The National Ready Mixed Concrete Association has completed a research project titled Optimizing Concrete Mixtures for Performance and Sustainability. The work leading to the report’s publication was conducted at NRMCA’s research laboratory. The primary findings of the study are:
- At a fixed water-to-cementitious materials ratio (w/cm), increasing cementitious materials (CM) content and thereby the volume of paste causes an increase in permeability and drying shrinkage of concrete mixtures. At a fixed w/cm, changing the quantity of cementitious materials did not impact compressive strength unless the quantity was reduced to a low level.
- For the combination of materials used in this study, a lower-bound paste volume was established for concrete mixtures to achieve optimum performance. The lower-bound paste volume is expressed as a ratio of the combined aggregate void content. The use of high-range water reducing admixtures was effective in reducing the lower-bound paste-to-void ratio.
- A mixture proportioning methodology based on the paste-to-void ratio is proposed to achieve concrete with required strength, improved durability and lower shrinkage with optimized use of cementitious materials.
The results of this research study indicate that the inclusion of minimum cementitious materials content in specifications is questionable and even counterproductive to the performance of concrete. The suggested mixture proportioning methodology can be used to produce concrete with specified performance, improved sustainability and lower cost when not constrained by a requirement for content of cementitious materials. This research work was funded by the RMC Research & Education Foundation. The complete report can be downloaded here.