- Author
-
Carino, N. J.
|
Guthrie, W. F.
|
Lagergren, E. S.
- Title
- Effects of Testing Variables on the Measured Compressive Strength of High-Strength (90 MPa) Concrete.
- Coporate
- National Institute of Standards and Technology, Gaithersburg, MD
- Sponsor
- Federal Highway Administration, Washington, DC
- Report
-
NISTIR 5405
October 1994
145 p.
- Distribution
- Available from National Technical Information Service
- Keywords
-
concretes
|
building technology
|
capping
|
high strength concrete
|
compressive strength
|
cylinder size
|
experiments
|
standards
|
statistical analysis
|
stress rate
|
testing machine
- Abstract
- A review is presented on the factors affecting the measured compressive strength of concrete specimens, with particular emphasis on the testing of high-strength concrete. A full factorial experiment was designed to examine the effects of cylinder size, end preparation, stress rate and type of testing machine on the measured compressive strength. Two concrete mixtures (45 MPa and 90 MPa) were used to determine whether there were interactions between strength level and the other factors. In addition, a 65-MPa mixture was required to allow testing four combinations of specimen size and testing machine. The cylinder sizes were 100 x 200 mm and 150 x 300 mm. The ends of the cylinders were either capped with sulfur mortar or ground flat. The stress rate was either 0.14 MPa/s or 0.34 MPa/s, which are limits currently specified in ASTM C 39 (AASHTO T 22). One hydraulic testing machine was of the manually-operated type with a capacity of 1.33 MN. The other hydraulic testing machine was of the servo-controlled type with a capacity of 4.45 MN. The general linear model technique and analysis of variance were used to analyze the results. Statistical analyses showed that all the factors had statistically significant effects on the measured compressive strength. On average, the 100-mm cylinders resulted in 1.3% greater strength, the faster stress rate produced about 2.6% greater strength, the ground cylinders were 2.1% stronger than the capped cylinders, and the 1.33-MN testing machine resulted in about 2.3% greater strength. There were significant interactions among the factors, so that the effects were greater (or smaller) than the average values depending on the particular factors 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% greater strength in certain cases. Besides the main test series, supplementary tests were done to investigate the effects of a defective spherically-seated bearing block. The defective bearing block had a concave depression within the central 100 mm. The maximum value of the depression was more than 0.2 mm, compared with the value of 0.025 mm currently allowed by ASTM C 39 (AASHTO T 22). Comparative tests with 68-MPa concrete showed no difference in mean strength due to the defective bearing block. Analysis of dispersion showed that the 100-mm cylinders had higher within-test variability, but the differences were not statistically significant. Recommendations for modifications to testing standards and future research are provided.