AGGREGATES IN SELFCONSOLIDATING CONCRETE

ir_71 · 03-04-2011, 06:38 PM

Self-consolidating concrete (SCC) is an advanced type of concrete that can flow through intricate geometrical configurations under its own mass without vibration or segregation. A research project was conducted to investigate the role of aggregates in SCC. Although SCC can be proportioned with a wide range of aggregates, the selection of favorable aggregate characteristics can significantly enhance the economy and performance of SCC. The objectives of the research project were to evaluate the effects of specific aggregate characteristics and mixture proportions on the workability and hardened properties of SCC, to identify favorable aggregate characteristics for SCC, and to develop guidelines for proportioning SCC with any set of aggregates. The effects of aggregate grading; maximum size; shape, angularity, and texture; clay content; and packing density were evaluated. Separately, the effects of mixture proportions, cementitious materials, and chemical admixtures were evaluated. In total, 12 fine aggregates, 7 coarse aggregates, and 6 microfines were tested. Tests were conducted on paste, mortar, and concrete. Paste measurements were conducted to evaluate the effects of cement, fly ash, microfines, high-range water-reducing admixture (HRWRA), and viscosity modifying admixture (VMA) on rheological properties. Mortar measurements were conducted to evaluate the effects of fine aggregates, microfines, and mixture proportions on workability and hardened properties. Concrete measurements were conducted to evaluate the effects of fine aggregates, coarse aggregates, microfines, and mixture proportions on workability and hardened properties. Target properties for SCC workability were defined as a function of the application and in terms of filling ability, passing ability, segregation resistance, and rheology. Seven workability test methods were evaluated extensively to provide sound, engineering justifications for their use and for the interpretation of their results. Specific tests for filling ability, passing ability, and segregation resistance were recommended. Based on the results of this research and well-established principles from the literature, a mixture proportioning procedure for SCC was developed. The procedure is based on a consistent, rheology-based framework and was designed and written to be accessible and comprehensible for routine use throughout the industry. In the procedure, SCC is represented as a suspension of aggregates in paste. In order to achieve SCC workability, the paste volume must be sufficient for the given aggregate blend and the paste rheology must be selected based on the aggregate blend and paste volume. The three-step procedure consists of selecting the aggregates, paste volume, and paste composition. Detailed recommendations are provided for each step. Aggregates are selected on the basis of grading, maximum size, and shape and angularity. The paste volume is set based on the aggregate characteristics. The paste composition is established to achieve workability and hardened properties. All required testing is conducted with methods standardized by ASTM International.

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