International Concrete Abstracts Portal

Describes the results of inspection of concrete operations and quality control of concrete at the Valley Bridge Project in Bluffs, Illinois, using a computer model, the CQC REPORT. The following variables affecting potential durability and strength of concrete in bridge structures were evaluated: accuracy of measuring of concrete materials, state of control and capability of batching operations, variations in SSD quantities of ingredients per cubic yard of concrete, yield, control over the water-cement ratio, properties of freshly mixed concrete, age of concrete at discharge, compressive strength of concrete cylinders, standards of control over concrete manufacture and concrete testing, amount of rejected concrete, and reason for rejection. Paper is concerned with the 6000 psi (41.4 MPa) concrete mix supplied for bridge spans from one of two batch plants used on the job. Paper also describes the structure of the CQC REPORT and statistical techniques used to analyze test data.

The Bureau of Reclamation has proportioned roller compacted concrete (RCC) mixtures since 1979, beginning with the Upper Stillwater Dam Project. Reclamation's research and project-related RCC investigations cover a variety of structures and varying site conditions. Reclamation proportions RCC mixtures to meet both fresh and hardened concrete properties. This assures construction of high-quality concrete structures. Ongoing research has continued to identify specific properties of RCC, such as bond strength, freeze-thaw durability, and thermal properties that significantly affect the performance of these structures. Paper summarizes RCC mixture proportions and properties of fresh and hardened concrete from Reclamation's RCC research and project activities.

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.

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.

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.