Quality Control Testing and Acceptance Criteria
You are the quality control inspector for a wet-mix shotcrete project. The specification requires a 28-day compressive strength of 35 MPa and a bond strength of 1.0 MPa. The contractor has placed test panels for each 50 m³ of shotcrete. The panels were cured in a moist room for 7 days and then stored at 20°C and 50% RH until testing at 28 days. The test results for three panels are: Panel A: compressive strength 38 MPa, bond strength 1.2 MPa; Panel B: compressive strength 32 MPa, bond strength 0.8 MPa; Panel C: compressive strength 40 MPa, bond strength 1.1 MPa. The contractor argues that Panel B is an outlier and should be disregarded. The nozzleman recalls that Panel B was sprayed at the end of the day when the air pressure was fluctuating.
1. How should the test results be evaluated according to typical acceptance criteria? Should Panel B be disregarded?
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According to ACI 506 and typical specifications, acceptance is based on the average of test results and the individual results. For compressive strength, the average of three panels is (38+32+40)/3 = 36.7 MPa, which exceeds 35 MPa. However, individual results should not fall below a certain percentage (often 85% of specified strength, i.e., 29.75 MPa). Panel B's 32 MPa is above 29.75 MPa, so it may be acceptable. For bond strength, the average is (1.2+0.8+1.1)/3 = 1.03 MPa, which meets 1.0 MPa. But Panel B's 0.8 MPa is below the specified 1.0 MPa. Many specifications require that no individual test be less than 80% of specified (0.8 MPa), so 0.8 MPa is exactly at the limit. However, some require all tests to meet the specified value. The contractor cannot simply disregard Panel B without justification. The fluctuating air pressure during spraying could have caused lower density and bond. The inspector should review the testing procedure and consider retesting or taking cores from the actual structure. If Panel B is confirmed low, remedial action may be required.
2. What actions should be taken if Panel B is representative of the actual shotcrete in the structure?
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If Panel B is representative, the shotcrete in that area may not meet the specified bond strength. The inspector should first verify the test results by retesting the panel or taking cores from the structure. If the low strength is confirmed, the following actions are recommended:
- Identify the extent of the affected area (e.g., the area sprayed at the end of the day with fluctuating pressure).
- Perform additional testing (e.g., core testing, pull-off tests) to delineate the low-strength zone.
- If the area is small, remove and replace the defective shotcrete. If large, evaluate structural implications and consider remedial measures such as additional reinforcement or a bonded overlay.
- Review the nozzleman's technique and equipment; ensure air pressure is stable and monitored continuously.
- Implement stricter QC for future placements, including real-time pressure monitoring and more frequent testing.
- Document all findings and corrective actions.