Exterior Building Restoration
Above grade waterproofing is completed at exterior building components to prevent moisture infiltration from entering the structure. Although above grade components are not exposed to hydrostatic pressure that is present in below grade conditions, detrimental weather effects, such as UV rays and precipitation (rain, snow, ice, dew, etc.), contribute to degradation over time. Water is the most destructive element to masonry, concrete and natural stone. Preventing moisture entry is a difficult task, primarily because water can penetrate above grade components in five distinctive methods:
- Natural gravity forces
- Capillary action
- Surface tension
- Air pressure differential
- Wind loads
Damage by each of these forces is consistent with water damage of all other building components. As water enters the exterior of the structure it damages the façade and the structural components on its way into the interior of the structure. Damage to the façade and structural elements can also occur from other weathering elements, such as exposure to repetitive freeze/thaw cycles. The wear and damage created in these areas must be properly repaired or restored prior to the application of above grade waterproofing.
Prior to restoration, an analysis of the above grade components should be conducted to determine the extent of damage caused by moisture infiltration or weathering elements. Façade repairs that do not address structural defects will fail to eliminate long-term problems. Analysis can be conducted with a variety of destructive and non-destructive testing methods. A trained investigator should complete these types of tests. Water tests may also be helpful in determining the source of the moisture infiltration, particularly since most building exteriors are constructed of several adjoining materials and components. The methods, materials and extent of restoration are determined by the results of the analysis.
PreparationThe most critical element of above grade restoration is proper surface preparation. Adhesion to certain surfaces will fail prematurely if the surface is not adequately free of all dust, pollutants and dirt accumulations. Failure from these conditions can result in direct substrate deterioration by corrosion to interior structural components (steel, concrete, etc.) when the water that enters the substrate is mixed with pollutants, such as salts, sulfates or carbons. Pollutants also build on the exterior surface and fill façade pores to entrap moisture, which slows down the natural breathing process. Moisture that can escape through the natural manner will lead to damage from freeze/thaw cycling. This type of damage includes cracking, disbanding of exterior materials and structural deterioration.
Proper preparation includes surface cleaning and eliminating all dirt and pollutants from the restoration substrate. The types of cleaning method(s) to employ are dependent on the surface condition, the types of exterior materials and the degree of damage to the material and substrate. It is a good recommendation to test the cleaning method on a small portion of the repair area prior to use. Typically, this type of testing can be conducted at an unobserved area of the building, such as at a lower portion of the building that is isolated from heavy traffic areas. This eliminates unsightly conditions that may occur if the test material is not compatible with the substrate.
Cleaning can be completed in one of four or a combination of methods:
- Water cleaning
- Abrasive cleaning
- Chemical cleaning
- Poultice cleaning
The most common form of exterior cleaning is water cleaning, which is done by pressure washing, water soaking or steam cleaning. Pressure washing is completed with pressure spray equipment that produces pressure from 300 to 25,000 pounds per square inch. The lower pressures are used for rinsing dirt and residue accumulation, and are the most commonly used methods. Higher pressures are used to remove paints and coatings along with residues. The pressure should be tested prior to use so that no structural damage or deterioration occurs and that no building joint sealants are displaced.
Another form of water cleaning is water soaking. Water soaking is conducted by installing specialty hoses or sprayers at the upper portion of the facility and allowing continual running water to flow down the building. This method, which is typically required on historical restoration projects, loosens the dirt and residue accumulation through the continual soaking process. The loose particles are than removed from the surface by rinsing with low-pressure sprayers. The major drawback to this method is that continual water soaking will allow further moisture infiltration into the substrate at existing leak locations. This contributes to further structural damage. In some cases, it may take up to a month for the surface to adequately dry.
Abrasive cleaning is the most extreme method because it not only removes dirt and pollutants but also takes a layer of the substrate. Abrasive cleaning is used for substrate restoration where there is identifiable surface damage. The deteriorated substrate is removed from the surface. Abrasive cleaning can be completed by sandblasting, wet aggregate blasting, sanding belts or wheel grinders. Safety is a concern with these methods and they should only be completed by highly trained individuals.
Chemical cleaning is conducted by adding acidic, organic or alkaline chemicals to water. This method is used when water cleaning does not produce the required results and abrasive cleaning is too severe due to possible substrate damage. Chemical cleaning can be conducted with pressure sprayers or water soaking. Due to the fact that the chemicals used can be toxic, proper safety requirements should be employed. Applicators should be properly trained and must be wearing the required protective clothing. In addition, all surrounding building envelope components (glass, metal, vegetation) should be covered to protect against potential chemical damage.
Poultice cleaning is completed by applying a specially formulated absorbent material over a stain that has penetrated the substrate. The pollutants are drawn into the poultice compound and absorbed. Spray washing of the surface is required to remove the poultice material. This method is completed when the other cleaning methods fail and the surface needs to remain intact so abrasive cleaning is not acceptable.
