Limestone is a broad term that refers to many types of sedimentary rock in which calcium carbonate is the major constituent. One hundred million to 500 million years old, limestone is derived from fossil deposits of marine animals. It generally has a uniform consistency and texture and is usually buff or off-white in color, but it can also be gray or very light in tone. Marble is primarily a metamorphic form of limestone, transformed under intense heat and pressure into a hardened mass of calcium carbonate. In general, marble has a much finer texture than limestone, which allows it to be polished. Limestone cannot be polished.
Importance of Repairing Historic Marble and Limestone
A building’s exterior provides protection from the elements and conveys its historic character. The age, style and significance of a building or structure can often be understood by analyzing exterior designs, features and materials. Changes in taste, fashion, architectural style and use may be evident. In any rehabilitation project it is critical to treat the exterior with great care.
Distinctive features, finishes and construction techniques are examples of craftsmanship that characterize a property and should be maintained. If possible, deteriorated historic features should be repaired. If replacement is necessary, the new feature should match the old as much as possible in design, color, texture and other visual qualities.
Where to Start?
The first step in the rehabilitation of stone at a building’s exterior involves the accurate assessment of that building’s existing system. In other words, before you can know how to fix something, you have to know how it was intended to work in the first place. Then a proper scope of rehabilitative work can be established.
As water is the primary cause of deterioration, understanding the management of water in the façade is absolutely essential. Mass wall systems must be allowed to breathe, cavity wall systems must drain, and barrier wall systems must be sealed. Repairs for each can be drastically different. The interaction of building components (roof, parapets, windows, doors, etc.) must also be understood.
What Is the Condition of Your Building?
A thorough assessment of your building’s exterior condition is the next step in the development of the scope of required work for a successful stone rehabilitation program. Identification of problem conditions early in the process is less costly by far than “righting” wrongs after your project has been completed. That condition assessment should include:
- Identification and/or classification of existing materials;
- Evaluation of existing façade systems including drainage, insulation, vapor barriers and structural supports;
- Thorough inspection of all elements documenting all signs of deterioration and defects;
- Investigation of hidden conditions through probes into exterior building components;
- Investigation of material deterioration through field and laboratory testing methods; and
- Identification of potentially hazardous and/or unsafe conditions.
The subsequent development of a realistic budget and project schedule, and the preparation of appropriate design documentation, materials and systems specifications, will ensure that a good stone rehabilitation program can be executed.
In For a Penny, In For a Pound
Proper repair of stone façade elements can seem costly. However, this cost can easily be eclipsed by the cost of correcting deferred repairs or ignored defects. Repairs that only address problems on the surface can mask deeper issues and often allow deterioration to continue unnoticed. In the long run, cost-effective repairs always correct the root problem first before restoring the damaged finish material.
It would be difficult to justify repairing a water-damaged ceiling below a leaking roof without first repairing the leaky roof. However, this type of repair happens with facades more often than you’d think. It’s important to understand the nature of the problem and the methods necessary to repair the defect. Be wary of solutions that don’t completely address the problem.
Removing a large piece of stone on the side of a façade any number of stories above the ground may seem daunting, but this might be the only way to access the defective condition. Specially trained design professionals and contractors are necessary to undertake such work.
Consider, for example, that a property manager discovers a crack at the base of a stone panel on a building façade.
Proper repair of the condition should start with removing enough of the stone to identify the root cause. Repairs could consist of removing all the spalled stone, preparing and painting the embedded steel and then repairing the stone with either a Dutchman or patching mortar to match the existing stone. If, however, prior to identification of the root cause of the stone spall (in this case, the rusted steel embedded in the wall), the crack and adjacent joints are sealed with patching mortar or caulking compound, water would still continue to enter the stone. The embedded steel would continue to rust and expand, displacing the patched stone once again.
Sources of Deterioration in Stone
There are many sources of stone deterioration. Damage occurs through weathering, the invasive action of plant growth, pollution, and dirt accumulation. Further damage can occur through erosion, as a result of movement in stone (thermal expansion/contraction), faulty or broken anchors, and even human error in design, engineering or construction.
