Cylinder Break Pattern and the Case for a Retest
A set of three 100 × 200 mm cylinders from a 35 MPa mix is tested at 28 days. Results: 31.2 MPa, 33.5 MPa, and 22.8 MPa. The low cylinder failed with a fracture that ran diagonally through the specimen and exposed a cluster of large voids near one end; the cap on that end was noticeably uneven. The specification requires the average of three cylinders to meet 35 MPa with no single cylinder below 30 MPa. The average is 29.2 MPa. The contractor argues the low cylinder should be discarded because of the visible defect, which would leave an average of 32.4 MPa - still below 35 MPa. The lab's records show the cylinders were stored in a moist room at 23 °C but that the low cylinder's cap was applied the morning of the test rather than 24 hours earlier.
1. Assess whether the low cylinder should be discarded, and whether discarding it changes the acceptance decision.
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Discarding a cylinder is justified only when there is documented evidence of a testing or specimen defect that explains the low result - not merely because the result is low. Here there is such evidence: a diagonal fracture through a void cluster and an uneven cap indicate a specimen/capping defect. The late cap (applied the morning of test) is a procedural nonconformity that can produce an uneven bearing surface and a low, erratic result.
However, discarding it does not rescue the lot: the remaining two average 32.4 MPa, still below the 35 MPa requirement, and with only two valid cylinders the set no longer satisfies the 'average of three' criterion. So the acceptance decision is unchanged - the lot fails the strength requirement - but the reason matters: the failure is now based on two cylinders, and the correct response is to obtain additional evidence (cores, additional cylinders from the same lot if available) rather than to declare the concrete deficient on a defective specimen.
The key judgment is separating 'this cylinder is invalid' from 'this concrete is nonconforming.' They are different conclusions and require different actions.
2. What does the late capping indicate about the lab's quality system, and what should be corrected?
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Capping the morning of test violates the standard requirement that caps be applied with enough lead time to develop adequate bearing strength (commonly at least 24 hours, and longer for some cap materials). A soft or uneven cap compresses under load, producing a low and often erratic break, and can also produce the diagonal fracture seen here.
The quality-system implications: (a) cylinder receipt and capping scheduling is not controlled; (b) the technician may not have been trained on cap cure time; (c) there is no check that caps are flat and sound before testing. Corrections: schedule capping at least 24 hours before test, use a flatness check (e.g., a straightedge or plate), document cap material and cure time on the test record, and flag any test where the cap was applied late so the result can be evaluated with that caveat. The lab should also review whether other recent results share the same defect.