Concrete is the most widely used building material worldwide because it has many unique properties that make it ideal for use in construction. Concrete is formable and is naturally resistant to UV, mold and insects. Concrete is noncombustible and is relatively inexpensive relative to other building materials.
Why Coat Concrete?
There are several reasons to coat concrete structures. There are coatings that are designed to help keep the moisture in the concrete during the concrete cure, as well as coatings to prevent moisture intrusion once in service. Maintaining the moisture content during the cure process assists in the development of the structural properties of the concrete. Coating types typically used during the curing of concrete include water-based styrene acrylic or acrylic emulsions. Additionally, surface applied coatings are beneficial for cured concrete. Although concrete is very durable under UV light and potable water, cured concrete is subject to several types of chemical attack that can degrade the surface or cause loss of structural integrity. The prevailing service environment, the expectation of the owner for the life-cycle of the structure and the aesthetics are all key parameters in identifying candidate coating systems. Concrete bridges may be constructed in areas of high visibility and aesthetic properties such as color and gloss retention are expected to be factored into functional coating performance characteristics. This article is specific to coatings intended to provide a barrier from the environment as a means of protecting concrete bridge structures.
Service Conditions, Owner Requirements, and Aesthetics
In addition to the type of concrete structure, (in this case bridges), the purpose or function and location of the concrete structure are the major factors in selecting the appropriate coating systems. The type of service environment (location) is a key parameter to establishing coating performance criteria. The temperature range, humidity, and contaminants found in the surrounding environment are all considerations. Concrete is a porous material and can absorb water, sodium chloride, carbon dioxide, acid precipitation, and other chemicals, in liquid or gas states from the surrounding environment that cause concrete degradation. The service environment may also produce mechanical damage such as impact, wear, and erosion.
Performance Characteristics and Challenges
Concrete bridges are subjected to a wide range of exposure conditions dependent upon their location, and the specific corrosivity category of the structure. Corrosivity categories can include atmospheric, water, and soil. ISO 12944-8 describes protective paint systems that are to be used on non-ferrous metals or concrete, to define suitable specifications. The general classification system is per ISO 12944 as excerpted in the table below:
There are several corrosivity categories to consider for the various elements of bridge structures. For example, the piers may be in a water or soil corrosivity category, or in an atmospheric corrosivity category. There may also be special system corrosivity categories such as splash zone areas. As such, there are challenges related to the selection of coatings for these structures.
Water can penetrate naturally through the capillary pore structures of concrete. Carbon dioxide reacts with calcium hydroxide in the pore liquid of the cement matrix and deposits as calcium carbonate, which results in carbonated concrete. Chlorides come from both de-icing salts that may be used in winter, or from salt water in marine environments. The expansion of free water in the capillary pores of concrete during freezing conditions result in internal stress. Concrete can spall and crack in areas that become carbonated, where there is high chloride content in contact with steel reinforcing bars, or due to the stresses associated with temperature cycling. A coating’s ability to prevent water, carbon dioxide, and chloride ingress is necessary for adequate protection of bridge structures. In addition to water resistance, staining resistance from water that has puddled is also needed to prevent discoloration. Coating products intended for concrete structures should also be blush resistant, which is often accomplished by keeping the particle size small, using specifically formulated polymerizable surfactants and keeping the level of other hydrophilic materials (such as surfactants) low.
Concrete Coating Systems
Concrete coating systems generally consist of concrete repair compounds, surface-applied coatings, and sealants. Since this article primarily addresses coatings intended to provide barrier protection on concrete bridge structures, concrete repair products will only be briefly described, and the performance of concrete repair compounds will not be discussed.
Concrete repair compounds – Mortars and bonding primers are used to prevent further corrosion of steel reinforcement and to repair areas of damaged or degraded concrete. Products include cementitious polymer-modified and epoxy-based products.
Surface-applied coatings – Typical surface-applied products include acrylics, epoxies, polyurethanes, polysiloxanes, and silanes.
Acrylics – Acrylic coatings have a great degree of variability due to the ability to formulate products with a high or low molecular mass. Higher molecular weight products are favorable for durability, while lower molecular weight materials favor cracking resistance and flexibility. For example, the harder acrylic polymers have better resistance to carbon dioxide infusion, but the softer acrylic polymers have better resistance to the effects of thermal cycling. The flexibility of a coating system is important for protection of concrete because concrete is expected to have some movement and cracking. However too much flexibility could mask excessive movement of cracking.
Epoxies – Epoxy coatings have a long history of use on concrete structures. Epoxy resins are often modified with reactive diluents to improve flow and viscosity for ease of application and improved wetting of the surface. Epoxy coatings can also be formulated to a desired range of flexibility as with the acrylic coatings. Epoxies can chalk on exterior exposure, and typically used with a topcoat that is resistant to solar radiation (sunlight).
