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Corrosion Control
Protection and Finishes for Concrete Floors and Walls in Food Processing Facilities By: Tom Murphy, General Polymers Abstract This article focuses on the design and selection of protective flooring and wall systems for food and beverage facilities. Concrete is the most commonly used building material in the construction industry. Food processing frequently presents a corrosive environment causing concrete to either erode leading to structural compromise or allow the contaminants to pass through, potentially contaminating the soil and groundwater. Protective coatings can be designed to provide flooring protection, enhanced physical properties, and chemical resistance to potential contaminants and corrosive agents. The design of a protective flooring system for concrete, requires an understanding of the existing concrete, the operating environment, and the conditions of installation and use. In order to successfully protect the concrete substrate and prevent facility damage, risk to personal health and legal liability, several factors must be taken into consideration. These include providing a sound substrate, selecting the best chemistry for chemical resistance, repairing surface imperfections, addressing cracks and joints, and preparing the surface to receive the system needed. Consideration of system design for food processing facilities is discussed including thickness, texture, reinforcement and aesthetics. Typical food and beverage processing facilities present some of the most aggressive environments for floors and walls. Protection of the concrete and maintenance of a clean facility is the focus of this article. Concrete is the most commonly used building material in the construction of food processing facilities. These facilities have a variety of extreme exposures such as, rapid temperature changes during steam or hot water washdowns; exposures of natural food acids, such as lactic acid (milk), citric acid, acetic acid, and others; plus constant traffic from foot, carts, and forklifts. Coatings are available to provide protection of the concrete floors and walls, while enhancing the physical properties, and chemical resistance to potential contaminants and corrosive agents. The selection of a protective flooring or wall system for concrete, requires an understanding of the existing concrete, the operating environment, and the conditions of installation and use.
Surface preparation is the single most important factor in the successful installation of a protective coating, lining or mortar system. The area to be protected must be inspected for damage, weak surfaces, cracks, spalls and surface contaminants. Oils, grease, waxes, chemicals and other surface contaminants must be removed prior to preparing the surface. Frequently this is accomplished using trisodium phosphate (TSP) or other detergent scrubbing, low-pressure water cleaning (less than 5,000 psi), steam cleaning, or chemical cleaning. In situations where concrete has been saturated with grease or oil, such as existing chicken processing facilities, the top 1 –2 inches of concrete may need to be removed. All concrete surfaces must be mechanically abraded to remove the weakest and upper most layer of the concrete containing laitance and curing compounds. Existing coatings must be removed to insure a secure bond to the substrate. All damaged areas must be removed and replaced. Polymer modified concrete or epoxy repair mortars are the best materials for repair providing superior strength and curing in relatively short periods of time. All cracks must be treated and repaired. In areas where cracks or control joints are moving due to vibration or temperature variation, a flexible epoxy is used to buffer this movement while providing an integral bond. Surfaces with these “active” cracks should receive a crack bridging membrane as part of the installation system. Most chemical resistant polymeric materials are extremely hard and will not maintain a seamless surface over moving cracks. Expansion joints are engineered into a structure to provide continuous movement. These joints must be honored through the protective surface treatment and filled with a chemical resistant joint compound to meet the exposure conditions. Moisture Vapor (MV) Transmission Concrete is a porous substrate, which contains water and allows moisture in the form of water vapor to migrate from below the slab to the area above the slab, depending upon the temperature and humidity. An impermeable protective coating applied to the concrete not only prevents the migration of chemicals in and through the concrete it also prevents any moisture from traveling out of the concrete. Although moisture vapor alone does not have the force necessary to disbond a resinous flooring system from concrete, it does carry ions contained within the slab to the surface which through a crystallization process will force the system to blister. There are several steps that can be taken to prevent this problem. Prior to installation of the concrete slab, a moisture vapor barrier should be installed directly under the concrete, in accordance with ACI 302.1. The slab edges and foundation drainage must be detailed to prevent water access to the sides of the concrete. Minimizing the amount of salts and water in the concrete mix will reduce the porosity and the amount of water residing within the concrete. Finally, wet curing the concrete will maximize paste formation and minimize porosity