 10-12 inches
- Chisels: Joint chisels or a standard mason's chisel with a 1-1/2 in. blade and a long narrow handle
- 5# stone dressing hammer
- 2# striking hammer
- "No-Bounce" hammer
- Full size and one half size brick hammers
- Joint Tools: (see 2.01 MANUFACTURERS above)
- 3/8"-1/4" raised beaded tool
- 3/8"-1/4" beaded striking tool
- 1/2" raised beaded tool with offset handle
- 1/2" flat joint iron
- Pointing tool should be about 1/16" narrower than the joint being filled to achieve good compaction
- Hawks: Plywood or steel hawk (mortar board)
- Natural bristle brushes
- Stiff bristle brushes (no wire)
- Spray bottle
- Measurement and Mixing: Measure cementitious and aggregate material in a dry condition by volume or equivalent weight. Do not measure by shovel, use known measure. Mix materials in a clean mechanical batch mixer.
- Consistently and accurately measure materials for each batch.
- Mix for at least five minutes in a mechanical batch mixer or mortar box.
- Mix trowel workable consistency for unit masonry setting and resetting.
- Mixer Pointing Mortar: Thoroughly mix cementitious and aggregate materials together before adding any water. Then mix again adding only enough water to produce a damp, unworkable mix which will retain its form when pressed into a ball. Maintain mortar in this dampened condition for 1-to-2 hours. Add remaining water in small portions until mortar of desired consistency is reached. Use mortar within 30 minutes of final mixing; do not retemper or use partially hardened material.
- Mix firm dry consistency for repointing.
- Discard mortar not used within two hours after mixing; do not retemper at mixer.
- Dispose of waste mortar and cleaning water in approved manner; do not contaminate the site, adjoining property, or waste to sewers.
- Colored Mortar: Produce mortar of color required by use of selected ingredients. Do not adjust proportions without RHPO's approval.
- Color Pigmented Mortar: Where colored mortar pigments are indicated, do not exceed pigment-to-cement ratio of 1-to-10, by weight.
- Match original color and texture for each kind of mortar and masonry work.
- Match to inner mortar color which has been protected from weathering and soiling, not face color.
- Conform to the dry mix color matching, "A Fast Method of Matching Mortar Colors Using Heat," APT Bulletin, Vol. X, No., 2, 1978, pp. 79-85, copy provided by RHPO; or other method approved.
- Do not use admixtures of any kind in mortar, unless otherwise indicated.
- Mortar Proportions:
- NOTE: VERIFY PROPORTIONS WITH MORTAR ANALYSIS TO MATCH OR RESEMBLE HISTORIC MORTAR.
- Pointing Mortar for Stone: One part white portland cement, 1 part lime, 6 parts colored mortar aggregate.
- Rebuilding Mortar: Comply with ASTM C270, proportion Specification, Type N, unless otherwise indicated with cementitious material content limited to portland cement-lime.
- Grout: ASTM C476.
- Surface Preparation: Where fresh masonry joins existing work, clean the exposed surface of the set masonry by removing loose stone and mortar and wet lightly to obtain the best possible bond with the new work.
- Carefully remove by hand, as indicated, any stone which has deteriorated, shifted, or is damaged beyond repair.
- Carefully cut out perimeter joints of stone by hand using a hammer and chisel.
- Break down the rest of the damaged stone with vertical saw cuts and break it up with a hammer and chisel.
- Carefully clean out the cavity to receive the stone. Remove mortar, loose particles and other debris from in preparation for resetting.
- Thoroughly wet the cavity with water; Dampen the new or salvaged stone with water.
- Spread a 1/2 inch thick mortar bed into the open cavity; the new mortar should consist of coarse sand and grit to match the original mortar. Butter vertical joints for full width before setting and set unit(s) in full bed of mortar, unless otherwise indicated.
- Repair and/or replace removed stone with salvaged stone, where possible, or with new stone to match existing, including size. Set the stone in the cavity.
- Fill the joints with a grout of lime and low sulphate pozzolanic fly ash (PFA) or lime and HTI powder.
- Tool joints after setting to match joints of surrounding stone.
- Repoint new mortar joint to comply with requirements for repointing existing masonry, except rake out joints before mortar sets.
- Wipe all excess mortar as the work progresses. Dry brush at the end of each day's work. 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, cracks, etc., after which wash down all masonry walls, leaving them clean and neat.