Concrete strength tests: how acceptance really works

Building · 24 Oct 2026

On most building sites, concrete is accepted on the basis of specimens made from samples of the delivered concrete. The rules for judging those results are simple, but often misread. This article explains the approach used in ACI-based specifications. Saudi Arabia's building code committee agreed in 2018 to use ACI documents in developing its own code.

The standards involved

• ASTM C172: sampling fresh concrete
• ASTM C31: making and curing test specimens in the field
• ASTM C39: testing cylindrical specimens for compressive strength in the laboratory
• ACI 318 and ACI 301: acceptance criteria for strength

Some specifications use cubes rather than cylinders, for example tested under EN 12390-3, Testing hardened concrete — Part 3: Compressive strength of test specimens. The specimen type and the acceptance rules then follow that specification. Do not mix the two systems.

What counts as "one test"

Under the ACI approach, one strength test is the average strength of specimens from the same sample, tested at the specified age (usually 28 days). That means either:

• two 150 × 300 mm (6 × 12 in.) cylinders; or
• three 100 × 200 mm (4 × 8 in.) cylinders.

A single broken cylinder is not a "test". Seven-day results show early strength, but they are not used for acceptance. Field-cured specimens are used to decide when to strip forms or load the structure. They are not used for acceptance either.

The acceptance rule

Concrete meets the specified strength (f'c) when both of these conditions are met:

1. The average of any three consecutive tests is at least f'c.
2. No single test falls too far below f'c:
3. where f'c is 5,000 psi (about 34.5 MPa) or less, no test is more than 500 psi (about 3.5 MPa) below f'c;
4. where f'c is above 5,000 psi, no test is below 0.9 f'c.

NRMCA notes that a properly proportioned mix will still fail this check about once in 100 tests, because of normal variability. One low result is not automatically a structural problem.

Why results come back low

NRMCA's guidance on low cylinder strength names improper handling, curing and testing of specimens as the most common cause. Typical problems include:

• specimens left on site for extra days, or exposed to heat or frost;
• initial curing above the allowed temperature;
• poor finishing, poor capping or uneven ends;
• damage in transport to the laboratory;
• careless breaking, or a poorly maintained test machine.

Real concrete problems also occur:

• water added on site to increase slump;
• high air content;
• production errors at the batching plant.

What to do when a test fails

1. Look at all the results, not just the low one. Check the sequence, the range between companion specimens, and the fracture patterns.
2. Check the specimen history. Who made them, when they were moved, and how they were cured and transported.
3. Check the laboratory. Look at machine calibration, capping or end preparation, and technician certification.
4. Check the delivery records. Look for water added on site, slump, air content, temperature and time from batching.
5. Only then investigate the structure. If the low result appears real and the strength matters structurally, an in-place investigation (for example cores) is the next step, agreed with the engineer.

Getting it right from the start

• Use trained, certified technicians to sample and make specimens.
• Provide a proper curing location on site, protected from sun and wind.
• Move specimens to the laboratory on time and pack them well.
• Label each specimen with the pour location, truck and time.
• Keep test results in sequence so the "three consecutive" rule can be checked.

Summary

Acceptance depends on correctly defined tests, judged by a two-part rule. Most low results come from how specimens were made and handled, not from the concrete. Investigate in order, starting with the paperwork and the specimens, before cutting into the structure.

General guidance only. The authority having jurisdiction and the official text govern.