during cure. If the slab is already in-place and exhibits a moisture vapor emission problem, as measured using ASTM E 1907 calcium chloride quantitative test kits, a surface applied remediation system is recommended. Recover 9000 is one such system which incorporates a specifically formulated primer to transform Calcium Hydroxide into Calcium-Silicate-Hydrate (C-S-H) densifying the surface and restricting ion movement, followed by a polymer modified slurry to further decrease surface permeability. Failure to measure and treat MV may ultimately lead to the failure of the protective flooring system resulting in manufacturing downtime and excess repair costs. Care must be taken to install the flooring system at use conditions. Facilities which will operate at elevated temperature due to heat generated in the process can experience moisture vapor drive if the floor was installed at lower temperatures. Selection of the System Chemistry Selecting the resin system chemistry to meet the corrosive conditions present will require an analysis of the specific chemicals potentially in contact with the flooring system. This includes testing for combinations of chemicals that may be exposed together at the surface. Vinyl ester and vinyl ester novolac systems have traditionally been used in highly acidic conditions. The advancement of epoxy novolac chemistry has allowed for these materials to be used in this environment as well, thus avoiding the limitations of moisture sensitivity, shrinkage and the pungent odor of the styrene based vinyl ester systems. The temperature and duration of chemical exposure is important in determining the ultimate permeability requirements of the system. Glass flake or graphite flake filled systems dramatically decrease the permeability and increase the service temperature within which these systems can be used. In general, flake filled systems are recommended for continuously wet or immersion conditions. Selecting the system to be applied for concrete protection not only requires a thorough understanding of the chemical resistance required, but is also driven by the conditions at which it will be installed. The most economical system to install is a system that cures at room temperature, has zero volatile organic content (VOC) and has low odor. In the real world, however, areas requiring protection may be in direct sunlight, underground in cool damp conditions or simply must be completed in a short window of time. These conditions will affect the chemistry recommended for the application. Epoxies and novolac chemistries are available in low odor, zero VOC and can cure at temperatures ranging from (35° to 150 °F). Vinyl ester, polyester, vinyl ester novolac, urethanes and methyl methacrylate are adversely affected in high moisture conditions. Urethane technologies have good UV stability but do not bond as well as epoxies. Polyurea chemistry can be extremely fast but has a history of poor adhesion. The best systems are frequently designed to take advantage of the benefits of each chemistry. For example, an exterior system can be based on a novolac epoxy for chemical resistance and a polyurethane enamel topcoat for UV stability. Selecting the best chemistry for the corrosion control application should be done in consultation with the manufacture. Test coupons should be provided for specific chemical resistance tests and the manufacturer’s experience at similar situations will help to identify the best solution. Thermal Shock After determining the most appropriate chemistry from which to build the protective flooring, the physical considerations must be addressed. Surface thickness will be driven by the need to provide thermal shock protection, resurfacing requirements and the traffic conditions expected. In general, chemical resistant systems are resin rich and utilize aggregate to build thickness, decrease the Coefficient of Linear Thermal Expansion (CLTE) of the system, provide for abrasion resistance, and to provide for conductive properties where required. As mentioned earlier, glass flake, mica, and graphite flake fillers are used primarily to reduce permeation, but have the side benefit of reducing the CLTE and provide for a marginal increase in the flexural and tensile strengths of the resin systems. In order to provide for dramatic additional flexural and tensile strength increase to resist shrinkage stress of the resin systems (particularly vinyl ester and polyester resins), thermal stress of the environment, and structural stress from substrate movement, these systems are typically laminated with fiber reinforcing fabrics. These fabrics are traditionally made up of fiberglass, nylon, and polyester synthetics to produce structural chopped strand mats, woven roving fabrics, open weave scrim cloths, and light weight veils. The chopped strand mats and woven roving fabrics are typically used as foundation layers for laminate resin systems or laminated into mortar applications. Surface veils are used on top of the resin saturated chopped strand mats and woven roving or on top of mortars to provide for a smoother less permeable surface. In situations where plant personnel use live steam or very hot water to wash down, special considerations must be given to the CLTE of the system and the flexibility of the bond interface to concrete. Recently, prolonged (12-24 hour) elevated heat (130F-150F) has been used for pest control due to concerns regarding the environmental effects of methyl bromide. These rapid rise and decline in temperatures can