Spot the Soluble Salt Risk Before Blasting
A bridge rehabilitation crew is scheduled to abrasive-blast 4,200 m² of heavily rusted structural steel. The structure sits 60 m from a tidal estuary, and the previous coating has been removed by the owner's prior contractor. A pre-blast walkdown reveals white crystalline staining in the weld toes and pitted areas of the lower chords. The specification cites ISO 8502-6 (Bresle method) and requires chloride below 20 mg/m² prior to coating, but no salt testing is listed in the blasting work plan. The contractor argues that blasting to Sa 2½ will remove all contamination, so testing is unnecessary. The inspector must decide whether to hold the job. Ambient conditions are 12 °C, 68% RH, steel 3 °C above dew point.
1. Should the inspector permit blasting to proceed without salt testing? Justify with the mechanism of contamination and the specification requirement.
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No - testing must be added before coating, and the inspector should not accept the 'blasting removes salts' argument as a substitute for verification.
Mechanism: Chlorides in weld toes and pits are hygroscopic; they draw moisture through a coating film by osmosis, causing blistering and undercutting corrosion even under a sound film. Blasting removes surface salts but can drive contamination into pits and can leave salt residues in crevices that no visual Sa 2½ assessment detects.
Specification logic: The spec explicitly invokes ISO 8502-6 and sets a 20 mg/m² chloride limit. A limit without a test method is unenforceable; the work plan omission is a nonconformity regardless of the contractor's opinion.
Action: Require Bresle extraction (ISO 8502-6) at representative locations, including weld toes and pitted lower chords, with conductivity measurement per ISO 8502-9. Test after blasting but before coating, and re-test any area that fails after re-cleaning. Document results in the inspection report with location, method, and value.
2. The steel is 3 °C above dew point and RH is 68%. Evaluate whether this is acceptable for blasting and for subsequent coating, and state what the inspector should record.
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Blasting: A 3 °C margin is generally acceptable for abrasive blasting because dry blasting does not require the same moisture control as coating application; however, condensation risk rises if conditions shift. The inspector should monitor and record temperature, RH, and dew point before and during work.
Coating: Most specifications and coating data sheets require steel to be at least 3 °C above dew point for application. At exactly 3 °C, the margin is at the minimum - a thin margin that can be lost as ambient conditions change. The inspector should verify the product data sheet requirement (often 3 °C minimum, sometimes higher) and confirm the margin is maintained throughout application and cure.
Record: Ambient temperature, RH, dew point, steel temperature, time, location, instrument used, and calibration status. Note that the margin is at minimum and that conditions must be rechecked before coating.
3. After blasting, a Bresle test returns 45 mg/m² chloride in a pitted lower chord area. What is the correct disposition, and what does this tell you about the original work plan?
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Disposition: The area fails the 20 mg/m² limit. The inspector should reject the surface for coating, require re-cleaning (typically re-blasting or high-pressure washing followed by re-blasting) of the affected area, and re-test until results are below the limit. Coating must not proceed on the failing surface.
Interpretation: A 45 mg/m² result after blasting confirms the contractor's assumption was wrong - blasting alone did not remove contamination from pits. This validates the decision to require testing and demonstrates why visual cleanliness standards (Sa 2½) are not a proxy for salt cleanliness.
Follow-up: Expand sampling in similar pitted areas, review the work plan to add salt testing as a hold point, and document the nonconformance and corrective action in the inspection report.