By Leo Procopio,

Bridges are critical to society because they facilitate connections between people and businesses. People need to see their friends and family, workers need to get to their jobs, and goods need to get to their market. Allowing the transport of people and goods across physical barriers such as waterways, ravines, and highways is necessary for our social and economic health.

Due to their importance, it is in the best interest of society to build new bridges when needed, keep existing bridges in safe and working condition, and protect them against excessive degradation from overuse, weather, and corrosion. However, therein lies the problem, as bridges are also very expensive to build and maintain.

Coatings certainly have an important role to play in the protection of bridges from the elements of the weather and corrosion, and effective coating systems can extend their service life and increase the time between maintenance cycles. This article will explore some of the challenges facing our bridges, the role of coatings in bridge construction and maintenance, and the types of coating systems that are currently used on bridges. CoatingsTech also reached out to several experts in the field of bridge coatings for their thoughts on topics such as challenges for bridge maintenance, trends in the bridge coatings market, and where the market is headed in the future. Their comments will be presented in the Q&A roundtable section of this article.

Figure 1. Number of bridges in the current U.S. bridge inventory and their condition, according to their age (year built). Based on data from the National Bridge Inventory as of June 2022.¹

The State of Bridges in the U.S.

According to the National Bridge Inventory (NBI), a database administered by the Federal Highway Administration, there were more than 620,000 road and highway bridges in the United States as of June 2022.¹ The inventory of bridges has been steadily growing over the decades. For example, 30 years ago, there were slightly more than 570,000 bridges. The inventory of road and highway bridges includes major bridges across mighty rivers, small bridges of at least 20 feet, as well as highway bridges and overpasses spanning other roads or other obstructions such as railroad tracks or gullies. The NBI database doesn’t cover bridges dedicated for rail traffic, and although there is not an exact accounting of railroad bridges in the United States, there are at least 61,000 bridges used for Class 1 rail traffic,² i.e., the largest rail carriers that account for about 94% of freight rail revenue and 67% of freight rail mileage.

The condition of infrastructure in the United States gets a lot of attention, especially when it’s not in good repair, and road and highway bridges are no different. The American Society of Civil Engineers provides an annual Infrastructure Report Card on various infrastructure segments, and in 2021, it gave the bridge segment only a C grade.³ We tend to take the condition of bridges for granted, but high profile bridge failures, such as the 2007 collapse of the I-35W Mississippi River bridge in Minneapolis or the more recent 2022 collapse of the Fern Hollow bridge in Pittsburgh, focus public attention on the state of our bridges.

Figure 2. Bridge condition for all US bridges and the subset of National Highway System (NHS) bridges, by bridge count (chart on left) and by bridge deck area (chart on right). Based on data from the National Bridge Inventory as of June 2022.¹

As might be expected, older bridges tend to have more wear and tear, and a higher percentage of older bridges are rated at a lower condition. Figure 1 shows data for the entire inventory of 620,669 bridges in the NBI, broken down by age (year built) and the overall condition of the bridge. The condition ratings of good/fair/poor are based on the National Bridge Inspection Standards,⁴ and they consider the combined condition of the deck (i.e., road surface), sub-structure, and super-structure of the bridge. A rating of poor does not necessarily mean that a structure is unsafe, but rather that it needs attention. Approximately 40% of all bridges were constructed before 1970 and are more than 50 years old. It is clear from Figure 1 that a higher percentage of those older bridges are in fair or poor condition compared with those built more recently.

Figure 2 shows the conditions of the entire inventory of U.S. bridges and a comparison with the subset of bridges that are part of the National Highway System (NHS). The National Highway System consists of roadways and major arteries deemed important to the nation’s economy, defense, and mobility. Bridges carrying NHS roads make up about 24% of all bridges by number. However, because these bridges tend to carry larger roads, they make up a larger percentage of the inventory, about 58%, when considering the deck area.

Most are owned by state, county, and local governments (Figure 3). States own about 48% of bridges in number, and local governments own about 50%. However, states generally own the larger and more heavily travelled bridges, and so own approximately 76% of bridges by deck area. The federal government owns a little under 2% of all bridges, such as those on federal lands.

Figure 3. Distribution of bridges by type of owner, according to bridge count (left) and bridge deck area (right). Based on data from the National Bridge Inventory as of June 2022.¹

Although the number of bridges characterized as in poor condition has been trending downwards over many years, there are still about 43,000 bridges listed in the recent NBI data as being in poor condition. Funding for bridge construction and repairs has been a perennial problem, and while state and local governments own most bridges, funding is largely from federal sources. In 2021, approximately $8.6 billion was obligated towards bridge projects from federal highway program sources. There had been no standalone federal funding sources dedicated to bridges since 2013, but the recent Infrastructure Investment and Jobs Act (IIJA), enacted at the end of 2021, now provides two funding programs dedicated to bridge projects. It is estimated that these new programs could roughly double the annual spending by states for bridge projects relative to 2021 levels (not adjusted for inflation).⁵

Painting Bridges

Coatings are utilized on bridges to improve their aesthetics, but protection is their most important function. Bridges in the United States are most commonly constructed from reinforced concrete (about 75%) and steel (about 25%). There are reasons for coatings both substrates.

Problems with concrete occur when water and soluble salts penetrate the concrete and cause the steel reinforcing bar to corrode. As the rebar corrodes, the expanding corrosion products can cause cracks in the concrete. The use of powder coated rebar can help mitigate the issue, and sometimes silane-based sealers and water repellants are applied to the concrete to prevent the water/salt penetration. Pigmented coatings such as epoxies, acrylics, and polyurethanes are also applied to concrete bridges for both aesthetics and barrier protection. However, concrete bridges are largely left uncoated.

Steel bridges, on the other hand, are almost always painted to protect the steel from corrosion. Some bridges constructed from a specific steel alloy, referred to as weathering steel, can be left unpainted, as the steel alloy develops a patina or protective layer during weathering. But in most cases, steel will corrode rapidly if left unpainted. Corrosion can be accelerated when soluble salts are present, such as from road de-icing salts.

Certain areas of bridges are more susceptible to corrosion due to their micro-climate. Steel located under leaking deck joints is in an aggressively corrosive environment. Splash zones, which are more prone to having water and salt splashed onto the steel surface, exist both above the bridge deck surface and sometimes underneath a bridge where another road passes. Any areas that can trap water/salt and remain wet for longer periods of time, such as bottom flanges on I-beams, also have an aggressive micro-climate.

Coatings for Steel Bridges

Table 1. Common Coating Systems for Painting Steel Bridges Note: IOZ = inorganic zinc rich, OZ = organic rich zinc, MCU = moisture cure urethane, CSA = calcium sulfonate alkyd

Historically and up to the mid- to late 1970s, many steel bridges were painted with multiple thin coats of alkyd coatings containing toxic corrosion-inhibiting pigments such as chromates and red lead and which were applied directly over the mill scale. Mill scale is a thin layer of iron oxides that forms on hot rolled steel during the milling process. It adheres to the steel surface, and as long as there is no break in the layer, protects the underlying steel. However, because it is electrochemically cathodic to the steel, any breaks in the mill scale can lead to corrosion of the underlying steel. Red lead alkyd coatings were effective, but eventually replaced due to regulation of the toxic lead and chromate pigments and recognition of the benefits of surface preparation (i.e., removal of mill scale via abrasive blasting).

