Failure Mechanisms for Epoxy On Concrete And How They Compare To Zirconia's Technology

There have been multiple questions we have been asked regarding the use of epoxy (bisphenol-A plastic) coatings over concrete in the last year.  Many of these questions relate to why epoxy coatings fail so quickly from concrete surfaces, and why are they different from Zirconia’s inorganic concrete surface treatment coatings.

Epoxy Is Too Dissimilar

The root cause of this general issue is that inorganic materials require an inorganic solution (like cures like); accordingly, cement needs another chemically compatible material that functions similarly enough to effect repair.  Trying to make an organic (epoxy) paint layer behave similarly to an inorganic matrix (concrete) for preservation cannot work.  Their behaviors are too different from each other, and therefore they are functionally incompatible.

Functionally, for epoxies, this means many failures (delamination from concrete surface), or the requirement for extreme preparation methods that support short-term performance, at greater economic cost to the asset owner.

Another thing to keep in mind is that epoxy was never designed to fix concrete as it was originally an adhesive resin. This is why it sticks well to surfaces initially, but is not robust against the environment, and delaminates from the interface of concrete when exposed to humidity, water, salt, etc., which is inevitable.

Zirconia's inorganic Concrete Surface Coatings are specifically designed for concrete restoration and preservation.  Critically, it is also inorganic and shares chemical and behavioral traits with concrete - for instance, both are silicate based.  These thin (6-8 mil) coatings that penetrate, cross-link, and form a sturdy ceramic polymer around concrete elements.  This process transforms the weak, chemically unstable hydrated calcium concrete into a stable (bound-up) alumina-zirconia-silicate composite.  The coatings can breathe-out humidity and dissipate water pressure from concrete (sometimes called “outgassing”) since it maintains a micro-porosity.  Also, coatings are molecularly attached to concrete elements (sand, aggregate and cement paste), so they are not able to detach from concrete.  In summary, Zirconia’s coatings do not form a “film layer" over the asset that is separate from the surface, or that can peel or flake away; instead they form a composite with the existing asset surface.  This composite is immune to UV and environmental factors, and has a lifetime measured in millennia.

Failure Mechanisms For Epoxy

Epoxies have multiple failure mechanisms because of their dissimilarity with concrete.  For clarity, it is best to look at the failure mechanisms in terms of the following two categories:

A. Initial placement/installation vulnerabilities, and; 

B. In-place durability.

To be clear, some surface preparation issues affect Zirconia's water-based products.  For instance, the failure to remove oily residue from concrete keeps both our water-based chemistry and epoxy from forming a durable bond. However, in general, the coatings from Zirconia are far less vulnerable to surface preparation and environmental conditions during installation, since their nano-scaled particles flow into the concrete and humidity cure together.  The tables below provide a quick summary of the various vulnerabilities for each product.

A. Initial Placement/Installation Vulnerabilities

Everything begins with surface preparation.  So any surface contamination that inhibits attachment is likely to affect both epoxy and Zirconia’s coatings.  The issue of surface cleanliness prior to coating is similar for all coating systems.  However, due to dissimilar chemistry, the surface of concrete must be etched to enhance mechanical bonding for epoxy.

Inadequate Preparation

Common acid etching of the surface removes cement paste at the surface allowing a physical surface profile (CSP2 or 3) for concrete attachment.  Failure to etch sufficiently leaves epoxy without sufficient mechanical strength to preserve the bond. 

a)    However, if not neutralized, the acid will delaminate the epoxy by attacking the interface once coated. 

b)    Acid etching will not remove oil stains, paint, wax, tire shine silicon or a variety of other petrochemical contaminants that are common to concrete and cause failure to bond.

c)    The common muriatic acid (HCL 18 -33%) leaves behind significant volumes of chloride ions that attack rebar and increase corrosion cell activity, even if this acid is neutralized.  So, unfortunately, the byproduct of acid etching, even if done properly, is increased potential for structural steel corrosion, and asset degradation.  

Zirconia’s coatings do not require acid etching for proper attachment.

