Evaluation of concrete mixing and its effects on portland cement concrete (PCC) properties
In this study, mixing methods and their effects on properties of pavement Portland cement concrete are investigated. Two mixing methods and twelve pavement concrete mix proportions, specified by the Iowa Department of Transportation (DOT), are studied. Four composition tests (unit weight, air content, coarse aggregate content, and water content of sieved mortar) and two performance tests (slump and compressive strength) are employed. The average, range, and coefficient of variation (CV) of these measurements from each given batch are evaluated, and the results are further used as responses for statistical analysis. The results show that concrete performance properties change with both mixing method and mix proportions. The CV of batch composition tests changes with each mixing method. Some mix proportions are sensitive to mixing method. An air voids analyzer (AVA) is used to evaluate the effects of materials, mixing time, mixing sequence, and mixer on the air void system of fresh concrete. Air content, specific surface, and spacing factor of three mix proportions, C4 mix, C4-C (with class C fly ash), and C4-WR (with water reducer), are used to reflect the material effect. It is observed that fly ash replacement for Portland cement and addition of water reducer both reduce the air void spacing factor of concrete. Insufficient mixing time (< 2 minutes) may not produce a desirable air void system. A multiple-step mixing sequence and a large mixer are more favorable in creating a better air void system in concrete.