Coating systems comprising a zinc-rich primer and solventborne vinyl finish coat replaced the red lead alkyds in the 1980s. However, the vinyl finish coats were eventually replaced with other topcoats due to their extremely high VOC levels. Today, the bridge industry uses many types of coatings systems, but the most common is a three-coat system comprised of a zinc-rich primer, an epoxy intermediate coat, and an aliphatic polyurethane topcoat.

There are various scenarios in which a steel bridge might be painted, and the choice of process has a large effect on the overall cost of the painting process. The cost of paint materials is typically only a relatively small percentage of the total project cost, with the main contributions coming from labor and the type of surface preparation. If removal of old lead-based coatings is required, the health and safety requirements for the lead abatement process will drive costs up further.

Figure 4. New bridge spanning the Mississippi River and carrying I-74 between Bettendorf, IA, and Moline, IL. Opened to traffic in 2021, all exterior steel surfaces were coated with a three-coat system using a zinc-rich primer, epoxy midcoat, and fluorourethane topcoat. Photo courtesy of AGC Chemicals Americas.

For new construction, it is common to paint the structural steel in a shop setting, where surface preparation and paint application are most easily controlled. After abrasive blasting in the shop, typically at least a zinc-rich primer is applied. Sometimes the full coating system including intermediate and finish coats is also applied in the shop, or alternatively the remaining coats can be applied in the field after the steel is erected.

For maintenance painting, several possible scenarios exist.⁶ Spot repair and touch-up is used when there are smaller areas of corrosion or paint failures, and those areas can be treated by various surface preparation methods and painted. Zone painting is used to remove and replace coatings in specific areas or zones, such as steel within a splash-zone or within a certain distance from expansion joints. Spot repair and overcoating is used when the original paint system is still in relatively good shape and well adhered. After partial removal of failed coating and rust, spot priming is done, and a full coat of topcoat is applied. Finally, full removal of the old coatings and replacement with a new multi-coat system can be done when the original coating is in poor shape.

Although less expensive on a square foot basis compared with spot repair and zone painting, full removal and replacement is generally the most expensive maintenance painting scenario overall, due to the large surface area being prepared and painted. Overcoating is a less expensive option than full removal/replacement because of the lower amount of surface preparation. Spot repair and zone painting have the highest cost per square foot, but are the lowest overall cost due to the smaller areas being repaired.⁷

Figure 5. The Tokiwa Bridge, located in a mountainous region near Hiroshima, Japan, was painted with a three-coat system in 1986. The coating system uses a zinc-rich primer, epoxy midcoat, and fluorourethane topcoat, and has performed well for over 30 years. Photo courtesy of AGC Chemicals Americas.

Table 1 lists some of the common coating systems used for steel bridges. The ultimate choice of coating system depends on factors such as the painting scenario (e.g., new construction, full removal or replacement, or spot removal or overcoating), expected durability, as well as immediate and/or life-cycle costs.

For new construction, an inorganic or organic (e.g., epoxy or moisture-cure urethane) zinc-rich primer is typically applied in a shop setting. For field-applied systems, organic zinc rich primers would be utilized. Sacrificial zinc-rich primers are used because they are the most effective primers at preventing corrosion of properly prepared (abrasive blasted), clean steel. There are several types of two- and three-coat systems used for new construction and full removal/replacement of old coatings, as shown in Table 1. As mentioned previously, the most common is a three-coat system using a two-component epoxy intermediate coat and a two-component aliphatic polyurethane topcoat over the zinc-rich primer. The zinc-rich primer provides the corrosion resistance, the epoxy midcoat protects the primer and adds to corrosion resistance via its excellent barrier properties, and the polyurethane topcoat provides a highly durable finish with excellent gloss and color retention.

There is a trend towards using ultra-durable topcoats with excellent gloss and color retention, and one approach is to use a polyurethane based on a fluoropolymer polyol, such as FEVE (fluoroethylene vinylether). An example of a recent bridge coated with such a system is shown in Figure 4. The bridge spans the Mississippi River, carrying I-74 between Iowa and Illinois, and was opened to traffic in December 2021. Protecting the steel on this $1 billion project is a three-coat system consisting of a zinc-rich primer, epoxy midcoat, and fluorourethane topcoat.

Similar coating systems based on fluoropolymers have been used elsewhere in the world, specifically in Japan, for many years. Figure 5 shows an example of such a bridge near Hiroshima, Japan, which was coated with a three-coat system using an FEVE-based fluorourethane topcoat in 1986.⁸

Table 2. Gloss and Gloss-Retention Data for the Fluorourethane Topcoat on the Tokiwa Bridge Near Hiroshima, Japan, Over a 30-Year Period

Table 2 shows some gloss and gloss retention readings made on the Tokiwa Bridge over the course of three decades, illustrating the excellent gloss retention with these systems. After wiping to remove dirt from the painted surface, it was found that the 60° gloss retention was 97% after 30 years. The photographs in Figure 6 illustrate the excellent gloss observed on an external-facing fascia beam, which is expected to receive the most exposure to sunlight and weather, over the 30-year exposure.

Another trend has been to use two-coat systems, in effect replacing the epoxy midcoat and polyurethane topcoat with a single finish coat.⁹ Utilizing a bridge coating system with fewer coats is advantageous for its lower cost of labor and faster return-to-service (e.g., less downtime for traffic lanes). Polysiloxanes are used directly over zinc-rich primers in a two-coat system and also provide superior durability.¹⁰

Figure 6. Photos of a steel fascia beam on the Tokiwa Bridge taken at various exposure times, and showing excellent gloss retention of the fluorourethane topcoat. The photo taken in 2016 represents 30 years of exposure. Photos courtesy of AGC Chemicals Americas.

Fluorourethanes and polysiloxanes are among the most UV-resistant and durable finish coats available today, and both have excellent gloss and color retention. Polysiloxanes are also used in other market segments where durability is important, such as marine coatings.

Polyaspartic finish coats,¹¹ based on amine-functional aspartate ester resins (e.g., Figure 7) crosslinked with aliphatic polyisocyanates, are also utilized in two-coat bridge coating systems.¹² Relative to polyurethanes, polyaspartic coatings can offer faster dry times with reasonable pot lives, can be applied at higher film thicknesses, and offer equivalent gloss and color retention. The thicker films facilitate moving to a two-coat system without sacrificing corrosion resistance.

Work on two-coat bridge coating systems continues. Figure 8 shows a highway bridge in Missouri that was recently painted with a proprietary 2-coat system based on new technology from Carboline.

Figure 7. Molecular structure of an amine-functional polyaspartic resin, which can be crosslinked with polyisocyanates (X = aliphatic or cycloaliphatic bridging group, and R = alkyl).

Waterborne acrylic topcoats also get some use in both two- and three-coat systems over zinc-rich primers for full removal/replacement scenarios, as well as find use in multi-coat overcoating scenarios. Acrylics have good exterior durability, the benefit of low VOC, and testing has shown that they can perform well in aggressive environments typical for bridges.^13,14

As regulations have pushed end-users towards lower VOC coatings, they have several options to move away from high VOC solventborne coatings, including the replacement of traditional solvents with VOC-exempt solvents and the use of either high solids solventborne coatings or waterborne coatings. A number of states, such as California and North Carolina, have specifications that allow the use of waterborne acrylics on steel bridges. California has recently pioneered the use of several waterborne coatings based on acrylic/FEVE blends as higher durability waterborne options.^15,16

State departments of transportation (DOTs) maintain qualified product lists (QPL) or approved product lists (APL) of coatings and coating systems that are approved for use on bridges in their state. A coating typically undergoes a long process of testing prior to being placed on a QPL. State DOTs can do their own testing, and there is also a national program administered by the American Association of State Highway Transportation Officials (AASHTO) that generates data on coating systems that states can use in determining whether a system can be approved for use in their bridge projects.

