Research raises questions on minimum CM specs, supports paste-to-void ratio rationale

A recent report on results of concrete mixtures testing at National Ready Mixed Concrete Association’s College Park, Md., research laboratory underscores optimized cement paste volume and paste-to-void ratio as key factors behind fresh and hardened specimens’ improved performance characteristics.

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Tests behind “Optimizing Mixtures” included Type II portland cement control plus Class F fly ash- or slag cement-dosed specimens. A control mixture with 0.55 water-cement ratio is shown here before and after dosing with a high range water reducer, the admixture imparting a workable concrete of 5.75-in. slump. Investigators observed the HRWA’s capacity to reduce specimens’ lower-bound paste-to-void ratio.

“Optimizing Concrete Mixtures for Performance and Sustainability” authors contrast the rationale for paste and void criteria against minimum cementitious materials (CM) content—prevalent in public and private project specifications. “Typically, specified minimum limits on cementitious materials exceed the quantity required for intended performance, such as workability, strength, and durability,” they observe. “This results in increased cost and higher carbon footprint of the concrete mixture. The performance implied or intended by the minimum cementitious material content may not be achieved or enforced.

“Minimum cementitious materials limits represent a significant restriction to a concrete producer towards optimizing concrete mixtures for performance and sustainability. Specifications that include these requirements do not provide any incentive for the concrete producer to invest in improved quality management systems and innovative technology … [and] prevent the evolution to performance-based specifications.”

The RMC Research and Education Foundation-funded testing spanned three phases involving evaluation of concrete mixtures designed with Type II portland cement, slag cement and Class F fly ash. Specimens were evaluated at three water-to-cementitious materials (w/cm) ratios, typical for most applications, with varying powder content. Coarse to fine aggregate volume ratio was maintained constant and paste volume varied to different levels relative to the void content of the combined aggregate. Cured 4 x 8 specimens were tested for compressive strength, chloride ion penetrability and overall permeability, revealing:

  • At a fixed w/cm ratio, increasing 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 CM quantity does not impact compressive strength unless the quantity was reduced to a low level. Inclusion of a minimum CM content threshold in specifications is questionable and potentially counterproductive to concrete performance.
  • For the combination of materials the researchers used, a lower-bound paste volume was established for mixtures to achieve optimum performance, and is expressed as a ratio of the combined aggregate void content. High-range water reducing admixtures were used advantageously to reduce the lower-bound paste-to-void ratio.
  • A proposed mixture proportioning methodology based on the paste-to-void ratio and embodying optimized CM volume can yield concrete with required strength, improved durability and lower shrinkage characteristics.

Testing and report preparation were led by NRMCA’s Vice President, Technical Services Karthik Obla, Ph.D., P.E.; Research Engineer Rongjin Hong; and, Senior Vice President/Division Head, Engineering Colin Lobo, Ph.D., P.E. Also contributing was Haejin Kim, Ph.D. of McLean, Va.-based SES Group & Associates. A pdf of the report can be obtained at