The new trend of using high-strength concrete in construction has caused a need for the use of 4 x 8 in. cylinders for assurance testing. A controlling factor that affects the size of specimen that can be tested in a compression machine is the strength of the concrete on evaluation. Some testing machines are not able to produce the force needed to break high-strength 6 x 12 in. concrete cylinders. If 4 x 8 in. cylinders are to be used in quality assurance testing, the relationship between fc4 and fc6 needs to be understood in order to ensure that concrete with sufficient strength is provided. If the average compression machine operates safely, rarely exceeding 80% of its capacity, and has a capacity of 250,000 lbs, the machine can test a 6 x 12 in. cylinder with a compressive strength of approximately 7,000 psi. The same machine can test a 4 x 8 in. cylinder of approximately 16,000 psi.
This research project was born from the need to determine a correlation between the strength of the standard size 6 x 12 in. cylindrical specimen and the strength of a 4 x 8 in. cylindrical specimen made from the same batch of concrete. The objectives of this study are to review the factors that may affect the compressive strength, those that may affect the strength obtained by 4 x 8 in. and 6 x 12 in. cylinders, and the variability associated with these tests. An extensive laboratory testing program was developed to evaluate the desired goals of the project. A total of 359 4 x 8 in. and 357 6 x 12 in. cylinders were tested.
The factors that were studied to evaluate the effect of cylinder size on concrete compressive strength were aggregate size, technician, compressive strength, and age of specimen at testing. It was determined that compressive strength was the only factor significant in affecting the ratio of 4 x 8 in. cylinder strength to 6 x 12 in. cylinder strength. Compressive strength was also the only factor significant in affecting the within-test variability of each batch of concrete. It is recommended that 4 x 8 in. cylinders may be implemented for quality assurance testing if the design strength of concrete is greater than 5,000 psi and the capacity of the testing machine will not allow the testing of 6 x 12 in. cylinders based on the design strength. However, if 4 x 8 in. cylinders are used, a correlation between the 4 x 8 in. and 6 x 12 in. cylinders should be determined using a capable machine for the project.