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Investigation of Critical Quality Assurance Parameter Variations for Concrete

The Indiana Department of Transportation is involved in preparing statistically sound specifications for strong and durable concrete used in quality assurance programs. Previous laboratory studies relating concrete strength to air content and concrete mix designs dealt with variation in compressive strength. This study searched for a statistically sound relationship between air content, concrete mix designs, and flexural strength. This study also developed a high-pressure method of hardened concrete air content determination. Sixty-four independent batches (combinations) of concrete were produced, each batch was subjected to a total of 24 tests—4 plastic and 20 hardened. The design factors were aggregate type and gradation, plastic air content, cement, and pozzolanic content and testing operator. After plastic testing, three flexural strength beams were cast from each batch of concrete. The experimental design response variables consisted of flexural, compressive, and split tensile strength along with pulse velocity. Analysis of variances, indicated that the optimum flexural strength could be obtained using as-received stone course aggregate and an air content of between 6 percent and 7.9 percent, with no fly ash. A high-pressure air meter, similar to the meter developed by the Army Corps of Engineers, was used. A strong statistical correlation of determination, r2 = 0.94, was obtained between plastic and the hardened concrete air content using this meter.