The heavy cost of corrosion and deterioration of concrete in pulp and paper mills has been widely recognized and addressed in recent years. Thus, the present article is not intended as a comprehensive tutorial for assessing the deterioration of concrete and some of the design considerations for the selection of repair-restoration materials because they are already thoroughly covered in previous TAPPI and JPCL articles.1,2

This article is much more limited in scope. It begins with a review of factors that affect the performance of concrete structures in conventional bleach plants. It also summarizes approaches to selecting protective materials, changes in the bleaching process that affect material selection, the effects of substrate compatibility and application conditions on the selection of repair materials, and the successful use of monolithic repair materials in bleach plants. Accompanying charts compare performance properties of various materials for protecting floors, trenches, pump foundations, and other concrete structures exposed to bleach plant chemicals.

Factors Affecting Performance of Concrete in Conventional Bleach Plants

Concrete floors and structures exposed to spills and leakage of aggressive chemicals in bleach plant operations have suffered extreme deterioration over the years, with some areas now requiring extensive and expensive repairs.3 The rate of corrosion and degradation of concrete from chemical and chloride ion attack varies with the age of structures. Listed below are some of the major factors affecting the performance of concrete structures exposed to highly oxidizing chemicals.

• Concrete design and the quality of installation and construction are the "foundation" for useful life and performance of structures.
• Selection of protective materials and the quality of application of coating, flooring, and lining products can significantly extend the life cycle and reduce the cost of concrete structures exposed to harsh environments.
• Because chemical service and exposure conditions vary widely depending on the bleaching processes being used, life cycles and costs of concrete structures can vary widely across the spectrum of old and new bleaching processes.
• Maintenance is necessary for early detection and correction of corrosion, which in turn can extend the useful life of concrete structures exposed to aggressive chemicals.

In the past, all too often in construction of a new bleach plant or maintenance of an old one, the major focus was the proper selection and detailing of "critical path" production or process equipment. The use of special alloys, fiberglass, and other corrosion-resistant materials for piping, tanks, vessels, and other equipment received primary attention and funding because the breakdown of such equipment can lead to very high costs through shutdown of critical path production. Design considerations for concrete and selection of appropriate protective materials for areas such as floors, trenches, and pump foundations were given secondary importance and limited funding for maintenance. The concrete structure was considered less critical to production capability than the production equipment itself.

The protection and maintenance of equipment has been so predominant a concern that it has often dictated the selection of protective materials for concrete and other structural elements. For instance, the resins considered good enough for protecting process equipment were considered good enough for the surrounding structures. The chemical-resistant resins used in the fabrication of solid fiberglass-reinforced plastic (FRP) piping and equipment have also been used in the formulation and application of coatings, floorings, and lining materials for steel and concrete structures in pulp bleaching process and other paper mill operations.

The performance of FRP in process equipment has led specifiers to rely on similar materials to protect concrete structures. After all, the resistance of various thermoset polymers to the pulp bleaching process and other chemicals is well documented by manufacturers of chemical-resistant resins, who furnish vinyl ester, polyester, epoxy, and other high performance thermoset resins for the fabrication of FRP process equipment.4 FRP equipment has been used successfully in most phases of the bleaching process. The most common and longest case histories have been in piping, storage tanks, hoods, and duct work.5 Vinyl ester resins are the most common thermoset materials used in paper mill FRP applications, followed closely by polyester resins.

But the selection of materials to protect concrete, when it is based on chemical resistance alone or on the chemical and temperature resistance properties of thermoset polymers (resins), has led to serious problems because exposure conditions are not always identical for equipment and structures, even though they are in the same process area. Accurate information on chemical service conditions such as temperatures, chemical compositions, pH levels, and the frequency and nature of upsets is as vital in the selection of lining and flooring materials as it is in FRP equipment for long-term, cost- effective performance.

