Grout Voids in a 20-Foot Reinforced CMU Shear Wall
A 20-foot-high, 12-inch CMU shear wall is reinforced with No. 6 vertical bars in grouted cells at 48 inches on center. The specified grout is 2,000 psi with an 8- to 10-inch slump. The crew places the first 10-foot lift in the morning and the second 10-foot lift after lunch. They drop grout from 5-gallon buckets from the scaffold, an 8-foot fall into the wall. They consolidate with a 2-inch vibrator whose hose is only 6 feet long. Cores taken later show voids at approximately 6 feet and 14 feet above the base. The contractor points out that the 12-inch units allow a 10-foot lift and refuses to remove the wall. The special inspector notes that no cleanouts were installed and that the grout was not reconsolidated.
1. Evaluate the contractor's claim that the 10-foot lift is acceptable. What does the lift-height rule actually require, and why does it not settle the issue?
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The contractor is treating a maximum permitted lift height as an automatic approval. A 10-foot lift may be permitted for 12-inch units under the project code, but only if the grout can be placed and consolidated without segregation or voids. Here, the 6-foot vibrator cannot reach the bottom of a 10-foot lift, no cleanouts were provided, and grout was dropped 8 feet from buckets. Those conditions make proper consolidation impossible. The cores confirm voids at two elevations. The lift-height provision does not excuse poor placement technique; it sets an upper bound. Because the wall is a reinforced shear wall, the voids are structurally significant until the engineer proves otherwise.
2. What caused the voids and what placement changes are needed?
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The voids are consistent with two failures: segregation from an 8-foot free fall and inadequate consolidation. Dropping grout from buckets causes aggregate to separate from paste, and a 6-foot vibrator cannot reach the lower part of a 10-foot lift. The lack of cleanouts also makes it impossible to remove debris or verify grout arrival at the bottom. To prevent recurrence, use a grout box, hopper, or pump line to limit free fall; use a vibrator long enough to reach the bottom of each lift; consolidate at the specified intervals; and provide cleanouts where required. The grout slump should be verified at placement, not just at the mixer. Reconsolidation requirements should be followed, especially in hot weather.
3. How should the wall be evaluated and repaired?
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The structural engineer should evaluate the wall using the core results and the wall's reinforcement and load path. Repair should not be chosen by the mason alone. Depending on the extent of voids, options include removing and replacing affected grout, removing and rebuilding portions of the wall, or an engineered repair such as supplemental reinforcement. Any repair procedure should be approved by the engineer and the special inspector, and verification should include additional cores or nondestructive testing at the repaired locations. The contractor should also submit a revised grouting plan that addresses lift height, free fall, vibrator length, cleanouts, and reconsolidation before work resumes. Acceptance based only on the contractor's reading of the lift-height limit is not adequate.