Polyurethanes – Polyurethane formulations can be modified both chemically and mechanically to incorporate desired properties. Polyurethanes can be formulated with aliphatic or aromatic isocyanates. Formulations that include aliphatic isocyanates produce color-stable products while aromatic formulations do not. However, aromatic formulations result in products that have better chemical resistance than the aliphatic counterparts. The other main constituent in polyurethane formulations is polyols. Common polyols are polyester, polyether, polycarbonate, and polyacrylate. The choice of polyol, and the associated molecular weight of the polyol affect the performance properties of the product significantly. Additionally, by modifying a polyurethane resin a fluorinated polyurethane can be produced that provides excellent UV light resistance and stability, maintaining long-term color and gloss retention far beyond a traditional polyurethane.
Polysiloxanes and silanes – The polysiloxanes and silanes are silicon resins that are highly cross-linked and typically inert chemically and physically. The products are typically promoted for water-repellency; however, they are generally more susceptible to alkaline conditions. Modifying the resin has lead to minimizing the effects of alkali for some formulations.
Sealants – Polymeric sealants can help prevent the chemical or physical degradation of concrete. Sealers are typically described as penetrating or film-forming and are different from cure sealants that are used during cure of the concrete to maintain moisture.
Silanes or Siloxanes are penetrating sealers. Benefits of use include protecting the concrete from liquid water and surface contaminants. They do not prevent water vapor loss of concrete during cure. The silanes have a low molecular weight while the siloxanes are pre-reacted and have a higher molecular weight. Because of the lower molecular weight, the silanes offer greater penetration into the concrete pores than the siloxanes. Since silanes penetrate beneath the surface, they are more abrasion resistant than the aqueous sealers that form a film on the surface.
Acrylic or styrene acrylic aqueous emulsions are typically used as sealants and prevent ingress of salts and moisture that can lead to degradation. They typically have good resistance to water but are breathable and allow vapor to pass through the film. These water-based sealers are typically designed to be applied at low dry film thicknesses and they are typically not high gloss finishes – usually satin or semi-gloss. Since the films are relatively thin and largely on the surface of the concrete, they usually need to be reapplied periodically to maintain protection.
Solvent-based acrylics are usually applied after the concrete has aged for a 30-days. These sealers are durable because of the acrylic or vinyl-ester backbone, and still low enough molecular weight to penetrate the concrete pores and provide a darkening of the surface (called a wet look), which can be desirable. They are applied at higher film thicknesses than the acrylic emulsions and can yield a high gloss once dry. These acrylics can be formulated with exempt solvents like acetone; however, the rapid drying and odor can pose significant challenges during application.
Polymer modified concrete mortar is also used as a thin overlayment to reduce the surface porosity of concrete. The polymeric admixes function by reducing the amount of water necessary for a given concrete fluidity, resulting in a lower water to cement ratio. In addition, as the concrete dries the polymer forms films in the pores and blocks the capillaries. The slower cure of the concrete and the dried polymer in the matrix results in reduced porosity. While the reduced porosity of the overlayment helps reduce the penetration of water carrying chloride ions, the polymer can also improve durability by increasing the tensile strength, resistance to abrasion and dusting.
Two-part epoxies are generally used to seal concrete in environments where a high level of chemical resistance is needed. These can be cured with polyamines, polyamides or polysulfides; although the polyamide-cured epoxies typically have lower chemical resistance. The epoxies have good adhesion to the concrete and can be formulated to cure at low temperatures.
Testing Concrete Coating System Performance
Coating performance on a concrete bridge structure cannot be predicted, as the variability in the substrate itself, the type of coating and the formulation can all influence the expected performance. Rather, performance testing is frequently required to determine acceptability for use.
The only global standard that covers the performance characteristics and key criteria for the selection of protective coating products for use on concrete is EN1504-2. This European standard outlines the performance characteristics that provide protection against ingress, moisture control, and provide physical and chemical resistance. The standard includes performance characteristics that are intended for all concrete structures and is not specific to bridge structures.
The American Association of State Highway and Transportation Officials (AASHTO) National Transportation Product Evaluation Program (NTPEP) for the Evaluation of Concrete Coating Systems (CCS) is intended to evaluate coatings for potential use on bridges (except bridge decks), walls, barriers, similar structural concrete, and other masonry surfaces, both new and existing, prepared by abrasive blast cleaning or high-pressure water cleaning. Tested concrete coatings are intended to enhance durability, and/or aesthetics of concrete structures that are subject to degrading atmospheric exposure, such as marine, industrial, deicing chemicals, and high humidity.