Environmental Impact: Trends in Protective Material Needs

In addition, the technology of bleaching, with increased temperatures and recycling of chemicals, has led to increased corrosion and, consequently, to a need for new or improved corrosion-resistant materials. There are 3 broad classes of traditional bleaching chemicals: chlorine-based chemicals, chlorine-free oxidizing chemicals, and chlorine-free reductive bleaching chemicals. The chlorine-based chemicals have traditionally been used, but because of environmental pressure and legislation, they will be used less in the future. Increased use of peroxide, hydrosulfite, and new processes and technology is anticipated.

For instance, in recent years, it has become more common to see U.S. mills substituting chlorine dioxide for elemental chlorine and installing oxygen delignification systems to reduce the precursors of dioxins in the pulp bleaching operation. One of the newer technology developments to solve the dioxin problem associated with pulp bleaching involved installation of a four-stage ozone bleaching system at a southern paper mill. The mill expects to recover and reuse a good deal of its effluent, thereby reducing the amount that will need to be treated. Another new development and technology involves incorporating diffusion washers in the hydrogen peroxide bleaching process to remove the peroxide from the product.

New processing chemicals and technology are changing the resistance requirements for protective materials. Conventional bleaching chemicals are alkaline and therefore demand that protective materials resist attack by alkalis. New chemicals in the bleaching process can result in wider swings in the pH and therefore a wider range of resistances in the protective materials. Hydrogen peroxide, for example, results in a lower pH, especially because its use includes the introduction of sulfuric acid. In addition, as temperatures rise with some of the new processing techniques, corrosion rates can rise on the caustic side.

The selection and application of protective barrier materials for the concrete structures in the pulp and paper industry in the future will require close attention to many details, including new technology and processes associated with reduced hazards to the environment.

Changing Approaches To Protecting Concrete

Because the design, protection, and maintenance of concrete has been neglected over the years, heavy costs are now being incurred by the paper industry for the repair and rehabilitation of old bleach plant building structures. Constant chloride ion attack from spills during the bleaching process has severely damaged the concrete and severely corroded its reinforcing steel. The repair and rehabilitation of old concrete structures under plant operating conditions also has resulted in significant increases in the costs of construction and maintenance. With the increased awareness of the high costs of deterioration, steps are now being taken to prevent or slow down corrosion by chemical and chloride ion attack.

For new construction, repair, and restoration of existing structures, some specifiers are detailing and specifying dense, high quality concrete with low-cement ratio and high early strength. They are also requiring the coating of reinforcing steel with adequate concrete cover suitable for the chloride exposure conditions and proper placement, finishing, and curing of the concrete.

For protection, some specifiers are selecting corrosion-resistant products on the basis of chemical exposure, compatibility with the substrate and/or repair materials, application and installation conditions, and previous field service performance histories.

And, because of the broad spectrum of chemical service conditions that prevail in pulp bleaching processes, combinations or composites of corrosion-resistant materials are now considered appropriate in some cases for FRP equipment as well as for concrete floors and containment systems. Some examples of composite or duplex corrosion-resistant systems are given below.

Thermoplastic-lined FRP equipment has gained wide acceptance for sodium hypochlorite and other chemicals where the temperature and pH conditions exceed the upper range of FRP.

Combinations of thermoset organic resin linings with brick/tile or thick inorganic cementitious monolithics have gained acceptance as viable lining systems that avoid the limitations of each when used separately.

Combinations of pre-fabricated FRP or thermoplastic trench liner systems with inorganic cementitious flowable grouts as base materials have proven successful where existing concrete conditions are too rough (large, exposed aggregate) or where the water flow or moisture cannot be controlled for trouble-free installation of bonded lining systems. The importance of substrate material bonding and application conditions are detailed in the next portions of this article. Differences between resin system performance in solid FRP and coatings for concrete in the more aggressive pulp bleaching processes are highlighted.

The Importance of Compatibility between the Substrate and Repair Materials

Poor compatibility with the substrate is one of the leading causes of failure of corrosion-resistant products applied to concrete. Poor compatibility is related to the inability of the protective materials/systems to adhere to the substrate and remain bonded when applied under varying environmental and plant operating service conditions. Key elements that can contribute to poor compatibility and bonding of products are described below.

