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Unlike steel, concrete is a porous material that frequently contains water, has an elevated pH, may contain surface laitance and has other inherent characteristics that make it challenging to paint. Like steel, though, surface preparation is a key factor in the long-term performance of any applied paint system. In this investigation, catastrophic peeling paint from tilt-up concrete walls was observed as application of the coating was still in-process, making the failure extremely premature. Was it moisture, alkalinity, dust or an incompatible coating system?
BACKGROUND
As part of a newly constructed, multi-building facility, the interior concrete block and tilt-up panel walls were to be painted. The outer building walls were typically constructed with pre-fabricated concrete panels (tilt-up) and the interior walls were constructed with concrete masonry unit blocks. There were no issues with the construction of the CMU walls.
The contractor was actively performing painting work at the facility at the time the failure was reported and investigated. It was reported that much of the painting was performed during periods of high humidity and freezing temperatures. Conversations with the painting contractor foreman indicated that surface preparation prior to painting consisted of dry sweeping the walls.
EXPECTATIONS (THE SPECIFICATION)
The project painting specification addressed surface preparation before coating. Surfaces were to be cleaned of all substances that could adversely affect the coating adhesion, including dust, dirt, oil and grease. Release agents, curing compounds, efflorescence and chalk were all to be removed. Additionally, surfaces were not to be painted if the moisture content or alkalinity of the surfaces exceeded the manufacturer's written instructions. No industry standards for surface preparation were referenced in the specification.
The specification did include a list of approved coating systems. For the interior faces of the concrete tilt-up walls, the primer coat used was reportedly an alkali-resistant primer formulated for use on concrete (including tilt-up panels) and was listed in the specification. The specification required a minimum of 1.2 mils' dry film thickness of primer while the product data sheets listed a DFT range of 1.2 and 1.5 mils. The topcoat was reportedly an interior enamel semi-gloss latex that was listed in...