Figure 8. Photo of bridge carrying I-270 over Dorsett Road in Maryland Heights, just outside of St. Louis, MO. The steel structure was painted with a new proprietary two-coat system. Photo courtesy of Carboline.

The AASHTO National Transportation Product Evaluation Program (NTPEP) evaluates materials and products across a variety of applications and includes a program for steel bridge coatings. The program helps prevent duplication of testing efforts across state DOTs and provides paint manufacturers with a single testing protocol rather than having to support testing done by each state separately. The NTPEP Structural Steel Coatings program evaluates products via an independent third party laboratory according to a consensus-based project work plan that describes the laboratory and field test protocols.¹⁷ Testing includes demanding accelerated weathering tests such as 5000 hours in ASTM B117 salt spray and 15 cycles (5040 hours) in ASTM D5894 cyclic salt fog/UV exposure, as well as slip coefficient testing for coatings applied to faying surfaces (i.e., surfaces being bolted together), among others. Results are shared with member states via an online database¹⁸ and used to make decisions about qualifying products for inclusion on a state’s QPL/APL.

Conclusions

The vast importance of bridges to society lies in the numerous social and economic connections that they facilitate. The United States currently has over 620,000 road and highway bridges that allow people and products to get over obstacles such as waterways, valleys, and roads which would otherwise be difficult to traverse. Unfortunately, many of them are rated as being in poor condition. Building new bridges and replacing unsafe or obsolete bridges is very expensive, so protecting and maintaining existing bridges against deterioration is incredibly important.

Coatings have a key role to play in protecting critical infrastructure such as bridges. Bridge coatings have evolved over many decades to become safer and more effective, and today, a three-coat system based on a zinc-rich primer, epoxy intermediate coat, and polyurethane topcoat is most common. However, many other systems are used, including ones utilizing more durable finish coats (e.g., fluorourethanes and polysiloxanes), environmentally friendly waterborne coatings, and systems with fewer coating layers. The challenges facing bridges and other infrastructure are many and daunting, but there is no doubt that future innovations in coatings will be part of the solution.

References

1. Information on the National Bridge Inventory database, maintained by the Federal Highway Administration, can be found at .
2. “How Freight Railroads Keep More Than 61,000 Bridges Safe,” Association of American Railroads
3. “2021 Report Card for America’s Infrastructure,” American Society of Civil Engineers, accessed at .
4. Info on the National Bridge Inspection Standards can be accessed at .
5. “Infrastructure Investment and Jobs Act: Highway Bridges,” Congressional Research Service, May 2022.
6. Ault, J.P.; Kimmer, C.; Shoyer, E., “Maintaining Modern Bridge Coatings Systems,” J. Protective Coatings & Linings, 40(1), pp. 18-27, January 2023.
7. Richards, G.; Grisso, B., “Maintenance Painting- Protective Coatings and Coating Systems for Bridges,” presentation at the 2013 Southeast Bridge Preservation Partnership annual meeting, 2013.
8. Darden, W., “Long life coatings for steel bridges,” Proceedings of AREMA Annual Conference, 2019.
9. O’Donoghue, M.; Datta, V.; Walker, S.; Wiseman, T.; Roberts, P.; Repman, N., “Innovative Coating Systems for Steel Bridges: A Review of Developments,” J. Protective Coatings & Linings, 30(1), pp. 34-52, January, 2013.
10. Calzone, T., “Ultra-Durable Finished for Zinc-Primed Steel Bridges,” Proceedings of the World Steel Bridge Symposium, 2005.
11. Squiller, E.P.; Reinstadtler, S., “Polyaspartic Coatings,” Chapter 14 in ASM Handbook Volume 5B: Protective Organic Coatings, K.B. Tator (Ed.), ASM International, 2015.
12. Olsen, A.; Williams, C.T.; Hudson, M.; Fleming, C.W., “Two-coat Polyaspartic Urethane Coatings Protect Virginia Steel Bridges for Over a Decade,” J. Protective Coatings & Linings, 33(1), pp. 56-63, January 2016.
13. Medford, W., “Testing Low VOC Coatings in Aggressive Environments: North Carolina’s Experience,” J. Protective Coatings & Linings, pp. 23-29, May 1995.
14. Peart, J.; Kogler, R., “Environmental Exposure Testing of Low VOC Coatings for Steel Bridges,” J. Protective Coatings & Linings, pp. 60-69, January 1994.
15. Marcks, B., “Improvements of Waterborne Acrylic Latex Finish Paint Properties by Incorporating Fluoroethylene Vinyl Ether (FEVE) Emulsion Technology,” Proceeding of the SSPC Coatings+ Conference, 2020.
16. For example, see State of California Department of Transportation Specification PWB-182B, “Dark Green Finish Paint Waterborne Acrylic Latex/ FEVE Blend Vehicle,” February 2023.
17. “NTPEP Committee Work Plan for Evaluation of Structural Steel Coatings (SSC-18-1),” AASHTO, 2019, accessed at https://ntpep.transportation.org/technical-committees/protective-coatings-ssc-ccs.
18. NTPEP DataMine for Structural Steel Coatings (SSC) can be accessed at .

Roundtable Q&A: Industry Experts Discuss Bridge Coatings

CoatingsTech asked several industry experts for their thoughts on the present and future of the bridge coatings market. Topics ranged from challenges facing the bridge construction and maintenance industry to trends affecting the market for bridge coatings. They also discuss the role of sustainability in this industry, as well as their thoughts on what the future holds for bridge coatings.

Participants in the Q&A roundtable discussion include experts in the bridge coatings industry from raw material suppliers, coatings manufacturers, engineering firms, and facility owners. The industry experts providing comments include:

• Peter Ault, president at KTA-Tator
• Andrew Birnie, North American market development manager for industrial coatings at Covestro LLC
• Winn Darden, business manager for AGC Chemicals Americas, Inc.
• Vijay Datta, technical leadership and business development manager at International Paints at AkzoNobel
• Justin Manuel, global product line director for Carboline
• Barry Marcks, associate chemical testing engineer with the California Department of Transportation (Caltrans)
• Kevin Morris, director of strategic segments and business development for protective and marine coatings at PPG

Q&A

Q1: From your perspective, what are some of the key challenges facing the bridge construction and maintenance industry?

Ault (KTA-Tator): At this time, the availability of skilled labor and supply chain issues are the prominent problems. As we digest the funding made available through the Infrastructure Investment and Jobs Act (IIJA), we will be back to the enduring challenge of balancing present-day cost with life-cycle costs. Often the life-cycle costs are too obscure to justify higher present-day expenditures.

Darden (AGC): Keeping older assets viable for longer periods of time is a key challenge. Also, new bridge construction has become extremely expensive, as has repainting.

Datta (AkzoNobel): The key challenge is the expense, length of time, and exclusion of emerging technology in the current approval protocol. This is applicable to both new construction and maintenance projects.

Manuel (Carboline): While there are many industries that face significant funding challenges, I believe the bridge construction and maintenance industry is actually in a prime position to thrive in this regard—particularly given the growing emphasis on tackling our nation’s infrastructure corrosion challenges. If more funding continues to be allocated to this industry, I expect that we will see an abundance of new bridge projects added to the DOT dockets in coming years. As a result, this would necessitate the hiring of more skilled labor in order to support these contracts.