Low Tensile Strength of Concrete

Compared to steel and heavy-duty chemical-resistant thermoset flooring and lining systems, concrete is very weak in tensile strength. Heavy-duty, thick-film protective barrier systems have higher tensile strengths than concrete and higher coefficients of thermal expansion. As a result, the systems can stress concrete at the bond interface. These stresses can cause a cohesive failure of the concrete during thermal cycles if the protective barrier system is applied at a thickness above the material manufacturer’s recommendations or if service temperature conditions exceed the upper limits of the product’s recommended temperature range.

Furan resin and certain types of vinyl ester resins are examples of high performance thermoset polymers that have been used successfully for solid FRP equipment but have limited success on field applications to concrete as flooring and lining products. Their limited success on concrete is due to the high shrinkage of the resins when they cure, which builds stresses at the interface. Under varying environmental and plant operating service conditions, it is extremely difficult, if not impossible, to control the cure shrinkage and stresses of furan-based resin products.

As noted earlier, composite or duplex corrosion-resistant systems are the best choice where the chemical and temperature exposures exceed the upper design-installation ranges and performance criteria for thermoset flooring and lining systems. Epoxies, particularly the new technology epoxies using aliphatic amine adducts and cycloaliphatic amine curing agents, are examples of thermoset materials that have been used successfully for protection of concrete floors and other structures in pulp bleaching processes. Also, bisphenol fumaric polyesters and certain types of vinyl ester resins have been used successfully in solid FRP and linings/floorings for concrete in bleaching plants.6

Concrete Conditions

The soil underneath on-grade slabs may be contaminated with caustic soda or other chemicals that have passed through the slab. Moisture moving through on-grade slabs and below-grade trenches or sumps by capillary action may carry these contaminants from the soil to a prepared surface, thus hindering the adhesion of a protective material. To resist capillary moisture intrusion and the seepage or flow of water into below-grade trenches under plant operating service conditions or short shutdowns, prefabricated FRP or thermoplastic "slip" liners can be used in conjunction with the repair and corrosion-proof surfacing of floors.

Evaluation of previous service combined with core drill tests and concrete tensile/bond (adhesion) pull-off strength tests7 on existing slabs contaminated with chloride and other chemicals may indicate that the concrete cannot be repaired and should be replaced.

A barrier beneath an existing slab to be repaired may influence the type of repair material and corrosion-proof floor surfacing or lining system to be used. If a significant amount of water is trapped in the concrete, it could cause problems during or after application of flooring and lining products. This concern with water entrapment is why general industry standards call for a minimum of 28 days for curing new concrete prior to coating. This cure time is particularly applicable to polyester and vinyl ester resin-based products that are sensitive to moisture on curing, and therefore, must be applied on dry concrete.

New hybrid epoxies using aliphatic amine adducts and cycloaliphatic amine curing agents in various forms offer advantages of high moisture tolerance and the capability to be installed under inclement conditions. They offer excellent resistance to strong caustics (alkalis), 98 percent sulfuric acid, other acids, and oxidizers.8

Selecting Materials for Application during Plant Operation

Plant operations generally are not shut down for major repairs; thus, applications of products to repair and resurface bleach plant floors are commonly carried out under very adverse environmental and operating conditions. This factor must be considered prior to selection of materials and the start-up of a project.

While polyester and vinyl ester resin-based flooring and lining materials offer the best overall resistance to most pulp bleaching processes and chemicals, they are a poor choice for plant maintenance work. They offer poor compatibility with the concrete when application and cure take place under varying environmental and operating conditions. Thus, they have limited value where application conditions, such as operating temperatures and pH, can vary widely.

