Surface Preparation Failure on a Contaminated Concrete Substrate
A grout crew is installing a precision non-shrink grout under a 900 mm × 900 mm pump base on an existing concrete floor. The substrate was previously used as a machine shop and shows dark oil staining across roughly 60% of the grout footprint. The crew wire-brushes the surface, vacuums it, and proceeds to place grout. The concrete surface is also smooth and glossy in the stained areas, suggesting a power-troweled finish. After 14 days, a tap test reveals hollow sounds along two edges of the base plate. The specification requires a minimum surface profile of CSP 3 and a saturated-surface-dry (SSD) condition before grout placement. The ambient relative humidity is 45%, and the concrete was not pre-wetted.
1. Identify the surface preparation deficiencies and explain how each contributed to the hollow sounding areas.
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Three deficiencies are present. First, oil contamination was not removed. Wire brushing and vacuuming do not remove oil that has penetrated the concrete pore structure; oil creates a bond-breaker between the grout and substrate, preventing adhesion and allowing debonding. Second, the surface profile is inadequate. A power-troweled, glossy finish is far smoother than CSP 3, which requires mechanical abrasion (e.g., shot blasting or grinding) to create a roughened profile. Without profile, the grout relies on adhesion rather than mechanical interlock, and adhesion is weak on a smooth surface. Third, the concrete was not pre-wetted to SSD. At 45% RH, dry concrete absorbs water from the grout mix, reducing the effective w/c at the interface, causing rapid drying and shrinkage, and creating a weak, porous bond zone. Together, these produced debonding at the edges, heard as hollow sounds.
2. What remediation and re-preparation steps should be specified before re-grouting, and how should the SSD condition be verified in the field?
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Remediation should begin with removal of the debonded grout, then mechanical surface preparation: shot blasting or diamond grinding to achieve CSP 3, followed by oil removal using a suitable degreaser or, if contamination is deep, a topical treatment or removal of the contaminated concrete layer. The surface should be checked for residual oil by a water-break test - water should sheet evenly without beading. Before grout placement, the concrete should be pre-wetted and allowed to reach SSD: the surface should be damp but with no free-standing water. Field verification: apply water, wait for absorption, and confirm the surface is uniformly damp with no sheen or puddles. A common field check is the plastic sheet test or simply observing that water no longer beads. The grout should then be placed while the substrate is SSD, and the perimeter should be checked with a tap test after curing.