Marcks (Caltrans): Some challenges in the construction and maintenance markets include the promotion of unqualified contractors through the low bid process and a lack of experienced QC/QA inspectors. A grey tsunami, involving the retirement of institutional knowledge, and a small pool of qualified new candidates and hires are other issues.

Morris (PPG): The two greatest challenges that I see right now are lack of funding (temporary correction with the IIJA bill) and the need to maximize life cycles so the budget money can address more needs.

Q2: What do you see as some of the important market trends and drivers affecting paints and coatings for bridges?

Birnie (Covestro): The top market driver on everyone’s mind is the potential elimination of exempt solvents, such as PCBTF, which is the most widely used exempt solvent in the industry. The challenge is keeping pace with the productivity demands of the industry and remaining compliant with current and potential future regulations. There are a few approaches coatings companies could take in anticipation of this change—finding a new exempt solvent or developing new coatings systems with waterborne or high solids resins which don’t require any at all. Coatings manufacturers are hesitant about finding a replacement solvent for fear that it could also end up being eliminated someday. Waterborne is an option in this case but could fall short of the productivity needed to be a true replacement. Finally, a high solids option, like polyaspartate chemistry can be formulated to most VOC levels and offers several other benefits, such as fast dry times, early property development, and a faster return to service. As lawmakers work to finalize their regulations, working with an existing chemistry with a proven record of performance is likely one of the most attractive choices for formulators.

Manuel (Carboline): One of the biggest drivers of trends within the coatings industry is a major push toward the development of more sustainable solutions that offer lower VOCs and a reduced environmental footprint. This has been a bit of a challenge for the bridge industry, which has predominantly used solvent-based, higher-VOC coatings in both field and shop environments for the past 50+ years. While this push for sustainability is still fairly new as it relates to the bridge industry, it will certainly be important for coating manufacturers and DOTs to anticipate this “green wave” based on other market trends we’ve seen in recent years.

Ault (KTA-Tator): Since the 1990s, coatings selection for bridges has largely been driven by issues surrounding lead-based paint. Even for new structures, the three-coat zinc-rich/epoxy/urethane (Z/E/U) became the standard replacement for coated steel bridges (uncoated steel also became a popular option). In the past 5 to 10 years, owners have begun optimizing their approaches to coated steel. As the initial Z/E/U bridges are approaching their first maintenance interval, the maintenance community is looking at new maintenance painting procedures—zone painting and new overcoating strategies are becoming common. For both new and maintenance painting, there is increasing interest in and use of color and gloss retentive topcoats. For new structures, the owners are beginning to refine their corrosion control strategies to use both less expensive systems (e.g., single coat inorganic zinc), and pricier systems offering more durability (e.g., duplex coatings consisting of a metallic coating in combination with organic coating(s)). Finally, owners have begun to combine coatings systems to optimize cost and performance. For example, on simple overpasses, several states will apply a full coating system on outside members and omit the finish coat on interior beams.

Marcks (Caltrans): Market trends affecting bridges coatings include the loss of raw materials and products used in making coatings. Supply chain disruptions have caused a lack of availability of some raw materials. Corporate takeovers and acquisitions of specialty materials suppliers is another trend. Inflation and rising costs affect the ability to plan projects. On the EH&S front, Go Green policies are advocating the removal and discontinuation of some products from use.

Darden (AGC): It appears that many DOTs and bridge authorities are more open to concepts like life cycle cost and trying to quantify those costs. Cost per gallon of paint isn’t the relevant metric for coatings, it’s applied cost per square foot per year of coating life. This means that more expensive products like fluoropolymer coating systems are more common today.

Morris (PPG): I think cost and budgets continue to be major impacts for bridges, and during this recent inflationary period several have speculated that the funds that will pour in from the IIJA Bill will only cover the inflationary costs that have been witnessed in the workplace.

Datta (AkzoNobel): The new upcoming trend could be surface prep and application by robotics. The inspection and maintenance can be done by drones. We also believe the current testing protocol should be replaced by ISO 12944 testing. I think ISO has classified environments based on severity, required durability, and expected service life. Their testing protocol is based on these requirements. This new testing/approval protocol should answer all the above questions.

Q3: What do you consider the top challenges for the bridge coatings industry today, and how are they being addressed?

Morris (PPG): What is unique in our world are the silos built around end use segments and the lack of data transfer from one segment to another. The bridge and highway market segment continues to look for ways to meet longer life cycle performance and to achieve the elusive “100-year design life” for bridges. Other branches of government, such as the Navy, looked for life-cycle improvements decades ago, and through technology, they realized improvements of 3x to 4x. Perhaps this rests as much on manufacturers to do a better job telling the story as it does on the bridge industry to become early adopters of new technology so that they realize improvements in performance. Two examples that I would give are edge retentive, ultra-high solids coatings, which could improve protection on areas where corrosion commonly starts on a bridge, and ultra-weatherable topcoats that are more resistant to degradation from UV light.

Datta (AkzoNobel): A key challenge is the cost of being a qualified contractor (QP 1 and QP 2 certifications from AMPP), including capital investments in equipment and containment and the ability to find the proper labor resources. In addition, recent supply chain constraints have either canceled or postponed several bridge projects.

Darden (AGC): One key challenge is trying to get new products qualified in the bridge coating market.

Manuel (Carboline): Although we have seen marked improvement in recent months, disruptions to the supply chain continue to be the biggest challenge that is faced by the bridge coatings industry. Because DOT specifications often follow rigid testing requirements, the approved list of coating solutions from which a contractor can select is very narrow and specific. As a result of ongoing raw material shortages, it has become increasingly difficult to manufacture these products. Ultimately, this can impact the completion timeline for bridge coating projects.

Ault (KTA-Tator): In addition to those mentioned above, the biggest challenge is deciding when new approaches make sense in the absence of long-term performance data to validate the decisions. Public perception often incentivizes near-term performance at minimal cost over solutions that are more cost-effective over the longer-term.

Marcks (Caltrans): Inflation with its rising costs has made everything more expensive. And a small pool of qualified labor has made it more difficult to get the people needed to complete projects. Supply chain issues have caused shortages, even allocation of products and materials. This delayed some projects. The problems seem to have been resolved for now, but will it happen again?

Q4: Are there any new and innovative products or technologies for the bridge coatings industry that you would like to highlight?

Darden (AGC): Newer concepts like duplex coatings can enable significant extension of coating system life. Using long-life fluoropolymer topcoats over galvanized or metallized surfaces can give coating system life beyond that offered by each technology alone.

Marcks (Caltrans): Caltrans has developed UV-resistant, one-component waterborne finish coatings using an acrylic/FEVE blend for use on our structural steel bridges.

Ault (KTA-Tator): As mentioned above, some key innovations include color and gloss retentive topcoats (e.g., polysiloxanes and fluoropolymers), less expensive systems (single coat inorganic zinc-rich, rapid-cure coatings), and pricier systems offering more durability (duplex coatings consisting of a metallic coating in combination with organic coatings).

Morris (PPG): This market segment is one in which it is difficult to innovate. The industry utilizes qualified and approved product lists (QPL/APL) that have to be met, and they either call for specific products/chemistries with minimum performance requirements, or they are formulary in nature.

Datta (AkzoNobel): To our knowledge, coatings innovation for the bridge market is somewhat stagnant due to the long and expensive testing procedures necessary for qualification, coupled with a low margin business. Adapting the new ISO 12944 standard for qualification of bridge coatings may inspire new technology development.

