Photos courtesy of CTL Group.
It’s never a good day when large chunks of a newly installed three-layer epoxy coating system begin to come off of any concrete floor — and even more troubling when they come off of a bridge deck.
Concrete is absorbent. It will hold chlorides, moisture, oils, greases, chemicals, gases and anything else that comes into contact with it. Cracks on a roadway, and more specifically on a concrete bridge, can provide a pathway to the rebar for contaminants, and particularly for chlorides from road salt. Rebar is wrapped in a figurative alkaline blanket approaching a pH of 11 when sitting in concrete. The alkalinity creates a thin patina on the rebar which, if maintained at that pH, will prevent corrosion for decades. However, if chlorides do enter the concrete and lower the pH, the corrosion rate of the rebar can skyrocket and therefore, preventing this phenomenon is critical.
Cracks and concrete go hand-in-hand. Concrete tends to crack for a variety of reasons and while almost always inevitable, instances of cracking can be reduced with proper design and engineering and if cracks do occur, they can be properly managed with appropriate coatings and other remedial solutions.
The Background
Fig. 1: An overall view of a lapped section of Core Sample No. 4.
The original specification called for an epoxy flood coat and two layers of an epoxy overlay with aggregate as well as the following surface preparation procedure.
“Prepare the surface in accordance to the manufacturer’s recommendation. At a minimum, ensure that the bridge deck surface is clean and free of any loose materials and unsound concrete or any other surface materials that, in the manufacturer’s and engineer’s opinion, would prevent proper bonding to and cure of the system. Abrade the entire deck surface by shot blasting to expose the course aggregate.”
The coating system applied was representive of industry standard materials and the specification was well-written. The system was designed to repair and protect a bridge deck and the associated sidewalk which was showing typical signs of wear, such as cracking and spalling.
The Flood Coat
Flood coats are self-leveling sealers and typically have very low viscosity — somewhere between corn oil and maple syrup. Their rheology is designed to penetrate deeply to fill cracks, voids and other imperfections, as well as allow for a level surface over which to apply successive overlays and topcoats. Flood coats are typically pure resin and designed to completely saturate the concrete but not necessarily leave a measurable film thickness, or at least a very thin one.
The Overlay
Overlays can be made from a variety of base resins and are typically aggregate-filled. In this case, the specification called for an epoxy mixed with aggregate.
FAILURE ANALYSIS
The failure in this case was disbondment and was clear and widespread, but trying to determine what was disbonded from what was the challenge.
Visual Inspection
The deck showed clear signs of distress in the form of disbonding sheets of what appeared to be the concrete overlay. Hammer testing (listening for changes in sound) was carried out which confirmed that the overlay was compromised.
A hammer and chisel were then used to test the overlay for adhesion, which further confirmed the ongoing disbondment. The concrete deck substrate appeared to be sound and exhibited cracking, which was anticipated.
Laboratory Analysis
Unlike coatings on steel, where a Tooke Gauge can be used to obtain a cross-section view of an entire coating system, the only way to examine coating systems and overlays on concrete is to extract a core sample taking care to avoid damaging the rebar.
Figure 1 is a side view of the top of the core sample which was roughly 5.5-inches thick. The view below the dark top band is of the concrete slab and the top dark area is the flood coat with the overlays on top of that.
Fig. 2: A close-up of a lapped section of Core Sample No. 4 showing the epoxy overlay/concrete substrate interface.
Figure 2 is a closer look at the top of the same core. The magnification is not yet sufficient to view the flood coat, but the resin in the two layers of the overlay are clear. Visually, the two coats of overlay appear as one and the border, or plane, between them is invisible, which is the desired outcome. Further, the overlay does not exhibit any air entrapment, blistering, discoloration, cloudiness or any other type of visual distress.
Figure 3 shows the magnification necessary to successfully view the flood coat. This photo shows what appears to be good adhesion between the flood coat and the overlay above, as well as good adhesion between the flood coat and the concrete substrate. Of some concern, however, is the apparent lack of penetration of the flood coat into the concrete substrate, as well as the surface appearance of the substrate.
Fig. 3: A close-up view of the separation between the epoxy overlay and the concrete substrate.
Conclusion
Figure 4 reveals the adhesive separation between either the flood coat and overlay or concrete substrate and overlay. The three coats of material appeared to have cured, become well adhered to one another and forming one monolithic system. Because the flood coat is so thin, it could be reasonably asserted that the failure is adhesive in nature between the monolithic coating system and the concrete substrate.
Fig. 4: Close-up view of separation (filled with blue epoxy) between the epoxy overlay and the concrete substrate under cross-polarized light.
Most coating failures, particularly those on concrete, are due to application error, and the main cause of application error is poor or inappropriate surface preparation.
Referring back to the specification for surface preparation, guidelines revealed a common but questionable practice of referring or deferring to the manufacturer’s recommendation for guidance. This is a common practice more often found in public works specifications than private sector specifications due to the requirement of specifying multiple products. The problem with this is that manufacturer application specifications are sometimes overly broad and lack specificity.
This specified surface preparation included abrading “the entire deck surface by shot blasting to expose the course aggregate” and herein lies a strong clue.
The photos fail to show any exposed aggregate of the concrete substrate, which can clearly be seen in Figure 2 where the concrete substrate is uniformly straight and lacks the hills and ruts often associated with abraded concrete. Also, inspection of the concrete underneath the disbondment showed substandard surface preparation.
Further analysis determined also that the surface preparation itself was not sufficiently aggressive to 1) provide sufficient surface area and expose enough course aggregate for adhesion of the flood coat and successive overlays, or B) remove any contamination which might prevent the penetration of the low-viscosity flood coat.
Both the driving deck and the sidewalk were coated, and while the sidewalk received similar surface profiling to the deck (roughly CSP 3-to-4) it was apparently sufficient enough to remove a dense concrete topping of some type, which allowed the flood coat to penetrate into the exposed pores of the concrete on the sidewalk, providing better adhesion and performance than on the bridge deck.
The findings report stated: delamination is occurring between the epoxy overlay and the original bridge deck concrete. The delamination is primarily the result of inadequate concrete surface preparation. The bridge deck concrete estimated surface profile is CSP 3-to-4. For epoxy overlay systems, a fairly rough surface profile is desired (CSP 7 with large aggregate exposed/partially uncovered). The observed surface profile of the existing concrete indicates that surface preparation was not aggressive enough to expose coarse aggregate to provide a proper surface profile for bonding.
Acknowledgment
The author would like to thank the CTL Group of Skokie, Ill for assistance with this article.
