International Concrete Abstracts Portal

The customary criterion for establishing grades of structural concrete is the crushing strength measured in an arbitrary manner on a standardized specimen stored in a stipulated fashion for approximately a month after making. In some parts of the world, the specimen is a cube, and in other parts of the world, the specimen is a cylinder. There have been debates as to how the strength at 28 days can be predicted from procedures performed at early ages, especially 24 hr, on the grounds that a month is not soon enough. The intent of this paper is to suggest that the only time that really is "soon enough" to know that the grade of the concrete in any batch is correct is before the concrete is discharged from the concrete mixer into the forms. It is suggested, therefore, that effort would preferably be expended upon insuring that the materials used are those intended and the proportions in which they are used are those that were intended. If this is the case, the grade of concrete will be proper at all ages, and testing at any age is merely confirmation.

The three power centers in the world today must support the tremendous concrete construction and building investments that are needed in the developing regions where 90 percent of the world's population lives. Concurrently, renovations and renewals are required in industrial countries. Profound updating of conventional concrete technology is necessary, recognizing the differences between the behavior of test samples of concrete under laboratory conditions and of field concrete. For example, the historic development of curing concrete is reviewed with emphasis on the methods for monitoring heat development during curing of modern concrete. Proposals for wider transfer of this technology are also presented.

In-place testing is used to estimate the compressive strength of concrete in a structure by measuring another related property. A strength relationship is used to convert the in-place test results to an estimate of the compressive strength. Statistical methods are needed for reliable estimates of in-place strength. Such methods should account for the uncertainties in the measured property, the uncertainty of the strength relationship, and the variability of the in-place concrete. Standard statistical procedures for dealing with these uncertainties have not yet been adopted in North American practice. Recommendations are provided for developing the strength relationship, and a reliable, easy-to-use approach is presented to estimate in-place characteristic strength.

Strength interpretation problems are created when standard practices and procedures for sampling and testing concrete mixtures were not followed during construction. Cylinder and core compressive strength records are reported for a project that required extensive coring due to low-standard 28-day cylinder strengths. Records are reported for 4000- and 6000-psi concrete mixtures. Over 80 core strengths correspond to the 6000-psi mixture that was used to cast columns, grade beams, pedestals, and shear walls. The described statistical methods were useful for analyzing the quality of core strength data and interpreting the significance of the results. Core strength results were analyzed by mixture, placements, and type of structural member. For the analysis, "Stem and Leaf" and "Box and Whisker" plots were used to identify outliers. "Analysis of Variance" was used to test for equality of mean strengths. "Fisher's" and "Tukey's" procedures were used in identifying significantly different mean strengths. The Chi-square test was applied to evaluate normality of distributions. The characteristic in-place strength was determined by using the tolerance factor. The analysis shows the importance of obtaining representative core strength samples when determining code compliance.

For some years, the Ministry of Transportation of Ontario (MTO) and the Ready-Mixed Concrete Association of Ontario (RMCAO) have separately run programs to measure the reproducibility of test results between commercial laboratories. The comparative test results were made on standard cured 150 mm diameter by 300-mm test cylinders. The reproducibility of test results obtained between testing laboratories on nominally identical test specimens is an important factor in the statistical evaluation of test data. Unless the reproducibility is good, concrete mixes need to be overdesigned, with resulting higher costs. In addition, poor reproducibility between laboratories increases the probability that some test cylinders will fail to meet specified strength requirements. Even if the concrete in the structure represented by these results probes to be adequately strong, disruption and economic loss result from the publication of erroneous results. This paper analyzes the data obtained by the RMCAO, derives repeatability and reproducibility indexes, and discusses their economic consequences. Some data from the MTO program are also reported. Recommendations for improvements are made.