Manuel (Carboline): Carboline will soon be launching an innovative, new two-coat system that will revolutionize the construction and rehabilitation of steel bridge structures. Developed to provide state DOTs with the ultimate corrosion protection, this system will significantly extend the asset life cycle over more traditional, three-coat systems. It will also eliminate the need for block-outs (masking areas where bolted connections are made, so only zinc-rich primers are on faying surfaces) as the entire coating system will adhere to Class B slip coefficient standards.

Q5: How is the bridge coatings industry dealing with the concept of sustainability? Are there any advances in surface preparation, coatings, application methods, etc., that show promise for improving the sustainability of the bridge coatings process.

Manuel (Carboline): While it has been fairly slow to evolve, the bridge industry is seeing a shift from solvent-based, higher-VOC coatings to more sustainable technology. Because new coating systems within the bridge industry are subject to rigid testing standards, some manufacturers may be hesitant to innovate new systems that compromise their current specification positions. That said, as the global coatings industry—and the world—increasingly push for more sustainable coating solutions, I expect that will accelerate the adoption of this trend within the bridge industry.

Ault (KTA-Tator): From my perspective, addressing sustainability is in its infancy. While coatings do have an environmental impact that is magnified with multiple maintenance cycles, the industry doesn’t have a firm grasp on the frequency of such intervals. Perhaps the more significant environmental cost is the so-called user impact. Slowed traffic, underutilized human resources, delayed deliveries are examples of costs associated with bridge maintenance that have a significant sustainability impact.

Morris (PPG): Sustainability is primarily addressed through VOC compliance such as but not limited to Ozone Transport Commission (OTC) Phase II regulations.

Birnie (Covestro): Over the last several years, sustainability has evolved well beyond a buzzword with theoretical implications for the distant future. The bridge coatings industry has begun to define sustainability and establish standards, goals and best practices that are all intended to push the industry toward a more sustainable existence.

Carbon neutrality is perhaps the most tangible example of sustainability drivers the industry is looking to improve. Paint companies are now shifting focus on carbon reduction from embodied carbon, making the choice of building materials, such as bridge coatings, even more critical than it has been in the past. Coatings derived from bio-renewable or recycled raw materials will not only reduce the amount of embodied carbon of a bridge project, but also help companies across the value chain achieve their carbon neutrality goals.

This progress on reducing environmental impact does not mean a sacrifice on performance specifications. Coatings made from bio-renewable raw materials will perform just like their fossil-based versions. When these coatings begin to prove themselves in the field, accelerated adoption is likely to follow.

Marcks (Caltrans): Sustainability is today’s buzzword, and many times it is a vaguely defined term.

Caltrans uses a waterborne FEVE/acrylic latex blend coating that lasts longer and doesn’t have to be repainted as often. This saves tax dollars and enhances sustainability through increased longevity of our coatings and steel structures.

Caltrans has looked into using laser blasting as a surface preparation method to remove existing coating and corrosion to achieve SSPC-SP 10 standards (Near White Metal Blasting). Caltrans is also utilizing UAS or Unmanned Aircraft Systems (drones) for bridge inspection and coating assessment.

Darden (AGC): The use of longer life coatings is one way to meet sustainability standards. Minimizing the need for repainting over multiple cycles reduces the amount of energy and carbon dioxide emissions from manufacturing of the coating, application of the coating, disruptions to traffic, etc.

Q6: Putting your futurist hat on, what do you see as the future of bridge maintenance and the role of coatings? What would you like to see the industry accomplish in the next 10 or 20 years?

Datta (AkzoNobel): We think the biggest barrier for the next 20-plus years is low return on investment due to R&D, testing costs, and final independent lab testing and approval.

Marcks (Caltrans): Coatings provide the primary corrosion protection system for steel bridges. Bridge painting is a cost-effective means of extending the service life of our infrastructure. I would hope to see more funding available for bridge maintenance programs and see the need for higher wages and better incentives to attract and hire painters.

Darden (AGC): The industry has started moving in the direction of more durable materials in coating systems. The ultimate goal should be to use coating systems that protect steel bridges for 50-plus years, moving as close as possible to 100-year coating systems.

Manuel (Carboline): Especially in more recent years, the bridge industry has increasingly shifted away from carbon steel bridges painted with the traditional three-coat system in favor of alternatives like concrete, weathering steel and metallizing (also known as thermal spray). I expect this to prompt a push for new and innovative advancements in bridge coatings technology and data to support these trends and bolster coating systems’ position as the ultimate corrosion control mechanism on steel bridges into the next generation.

Morris (PPG): The future of bridge maintenance and role of coatings does not show much in the way of expected change going forward. The one exception would be the potential addition of ultra-durable topcoats. I would like to see an acceleration of trials/demos for new technologies from coatings manufacturers to prove increased life cycles.

Ault (KTA-Tator): In 10 to 20 years, major bridge rehabilitation will be limited to functional issues (capacity constraints, road realignments, etc.). A properly designed bridge will be able to endure its design life with minimal coating maintenance.

About the Author

Leo J. Procopio, Ph.D., is president and owner of Paintology Coatings Research LLC. For more information, visit or email leo.procopio@scienceofpaint.com.

INTRODUCTION

Concrete is one of the most used construction materials due to its strength, durability, resilience, safety, and low cost. In the flooring applications, demand for concrete for interior and exterior use in residential and commercial settings is growing significantly thanks to the strong construction market around the globe.

Coatings for concrete floor not only protect concrete from wear, deterioration, and contamination, but also enhance the physical performance, provide chemical resistance, and improve aesthetics. Today, architects, specifiers, and end-users can select from a broad range of technologies and finishes to protect and enhance the aesthetics of the concrete floor. Each technology or finish brings specific benefits and introduces trade-offs in other areas. Polymer technology is among the most popular choices for concrete flooring and include thermoset chemistries such as amine-cured epoxy, urethane, urea, methacrylate, and acrylic. The preferred option depends on the type of application and the required performance properties.

High-performance floor installation using polymer technology requires at least two stages (primer and topcoat), and preferably three (including a midcoat) as shown in Figure 1. With each stage needing time to cure, fast-cure speed to reduce the total installation time translates into minimum downtime and fast return to service. This has been a key market driver across the coating industry. Yet, fast-cure speed often comes as a trade-off for short working time. Balancing fast cure and good working time remains a challenge for coating industry. This article describes fast-cure amine technologies using a system approach that enables the installation of a multistage floor system within one day while maintaining good working time at each stage and high-performance of the entire system.

FIGURE 1—Representative examples of a high-performance flooring system.

As shown in Figure 1, the concrete primer serves to penetrate and seal the concrete pores because concrete is a porous and permeable material. Primer establishes good adhesion and bonding between the concrete substrate and the polymer overlayment. Cured concrete traps various amounts of moisture, about 1% to 2% in ambient dry concrete, and 4% to 5% in damp and wet concrete; however, the corresponding relative humidity inside concrete is as high as 75% to 95%. When a primer is applied at low temperature, applicators also need to consider the dew point to avoid moisture condensation during or shortly after the application of the primer. Two-part amine-cured epoxy primers have proven to tolerate the challenges introduced by concrete as a substrate and provide good adhesion to dry and damp concrete even at low temperature and high humidity. In addition, advancement in waterborne technology equips the formulators with waterborne epoxy system as a new tool to meet performance, low volatile organic components (VOC) and low emission requirements.^1–3 It has gained wide acceptance as an environmentally friendly alternative to the solventborne system due to the improved performance made during the past two decades.