The performance tests included in this program were selected by a Technical Panel consisting of several state Department of Transportation (DOT) representatives, coating manufacturer’s, and members of the AASHTO NTPEP council. The tests were selected to specifically address the performance expectations for bridge coatings. The program addresses six performance characteristics, including: chloride ion penetration, moisture vapor transmission, weatherability, resistance to thermal cycling, crack-bridging (optional; not discussed herein) and graffiti resistance. The following test descriptions outline the procedures that are used to evaluate the associated performance characteristics:
Chloride Ion Penetration Test (AASHTO T259 – Standard Method of Testing for Resistance of Concrete to Chloride Ion Penetration). Protective coatings enhance the durability of concrete structures by providing resistance to penetration of chloride ions into the concrete matrix. This property is tested through saline ponding (3% chloride solution) of concrete slabs for 90 days and measuring chloride ingress at four depths, as shown in Figure 1.
Moisture Vapor Transmission: This test evaluates the relative ability of a coating to pass moisture from within the concrete to the outside, as depicted in Figure 2. Approximately 24 hours after coating application to the test cubes, individual initial weight (W0) of the coated and uncoated test cubes is determined to the nearest 0.1 g. The test cubes are maintained at 25 ± 2ºC and 50 ± 5% RH, minimizing the effects of air currents, for 14 days. Each cube is weighed at 7 days (W7) and 14 days (W14). The moisture vapor transmission is determined as the weight loss between the 7th and 14th day of drying for both the coated and uncoated cubes using the formula:
VT = (W7 – W14) / (168 h x 0.062 m2), in g/ (m2 x h)
Weatherability: ASTM D4587, “Standard Practice for Fluorescent UV-Condensation Exposures of Paint and Related Coatings,” is the test standard employed to evaluate weatherability. Tests are performed on mortar test panels placed in a QUV cabinet (UV light/condensing humidity cycle) for 2500 hours (approximately 15 weeks) total test time. Coating durability is evaluated by resistance to blistering, coating thickness erosion, and adhesion loss, and gloss and color retention. In addition, pores and bug holes frequently filled with a repair compound (prior to coating) must be compatible with the coating system and must be resistant to the internal stresses created by weathering. Figure 3 represents the panel used for weatherability testing as well as the evaluation of the adhesion of the coating system to the repair compound following the weatherability test.
Additionally, coatings are evaluated using the same panels shown in Figure 3 for resistance to transmission of efflorescence in accordance with ASTM D7072, degree of dirt pickup in accordance with ASTM D3719, and fungal resistance in accordance with ASTM D3273.
Resistance to Freeze/Thaw Cycling and Adhesion Strength: Coated mortar test panels are exposed to freeze/thaw resistance testing in accordance with AASHTO T 161, Procedure A for 300 cycles. Prior to cycling, adhesion tests are performed on coated panels both with and without intentional defects in accordance with ASTM D7234, Standard Test Method for Pull-Off Strength of Coatings on Concrete Using Portable Pull-off Adhesion Testers. Adhesion is reassessed at 50, 150 and 300 cycles.
Overcoatability/Graffiti Resistance: Resistance to graffiti and the ability to overcoat tagged coating systems is evaluated by subjecting the weathered coated test panels to aerosol spray paint and permanent marker. After 7 days drying of the spray paint and marker, a topcoat of the test system is applied to hide the simulated graffiti. After a 30-day curing period, adhesion testing is conducted on the overcoated area per ASTM D 7234. Hiding power of the coating is also evaluated per ASTM D 2805.
Protective coating systems will not infinitely protect concrete bridges, but if properly formulated and applied will delay substrate degradation and extend the useful life of the bridge, provided the coating system is maintained. Not all coating systems are created equally. Coating performance testing provides a mechanism for evaluating coating system performance prior to installation to reduce the probability of performance issues in a demanding environment.
 Standard Practice for Evaluating Accelerated Efflorescence of Latex Coatings
 Standard Test Method for Quantifying Dirt Collection on Coated Exterior Panels
 Standard Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber
 Standard Test Method for Hiding Power of Paints by Reflectometry
About the Author:
Robert Leggat is the KTA Laboratory Services Manager and has over 15 years of experience in the protective coatings industry. He holds a PhD in Materials Science and Engineering from the University of Virginia and successfully completed the KTA Level I Basic Coatings Inspection training course. Mr. Leggat joined KTA in August 2016 as the Laboratory Services Manager overseeing the operations of the Analytical and Physical Testing Laboratories. In this position, he oversees all laboratory services which include paint, corrosion and material testing services, coating failure investigations, coatings research, and compositional analysis. Under his oversight, senior chemists, chemists, and research and development specialists provide clients with independent, accurate analyses of coating problems and advance the industry’s understanding of the performance characteristics of protective coatings and abrasive media. Prior to joining KTA, Mr. Leggat held various senior research and technical management positions with the United States Steel Corporation in Pittsburgh, PA.