Special Considerations forSodium Hypochlorite Service

An area that requires special attention is sodium hypochlorite service. The successful use of FRP and thermoset lining materials in hypochlorite applications is very dependent on operating temperature and chloride level. For best results, operating temperatures should be maintained at or below 120 F (49 C), and pH should be maintained at a minimum of 10.0.9

Heavy-duty flooring systems incorporate aggregate fillers to build thickness, increase durability, and reduce the differences in coefficients of thermal expansion that exist between the flooring material and the concrete substrate. Certain chemicals such as sodium hypochlorite and sodium hydroxide at elevated temperatures not only affect the polymers but also can attack the aggregate fillers used in flooring and lining products. Straight silica aggregates are a poor choice for hypochlorite and high temperature caustic service.

While epoxies, particularly the new hybrid technology systems, offer the advantages of high moisture tolerance and excellent bonding under less than ideal conditions, they have poor resistance to sodium hypochlorite and some of the strong oxidizing chemicals in pulp bleaching processes. Modified amine adduct-cured epoxies have, however, provided excellent service performance in several bleach plant floor repair and resurfacing projects and installations. This trade-off between superior bonding and limited or poor resistance of the top trowel floor surfacing system finish has worked well on several projects.

Successful Use of Monolithic Materials

The use of the term monolithic (joint-free) to describe corrosion-proof product and installation work actually originated over 30 years ago through the development of "monolithic linings" to solve the problem of mortar joints in immersion service in aggressive chemicals. These original linings, still in use today, are resin-based formulations (modified epoxy or polyester) combined with special fillers and chemical hardeners, and are usually reinforced with fiberglass cloth or mat. Applied to concrete or steel surfaces, monolithics form a continuous protective barrier surface that resists attack by a multitude of chemicals.10

While brick and tile have been widely used as materials of construction for protecting concrete floors and vessels against a wide range of chemicals, temperatures, and physical impositions, they are not monolithic materials. The weak point is the mortar joint, which prevents the formation of a continuous protective barrier surface.

Developed especially to meet the severe conditions in the chemical industry, heavy-duty, trowel-applied, reinforced monolithic lining systems are now being used in a wide range of pulp and paper industry flooring and lining applications. Developments in thermoset polymers (vinyl ester and epoxies) over the last decade have been incorporated into flooring and lining products to improve their capabilities to meet the user’s specific operating and field application requirements.

Tables 1, 2, and 3 compare the performance, application, and costs of various corrosion-proofing systems. Cost estimates are based on projected service performance when exposed to aggressive chemicals.

Examples are given below of specific paper mill bleach plant floor repair and resurfacing projects utilizing some of the materials and installation methods described.

Examples of Bleach Plant Floor Repair and Resurfacing Projects and Performance

Three repair projects are described below, based on the author’s experience. It should be noted that all projects involved major floor and concrete building structure repair-rehabilitation work due to chemical and chloride penetration of concrete and chloride ion attack of the reinforcing steel over time.

All projects also involved repairing and restoring the concrete and applying corrosion-proof surfacing materials during plant operations.

In all projects, work began on the top washer operating floor, where the spillage and leakage of chemicals and chlorides started. Chemicals and chlorides then penetrated the building structure over time.

The concrete repair and restoration materials were Type I cement with synthetic chopped fibers; latex-modified structural overlayment; and epoxy repair mortars or grouts. The corrosion-proof flooring system was a glass cloth-reinforced, trowel-applied, modified epoxy flooring system.

Project objectives were to repair the structural damage and extend the useful life of the building through improvements in concrete quality, construction, and use of corrosion-resistant materials. Criteria for selecting a corrosion-proof flooring system included the following:

• resistance to the chemicals;
• ability to be installed under the expected environmental and plant operating service conditions;
• compatibility and capability of bonding to concrete repair materials with short curing times and minimum downtime for the work;
• ability to provide a total monolithic installation to seal as well as protect the floors, thus preventing leakage of chemicals and chlorides through spills into the structures;
• ability to resist mechanical or physical abuse during mill maintenance shutdowns; and
• ease of repair.

Several floors of a bleach plant at a southern paper mill were restored and resurfaced over a three-year period using the concrete repair and corrosion-proof flooring system materials described above.

