Concrete is the most widely used building material in the world because it has many unique properties that make it an excellent choice for construction. Concrete is formable and naturally resistant to UV rays, mold and insects. Concrete is noncombustible and relatively inexpensive compared to other building materials.
Why paint concrete?
There are several reasons for coating concrete structures. There are coatings that help retain moisture in concrete while it cures, and there are coatings that prevent moisture intrusion. Maintaining the moisture content during curing contributes to the development of the structural properties of the concrete. The types of coatings commonly used during concrete curing include water-based styrene acrylics or acrylic emulsions. In addition, the surface-applied coating benefits the 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 a loss of structural integrity. Prevailing service environments, owner expectations for structural life cycle and aesthetics are key parameters in identifying candidate coating systems. Concrete bridges can be constructed in high visibility areas, and aesthetic properties such as color and gloss retention are expected to factor into functional coating performance characteristics. This article specifically addresses coatings designed to provide an environmental barrier as a means of protecting concrete bridge structures.
In addition to the type of concrete structure (in this case a bridge), the purpose or function and location of the concrete structure are major factors in choosing an appropriate coating system. The type of service environment (location) is a key parameter in establishing coating performance criteria. Temperature ranges, humidity, and pollutants in the surrounding environment are all considerations. Concrete is a porous material that can absorb water, sodium chloride, carbon dioxide, acid deposits, and other chemicals from the surrounding environment, causing concrete to degrade. The service environment can also generate mechanical damage such as impact, abrasion and corrosion.
Concrete bridges are subject to a variety of exposure conditions, depending on their location and the specific corrosivity category of the structure. Corrosive categories can include atmosphere, water and soil. ISO 12944-8 describes protective paint systems for use on non-ferrous metals or concrete to determine suitable specifications. The general classification system complies with ISO 12944 and is shown in the table below:
There are several corrosivity categories to consider for various elements of bridge structures. For example, a bridge pier might be in the water or soil corrosive category, or in the atmospheric corrosive category. There may also be special system corrosive categories such as splash zones. Thus, there are challenges associated with coating selection for these structures.
Water can naturally permeate through the capillary structure of concrete. Carbon dioxide reacts with calcium hydroxide in the pore liquid of the cement matrix and deposits calcium carbonate, which results in carbonated concrete. Chlorides come from de-icing salts that can be used in winter, or from salt water in marine environments. Under freezing conditions, the expansion of free water in the pores of concrete causes internal stresses. Concrete can spall and crack in areas of carbonation, where there is a high chloride content, where it is in contact with steel reinforcement, or due to stresses associated with temperature cycling. The ability of the coating to prevent the ingress of water, carbon dioxide and chlorides is necessary to adequately protect the bridge structure. In addition to water resistance, stain resistance from water with agitation is also required to prevent discoloration. Coating products for concrete structures should also be anti-blushing, which is usually achieved by using specially formulated polymerizable surfactants to keep the particle size small and keeping the content of other hydrophilic materials (such as surfactants) low .
Concrete Coating System
Concrete coating systems typically consist of concrete repair compounds, surface coatings and sealers. Since this article is primarily concerned with coatings intended to provide barrier protection to concrete bridge structures, concrete repair products are briefly described and the properties of concrete repair compounds are not discussed.
Concrete Repair Compounds - Mortars and bonding primers are used to prevent further corrosion of reinforcement and to repair areas of damaged or degraded concrete. Products include cementitious polymer modifications and epoxy-based products.
Surface Coatings - Typical surface coating products include acrylics, epoxies, polyurethanes, polysiloxanes and silanes.
Acrylic - Due to the ability to formulate products with high or low molecular weight, acrylic paints have a large degree of variability. Higher molecular weight products are good for durability, while lower molecular weight materials are good for crack resistance and flexibility. For example, harder acrylic polymers are more resistant to carbon dioxide infusion, but softer acrylic polymers are more resistant to thermal cycling. The flexibility of the coating system is important to protect the concrete where some movement and cracking is expected. However, too much flexibility may mask excessive crack movement.