Restoration workOnce the preparation cleaning has been completed, the substrate must be examined for structural deficiencies that require restoration. All exterior waterproofing materials require a sound substrate for application. Any substrate deterioration that has occurred-including spalling concrete, damaged structural components and oxidized reinforcement steel-should be repaired. Repairs are also required at all building joints (masonry, concrete, windows, etc.) and all points of possible moisture infiltration should be repaired. The types of proper repairs depend on the substrate and exterior finish materials.
Masonry Repairs: Leaks in masonry walls are typically attributed to mortar joints. This is due to the fact that any moisture that passes through the masonry, unless the material is excessively porous, will not enter the facility because of flashings, weep holes and/or damp proofing. Moisture enters at mortar joints as the mortar breaks down over time. Deterioration from fractures and cracks occur from weathering factors, swelling of the masonry, freeze/thaw cycling, thermal movement and chemical deterioration from pollutants, such as sulfates and chlorides. When the deterioration of the mortar joints reaches the point where moisture entry exceeds the capabilities of the waterproofing protection, repairs are required.
An investigation of the masonry wall should be conducted to determine the extent of required repairs. If mortar joint damage is localized or is only sporadic throughout the structure, then tuck-pointing should be completed. If large amounts of joints are deteriorated-or if all of them require repairs-it may be economical to conduct another form of repair method.
Prior to the tuck-pointing process, all surface containments should be removed. The existing mortar should be removed to a minimum of 3/8 to 1/2 inch. A maximum 1-inch opening should be removed on severely damaged joints. Mortar removal can be completed with hand or power tools, such as hand grinders. It is imperative that joint lines are maintained in a straight condition and that no masonry is damaged during the process. This may eliminate the use of power tools, particularly on historic structures.
Once the old mortar is removed, the surface should be thoroughly cleaned of all dust, residue and contaminants. Loose particles of residue left on the joint surface will have a negative effect on mortar bonding and will contribute to loss of adhesion. The most effective form of residue removal is by spraying the joints with compressed air. When the surface is adequately cleaned, the existing mortar cavities should be wetted. This prevents premature drying and curing, which results in structurally weak joints.
Tuck-pointing should be completed using pre-mixed materials that are specially formulated for these types of applications. Regular masonry mortar may not provide sufficient performance. This is important for effective bonding, waterproofing and to control the compressive strength of the material. Materials higher in compressive strength than the masonry units are detrimental because if the joints are stronger than the masonry units, spalling of the masonry units will occur during movement. The mortar should be prepared with dry mixed cement, sand-based products, and mixed with only clean water in the amount specified by the manufacturer.
The application should be completed with a convex jointing tool that compresses the mortar tightly against the sides of the masonry units and in the joints. Excess mortar should be applied into the joints to ensure proper fill to capacity. Excess materials and fins are brushed or scrapped off the joints right after application and the joints should be struck to provide a concave finish. Tuck-pointing should not be completed in wet conditions or when the outside temperature is lower than 40 degrees F.
If the entire masonry surface requires joint repair or if there is excess damage to the masonry units, than face grouting should be considered. In this process, a cementitious waterproofing material is applied over the mortar joint and masonry façades and removed prior to complete cure. The grout that is brushed over the surface fills voids in the mortar and masonry units, effectively waterproofing over all openings, cracks and fissures.
The grout materials are cement- and sand-based products added to proprietary chemicals and waterproofing agents. The grouting process should not change the color or the aesthetics of the surface. However, some grout materials contain metallic additives that may change the surface color. A test area should be completed prior to application, particularly if maintaining the original color is an important issue.
The surface should be thoroughly cleaned of all residue and contaminants prior to grout application and all severely deteriorated mortar joints should be repaired with tuck-pointing. The grout should be applied over the prepared surfaced with a scrub brush in a continual circular motion. The exterior surface should be kept damp during application to prevent the grout from drying prior to the required removal. When the grout is near cured, it should be removed using a stiff bristle brush. Proper removal should leave the masonry surface free of all grout deposits with no streak or change in color.
Concrete Repairs: Concrete repairs are completed with a variety of cementitious patching compounds. Cementitious patching methods include high strength patching, hydraulic cement, Shortcrete or full overlays. These applications are primarily completed to restore the substrate to a sound condition prior to application of an exterior waterproofing material or coating. The compounds are either cement-based or pre-mixed products. The cement-based products are manufactured from high strength, dry mixes with bonding agents. The pre-mixed products are also manufactured with high strength mixes. They are fast setting and can be applied to damp surfaces. These products can be used as concrete or masonry overlays or to patch voids, add strength or repair honeycombs.
Preparation of the substrate is as critical on concrete surfaces as it is with masonry repairs. The substrate should be removed of all dirt and residue accumulation. It is also critical that all unsound substrates are properly repaired. This includes removal (chipping off) of all loose concrete and honeycombed areas. All cracks in excess of 1/8-inch should be properly filled with the required grout or fill material. Repairs to interior structural members (reinforcing steel) must be completed prior to exterior patching. This may include the addition of new steel reinforcements or removal of any accumulated corrosion to the steel. Patching applications should be completed in accordance with the material manufacturers latest printed specifications.
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