The work was started on the top washer operating floor level. Additional phases included screen decker floor areas, seal tank floor areas, and ground floor areas between the bleach plant and screen room.

The flooring system installed on the top washer operating floor and other floor levels is in excellent overall condition after approximately 8 years of service. A reinforced polyester or vinyl ester floor topping installed around the ClO2 towers on the ground floor started to fail after less than 1 year of service due to poor bonding to the concrete.

The project is an example of the trade-off between good compatibility (bond ability) and some limited or poor resistance to some chemicals in the pulp bleaching process. Spot maintenance repairs to the modified epoxy flooring system in the areas exposed to constant drippage of sodium hypochlorite or ClO2 represent less than 5 percent of the total surface area covered.

Duplex, modified epoxy/vinyl ester flooring materials are being installed as an improved system for the areas exposed to constant hypochlorite or ClO2 spillage or drippage.

Top Washer Floor Restored and Sealed

The top washer operating floor at a second southern paper mill was restored and completely sealed with the materials described above. Additional bleach plant floors at this mill are scheduled for total replacement in the future due to extreme chloride ion attack and corrosion of the reinforcing steel.

The U-drain trenches on the ground floor around the ClO2 towers at this mill were "slip-lined" with a prefabricated FRP vinyl ester liner with latex-modified grout. The site and plant conditions were considered too difficult for a bonded lining system. Both the corrosion-proof floor system on the top floor and the FRP vinyl ester lining system on the ground floor show excellent performance after 2-3 years of service.

Tile Floors Replaced after Chemicals Saturate Grout

The scope of work and accomplishments at a third southern paper mill were very similar to the first project. The existing tile floors on the top washer levels of the 2 bleach plants were completely removed.

Through time and age, the tile mortar grout and bed were saturated with bleach plant chemicals, and the tiles were loosely bonded to the substrate. After the tile floors and the contaminated mortar bed and concrete were removed, the floors were restored with a combination of latex-modified structural overlayment materials and Type I cement with chopped fibers. The choice of repair materials depended on the size and the depth or thickness of the repairs. The floors were then surfaced with a glass cloth-reinforced modified epoxy flooring system to achieve monolithic installation and positive containment of spills.

Some of the U-drain trenches on the ground floor of the bleach plant at this mill will be lined with a prefabricated thermoplastic slip liner system in the future. The site and plant conditions are considered too difficult for a bonded lining system.

This project is a good example of why old tile floor materials and methods (grout bed without membrane) do not work and should not be considered for pulp bleaching process floors or other aggressive chemicals service areas. In this case, after the mortar grout and bed failed through time and age, the chemicals and chlorides penetrated the concrete, eventually causing extensive damage and costly repairs to the building structures.

The ongoing damage to the floors and the building structure from chloride ion attack was concealed by the tile floor covering. One of the major benefits of monolithic flooring systems is that it has no joints. In addition, because the monolithic has no floor covering, if the flooring system starts to fail, you can see the damage and fix it.

Conclusion

In summary, the selection and successful application of protective barrier materials for concrete requires close attention to many details of chemical exposure conditions, expected field conditions on applications of products, surface preparation, monolithic installation, and compatibility of products with concrete substrate or repair materials.

There is no universal product that will handle the multitude of pulp bleaching processes, chemicals, and corrosion problems. Some of the lessons learned from the use of FRP process equipment in pulp beaching apply to the selection and application of lining and flooring materials to concrete, while in other cases, the performance of FRP tells little about the materials suitable to protect concrete. Combinations of products or duplex corrosion-proofing should be considered when the chemicals or temperatures exceed the limits of single products.

In reference to pulp bleaching processes, particularly the top washer operating floors, monolithic design and installation of flooring system materials is essential to prevent penetration of chemicals and chloride ion attack of floors and concrete structures. The goal should be to seal as well as protect concrete floors and containment areas to prevent severe deterioration and expensive repairs.