Low Cylinder Breaks and Core Acceptance on a Two-Way Slab
A 300 mm thick two-way slab with fc' = 35 MPa is placed from a single ready-mix truck. Six 150 mm × 300 mm cylinders were cast, and the 28-day breaks from four sets, each reported as the average of two cylinders, are 36.5, 34.0, 30.0, and 32.0 MPa. The 30.0 MPa set came from concrete placed near the end of the truck; its batch ticket shows 110 mm slump and 5.5% air. The specification requires ACI 318 acceptance criteria. The contractor argues that most results exceed 35 MPa and that the low set is only 5 MPa below fc'. The inspector must decide whether to require core testing. Cores are later taken from the low-strength locations: 31.0, 33.0, and 29.5 MPa, using 100 mm diameter cores tested per ASTM C42. The EOR asks whether the slab can be accepted and what else should be verified before closing the issue.
1. Apply the ACI 318 strength-test acceptance criteria to the four test sets. Does the concrete pass, and is core testing required?
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Each reported test is the average of two cylinders. For fc' = 35 MPa, the individual-test limit is fc' − 3.5 MPa = 31.5 MPa, and the average of any three consecutive tests must equal or exceed fc'. Here the individual result 30.0 MPa is below 31.5 MPa, so the individual criterion fails. The three-test averages are (36.5 + 34.0 + 30.0)/3 = 33.5 MPa and (34.0 + 30.0 + 32.0)/3 = 32.0 MPa; both are below 35 MPa, so the consecutive-average criterion also fails.
The contractor's argument that 'most' results exceed 35 MPa is not the acceptance rule. One failing individual test and failing three-test averages are enough to make the lot nonconforming under the standard criteria. Core testing is therefore required before acceptance, unless the EOR directs a different strength-evaluation procedure. The 30.0 MPa result should also be checked for cylinder handling, break type, and whether the cylinders were sampled from the same concrete as the low area.
2. Evaluate the core results. Can the slab be accepted on the basis of these cores?
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For core acceptance, the average of the three cores must be at least 0.85 fc' = 0.85 × 35 = 29.75 MPa, and no individual core may be below 0.75 fc' = 0.75 × 35 = 26.25 MPa. The measured cores average (31.0 + 33.0 + 29.5)/3 = 31.17 MPa, which exceeds 29.75 MPa. The lowest core is 29.5 MPa, which is above 26.25 MPa.
On those numbers, the cores satisfy the standard core acceptance criteria, so the slab may be accepted from a strength standpoint, provided the cores are representative of the concrete in question and were obtained and tested properly. A core below 0.85 fc' is not automatically a failure; the limit is that no core falls below 0.75 fc'. If the core average had been below 29.75 MPa or any core below 26.25 MPa, the EOR would need a structural evaluation or further investigation rather than simple acceptance.
3. What else should the inspector verify before accepting the core results and closing the nonconformance?
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Verify that the cores were taken from locations selected by the EOR to represent the low-strength concrete, not from convenient high-strength locations. Check that the cores were 100 mm diameter or at least three times the maximum aggregate size, that they did not contain reinforcement, and that they were tested per ASTM C42 with proper length-to-diameter correction and moisture conditioning. Confirm the cores were not damaged during drilling or handling.
Review the original cylinder records for sampling and curing compliance, because the cores are the referee only if the cylinders were made and handled properly. Check batch tickets, truck times, slump, air, and any water added at the site. Confirm that the low test was not caused by a cylinder that was improperly consolidated or stored.
Finally, record the core locations, test results, and the EOR's written acceptance. If the cores had failed, the EOR would need to perform a structural evaluation or require remediation. Acceptance of strength does not automatically resolve durability or curing concerns, so also verify curing and finish condition.