Field Testing Dispute and Acceptance Criteria for Grout
A grout contractor has placed grout under a 1,500 mm × 1,000 mm base plate. The specification requires 28-day compressive strength of 50 MPa, tested on 50 mm cubes, with a sampling frequency of one set per 5 m³ or per placement, whichever is greater. The total grout volume is 3 m³, placed in one continuous operation. The contractor cast three cubes and tested them at 28 days, obtaining 52, 48, and 46 MPa. The average is 48.7 MPa. The specification states that no individual result may be more than 10% below the specified strength, and the average of any three consecutive tests must equal or exceed the specified strength. The contractor argues the grout is acceptable because the average is close to 50 MPa. The owner's inspector disagrees.
1. Apply the acceptance criteria to the test results. Is the grout acceptable? Show your reasoning.
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The criteria are: (1) no individual result more than 10% below 50 MPa, which is 45 MPa; and (2) the average of any three consecutive tests must be at least 50 MPa. Individual results: 52 MPa (pass), 48 MPa (pass, above 45), 46 MPa (pass, above 45). So criterion 1 is met. Criterion 2: the average of the three tests is (52 + 48 + 46)/3 = 48.7 MPa, which is below 50 MPa. Therefore the grout fails the acceptance criterion. The contractor's argument that the average is 'close' is not sufficient; the specification requires the average to equal or exceed 50 MPa. The inspector is correct. The contractor should review mix design, batching, and curing, and may request retesting or a concession, but the current results do not meet the specification.
2. What testing or sampling errors could have affected the results, and what should be done before deciding on remediation?
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Potential errors include: improper cube casting (inadequate compaction, incorrect mold filling, or poor curing of cubes), testing at the wrong age, cube storage at incorrect temperature or moisture, and sampling that did not represent the placed grout (e.g., samples taken from a batch with added water). The contractor should review the cube casting and curing records, verify the testing laboratory's calibration and procedures, and confirm the cubes were representative. If errors are found, retesting may be justified. If not, the contractor should consider additional testing (e.g., cores from the placed grout, if permitted) and consult the engineer. Remediation options depend on whether the in-place grout is deficient; if it is, removal and re-grouting may be required. The decision should be documented and agreed with the owner before any remediation.