Because marble and limestone are carbonates, both stones present several key maintenance and preservation problems, for example:
- Unlike granite, marble and limestone are highly sensitive to and soluble in acid (think baking soda and vinegar). The natural acidity of rainfall – even without the compound effects of acid rain, automobile exhaust and other airborne pollutants – will cause the stones to deteriorate over time.
- Marble and limestone are porous and absorb water readily. The rate of absorption and the level of porosity vary. Water trapped within the stone will exacerbate deterioration. Therefore, it is important to ensure that moisture infiltrating the stone can evaporate.
- Marble and limestone are relatively soft stones and can be easily scratched and marred. This softness means chemical cleaners, sandblasting and wind-driven grit will take their destructive toll.
The underlying causes, as well as the indicators themselves, should be thoroughly investigated before any rehabilitative work is performed. There is no value, for example, in patching a cracked marble wall panel if the cause for the crack lies behind the panel at its anchorage system.
Common Repair Techniques
Cracked, spalled or exfoliated limestone and marble should be repaired in a timely manner to prevent further damage. Typical repair methods include the following:
Repointing requires cutting out failed joint mortar and applying new mortar that is finished to replicate the original mortar style. There are two important points to take into consideration when repointing: the new mortar should be of a weaker compressive strength than the surrounding masonry to avoid spalls at the joints, and joint sealant is not recommended, as it prevents the migration of damaging moisture from within the wall system.
Re-anchoring is usually necessary when movement of the stone from its original position has broken existing anchors or fractured the back of the stone.
Pinning stonework is performed by securing non-corrosive rods through the stone into the masonry backing.
Patching is used to repair small of damaged stone with a matching cementitious material that is applied and built up in layers until it matches the original profile. For larger patches, pins might be needed to secure the new material.
Sculpting is the process of re-carving badly eroded stone or re-accentuating indistinct detailing. This method is frequently employed to repair ornamentation.
Resetting returns displaced stone to its original position.
Stone replacement can be the most viable approach for restoring large areas of damage. A new anchoring system may be required.
Crack repair can be accomplished by injecting an adhesive material into the fissure to provide water impermeability and prevent further cracking. A minor crack can be successfully repaired by routing the crack and filling it with patching material.
Consolidation is a time-tested technique of reconstituting softened or deteriorated stone by filling the voids and binding the grains within the stone. Consolidation has many applications for the preservation of historic structures, but requires thorough testing and evaluation to ensure that the consolidate leaves a compatible vapor-permeable surface.
A Word about “Protective” Coatings
Waterproof coatings should rarely, if ever, be used on stone. Although these coating prevent water infiltration, they have a tendency to trap moisture within the stone and the masonry in general. Even water-repellent sealers will prevent the natural migration of water from stone and should be used with great caution, if at all.
Ongoing Maintenance and Cleaning
Ongoing maintenance is essential to preserve exterior stone surfaces and to prevent future deterioration. A good maintenance program includes routine inspections to detect early signs of stone damage and joint failure.
Regular washing keeps dirt and pollutants from accumulating on stone, helping prevent deterioration. You should correctly identify stone before cleaning, because certain cleaning agents and treatments, if improperly applied, may cause or accelerate physical deterioration in stone. For that reason, it’s best to use the mildest cleaning method possible to get the job done.
Because marble and limestone are especially sensitive to acid-based cleaners, water washing is usually the best treatment. Washing by bucket and brush is a well established method of cleaning marble and limestone surfaces. Spraying and misting (soaking) is effective on marble and limestone for removing heavy accumulations of soot or crusts that have a tendency to form in protected areas that are not regularly washed by rain.
A purification system might be needed to eliminate the corrosive effects of impurities in the water. And it goes without saying: the surface you’re cleaning should be watertight.
Who is Qualified To Do the Work?
Walk down any street, in any city, and you’ll more than likely see good stone rehabilitation intentions gone bad… stone “rehabilitation” work that has caused more harm than good. At the Acropolis in Athens, Greece, for example, a botched 1930s marble restoration project involved the use of iron clamps that rusted over time, causing the structures to further crack and crumble. The outcome was unfortunate and avoidable.