Epoxy - 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 surface wetting. Like acrylic paints, epoxy paints can be formulated to the desired range of flexibility. Epoxies can chalk on exterior exposure and are often used with topcoats that are resistant to solar radiation (sunlight).
Polyurethane - Polyurethane formulations can be modified chemically and mechanically to combine desired properties. Polyurethanes can be formulated with aliphatic or aromatic isocyanates. Formulations containing aliphatic isocyanates yielded color-stable products, while aromatic formulations did not. However, aromatic formulations yield products with better chemical resistance than their aliphatic counterparts. Another major ingredient in polyurethane formulations is polyols. Common polyols are polyesters, polyethers, polycarbonates and polyacrylates. The choice of polyol and the relative molecular weight of the polyol significantly affects the performance of the product. In addition, by modifying polyurethane resin, fluorinated polyurethane with excellent UV resistance and stability can be produced,
Silicones and Silanes - Silicones and silanes are highly cross-linked silicone resins that are generally chemically and physically inert. Typically promotes water resistance of products; however, they are generally more susceptible to alkaline conditions. For some formulations, modified resins can reduce the effect of alkali.
Sealers - Polymeric sealers can help prevent chemical or physical degradation of concrete. Sealers are often described as penetrating or film-forming, and are distinct from curing sealers that are used to retain moisture during concrete curing.
Silanes or siloxanes are penetrating sealants. Benefits of use include protection of concrete from liquid water and surface contaminants. They do not prevent the loss of water vapor from the concrete during curing. Silanes have low molecular weight, while siloxanes are pre-reacted and have higher molecular weight. Due to the lower molecular weight, silanes provide greater penetration into concrete pores than siloxanes. Because silanes penetrate below the surface, they are more wear-resistant than water-based sealers that form a film on the surface.
Acrylic or styrene acrylic water emulsions are commonly used as sealants and prevent ingress of salt and moisture that can cause degradation. They usually have good water resistance, but are breathable and allow vapor to pass through the film. These water-based sealers are usually designed to be used at low dry film thicknesses, and they're usually not a high-gloss finish—usually satin or semi-gloss. Since the film is relatively thin and sits mostly on the concrete surface, it usually needs to be reapplied periodically to maintain protection.
Solvent-based acrylics are typically used after the concrete has aged for 30 days. Due to the acrylate or vinyl ester backbone, these sealers are durable and still have a low enough molecular weight to penetrate concrete pores and darken the surface (known as the wet appearance), which is desired. They are applied at a higher film thickness than acrylic emulsions and can develop a high gloss once dry. These acrylics can be formulated exempt from solvents such as acetone; however, the fast drying and odor can present significant challenges during application.
Polymer-modified concrete mortar is also used as a thin cover to reduce the surface porosity of the concrete. The polymer mix works by reducing the amount of water needed for a given concrete fluidity, resulting in a lower water-to-cement ratio. In addition, when the concrete dries, the polymer forms a film in the pores and blocks the capillaries. Slower curing of the dry polymer in the concrete and matrix results in reduced porosity. While the reduced porosity of the cover layer helps reduce the penetration of water carrying chloride ions, the polymer also improves durability by increasing tensile strength, abrasion resistance and dust resistance.
Two-component epoxies are commonly used to seal concrete in environments where high levels of chemical resistance are required. These can be cured with polyamines, polyamides, or polysulfides; although polyamide-cured epoxies generally have lower chemical resistance. Epoxy resins have good adhesion to concrete and can be formulated to cure at low temperatures.
Testing Concrete Coating System Performance
Coating performance on concrete bridge structures cannot be predicted because variations in the substrate itself, coating type and formulation can all affect expected performance. Instead, performance testing is often required to determine acceptability for use.
EN1504-2 is a global standard covering the performance characteristics and key criteria for the selection of protective coating products for use on concrete. This European Standard outlines performance characteristics for preventing ingress, protecting against moisture and providing physical and chemical resistance. The standard includes performance characteristics applicable to all concrete structures and is not specific to bridge structures.
