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| Title | Extending the Life of Concrete Repair | |
| Author | Jay H. Paul | |
| Jay H. Paul, S.E., is
the CEO of Klein and Hoffman, Inc., Chicago, Illinois,
where he heads the restoration engineering group. He has over 17 years
experience in the evaluation and repair of concrete structures. The cost of repairs to concrete high-rise structures and parking decks can be enormous. A typical project budget might be hundreds of thousand of dollars, and could even run into the millions. However, a repair program is not complete without considering a protection system. While the results of a repair program are easy for owners to see, the benefits of a protection system may be a difficult sell. After spending such a large sum on repairs, it is no wonder owners are generally reluctant to spend even more money to protect that investment. An owner may ask: "Why, if the restoration program is a success, do I need to protect the repairs?" Why protect repairs? Under most circumstances, concrete repairs have a finite life. So, the primary reason for a protection system is to extend the life of those repairs. Without one, an owner may be repairing the structure again long before it was ever anticipated. The truth is it is difficult, if not impossible, for repairs to actually change the conditions that lead to concrete deterioration. Protection systems are designed to improve the performance of repairs by moderating the underlying causes of concrete deterioration. An added benefit is that some protection systems mask repairs and can improve appearance. Protection methods There are many methods and certainly a multitude of products available to protect concrete. The objective is to reduce corrosion of metals in concrete and related problems, as well as improve other characteristics of the concrete matrix that cause various types of deterioration. This is generally accomplished by limiting the intrusion of moisture, chlorides, carbon dioxide, and other contaminants into the concrete substrate by surface treatments or by electrochemical principles. (Photo not included) Caption: Repairs failed after only 3 years. Protection system did not address inadequate coverage or carbonation. Protective systems also include materials and methods that increase the ability of the concrete surface to resist abrasion, impact resistance, or other deleterious influences (see end note 1). Following is an overview of various systems available for protecting concrete, along with a brief description of the characteristics of each system. Surface treatment For years, surface treatments have been the most common method of protection. The objective of a surface treatment is to limit corrosion by minimizing free water in the capillaries of the concrete. At the same time, surface treatments prevent further moisture (and chloride intrusion for parking or bridge decks) from migrating through cracks and reaching the reinforcing. The general classifications for surface treatments, used in the AConcrete Repair Guide (ACI 546R-96), are:
By definition, penetrating sealers lie within the substrate of the concrete. Depth of penetration will vary by product, the properties of the concrete, the existence of contaminants and, to some extent, the surface preparation. Penetrating sealers are not subject to abrasion, generally do not degrade due to ultraviolet (UV) exposure, will not bridge nonmoving cracks, and do not appreciable alter appearance. They will not hide concrete repairs, nor will they stop water penetration through cracks. Because these products lie beneath the surface of the concrete, they are excellent for use on parking decks. Often, these products are used in conjunction with coatings to improve durability. Included in this group are: boiled linseed oil, silanes, siloxanes, migrating corrosion inhibitors, certain epoxies, and high molecular weight methacrylates. Because most of these products are intended to reduce moisture and thus chloride intrusion, they may reduce or delay the onset of future corrosion and freeze-thaw degradation. The specifier must check volatile organic compound (VOC) emissions with the manufacturer because that could be an issue in some applications. Surface sealers and high-build coatings (Photo not included) Caption: Halo effect: Existing reinforcing extends from the parent concrete into new concrete, fostering corrosion where the two meet. Failure occurred just beyond the bond line. (Photo not included) Caption: Repaired slab with a patchwork pattern. The owner should be informed about how repairs will affect the final appearance. Depending on their application, there are many products that could be classified as a surface sealer (10 mils or less dry film thickness) or as a high-build coating (10 to 30 mils dry film thickness). Surface sealers are used for protecting decks and vertical surfaces. Because concrete repairs and blemishes may mirror through, a high-build coating may be more appropriate where appearance is a major criteria, such as on facades. Many of these products will effectively waterproof the surface. Some manufacturers of elastomerics have data to indicate their products provide good anticarbonation resistance which could be beneficial where concrete coverage over reinforcing is insufficient. In many instances, water and vapor permeability are important parameters for product selection. Although most elastomers will not bridge moving cracks, they may be effective in bridging nonmoving cracks. However, there are some elastomeric coatings that will bridge small moving cracks if properly detailed. Included in this group are: epoxies, polyurethanes, methyl methacrylates, moisture-cured urethanes, acrylic reslins, certain paints (oil-based and latex) and silicone water-based elastomers. Selection of an individual product may depend on its ability to breathe (or in some cases to act as a vapor barrier), as well as to provide sufficient resistance to water penetration. Many of these products are affected by UV and will wear under surface abrasion. Skid resistance may be reduced unless fortified with an appropriate aggregate. Membranes Membranes are generally applied to the surface of the concrete for horizontal applications. These applications significantly alter the appearance of the concrete, can bridge small moving cracks, and will mask most repairs. With the introduction of appropriate aggregates, membranes provide adequate skid resistance and durability when required. Because they do reduce the intrusion of moisture and chlorides into the concrete, the onset of future corrosion may be delayed significantly. When exposed directly to the weather, UV degradation is a potential problem. Because of VOC regulations, most manufacturers have developed products with no or low emissions. However, in almost every instance, they do not have a long track record of in-service