Corrosion Control of Concrete Surfaces

By: Tom Murphy and Bob Johnson, General Polymers

Abstract This article focuses on the design and selection of a protective barrier system for concrete surfaces. Concrete is the most commonly used building material in the construction industry. In a corrosive environment concrete will either erode leading to structural compromise or allow the contaminants to pass through, potentially contaminating the soil and groundwater. Protective coatings and linings can be designed to provide barrier protection, enhanced physical properties, and chemical resistance to potential contaminants and corrosive agents. The design of a protective barrier 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 and application, repair surface imperfections, address cracks and joints and prepare the surface to receive the system needed. Consideration of system design is discussed including thickness, texture, reinforcement and aesthetics.

Concrete is the most commonly used building material in the construction industry. Throughout the years it has proven its value as a low cost, high strength, structural building material for a variety of applications. Concrete mix design modification, to include the use of both chemical pozzolanic admixtures, has been used to improve the properties of the plastic concrete mix and the hardened concrete. A properly engineered mix design can offer control of workability or slump, ease of placement and finishing, control the time of initial and final set, and define the important hardened properties of compressive, flexural and tensile strength, density, and permeability. Traditional concrete composed of graded fine and coarse aggregate, cement, and water remains a porous matrix and is susceptible to degradation by acids, alkalis, organic solvents and oils. In a corrosive environment concrete will either erode leading to structural compromise or allow the contaminants to pass through, potentially contaminating the soil and groundwater. Current Environmental Protection Agency (EPA) and other governmental dictum impose severe penalties to corporations and individuals that neglect their responsibility to environmental protection.

It is not necessary to substitute more expensive building materials in place of concrete in areas where concrete alone is exposed to corrosive agents. Protective coatings and linings can be designed to provide barrier protection, enhanced physical properties, and chemical resistance to potential contaminants and corrosive agents. The design of a protective barrier system for concrete, requires an understanding of the existing concrete, the operating environment, and the conditions of installation and use.

  • What is the existing condition of the concrete, (cracks, spalls, degradation, slope, contamination, etc.)?
  • Is there a vapor barrier below the on-grade concrete installed in accordance with ACI 302.1?
  • What are the conditions during which the protective surface will be installed?
  • What chemicals are present?
  • What is the duration of actual and potential exposure(splash and spill, 72 hour, or immersion)?
  • At what temperature will the chemicals be during exposure?
  • Will the area be exposed to rapidly changing temperature extremes (thermal shock)?
  • What physical stress will be applied to the area relative to traffic, impact and abrasion?
  • Are there any special considerations required for the area (i.e. static charge dissipation, texture, etc)?
  • What maintenance procedures will be followed?
Existing Surface Condition

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 TSP or other detergent scrubbing, low pressure water cleaning (less than 5,000 psi), steam cleaning, or chemical cleaning. 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 is the best materials for repair as they provide superior strength and cure 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 such as a polysulfide or fluoroelastomer.

Moisture Vapor (MV) Transmission n

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 (Dew Point). 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 polymer from concrete, it does carry ions contained within the slab to the surface which through a crystallization process will force the coating off the concrete. 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. 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 already exists and has a moisture vapor emission problem, as measured using ASTM E 1907 & F1869 calcium chloride quantitative test kits, a surface applied remediation system can be applied. Recover 9000 is one such system which incorporates a 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 barrier system.

Affects of Moisture and Temperature During Cure

Selecting the resin system chemistry to meet the corrosive conditions present will require an analysis of the specific chemicals potentially in contact with the barrier 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 these styrene based 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.

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. Ideally, a system that cures at room temperature, is 100% solids, has zero volatile organic content (VOC) and has low odor, will be the most economical system to install. In the real world, however, areas requiring protection may be outside 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 and methyl methacrylate chemistries do not cure well in high moisture conditions. Urethane and polyurea technologies have good UV stability but have relatively poor adhesion properties. 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 but uses a urethane topcoat for UV stability. Selecting the best chemistry for the corrosion control application should be done in consultation with the manufacture. Test coupons can 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 barrier, 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, corrosion control 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 providing 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 topping mortars to provide for a smoother less permeable surface.

To protect the system from movement of the concrete substrate itself, a flexible epoxy membrane is recommended under the system. The open weave scrim is placed on top of the membrane to serve as a hinge point for absorbing crack and surface movement.

Impact and Abrasion Resistance

The type, frequency, and duration of traffic expected on the protective system provide information necessary for the selection of thickness, texture, aggregates and reinforcement. For example, in secondary containment areas, where only light pedestrian foot traffic is expected, a protective coating system that is designed for chemical resistance, texture and atmospheric exposure will serve the user well. Areas within a production facility or food processing area may experience constant vehicular traffic from forklifts and heavy foot traffic. These areas will require thicker applications, textured surfaces, abrasion resistant aggregates and possibly fabric reinforcement. 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. These situations generally call for a flexible membrane component of the system, fabric reinforcement, maximum thickness, and a textured surface.

Special Considerations

The final step in system design is to address special conditions and requirements. These may include the need for a conductive system, textured surface, slope and fill, 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 barrier systems are designed for service conditions and chemical exposure performance. The resin components are not designed for high gloss, light colors, or color stability. These resins are 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 barrier 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 barrier system will continue to provide the functional performance required regardless of the topcoat selection.

Installation

Professional installation of the corrosion control 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. Depending upon the exposure, wear and use, all systems will require periodic upkeep.

Summary

Concrete requires protection in some environments. Polymeric coatings can be used to provide corrosion protection 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:

  • Provide a sound substrate
  • Select the best chemistry for chemical resistance and application
  • Repair surface imperfections
  • Address cracks and joints
  • Prepare the surface to receive the system needed
During the selection process consult with the coating manufacturer for recommendations and assistance in testing various conditions. Take the time to select the best contractor to install the system and understand the long-term service to be provided. After the system is in place, maintain the system to the original selection criteria and keep on-going records of repair and exposure incidences.