When Concrete Cylinders Lie

A 28-day cylinder breaks at 3,150 psi. The specified strength is 3,500 psi. The testing lab stamps the report: "Does not meet project specifications." The architect calls an emergency meeting. The contractor is told to prepare for removal and replacement.

But here is what most people in that room do not know: the concrete may be perfectly acceptable.

ACI 318 does not reject concrete based on a single test result. Under Section 26.12.6, acceptance is based on a running average of three consecutive tests. No individual test needs to equal f'c. It only needs to stay within 500 psi of f'c (for concrete specified at 5,000 psi or below). A result of 3,150 psi against a spec of 3,500 psi is only 350 psi below. It does not trigger a failure.

According to the American Society of Concrete Contractors (ASCC), these acceptance criteria have been the same since the early 1970s, yet they have been "often interpreted incorrectly" for over 40 years.

When test results truly fail, the next step is core testing per ASTM C42. If the average of three cores is at least 85% of f'c and no single core falls below 75% of f'c, the concrete is considered structurally adequate (ACI 318-19, Section 26.12.6).

But what if the problem is not the concrete at all, but the testing?

In February 2010, a New York jury convicted Testwell Laboratories of enterprise corruption for fabricating concrete and steel test results on nearly 120 construction projects, including One World Trade Center and the new Yankee Stadium. The company's president was originally sentenced to 7 to 21 years in prison (later reduced on appeal to 16 months to 4 years). A second lab, Stallone Testing Laboratories, pleaded guilty in April 2010 to falsifying results for the World Trade Center memorial and LaGuardia Airport control tower. A third, ASTC Laboratories, pleaded guilty in December 2012 to enterprise corruption charges involving the Jacob Javits Convention Center, Memorial Sloan Kettering Cancer Center, the Lincoln Tunnel, and the Second Avenue subway.

The lesson: When your cylinders come back low, the concrete is not always the problem. Before you reject a pour, verify the test. Before you trust a lab, verify their accreditation. And before anyone in the room makes a decision, make sure they understand what ACI 318 actually requires.

Read more: Concrete Acceptance Criteria for Dam QA/QC →