use. Included in this category are: urethanes, acrylics, epoxies, neoprenes, cement, polymer concrete, certain methyl methacrylates, and asphaltic products. Frequent maintenance of exposed membranes, especially in parking structures, is required although it is usually inexpensive. There are membranes available that are self-healing, which would be beneficial under buried overlays. Avoid situations where the concrete is encapsulated with nonbreathing membranes on each side. In general, membranes offer good elongation properties and excellent water permeability, but only marginal vapor permeability. Membranes over slab-on-grade or in situations where there is a vapor drive from below must be applied with the greatest of caution. There have been instances where some membranes are not compatible with certain proprietary concrete repair products. Overlays Overlays are generally bonded to the concrete and will add weight proportional to their thickness which must be considered in the analysis of existing structures. Additional reinforcement can be added. If installed to act compositely with the existing structure, overlays can increase strength. Overlays can be formulated to reduce moisture intrusion, to improve durability and corrosion resistance as well as prevent the intrusion of chlorides. Many of the available products are fiber reinforced to reduce plastic shrinkage. Because of the additional thickness, the overlay does afford the opportunity to improve drainage. Although overlays will initially bridge cracks, moving cracks may mirror through. Very often, overlays are used to enhance appearance and are very effective in masking repairs below. Included in this category are: low slump concrete formulations, polymer concrete, epoxies, certain methyl methacrylates, and polymer-modified concrete. For bonded overlays, it is best to select a product that has properties similar to the parent concrete to minimize compatibility problems. Because many of these products are cementitious, vapor permeability problems can be easily avoided. Products containing epoxies and polymers should be evaluated for potential UV degradation. Cathodic protection The concept behind this method to control corrosion is to make the embedded reinforcing cathodic - as opposed to anodic where corrosion occurs. Reinforcing steel is electrically connected to a sacrificial metal that becomes the anode. This can be done with power (impressed current system) or without power (passive system). There are several types of cathodic protection systems available. The primary difference between the systems is the anode system and its use (see end note 2). Cathodic protection will not replace corroded reinforcing. Any required structural repairs would still have to be performed. Currently, cathodic protection is not recommended for use on prestressed concrete because hydrogen embrittlement of high-strength steels might occur. If there is epoxy-coated reinforcing in the structure, a determination of electrical continuity must be conducted to establish whether cathodic protection would be effective. Cathodic protection only addresses the control of future corrosion. Other techniques will need to be used on such issues as appearance, durability, and performance of the repaired concrete. Unlike some techniques and products now entering the market, cathodic protection has been in use in one form or another for years. Bridge piers were among the initial applications. Most manufacturers can provide at least a partial track record of similar applications that are now commonly in use. Corrosion inhibitors New technologies are being developed to add to or improve the arsenal of protection systems. One such product is corrosion inhibitors which are added to the concrete. Corrosion inhibitors are meant to supplement the concrete’s natural ability to protect the embedded reinforcing by forming a passibating oxide layer on the steel. This will normally occur when alkalinity of concrete is maintained at a pH of about 12. The most commonly used product contains calcium nitrate. The amount added to the concrete is based on the anticipated chlorides that the concrete will be exposed to over a given period of time. Having been used successfully for about 20 years, this product is appropriate for use in full slab replacements as well as concrete overlays for bridge and parking structure decks. Recently, a new generation of products (with two different chemistries) has been introduced. These are surface applied to existing concrete and designed to migrate to the embedded reinforcing to protect it against future corrosion. These products are generally referred to as migrating corrosion inhibitors or MCIs. (Photo not included) Caption: Quality control testing on silane sealer. A core is exposed to dye. Depth of penetration is roughly 0.5 in (13mm). One product, based on calcium nitrite technology, works in the same manner as the additive already in use. The other is a water-based blend of surfactants and amine salts. The manufacturer claims it migrates as a vapor through the concrete to form a thin, protective monomolecular film on the reinforcing steel. They claim the corrosion inhibiting chemicals can migrate as much as 1.6 in. (40mm) in 24 days (see end note 3). Extensive testing of these products has indicated both products are effective at reducing corrosion rates in chloride-contaminated concrete (see end note 4). However, it is still far too early to ascertain the full benefits that can be achieved by incorporating MCIs as part of the protection system. If they do prove to be effective, they might solve the "halo" problem (see photo mentioned above). Protection selection factors Just as one evaluates the cause of the problem before selecting a repair technique, the requirements for each project must be determined and carefully evaluated prior to selecting a protection system. Although several different systems might be suitable, the parameters unique to a particular structure might make one system more attractive than another. The implications – that a system might have on a particular project including the potential for failure and future maintenance – need to be taken into account prior to selection. Factors to consider are listed following. Track record A number of new systems for protecting concrete have almost no service record. Although test results might be encouraging, most engineers and owners are reluctant to incorporate such systems on their projects. There are circumstances, however, where the problems are so acute that new technology may be the only answer. It is crucial that the owner is aware of the risks. The engineer must attain the full consent of the owner for whatever risk is acceptable regarding use of a system without a long track record. Initial and long-term costs Cost is a factor an owner can’t ignore. Most protective systems have maintenance