potentially create a problem between any two bonded materials with significantly different rates of thermal expansion. Resinous flooring will attempt to expand at ten times the rate of concrete or steel. This is also true of a resinous coating on a block or concrete wall. System failure will result in blistering and disbondment. In order to minimize the detrimental effects of this treatment flooring systems should be at least ¼” thick and using a flexible membrane beneath the flooring system will help insulate the temperature differential at the bond line. Wall system can also be constructed using flexible epoxies. Impact and Abrasion Resistance The type, frequency, and duration of traffic in the food processing facility will help determine the selection of thickness, texture, aggregates and reinforcement required of the protective flooring. For example, in secondary containment areas, where only light pedestrian foot traffic is expected, a protective coating system that is designed for chemical and atmospheric exposure will serve the user well. Areas within a bottling facility or food processing area may experience constant vehicular traffic from forklifts and foot traffic. These areas will require thicker applications, textured surfaces, abrasion resistant aggregates and possibly fabric reinforcement. These areas and floors exposed to impact, heavy loads and vibrations will benefit from the use of a flexible membrane installed under the flooring system. Special Considerations The final step in system selection is to address special conditions and requirements. These may include the need for a conductive system, textured surface, slope, details around drains and trenches, cove base details and details of transitions to other surfaces. The ultimate success of installation is dependent upon the proper terminations. Protective flooring systems are designed specifically for service conditions and chemical exposure performance. These resin components are not usually designed for high gloss, light colors, or color stability. Systems may be subject to staining (without degrading overall performance) from some chemicals, and chalking from UV exposure. The result may be a completely functional, albeit mottled looking, chemical resistant flooring system. If aesthetics are an important issue, sacrificial topcoats of less chemically resistant resins may be installed. These topcoats may not provide the necessary protection for long term exposure or even splash and spill situations, but can easily be replaced on a routine schedule or after an exposure incident. The underlying protective flooring system will continue to provide the functional performance required regardless of the topcoat selection. Wall Systems In food and beverage processing facilities frequent hot water or steam cleaning maintenance procedures are conducted. This environment not only requires a flooring system to handle the chemical exposure, thermal shock and constant traffic, but also require a wall system that will handle the wet environment without creating seams and uncleanable areas at floor to wall transitions. Ideally, the walls will be coated with a seamless epoxy or urethane material to prevent moisture migration into the wall. Standard epoxies provide excellent barriers but if presented with thermal shock environments must be reinforced with fiberglass cloth. A flexible epoxy as a base material may be used as a substitute providing a moisture barrier, thermal shock resilience and impact resistance wall coating. This product is topcoated with a urethane to insure long lasting color stability. The transition to the floor is typically handled using a cove base followed by a featheredge under the wall system. This detail sequence results in a thin mil edge facing down leaving no shelf to collect dirt and potential microorganisms. Installation Professional installation of the high performance system is as important as the design of the system itself. When selecting a specialty contractor, check with the material manufacturer for recommendations of certified installers. Obtain a reference list of existing installations of like-kind systems. Discuss the schedule and coordinate mobilizations with available areas to be installed. Understand the contractor's ability to provide service both immediately after the installation and in the years to come. These installations are generally not inexpensive, but the cost of failure is greater. Don’t be tempted to accept a lower cost system/installation after you have done your homework with respect to what is really needed. Maintenance and Warranty After the system has been installed, all inspections are completed and the job is accepted, obtain the warranty. Work with the manufacturer and installer to determine how the area will be maintained and inspected on an on-going basis. Maintaining the integrity of the system by repairing damage is a necessary part of preserving the warranty, and more importantly, peak performance. Depending upon the exposure, wear and use, all systems will require periodic upkeep. Summary Concrete requires protection in food processing environments. Polymeric coatings can be used to provide corrosion protection and sanitary conditions when selected and applied correctly. In order to successfully protect the concrete substrate and prevent facility damage, risk to personal health and legal liability, several factors must be taken into consideration. These include the following:
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