White Metal on a Concrete Floor
A contractor must remove a failing epoxy coating from 4,000 sq ft of concrete floor in a food-processing plant. The specification says 'white metal blast,' but the substrate is concrete. The crew has a 20,000 psi, 8 gpm rotary surface cleaner with a 15-degree fan tip. The plan is a 4-in. standoff and one pass at 3 ft/min. Ambient temperature is 50 degrees F, and the floor is damp from washdown. A test patch at 20,000 psi and 6-in. standoff removed the coating but eroded the cement paste and exposed aggregate in spots; a patch at 10,000 psi smeared the coating and left a gummy residue. The client wants 'no visible coating' and a rough profile before a new epoxy is applied. The crew has 6 hours of production time and one spare tip.
1. How should the crew interpret the 'white metal blast' requirement for this concrete substrate?
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White metal blast is a steel surface preparation standard, not a concrete standard. Concrete cannot be brought to white metal because there is no steel surface and the cement paste would be destroyed. The correct reference for concrete is a concrete surface profile, typically described on the ICRI CSP scale. The specification is therefore ambiguous or copied from a steel job, and the crew should request clarification before production.
The test patches show why the distinction matters: at 20,000 psi the coating came off but the paste eroded and aggregate exposed, which is over-preparation; at 10,000 psi the coating smeared, which is under-preparation or a pressure/heat problem. The acceptance target should be a defined CSP range and no visible coating, not a steel blast standard.
2. Recommend a test-patch plan and operating parameters to meet the client's requirement without damaging the concrete.
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Use the existing patches as endpoints, then bracket the acceptable range. Run additional patches at approximately 8,000, 10,000, and 12,000 psi with the same 15-degree tip, a 6-8 in. standoff, controlled overlap, and a travel speed adjusted so the cleaner does not dwell. Keep the surface damp but not flooded; at 50 degrees F, water temperature and dwell time affect smearing.
Evaluate each patch after it dries: coating removal, uniformity of profile, exposed aggregate, and paste erosion. Choose the lowest pressure and fastest travel speed that removes the coating without eroding paste. If 10,000 psi smears, try a faster pass or a slightly greater standoff before increasing pressure; if 20,000 psi erodes, do not use it as the production setting. Document the selected pressure, standoff, tip size, and travel speed, and get client approval of the patch before the 4,000-sq-ft run.
3. What acceptance criteria and documentation should be required before the new epoxy is applied?
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Acceptance should be based on the approved test patch: no visible coating, a uniform concrete surface profile within the agreed CSP range, no laitance, no fractured or loose aggregate, and no standing water. The surface should be checked for moisture and pH if the coating manufacturer requires it, and the profile should be verified with a replica tape or comparable method in representative areas.
Documentation should include the approved patch location and photographs, the pressure and travel speed used, tip size and standoff, ambient and surface temperature, and any deviations approved by the client. The crew should also record where over-erosion occurred and how it was corrected. The reasoning is that a visual 'no coating' check alone would accept both the smeared patch and the eroded patch; the profile and paste condition are what determine whether the new epoxy will bond.