Coated on top of the primer is the polymer midcoat and then topcoat. Until recently, topcoats have been dominated by two key technologies: amine-cured epoxy and polyurethane based on polyol-cured isocyanate. Lately, another two-component (2K) aliphatic polyurea, referred to as polycarbamide or polyaspartic technology, has been developed to practical industry use. It is derived from polyamine cured isocyanate chemistry.^4–7

Continue reading in the �Ǵ���CoatingsTech.

It is often assumed that solvent-based sealers give better performance than water-based formulations on concrete because the solvent-based sealers are thought to better penetrate into the concrete matrix. In practice, it can be difficult to measure actual penetration of a sealer into concrete, as dyes and colorants used to highlight the sealer may show different migration properties than the polymeric binder. Confocal Raman spectroscopy mapping has been used to map the depth of penetration of solvent-based, 100% solids, and water-based sealers that employ acrylic and epoxy binders. This mapping shows that neither solvent-based nor water-based formulations showed any significant penetration into the concrete substrate beyond the first few microns of the open surface. A study has also been carried out using a model waterborne acrylic sealer formulation to determine whether additives might influence the penetration of the sealer into the concrete or other factors that could affect the performance of the waterborne coating. This work has shown that the addition of anti-foaming, coalescing surfactants can improve the protective properties of the coating by improving air release and increasing film network formation at the concrete surface.

Introduction

Sealers are an important part of protecting concrete against surface damage, corrosion, and staining. They work by either blocking the pores in the concrete to prevent the ingress of water and water-soluble salts or by forming a polymeric barrier that prevents such materials from passing through to the substrate. Penetrating sealers contain reactive species, such as silanes, siliconates, and silicates that can enter the concrete matrix and react with minerals present to block the pores and create a hydrophobic and oleophobic barrier. Penetrating sealers have excellent durability but do not change or enhance the surface appearance of the concrete surface. They can also make further treatment of concrete difficult.^1,2

Topical sealers are coatings that form a polymeric film and barrier at the concrete surface. They are prepared using many different chemistries, although acrylic, epoxy, and polyurethane binders are most common. Topical sealers may not last as long as penetrating sealers, but they can enhance and decorate the concrete surface and prepare it for additional treatment. Sealers are typically low viscosity coatings and contain either solvent or water as a diluent. Solvent-based sealers give a more glossy, wet-look finish, whereas water-based sealers often give a more natural-looking appearance. Many states now restrict the use and sale of solvent-based sealers for environmental, health, and safety reasons.

Concrete is a highly complex substrate, comprising mineral aggregates and fillers like sand bound together in a crystalline, inorganic matrix based on hydrated calcium and aluminum salts.^3-4 Concrete is also porous, containing many natural air voids as well as capillary pores created during the release of water during setting and cure (Figure 1). The exact nature of the concrete and concrete surface will vary, depending on the supplier and local raw materials, although it is essentially a porous, mineral substrate for the sealers.

Solvent-based sealers are claimed to give better performance than water-based sealers,⁵ although it is acknowledged that the performance of water-based sealers is improving.^6-7 One explanation often cited for the improved performance is that solvent-based sealers can better penetrate the concrete surface, providing some pore blocking and developing enhanced adhesion through mechanical interlocking.^{2, 8-9} The improved adhesion is also cited as evidence for the improved penetration into the concrete, but in practice, there has been little research on the actual penetration of the sealer into the concrete. Simple dye or color penetration tests may be misleading as the color spread may not show the real spread of the polymeric binder needed for performance (Figure 2¹⁰).

Wood is a porous substrate for which several investigations into the penetration of coatings have been conducted. De Meijer, et al., observed that loss of water or solvent into the capillaries between wood cells caused a rapid increase in binder viscosity that limited penetration of the coating into the wood capillaries.¹¹ They noted that the binder particles were too large to penetrate the cell walls of the wood itself. Alberdingk Boley Inc. also observed that the penetration of the coating into the wood was effectively prevented by the cell walls of the wood itself, regardless of diluent, polymer chemistry, molecular weight, or particle size.¹²

Penetration of Coatings into Concrete

The penetration of two different sealers into concrete substrates was studied by two different methods. The first method employed simple UV fluorescence to visualize the depth that two epoxy sealers penetrated the substrate. Simple stoichiometric mixtures of liquid epoxy resin (EEW 190) with either a liquid amidoamine hardener or a water-based hardener (Evonik’s Anquawhite^® 100) were prepared, and applied by roller onto cleaned, shot-blasted concrete pavers purchased from a local DIY store. The pavers were about 6 x 12 in. (15 x 30 cm) in size and 2 in. (5 cm) thick, and the coating was applied to the shot-blasted side of the paver. The coated panels were cured for seven days under ambient conditions and then carefully cut in half to expose the concrete substrate. The cut surfaces were illuminated with short wave, UV radiation (“black light”), and the resulting fluorescence from the aromatic epoxy binder was photographed looking at the cut edge of the panel (Figure 3). In both cases, fluorescence was only visible at the coated surface and 1–2 mm below the surface, with almost no fluorescence visible further into the body of concrete paver, indicating that neither coating penetrated the concrete substrate.

Unfortunately, not all binders fluoresce, so an alternative method was needed to measure penetration of acrylic-based sealers. Confocal Raman microscopy couples a Raman spectrophotometer with an optical microscope together with a spatial filter to chemically analyze the volume of a sample in three dimensions. The limits of the spatial resolution are dependent on the laser and microscope objective, but it is possible to identify individual particles smaller than 1 µm.¹³ Raman spectroscopy is similar to infrared spectroscopy to the extent that it measures the internal molecular vibrations via a different mechanism. Infrared and Raman spectra often provide complementary information, but Raman spectroscopy is suitable for aqueous samples and samples containing high water content that can interfere with infrared spectroscopy.

Two commercial acrylic sealers were used (Behr Premium^® Low-Lustre Sealer Water-based masonry sealer and Increte Systems’ Clear Seal 400 Solvent-based masonry sealer), and they were applied by both roller and brush onto cleaned, shot-blasted concrete panels. The coated panels were allowed to cure for seven days under ambient conditions before the coated concrete plates were fractured into manageable pieces, and one face on each section was polished using 600-grit abrasive film.

Raman spectra were obtained using a Horiba LabRAM HR Raman confocal microscope system and the operating conditions shown in Table 1. Spectra of the coating/concrete interface cross sections were obtained in a x = 19 µm by y = 24 µm grid, with 1 µm steps using a 1 x 1 µm pixel for complete overlap between steps. A small portion of each liquid coating sample was allowed to air dry on a glass slide, and Raman spectra of the resulting films were obtained.

Both sealers had Raman spectra characteristic of acrylic polymers (Figure 4). The C–H stretching band marked at 2931 Rcm^-1 was the strongest feature in the spectrum of the water-based sealer and selected to monitor the coatings on the concrete cross sections, together with a second sharp shoulder marked at 2881 Rcm^-1, assigned to C–H stretching in a linear hydrocarbon chain, possibly a hydrocarbon wax. The C–H stretching band marked at 2936 Rcm¹ was the strongest feature in the spectrum of the solvent-based sealer and selected to monitor the solvent-based coatings on the concrete cross sections.

A sample image of a cross section of the roller applied, water-based sealers is shown in Figure 5a. The green rectangles in the images indicate the area mapped by Raman spectroscopy. The intensity of the C–H stretching band at 2931 Rcm^-1 (integrated from 2818–3054 Rcm^-1 with baseline correction) is represented by imposing saturation of green superimposed on expanded scale micrographs in Figure 5b. The green areas on the images end at the visible boundaries between the coating and concrete, indicating the coating is concentrated on the surface of the concrete. The nearly identical results with the solvent-based sealer are shown in Figures 6a and b.

