Concrete is a very utilitarian construction material used to create highways; bridge decks; construction blocks; tilt-up wall panels; floors on grade or elevation; tanks; secondary containment structures; kitchen countertops; textured, stamped or molded reliefs; and more … amazing. Consider the overall “natural” appearance of concrete and how dull it would be if all concrete structures were available only in various shades of gray. So concrete is often treated in a variety of colors. These treatments include stains (mineral acid and water-based), dyes, integral colorants and dry shake hardeners. Each of these options, however, has limitations, particularly in uniformity of appearance, initially and with age. Application of paints and coatings to concrete, therefore, remains the most common means for achieving a pleasing appearance through color, as well as functions such as secondary containment and protection from the environment. Painting concrete may be considered by some to be an easy, straight-forward process, and for the most part, it is. However, just as it is true for steel, wood, and plastics, it is easy only after the surface has been properly constructed and prepared.
BOND BREAKERS AND CURING AGENTS
When they are good they are very, very good… The function of bond breakers and form release agents is to minimize the effort (load) required to lift cast, tilt-up panels or slabs and to strip forms from cast-in-place concrete construction. Concrete for tilt-up panels and cast floor and roof slabs is placed into a form having the dimensions of the item. The forms for tilt-up panels are commonly poured on a concrete floor base, allowed to set, and, upon adequate curing, are lifted/tilted into place (such as tilt-up walls) or placed in stacks for subsequent lifting and placement (floors and roofs).
|Poured cast-in-place walls have forms of reinforced side walls constructed of boards, plywood or steel panels and tie-rods between the two sides helping to hold the forms in place against the outward pressure of fluid concrete. Where tilt-up or lift panels must be raised from their forms, the cast-in-place have to be stripped from the wall or face. The bond between the form and concrete can damage the concrete face, cause cracking, or break the lifted elements. Clearly, in both cases, there is benefit to minimizing the bond between the fresh concrete and the solid surfaces that serve to give it shape. Added benefits include panel faces that are generally smoother, textures that are more uniform and sticking that is greatly reduced. Originally, heavy sodium-based greases were used as release lubricants, but they gave way to oils, waxes, solvent-borne oleoresins and synthetic hydrocarbons. Treated plywood faces and plastic liner sheets were used for cast concrete, but often left impressions and wrinkles that were undesirable. Chemical engineering technologies have spawned a number new release products, which may be solvent- or water-based, include silicone moieties, silicone-free solutions and organic resins/solvent emulsions and more recently, chemistries based on “dissipating resinous” films. The dissipating membranes are susceptible to sunlight and weather exposure that degrades the resin, causing it to separate and flake from the surface. Degradation of these resins can begin as early as seven days after exposure and can take as long as 55 to 60 days to be fully degraded.|
|Welcome to St. Petersburg, FL, concrete monlith.|
These membrane films can also serve as curing agents that slow the release of water from concrete, thus allowing more complete hydration and greater strength. Techniques that have been used in the past include misting with water and wet burlap sacks to aid hydration and concrete curing. Simply stated, the bond release agents hinder movement of water from new concrete and serve to minimize adhesion between new concrete and the molds (forms) into which it is placed.
REVEALS, MOLDS AND FORM-LINERS
The use of relief materials, molds and form-liners for construction of architectural concrete also relies on the use of form release agents. Form liners are commonly constructed of elastomeric urethane rubber, plastic, fiberglass or polymer foam molds. These are stripped from the concrete leaving a three-dimensional relief in the concrete. Fluted panel walls are probably one of the most common examples along major metropolitan highways; but now, primarily in the southern United States, roadway walls and abutments have become adorned with logos and creatures. Even these forms require release agents to strip easily and prevent damage to the concrete face. Complex relief molds may limit the size of the panels cast so the molds can be removed. In some complex geometries, foam forms may be removed by dissolving them with solvent. When not integrally colored, the concrete can be prepared for painting in a variety of colors to highlight the design developed by the architect (or artist).
RELEASE AGENT TRANSFER
… and when they are bad, they are awful. Bond breakers are applied to the forms prior to concrete placement. Curing agents are applied to the concrete surface after placement. In both cases, the disadvantages of using bond breakers and curing compounds are related directly to the advantages they offer, the low (or no) stick, impervious surface.