Surface Contamination

There are a great many contaminants associate within wastewater treatment and sewage infrastructure, including: fatty acids (oils from cooking), non-point source petrochemical residues from roads and driveways, consumer paints and silicones, all of which require removal from the concrete surface.  If the surface is deeply saturated and it is not practical to clean, then the surface needs to be abraded or removed.  This may require up to CSP-4 shot blasting or removal of the concrete and replacement. 

Unfortunately, given the large amount of space inside concrete for contaminants to be stored (up to 10% void space), and the unstable calcium chemistry, the surface may not survive aggressive cleaning.  This is especially true as the concrete will store petrochemicals that will "wick" to the surface initially, and over time will attack and destroy the epoxy bond.  This issue affects all coating systems because it disallows initial attachment no matter how much you clean.

Poor Bond Strength

Epoxy is a composite that must be mixed on site.  Heat, and humidity can affect curing rate. Uneven mixing can cause thermal variations and uneven curing which stresses the plastic coatings resulting in poor bonding. Also, low quality epoxies with too many fillers or additives will not have sufficient bond strength to be durable.  This issue is in addition to the problems with fundamental surface incompatibility.

These issues do not affect Zirconia’s coatings.

Insufficient Wicking/Wet-Out

Epoxies that are not designed specifically for the surface preparation and specific surface characteristics, will fail to fill in the voids created by surface preparation, which creates adhesion (commonly called "wetting out"). Epoxies that are too thick, cure too fast or shrink too drastically will not wet-out the concrete.  Quick cure epoxies, hot/humid weather can lead to rapid curing and failure to surface adhere.

These issues do not affect Zirconia’s coatings.

Poor mixing

Epoxies require mixing in the right ratios for sufficient time periods, or they cure unevenly.  Also, air entrainment will create spot-failure locations.  Either of these issues leads to disbondment.

These issues do not affect Zirconia’s coatings.

Humidity

Humidity is a complex issue that can affect the bonding of many coatings, including epoxies and Zirconia’s coatings.  In areas like high desserts or during extreme hot/cold, the humidity may drop below 30% and require additional humidity and temperature buffering to augment the immediate environment.  

Overall, the use of organic chemistries like epoxy that are dissimilar, create additional burdens on the applicator to adjust the surface for compatibility. This requirement means additional failures from a variety of causes, short and long term.  In general, dissimilar materials make a building system more fragile, and elevate the cost of maintenance.

 

Table 1. Factors Affecting Surface Adhesion Failure: Zirconia vs. Epoxies

 

B. In-place Durability

Failure of adhesion in-place occurs for reasons in addition to those that affect initial bonding.  Because epoxy forms a dissimilar film, not a composite, there are a large number of failure mechanisms that can affect the film's bond.  

For instance, the expansion-contraction coefficient is different than concrete.  Over time, epoxy does not deal well with compression (cold), which accelerates delamination.  

Also, epoxy cannot deal with vapor pressure from humidity from behind because it does not breath, so that rain events or groundwater-related humidity from behind or under the concrete asset can cause delamination.  

Fundamentally, when placed out into the environment, almost any contaminant that interacts with the bond area at the concrete interface can dislodge the "sticky" connection between epoxy and the concrete since there is no molecular cross-linking, and mechanical bonding is really not sufficient to effect long term durability.  

Lifespan depends on the durability of the coating during standard operations and in real world conditions.Because the dissimilarity of materials, non-breathing film formation, and inadequate bond durability, the ways in which epoxies can fail (delaminate) from the surface go well beyond this document.Here are a few:

 

Table 2. Durability Factors: Zirconia vs. Epoxies

 

Conclusion

Zirconia's Concrete Surface Coatings eliminate the problem of dissimilar materials, and thus greatly diminish failure pathways. They physical and chemical durability of the coatings greatly exceeds and dominates the weaker calcium chemistry of portland concrete, which preserves the asset in standard operations, and over long-term environmental exposures.

The non-toxic and extreme durability under harsh conditions of Zirconia's Concrete Surface Coatings lower the cost of asset preservation dramatically and preserve human health and the environment.