Question: I recently had a customer contact me regarding a low cylinder break result from his job. What does a low break mean and how should I respond?
Answer: Frequently, break results are assumed to represent the in-place strength of the concrete that was placed. A low break report excites everyone (owners, engineers, contractors, and producers). Quick actions are sometimes taken that are irreversible and often unnecessary, including removing perfectly acceptable concrete. The response depends on the actual contract documents, but understanding how code, specifications and standards work and what they say to undertake is important.
In Virginia, the International Building Code (IBC) is the legal code. ACI 318 is cited in the code and thus governs most things concrete. IBC also references specific ASTM standards which define how to conduct testing. The key to determining what to do with a low break result is to know what the purpose of the cylinders was in the first place, as that will determine how to interpret the results.
Most of the cylinders on commercial projects are cast for the sole purpose of determining whether the concrete was acceptable per the design strength (F’c) as specified in the contract. In ASTM C31 (Making and Curing Concrete Test Specimens in the Field) specific requirements are spelled out for cylinders to be used for acceptance, including the mandatory requirement that they be “standard cured”. In a low break situation, Step 1 would be to verify that the cylinders met all the standard cured requirements, for if they do not then they cannot be used for acceptance testing of the concrete. Initial curing is not optional, and without documented verification of the conditions, improper conclusions can be drawn.
If standard curing was followed properly, ACI 318 spells out actions to take depending on how low the individual break result is.
Case One (when the result is less than F’c but above F’c-500 psi) requires no further action. This is like running onto the rumble strips of a highway; it should raise awareness but does not indicate a failure. Combined averages of consecutive sets of three tests should also be monitored, and changes should be made to boost future results if any of these three set running averages fall below F’c. These changes may include raising the strength of the mix but could also mean improving the testing and/or handling, curing and testing of the strength cylinders. The intent is to avoid future failures by raising the average strength test results.
Case Two (when results show breaks below F’c – 500 psi) by code requires that the building official be involved. As all situations are not the same, the building official should apply judgement as to the results and their significance. If an investigation indicates the load carrying capacity of the structure is potentially compromised, it may result in the need for further testing, from non-destructive or in extreme cases to the taking of cores from within the structure.
Unfortunately, when acceptance cylinders are taken and proper testing requirements are not followed, the cylinders cannot be used for acceptance determination. Every effort should be made to avoid this situation, as additional testing of the structure may become necessary. This is one of many reasons why a pre-pour conference should be held, and proper testing should be emphasized. It’s important to not confuse the acceptance testing of the concrete with the structure, the results (good or bad) measure strength potential of the delivered mix, not the actual strength of the structure.
George R. Kuhn, Jr.
Chairman – VRMCA Technical Committee