Effects of Extended Discharge Time and Revolution Counts for Ready-mixed Concrete.

摘要

Specifications for ready-mixed concrete limit drum revolution and mixing time to 250 drum revolutions and/or 1.5 hours before discharge. These specifications have been in place for many years with the objective of ensuring the quality and performance of the finished concrete product; however, these limitations could potentially increase construction costs without additional benefits, especially when longer transport distances are required. Because there have been significant changes in concrete constituent materials (e.g., chemical admixtures) and equipment since these limits were first implemented, research is needed to assess the effects of time to discharge and drum revolution counts on concrete performance. The objective of this research program is to evaluate and verify whether existing specifications for mixing and transporting concrete are applicable for today’s materials and equipment and if not, to identify key variables or tests that can ensure adequate placeability and good mechanical and durability characteristics. This study evaluated the influence of coarse and fine aggregate, supplementary cementitious materials quantity and type, chemical admixtures, temperature and mixing time and drum revolution on the mechanical and durability characteristics of concrete. Results indicate that existing specifications may be appropriate due to loss of workability. However, mixing time and drum revolution counts had no significant effects on the majority of mechanical properties and durability characteristics of ready-mixed concrete and results from this research indicate that existing specification limits are very conservative.