|Dragon design in relief.|
Application of coatings to concrete requires that foreign matter and interference materials be removed from the surface. In this context the bond breaker or curing compound is a foreign material. It must be removed. Further complications can occur if the bond breakers are not applied uniformly and within a thickness tolerance. Some of these include increased frequency of pinholes, increased pinhole size, development of bug holes, surface stains and increased amount of bond breaker transferred from the forms to the concrete panel faces. The most commonly recommended removal process for the dissipating bond breakers and curing compounds is pressure washing. The typical pressure cited is 3,000 psi to “waterblast” these membranes from the concrete surface. During the process of allowing new concrete to age 28 days prior to painting, this period also allows for dissipating bond breaker membranes to oxidize (degrade) and become more easily removed. Unfortunately, 28 days are not always allowed to pass before painting, or, under some circumstances, the delay time is insufficient for the bond breaker resin to degrade. Pressure washing may not be sufficient to remove resins that are only partially degraded. Matters can become more complicated when the bond breaker or curing compound requires a cleaning agent to be used for effective removal. Manufacturers of coatings and bond breakers frequently offer cleaning agents that aid in removal of the bond breakers during pressure washing. Even so, there are some steps that can be taken to confirm removal of release agents, such as the water test. Water sprayed onto the surface of concrete that is free of surface films should not bead up, but rather, the concrete surface will become somewhat darker in color. Note that the concrete surface must be allowed to dry before painting.
BOND BREAKERS AND COATING FAILURE
Concrete is a porous matrix of cement paste, aggregate and additives. The hydration process consumes mix water and, in doing so, leaves microscopic capillaries containing excess water and/or air. During the recommended 28-day waiting period, excess moisture is typically released and the surface becomes sufficiently dry to coat after surface cleaning. The presence of a surface membrane (e.g., release or curing compound) retards water release, and excess water content can exacerbate already compromised coating adhesion. Movement of free water through a curing membrane/bond breaker is retarded except in some instances where unsealed pinholes or bug holes are present. Water vapors, on the other hand, can slowly pass through the surface of the film, more rapidly where unsealed pinholes or bug holes are present. Application of a paint or coating over the bond breaker or curing membrane will inevitably fail. Why? First, there is low adhesion between the coating and membrane. Liquid vehicle (including resins in water/solvent) is inhibited from passing into the concrete texture or capillarity. No mechanical bond is established. Vapor pressure on the back surface of the film can cause vapor blisters to form and grow due to increasing substrate temperature. Such blisters commonly shrink due to decreasing substrate temperatures. Rapid temperature cycling can cause free water to collect behind the coating film due to condensation as the temperature decreases. Excess mix water may remain or even collect until the blister ruptures. Vapor blisters can shrink back to the surface (often wrinkled), while liquid filled blisters do not. Tracks may form where water migrates beneath the coating layer due to gravity whenever the “weight” exceeds the adhesion force at the paint-membrane interface.
A CASE IN POINT
A tilt-up concrete panel building was constructed in the Carolinas. The exterior faces of the panels consisted of two types of finishes: Smooth finished painted surfaces and a thin brick veneer cast into integrally colored wall panels. The painted concrete included beige and chocolate brown finish colors. The wall panels were 7.5-inch thick with rigid foam insulation panels attached directly to the interior face of the wall. The exterior had “wing walls” that extended outward past the corners of the building. The concrete mix design used for the building walls included cement, aggregates, sand and a Type A water reducer. No fly ash was included in the mix design for the tilt wall panels.
|Building elevation with integral color brown brick and painted panel faces.|
The bond breaker reported to have been used on the casting slab was a chemical reactive organic curing agent and bond breaker (without wax or hydrocarbon resins). The product and aqueous emulsion reacts with calcium hydroxide (Ca (OH) 2) in the concrete and forms a gel which seals concrete surface pores, restricting moisture entry and exit. The construction sequence had the panels erected soon after they were cast with the building walls, being the first panels erected a minimum of 30 days before exterior coatings were applied. The tilt-wall subcontractor was reported to have washed the exterior face of the tilt walls with a pressure washer using pressurized hot water without additives or cleaning agents. Approximately five weeks after erection, the painting contractor’s activities during the first week onsite included pressure washing the exterior wall surface with plain potable water. During the second week onsite the primer coating arrived and priming began. The primer was a fast drying waterborne acrylic masonry surface sealer used to seal chalky porous concrete or masonry walls prior to painting (0.7 to1.3 mils DFT).
|Blisters in the applied coating system.|
The finish coat was a high-build waterborne acrylic topcoat, an alkali- and efflorescence-resistant product (3.2 to 5.8 mils DFT per coat). Other properties reported included water resistance (two pinhole-free coats) and breathability. Within a year of application the coatings on the concrete panels exhibited blisters and areas of delamination. The blisters generally ranged from 1/8” to 1.5” in diameter with a higher frequency of blisters on the north and east walls compared to the south and west walls. Three of the four wing walls on the building had a blister frequency comparable to the north and east walls. The blisters did not contain liquid. When blister caps were removed a white film, the primer, remained adhered to the surface. Blister separations were between the finish coat and primer. Beyond the area of the blisters and in locations of delamination, the topcoat and primer separated from the concrete together. Topcoat removal revealed the pattern shown in the figure below.
