One of the common problems in commercial construction is an indication of low concrete strength, based on concrete cylinder tests. It is unfortunate that concrete cylinder tests are used as a basis of "passing or failing" concrete on a project. The results are often inaccurate and variable even if all parties are performing their respective duties to the best of their abilities. The greatest majority of the time it is proven that there is not a concrete strength problem. The concrete cylinder test is intended to give an indication of conformance to compressive strength requirements during the course of a project. If a cylinder test result is below design strength requirements, it does not mean that there is bad concrete. A low cylinder strength is just a red flag to indicate there is potential for a problem and further investigation is needed. Unfortunately the producer is typically found to be guilty until proven innocent.

If a compressive cylinder test is low, several actions should be initiated. The first concern will be for the concrete structure. The structure can be evaluated based on the cylinder strength to determine if adequate strengths have been attained. This can be done using the ACI 318 running average of three method and/or actual formulas to determine structural needs. Tests can be performed on the in-place concrete (Windsor Probe or cores) to determine if the cylinders were indicative of the in-place strength. Load testing can be performed to determine capacity of concrete.

The ready-mix producer should look at batch records, plant performance, material quality, testing procedures and batch procedures to determine areas in need of improvement.

The testing lab needs to look at all procedures pertaining to sampling, making cylinders, curing, transporting, handling, capping and testing. Minor variations in any of these procedures can produce erratic test results.

The contractor needs to provide a stable secure area for the testing to be performed, including initial curing of the samples. The contractor also should communicate with all trades involved the importance of spacing of concrete truck deliveries, slump control and protection of cylinders.

Many of these issues will be discussed prior to the first concrete place if a pre-pour conference is held. Please refer to the pre-construction checklist and use this whenever possible.