Title: Effects of Testing Variables on the Strength of High-Strength (90 Mpa) Concrete Cylinders
Author(s): N. J. Carino
Publication: Symposium Paper
Appears on pages(s): 589-632
Keywords: capping (of concrete test specimens); compressive strength; cylinders; high-strength concretes; standards; statistical analysis; stresses; test equipment; Structural Research
A full factorial experimental design was used to investigate the effects of the following variables on cylinder strength: end preparation (sulfur capping versus grinding), cylinder size (100 versus 150 mm diameter), type of testing machine (1.33-MN capacity versus 4.45-MN capacity), and nominal stress rate (0.14 versus 0.34 MPa/sec). Two levels of strength were used (45 and 90 Mpa), and three replicates were tested for each run. Specific gravities were measured to check on the consistency of cylinder fabrication. Statistical analyses indicated that all the factors had significant effects on the measured compressive strength. On average, the 100-mm cylinders resulted in about 1.3 percent greater strength, the faster stress rate produced about 2.6 percent greater strength, the ground cylinders were 2.1 percent stronger, and the 1.33-MN testing machine resulted in about 2.3 percent greater strength. There were significant interactions among the factors, so that the effects were greater than the average values for particular factor settings. For example, the effect of end preparation depended on the strength level. For the 45-Mpa concrete, there was no strength difference due to the method of end preparation, but for the 90-MPa concrete, grinding resulted in as much as 6 percent greater strength in certain cases. Analysis of dispersion indicated that the 100-mm cylinders had higher within-test variability, but the differences were not statistically significant. Recommendations for modifications to testing standards are provided.