Dr. Heather Brown, Ph.D. Associate Professor, Concrete Industry Management Program Middle Tennessee State University

What could possibly go wrong? The better question to ask when talking about accurate cylinder breaks is what can’t go wrong. 6” by 12” concrete cylinders are the standard of checking in-place compressive strength for job performance, acceptance, checking mix proportion adequacy and gathering strength data for future quality control. The measured results are dependent upon adhering strictly to standardized uniform procedures, as outlined in ASTM C31 (casting cylinders), ASTM C617 (capping cylinders), ASTM C39 (breaking cylinders) and various other prescribed testing protocols. It is not surprising that problems can arise from failure to adhere to these various specifications and it is very important to realize that nearly all testing errors produce lower strength results. Some consequences of falsely low results are unnecessary delays, costly follow up testing, wasteful over-design and rejection of good concrete.

Here are some key steps that must be properly completed to insure accurate 28-day cylinder breaks.

• Sampling – technicians are required to obtain at least two portions from the middle third of the load. Remixing ensures consistency of the sample. Adhering to the maximum interval of fifteen minutes between obtaining the sample and casting the cylinders is important. Each set of cylinders must come from a single truck.

• Casting – technicians must choose the appropriate mold material, properly fill and consolidate cylinder molds, and ensure both good end condition and cylinder uniformity.

• Initial curing – this is critical to the accuracy of a cylinder. Temperatures lower than the specification (60-80 degrees F) can cause up to 7% loss in strength while exposure to freezing temps can cause up to 56% strength loss. High temperatures may boost early strengths but 28-day strengths will suffer. Cylinders must be properly stored in the field, in an environment that insures both proper temperature and moisture conditions and should be brought into the lab within 48 hours of casting

• Transporting cylinders – improper timing of moving cylinders can cause a loss of up to 7% in compressive strength. Proper handling during transport also prevents loss of cylinder strength.

• Laboratory curing – Wet curing in a 73 +/- 3 degree F environment maximizes the hydration of the cement, and is required by the specifications. This curing should begin as soon as cylinders are received in the lab and continue until the time for breaking the cylinder.

• Capping – this involves paying attention to the end condition and the capping material. The capped ends must be flat, perpendicular to the specimen axis, parallel to each other and not excessively thick. The capping material should not induce a stress distribution in the cylinder that would cause inaccurate measured results.

• Testing – this step involves attributes of the compression machine such as loading rate, specimen misalignment, loading platens, seating behavior and machine calibration. To ensure a properly working machine, refer to ASTM C39 for rules to check compliance.

• Reporting – the last step is to properly record the results of the test, including a post-failure inspection report on the type of cylinder break. Dissemination of the results to all parties, including the ready mix producer, is essential to identify any problem areas and to confirm successful mix performance.

The table shown below is an example of how many things can actually affect the strength of a cylinder. To get more detailed information about these variables, there is information in an NRMCA Publication No. 179 by David N. Richardson.