This limited study suggests that there is limited sealer penetration into the concrete substrate, beyond the initial surface roughness and open capillary structure at the surface. Therefore, to address the real or perceived performance gaps between water-based and solvent-based sealers, more study is needed. Most sealers are unpigmented or only contain low levels of pigments and fillers, so improved resin technologies will be key to bridging the performance gaps,^14-16 but other formulation components can also affect sealer performance.

Effect of Additives on Water-based Sealers

Water-based coatings often have higher surface tensions than solvent-based formulations due to the higher surface tension of water. Flow into capillary pores can occur when the adhesive forces of the liquid to the pore material are greater than the cohesive forces of the liquid to itself, so low surface tension is desirable for capillary flow and penetration.^{2, 11} Wetting agents (surfactants) are additives used to lower the surface tension of aqueous coatings; therefore, a number of different surfactants were tested in a water-based sealer formulation (Table 2) to determine whether they could improve performance and penetration into the concrete substrate.

The surface tension of formulations containing different surfactants was measured using a Krüss Bubble Pressure Tensiometer–BP2 and the results are shown in Figure 7. The sealer without additives has a relatively low surface tension, but many of the surfactants tested lowered this considerably. The most effective products for reducing surface tension were the advanced acetylenic glycols.

The different sealer formulations were also applied by both roller and brush onto cleaned, shot-blasted concrete panels and the panels tested for substrate penetration using the Confocal Raman mapping method described above. However, the results from the Raman mapping were nearly identical to the previous results, with almost no penetration into the concrete substrate regardless of the surfactants used (Figure 8).

The water sensitivity of the sealed panels was also tested using a simple water spot method that measures the time for water to penetrate the sealer and become visible by a darkening of the concrete below the coating. A minimum of five tests per panel were used, and the tests were repeated with different panels, and the averaged results are summarized in Figure 9. This is a simple, visual test; the time to start penetration result is much more reproducible and thought to be more significant as it relates to barrier properties of the coating to the liquid water. The area of penetration is less reproducible as the can might spread both horizontally across the surface (which would be visible) and vertically into the depth of the concrete, which cannot be seen.

The best results were obtained with the formulations containing non-ionic, coalescing surfactants, although these did not give the lowest surface tension. The dried sealer gave a relatively low gloss film on the concrete surface that made visual and photographic comparison difficult, except when the surfactants were very foamy and surface bubbles were visible. The formulations containing the non-ionic, coalescing surfactants also gave the best appearance, both visually and under the microscope, especially when looking for dry, uncoated areas on the concrete, especially in some of the deeper holes in the concrete surface.

The choice of defoamer also affected the water sensitivity of the sealer. Both strongly incompatible defoamers that were highly effective at reducing foam and highly compatible defoamers (less effective at controlling foam) produced sealers that had poor water resistance. The strongly incompatible defoamers gave very poor surface appearance (craters or dewetting) when the sealer was applied over Leneta charts, and it is thought that these might also disrupt the film on the concrete surface allowing water to penetrate. However, it was not possible to confirm this visually. Similarly, defoamers that were too compatible left residual foam and bubbles in the dry films when the sealer was applied onto Leneta charts, and these bubbles are likely to remain when applied onto concrete; however, these also could not be seen visually or under a microscope. The optimum defoamers gave significantly improved water resistance (Figure 10).

Conclusions

There are many different types of concrete sealers available, but the two most commonly used are film-forming acrylic sealers and penetrating sealers. Although they both seal concrete, they do so in different ways. Film-forming sealers work by forming a protective film atop the concrete and, whether water-based or solvent-based, do not appear to penetrate the concrete substrate.

The use of water-based sealers has increased as regulation of volatile organic compounds has restricted the availability and use of solvent-based sealers; however, some concern remains about the performance of water-based sealers. This performance is primarily affected by choice of resin, but the choice of additives can also affect the sealer performance. De Meijer noted that significant amounts of water can be lost to the substrate when coatings are applied to porous substrates, and this can affect both rheology and viscosity build, but potentially also film formation.¹¹ Coalescing surfactants can help lower the surface tension of water-based sealers as well as aid film formation at the concrete surface to improve film properties.

Acknowledgments

The author wishes to thank Paul Marcella for helping to create the data and Ingrid Meier and Maria Nargiello for editorial assistance.

References

1.  �ąhttps://en.wikipedia.org/wiki/Concrete_sealer.
2. Understanding Concrete Sealers and Chemicals, Seminar TH140, WoC2011.
3. https://www.understanding-cement.com/hydration.html.
4. https://www3.imperial.ac.uk/concretedurability/researchprojects/predictingmasstransport.
5. Comparison of Waterborne and Solventborne Resins for Concrete Sealers, Wexler, �鶹��ƵCoatings for
   Concrete (2014).
6. https://waterbasedsealers.com.au/water-based-concrete-sealers-vs-solvent-based-concrete-sealers/.
7. https://www.concretecamouflage.com/water_based_concrete_sealer.cfm.
8. Understanding High Performance Coatings for Decorative Concrete, Seminar WE27, WoC2011.
9. https://indecorativeconcrete.com/?page_id=644.
10. https://www.amshieldcorp.com/capillaryWaterproofing.htm.
11. de Meijer, M., de Velde, B., and Militz, H. “A Rheological Approach to Understanding Capillary Penetration of Coatings into Wood,” 5th Nurnburg Congress (1999).
12. Alberdingk and Boley, Lignocure VP2010 Product Information.
13. https://www.horiba.com/us/en/scientific/products/raman-spectroscopy/raman-academy/raman-faqs/what-is-confocal-raman-microscopy/.
14. Flecksteiner, Matranga, and Lawhorn, “Development and Properties of a Waterborne Wet-Look Sealer at 50 g/L VOC,” �鶹��ƵCoatings for Concrete (2014).
15. Goldschlager, B., “A Unique Acrylic Latex for Improved Chemical and Hot Tire Resistance in Low VOC Garage Floor and Masonry Applications,” Coatings Trends and Technologies (2016).
16. https://www.epscca.com/opencms/export/sites/epscca/galleries/pdfs/articles/ECS-Garage-Floor-Paint-Andrew-Hearley.pdf.

*Presented at the 2019 Waterborne Symposium, February 24–March 1, in New Orleans, LA.

CoatingsTech | Vol. 16, No. 10 | October 2019

Demand for concrete for both exterior and interior flooring applications in residential and commercial settings is growing significantly due to the strong health of the construction market, including both new housing starts and remodeling work, in most regions around the world. In the United States alone, over 214 kilotons of concrete was used in flooring applications in 2016, and demand is expected to expand at a compound annual growth rate (CAGR) of 6.3% from 2016 to 2025, according to market research firm Grand View Research. All of that concrete requires protection of some sort, most often in the form of concrete sealers. Much of that concrete is also beautified in some way using decorative coatings designed for that purpose. Consequently, the overall concrete floor coating market was valued by the same firm at $645.9 million in 2016 and expected to grow to $1.15 billion by 2025. Basic epoxy coatings are projected to grow at a CAGR of 6.0% from 2017 to 2025 due to their durability and toughness. The market for polyurethanes used on concrete floors is expected to be worth $268.3 million by 2025 as a result of their excellent corrosion, abrasion, and chemical resistance. Polyaspartics are a third important segment of the concrete flooring market and are used where rapid curing and high performance are required.