costs along with their initial cost (e.g., sealers and paints need to be reapplied; cathodic protection systems need to be monitored and certain components replaced). In some cases, the initial cost may appear high. However, when evaluated over the method's life cycle, its use on a repair project may be very cornpetitive. Moreover, if down time for a structure is a major concern, the large initial cost of a method that extends use of the facility without interruption for future concrete repairs might be well worth it. Appearance As with the repair program itself, owners need to know upfront what they are buying, i.e., how it will look. The location of a repair may be of concern when considering several protective systems. Engineers must also look to the long term. They need to ask: How will the system appear after it has been in place for a few years? Are there environmental factors that will affect how a protection system will hold up over the years? VOC compliance For regulatory purposes, the Environmental Protection Agency (EPA) defines a volatile organic compound as "any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium, which participates in atmospheric photo- chemical reactions." Compliance with current VOC regulations is an important factor that must be considered when selecting a protection system. There are two concerns regarding VOCS: (1) the strong unpleasant odors solvent-based products emit are an issue in poorly ventilated indoor spaces, and (2) the photochemical reaction between nitrogen oxides and VOCs which produces ozone. Unfortunately, many of the products that engineers have relied on in the past are no longer available in their original formulations. To meet the regulations of several states that have such laws, low VOC products have been introduced to the marketplace. The latest versions of some of these products really don't have an acceptable track record. Again, it is critical that the owner understands this dilemma. Since 1991, the EPA, the coatings industry, and government agencies have been trying to reach mutual agreements on limiting VOC emissions for the entire nation. The objective is to establish some consistency that will enable specifiers to compare products and claims made by the various manufacturers (see end note 5). Compatibility It is very important to review the condition of the surface when considering any protection system. Many of the proprietary products commonly used in repairs might have a chemistry that is incompatible with coatings or sealers specified for use over them. Thus, the potential for compatibility problems must be researched. Prior to the start of production work, the selected system should be tested on the job site. In addition, the efficacy of penetrating sealers is greatly influenced by the presence of existing sealers or coatings. Most manufacturers require approval of a coating's substrate before granting a warranty. Removal of existing coatings is very expensive. Sometimes only partial removal is required, which could result in a significant cost savings on the project. Durability/performance When evaluating a protection system, the engineer must determine the owner's expectations of the life of the repair system. Among other factors to consider are environmental conditions (expo- sure to wind-driven rains, temperature variations, acid rain, carbon dioxide exposure, etc.), exposure to UV, extent and nature of traffic (for decks), and use of the structure (potable water reservoirs have certain regulatory requirements). Installation requirements Prior to selecting and installing a protection system, especially surface treatments, there are certain basic conditions that need to be met. These include:
The best way to check for compatibility, as well as other project conditions, is to include mock-ups as part of the repair program. This can be done prior to preparation of the project documents or included as the initial work to be performed under the construction contract prior to implement the production work. The latter approach is quite common and presents the added opportunity for the contractor working in conjunction with the engineer to "fine tune" the repair/protection techniques and possibly improve the quality of the project and/or reduce costs. All the basic repair techniques should be tested. Of particular importance is surface preparation, especially for coatings. If there are existing coatings on the building, the extent of removal must be determined at this time. Regarding product warranties, it is important for manufacturers to become involved, and that they in fact approve the surface preparation to be satisfactory to warrant their products. Testing should be done as part of this procedure to confirm that the bond and other significant criteria in the specifications as well as manufacturers’ conditions are generally being met. Those procedures that meet with the approval of the manufacturer, the engineer, and the owner then become project standards. Sometimes mock-ups are left in place for the duration of the project as a reference to judge the production surface preparation. Besides allowing one to scrutinize the technical aspects of the work, mock-ups provide the opportunity for the owner to judge the appearance of each of the techniques and, most importantly, the final coating. It is recommended that specifications require the owner to approve the appearance of the repairs, including the final color and texture of the coatings, before the production work commences. (Photo not included) Caption: Moisture permeability test being conducted. The choice of a protection system may be influenced more by the installation conditions than other factors. For example, products with low VOC emissions would need to be selected for use in poorly ventilated interior spaces to comply with the current regulations. Yet, this method might be more expensive and lack the track record of traditional products that the engineer has probably used successfully in the past. Another example of influences that affect the choice of a coating on an exterior exposure might be low temperature limitations. Perhaps the work schedule indicates it will be necessary to apply the final coating in the fall. A factor such as this, having a potential for significant effect on the project's success, would have to take precedence over other parameters. Summary Concrete protection systems are a necessary and valuable part of any successful repair program. Because of improved technology as well as governmental regulations, techniques are changing. The specifier must keep up to date with the newest developments and carefully evaluate the products in light of each individual project. Only then can the useful life of the repairs be extended and the owner achieve the cost benefits. Guides for selecting a protection system
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