Coating Failure Due to Poor Adhesion
A newly applied epoxy coating on a concrete floor is failing by peeling after 6 months. The concrete was prepared by diamond grinding, and the moisture content was 4% when coated. The coating was a high-build epoxy at 300 microns DFT. The floor is in a warehouse with forklift traffic. The failure shows cohesive failure within the concrete substrate. The owner claims the coating was defective.
1. Determine the likely cause of failure and explain.
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The cohesive failure within the concrete indicates that the coating adhesion to the concrete exceeded the tensile strength of the concrete itself. This suggests that the concrete surface was weak or contaminated. Possible causes: 1) High moisture content: 4% moisture may be too high for epoxy; moisture vapor transmission can cause blistering and debonding, but here failure is cohesive in concrete, so moisture may have weakened the concrete. 2) Insufficient surface preparation: Diamond grinding may not have removed laitance or contaminants, but cohesive failure implies the concrete itself failed, so the coating bonded well. 3) Concrete strength: The concrete may have low tensile strength due to poor curing or high water-cement ratio. 4) Application over contaminated concrete: Oil or dust could weaken the bond, but cohesive failure suggests the bond was strong. The key is that the coating adhered well, but the concrete could not withstand the stress from forklift traffic or thermal movement. The owner's claim of defective coating is likely unfounded; the issue is substrate condition.
2. What remedial actions and preventive measures should be taken?
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Remedial actions: 1) Assess concrete strength: Perform pull-off tests to determine tensile strength. If low, repair or replace concrete. 2) Remove failed coating: Grind or blast to remove coating and weak concrete. 3) Surface preparation: Use mechanical preparation to achieve a profile and remove laitance; ensure moisture content is below manufacturer's limit (typically <4% or use moisture mitigation). 4) Primer: Apply a suitable primer to enhance adhesion and accommodate moisture. 5) Reapply coating: Use the same or compatible system with proper thickness. Preventive measures: 1) Specify concrete strength and moisture testing before coating. 2) Require mock-up to verify adhesion. 3) Quality control during surface preparation. 4) Consider moisture mitigation system if moisture is high. The owner should be informed that the failure is due to substrate, not coating defect.