|White spots and lines (primer) on the tilt-up panel face following topcoat removal.|
The pattern on the concrete surfaces was limited to the areas having blisters and delaminations. Intact, adherent coating removed from non-failing areas did not exhibit the pattern and were generally difficult to remove cleanly from the substrate. One observation regarding the pattern was that it could represent the distribution of various amounts of the bond breaker products that transferred from the form to the face of the poured tilt-up panels. The extent of bug holes and pin holes could also be related to application of bond breaking materials. Heavy builds of the bond breaker would be more difficult to remove by pressure washing alone and are typically not a visible film. However, a grayish color residue was found on the concrete surface that was not related to the coating. Laboratory analysis by Fourier transform infrared spectroscopy produced a spectrum of the gray material that was consistent with calcium carbonate. When the back side of a poorly adhered coating film was rinsed with hexanes, the solution spectrum showed evidence of hydrocarbons sometimes found in release agents. The spectra was also compared to a spectrum of the release agent reportedly used, which was similar to the spectrum from the hexane rinse on the back side of the poorly adhered coating sample. Testing of the exposed surface of concrete using water mist yielded some contradictory observations: the concrete appeared to darken with minimal water beading when misted. This was not expected.
|The image on the left shows an acid test producing no reaction, while the image on the right shows an acid test with little reaction.|
Water did bead on the white spots of primer/sealer applied to the surface which was expected. When the surface of the concrete was exposed to an acid solution, some concrete surfaces did not “fizz,” indicating that the concrete was protected from the acid. The investigation established that the bond breaker compounds used were still on the surface after an initial hot water pressure washing and ambient temperature pressure washing over 30 days later. Additional relevant building issues were observed in addition to the bond breaker on the surface. Bug holes were visible in various areas on the wall, creating pinholes in the coating.
|Acid test with a strong reaction at the crack in the concrete surface.|
After removal of the coating, some bug holes appeared to have a sandy debris on the interior surfaces of the void. The foundation waterproofing stopped short of grade approximately 5 inches on the north wall and gradually decreased on the east and west walls to the point where the foundation waterproofing daylights on the south wall. The ground on the north and east walls appeared to stay damp around the drainage grates near the base of the wall. Random cracks were visible in various areas in the wall on all sides, and efflorescence had erupted through the coating along portions of the lines and in spots on the walls. Discontinuities were discovered in the coating film at several locations: The skyward side of the recessed accents had discontinuities in the recessed band at the top of the parapet wall, and inconsistences in the coating were observed at some of the window sills and ledges. These observations supported the suspicion that some free water was able to enter the concrete and exacerbate the failures associated with the presence of the bond breaker. Ultimately, the solution was to remove the coating system that was applied to the tilt-up panels, then test for the presence of the bond breaker on the surface (water and acid solutions). Subsequent to removing the coating, it was estimated that as much as 1 mil of the bond breaker or residual primer coat remained on the surface. Prior to coating application, surface cleaning by pressure washing (using water with cleaning agents) was necessary. Wet blast cleaning using sand injection was also proposed as a final means to prepare the surface.
|North elevation of the building after the coating system was fully removed.|
Careful review of the bond breaker product data sheets is critical to recognizing the possible difficulties that must be overcome to remove them from the surface. In this instance, seven “Not Recommended for…” conditions were listed on the data sheet. The cleaning instructions from the product manufacturer recommended pretreating tilt up panels with a tri-sodium phosphate solution prior to “rinsing” off the surface by pressure washing with a minimum of 4,000 psi water. Following the cleaning step, the manufacturer recommended further surface testing to determine the adequacy of cleaning using appropriate site tests.
The benefits of bond breakers, form release agents and curing compounds help to make concrete construction truly utilitarian and attractive. The use of coatings is critical to appearance and function. Millions of square feet of concrete have been cast-in-place, cast and lifted or poured into forms for shape and design — much of it painted. However, surface preparation is still the most critical step to successful painting. This article discussed why bond breakers, form release agents, curing membranes and sealants are an anathema to painting concrete. You certainly can think of other surface preparation issues in the same light. D+D
|ABOUT THE AUTHOR: Richard Burgess is a senior coatings consultant with KTA-Tator Inc., where he has been employed for more than 18 years. He has experience in coating condition assessments; failure analysis; specification preparation; expert witness; and environmental, health and safety consulting. Burgess has further served as project manager/inspector supervisor on a number of projects. He is an SSPC-Certified Protective Coatings Specialist and a NACE-Certified Coatings Inspector Level 3. He also serves as an instructor for a variety of KTA-offered training seminars. Burgess holds a bachelor’s degree in environmental science from Rutgers University and a master’s of science in operations management from the University of Arkansas.|