International Concrete Abstracts Portal

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.

The Bureau of Reclamation has produced mass concrete since 1904. Since then, quality assurance of mass concrete has evolved from the rudimentary measurement of batch volumes to using computers to batch and evaluate concrete. The construction of Hoover Dam in 1931 was a major turning point in quality assurance of mass concrete, with the initiation of scientific evaluations of the physical properties of concrete and concrete-making materials. Reclamation's goal is to provide the most economical concrete mixture that will meet the design and construction requirements. Over the years, several effective procedures have been developed to meet that goal. The primary focus is to keep the amount of cementitious materials low. This is done in several ways. Large, nominal, maximum-sized aggregates are used, multiple coarse aggregates are blended to reduce the required mortar volume, and the fines content is kept very low. The next step is to use high quality materials. Reclamation's specifications for fly ash and aggregates are more stringent than the ASTM standards. To minimize overdesign, close control is maintained over the batching process, helping to keep a low standard deviation. Paper discusses the different methods the Bureau of Reclamation uses to produce economical, high-quality mass concrete mixtures.