The National Transportation Product Evaluation Program (NTPEP) of the American Association of State Highway and Transportation Officials (AASHTO) for evaluating concrete coating systems (CCS) is designed to Cleans prepared similar structural concrete and other new and existing masonry surfaces. The tested concrete coatings are designed to enhance the durability and/or aesthetics of concrete structures subjected to degraded atmospheric exposures such as marine, industrial, deicing chemicals and high humidity.
该计划中包含的性能测试由技术小组选择，该小组由几个州交通部（DOT）代表，涂料制造商和AASHTO NTPEP理事会成员组成。选择这些测试是为了专门解决桥梁涂层的性能预期。该程序解决了六个性能特征，包括：氯离子渗透，湿气透过，耐候性，耐热循环，裂缝桥接（可选;本文未讨论）和涂鸦抗性。以下测试说明概述了用于评估相关性能特征的过程：
氯离子渗透试验（AASHTO T259-混凝土抗氯离子渗透性的标准测试方法）。保护涂层通过提供抗氯离子渗透到混凝土基质中来增强混凝土结构的耐久性。通过盐水积水（3％氯化物溶液）混凝土板测试该性能90天，并测量四个深度的氯化物侵入，如图1所示。
湿气透过率：该测试评估涂层将水分从混凝土内部传递到外部的相对能力，如图2所示。涂覆到测试立方体上大约24小时后，涂层的单独初始重量（W0）测定未涂布的测试立方体，准确到0.1克。测试立方体保持在25±2℃和50±5％RH，最小化气流的影响，持续14天。每个立方体在7天（W7）和14天（W14）称重。使用以下公式将湿气透过率确定为涂布和未涂布立方体在干燥的第7天和第14天之间的重量损失：
VT =（W7-W14）/（168 hx 0.062 m2），单位为g /（m2 xh）
耐候性：ASTM D4587，“涂料和相关涂料的荧光紫外 - 冷凝暴露的标准规范”是用于评估耐候性的测试标准。在置于QUV柜（UV光/冷凝湿度循环）中的砂浆测试板上进行测试，总测试时间为2500小时（约15周）。涂层耐久性通过耐起泡性，涂层厚度侵蚀和粘附损失以及光泽和颜色保持性来评估。此外，经常填充修复化合物的毛孔和虫洞（涂覆前）需要与涂层系统相容，并且需要能抵抗风化产生的内应力。图3表示用于耐候性测试的面板以及在耐候性测试之后评估涂层系统与修复化合物的粘附性。
此外，涂料是用ASTM D7072在图3中示出了用于耐风化的传输相同的面板评价[1] ，积垢的ASTM D3719度[2]ASTM D3273，和真菌抗性[ 3]。
Freeze/Thaw Cycle Resistance and Bond Strength: The coated mortar test panels were exposed to freeze/thaw resistance testing for 300 cycles according to AASHTO T 161, Procedure A. Prior to cycling, adhesion testing was performed on coated panels with and without intentional defects according to ASTM D7234, Standard Test Method for Pull-off Strength of Coatings on Concrete Using a Portable Pull-Off Adhesion Tester. Adhesion was re-evaluated at 50, 150 and 300 cycles.
Overcoating/Graffiti Resistance: Graffiti resistance and ability to apply marking coating systems were evaluated by aerosol spraying and permanent marking of weathered coated test panels. After the spray paint and markers had dried for 7 days, the top coat of the test system was applied to hide the simulated graffiti. After a curing period of 30 days, adhesion testing was performed on the overcoated areas according to ASTM D 7234. The hiding power of the coatings was also evaluated according to ASTM D 2805 [4].
[1] Standard Practice for Evaluating Accelerated Fluorescence of Latex Coatings
[2] Standard Test Method for Quantifying Dirt Acquisition on Coated Exterior Panels
[3] Standard Test Method for Resistance of Coated Surfaces to Fungal Growth in Environmental Interiors
[4] Standard Test Method for Hiding Power of Paints by Reflectance