ENHANCING THE LOOK OF CONCRETE

Concrete floors—from concrete walkways, patios, driveways, and pool decks to garage floors and concrete interior floors—do not have to retain that original gray look, however. Decorative concrete coatings comprise a special segment of the concrete coatings market designed to add color and/or texture, enabling the transformation of horizontal concrete surfaces to a wide range of finishes and appearances that mimic everything from Venetian plaster to marble and natural stone.

Flooring is a dominant part of a room or space’s color scheme and can dictate the palette or provide the perfect complement, according to Dee Schlotter, PPG senior color marketing manager. Concrete floors are no exception. “Natural concrete is popular and is partly responsible for the prevalent industrial look that is seen today in many commercial and residential designs. The industrial design has evolved to be cleaner and more contemporary, and concrete floor accents in these designs are following suit with subdued colors or sealing with a matte or glossy finish,” she says.

Demand for concrete for both exterior and interior flooring applications in residential and commercial settings is growing significantly.”

The growing number of consumers looking for more sustainable solutions are also driving the growth of the concrete flooring and floor coating markets. “People opting for decorative concrete in their homes are inspired by what they see on social media,” according to Johnnie Elliott, manager of Rust-Oleum’s architect and engineering team. Popular choices include the use of decorative chips and metallic coatings to create unique looks. Exterior applications remain the largest part of the market, though. “Exterior decorative applications are growing as homeowners look to extend the inside to the outside. They want the same decorative look outside as they expect inside, and they see the value in protecting concrete,” he adds. The natural state of concrete is very relevant, especially in multifamily spaces, as people want a seamless look from inside to the outside of the home. Agrees Schlotter, “It’s becoming clearer in today’s design that homeowners want to bring the outdoors in—with plants and natural elements. We are also seeing the ombre look on concrete, which creates a haphazard, imperfect look,” she says. For contractors, ease of use and repeatability are the key decision criteria when choosing a partner, according to Elliott.

Both old and new concrete can be modified with decorative coatings. Application of decorative coatings is, in fact, a way to revive old concrete and avoid the need to remove and replace it. “Most often, decorative coatings are applied to substrates that pose a problem to remove or are too expensive to remove/replace,” observes Elliott. In addition to ceramic tile, he points to stamped concrete patios as an idea for reviving with decorative coatings, which avoids the need to re-pour and stamp, color, and stain. “Applying decorative coatings to floors previously covered with carpet is another way building owners can introduce industrially modern, creative, and unique looks that are easy to clean and will far outlast their predecessors,” he notes.

OVERLAYS AND STAINS

There are two major categories of decorative concrete coatings: overlays and stains. Overlays are applied on the top of the concrete floor. They include polymeric coatings such as acrylic, epoxy, and polyurethane systems formulated in a range of colors. They generally are composed of a mix of cementitious and acrylic materials, but some products are based on natural crushed limestone or recycled solid surface fillers. Overlays are used to add both color and texture and are typically applied in thin layers, most often using a trowel, but in some cases by spraying. Some come pre-mixed except for the desired liquid color, while others are sold as powders that require mixing with colorant and water/solvent.

Stains penetrate the concrete. Acid stains are typically a mixture of water, hydrochloric acid, and acid-soluble metallic salts. They are available in approximately 10 different earth tones that can be diluted to create a range of shades. They react with the lime in the concrete, generating a mottled appearance that is determined by the composition, age, and condition of the concrete. Water-based stains are typically acrylates designed to penetrate the concrete and deposit pigment particles in the pores of the floor. Because they do not react with the concrete, the color of an applied water-based stain is the same as that in the bottle. The range of colors is also much broader than acid-based stains. In addition, water-based stains do not present the hazards associated with acid-based systems.

Both stains and overlays can be used on new concrete. For worn concrete areas, resurfacing with an overlay provides numerous options for adding colors, stamped patterns, and textures. Staining and stenciling are appropriate for older concrete that remains in good condition. In addition to the desired appearance and the state of the concrete floor, the appropriate decorative coating will be dictated by the application conditions (time, temperature, location, etc.), the conditions to which it will be exposed, the desired life expectancy for the coating, and the cost limitations.

PREPARATION OF CONCRETE SUBSTRATES

Whether the concrete floor is old or new, its surface must be in reasonably good condition. Decorative coatings applied to existing floors with extensive cracking or gravelly surfaces will not last. In these cases, it is better to replace the floor—or portion of the floor—that is damaged prior to applying any type of coating. It is also crucial that floors in appropriate condition be properly prepared prior to coating application. To achieve both the desired appearance and a durable decorative coating, dirt, old peeling paint, and other materials should be removed first. In addition, different types of decorative coatings may require different preparation methods or techniques. Choices include sand or water blasting or grinding or cleaning with special cleaners and/or a mild acid wash. The goal is to remove the unwanted materials and open the pores of the concrete. Minor cracks should also be repaired, and the floor patched and leveled as well. Compatibility with the decorative coating system should be considered when selecting the materials used for any of these repairs. Even new concrete requires preparation, such as the removal of curing compounds and any adjustment of the surface profile/roughness as dictated by the decorative coating to be applied.

All decorative coatings applied to concrete floors require protection of some kind. Concrete floors, particularly exterior surfaces, can be exposed to impact, abrasion, chemical attack, and thermal shock. A concrete sealer should be applied over the top of the stained or overlayed surface to reduce wear and tear and extend the lifetime of the decorated surface. Different sealers are available depending on the level of abuse expected for a given concrete floor.

MEETING TODAY’S CHALLENGES

One of the biggest challenges for formulators of decorative concrete coatings today is the development of products that reduce the application time and enable faster return-to-service. “Our biggest challenge is providing the fast return-to-service that customers are coming to expect while also providing enough working time for contractors to ensure a consistent result each time,” Elliott says. Rust-Oleum is working closely with its key suppliers to develop custom resins and co-develop products that address this challenge. Offering a primer/basecoat in one product, for example, provides a quicker return-to-service, with floors being completed in one day. Longer working time polyaspartic coatings with a rapid cure time are another. “These products also enable same-day return-to-service, which is key in most industrial and light commercial environments,” comments Elliott.

“One of the biggest challenges for formulators of decorative concrete coatings today is the development of products that reduce the application time and enable faster return-to-service.”

Environmental regulations, particularly those relating to volatile organic compound (VOC) content, can create some difficulties as well. “VOC restrictions and certain chemical shipping and use regulations have had some effect on product development. These regulations have limited the shipment to and use of some decorative concrete coatings (e.g., acid stains and traditional solvent-based coatings) in certain areas, which has driven the development of alternative products. In addition, as the industry becomes more heavily regulated, costs to the end user continue to rise,” observes Elliott. Rust-Oleum continues to push the limits of the technology to offer contractors the best products that are the most repeatable. One notable example is expansion of its portfolio of nonisocyanate-based products that still offer true UV stability, which will provide contractors with the ability to offer their customers very unique finishes for all exterior concrete surfaces, according to Elliott.

Rust-Oleum has two primary focus areas—working closely with contractors to ensure that their needs are met and listening to the market trends regarding floor coverings as a whole.  “Most decorative concrete coatings can mimic a look or trend, so keeping up-to-date on these developments will be key to our success moving forward. Our goal is to always have products/finishes that will wow the consumer,” Elliott states.

CoatingsTech | Vol. 15, No. 10 | October 2018