When it comes to stone rehabilitation, you can count on one thing. What you don’t know will hurt you. It’s essential to work with design professionals and restoration contractors who are knowledgeable, trained and experienced in restoring stone structures and surfaces.
Although deterioration in limestone and marble is inevitable, there are ways to slow the process and prevent serious damage. Routine maintenance and inspections can reveal surface damage, displacement and joint failure that can signify more serious underlying conditions. Early detection and prompt, careful attention are key.
How to Repair Cracked Limestone
Limestone is a soft, sedimentary stone made up of calcium. It may contain small cracks known as fissures at or beneath its surface. Over time, normal wear and tear can cause these fissures to open up wider into larger cracks. If a crack appears in limestone, it may detract visually from the stone, and possibly weaken it as well. Filling the crack with epoxy resin tinted to match the color of the limestone both masks the crack and gives the stone back its tensile strength.
Vacuum or sweep the crack in the limestone to remove any loose stone dust or debris. The crack has to be completely clean and free of any loose material for the repair to hold.
Wipe acetone over the crack with a soft cloth. This cleans the crack and prepares the surrounding stone to accept the epoxy repair.
Mix up the epoxy resin with the pigments until it matches the color of the limestone. Put some of the mixed epoxy into the syringe.
Inject the epoxy into the crack with the syringe. Start at the far end and pull the syringe toward yourself as you push the plunger. Fill the crack until it is level with the surrounding stone. If you overfill the crack, scrape away the excess epoxy immediately with a craft stick or razor blade. Let the repair harden for 24 hours.
Things You Will Need
- Vacuum or broom
- Soft cloth
- Epoxy resin made for limestone repair, in a color that matches the stone
- Craft stick or razor blade (if needed)
- Limestone repair kits are available with pigments you add a little at a time until it reaches the shade of the stone surrounding the crack.
Place a strip of plastic film on your work bench to prevent the repaired block from sticking to the surface. Use a heavy duty caulk gun fitted with a tube of premixed exterior limestone epoxy glue. Clean the broken ends of block with a rag soaked in acetone and squeeze out a short bead of epoxy before starting. Coat one of the faces of block with a layer of glue, clamp the two halves firmly together and allow 30 minutes curing time. Remove any excess epoxy and sand the surface carefully with a sanding block or rotary sander and medium grade abrasive.
Use an exterior polyurethane limestone putty and caulk gun with a specialized nozzle and an injection technique to repair cracked, installed blocks. Cut a 3/8 inch deep “V” through the surface of the crack with a rotary tool, then use a drinking straw-sized masonry bit to drill holes halfway through the center of the crack, spaced 4 to 5 inches apart. Insert short plastic drinking straws into the holes. Seal both the straws and the V with the limestone putty. However, if the stone is rough-hewn, use Historic Restoration Mortar (HRM) and blend the material in as neatly as possible before allowing adequate setting time. Fit the injection nozzle onto the first straw and inject epoxy into the crack by pressing the handle several times until you feel resistance. Repeat with each straw, and sand the repair with a rotary power sander and medium grade abrasive.
Repairing Broken Corners
Use exterior polyurethane limestone putty to repair broken corners in cut limestone blocks. Clean the broken surface with a rag soaked in acetone. Mix hardener to a wad of putty on a pane of glass as directed by the manufacturer’s instructions; in cold weather, add a small amount of accelerator to the mix. Form a mold by holding strips of stiff plastic against the side corners. Wear latex gloves and press putty into the corner while molding the upper face slightly higher than the surrounding block by hand. Allow adequate curing time, remove the plastic pieces, and test the putty for hardness. Finally, sand all three corner surfaces smooth with a medium-grit sanding block or rotary sander fitted with medium grade abrasive.
End Result — A Perfect Match
Done correctly, the break on the stone will be invisible and the joint will be stronger than the original limestone. Once it dries, both the putty and HRM will match the surface color of the limestone, and both materials will absorb water and darken at the same rate as the surrounding limestone. In addition, after rebuilding missing corners and sanding, the repaired corner will be as sharp as the unbroken corners of the rest of the block.