SCM dosage, w/cm limits lead producer beefs with prescriptive mix designs

Sources: National Ready Mixed Concrete Association, Silver Spring, Md.; CP staff

A survey on prescriptive mix designs reveals requirements NRMCA Research and Engineering Standards Committee members find most onerous: maximum water/cementitious material ratio when not applicable, minimum cementitious material content, plus restrictions on supplementary cementitious material dosages, aggregate characteristics like grading, and SCM type (i.e., Class F fly ash only) or characteristics.

RES Committee members cited 102 CSI MasterFormat 03300 Cast-in-Place Concrete specifications. NRMCA Engineering Division staff gauged each against the top onerous mix design requirements; a specification with all five was calculated as 100 percent prescriptive. Over the 102 specifications, 51 percent proved prescriptive. A typical 03300 spec has two to three prescriptive requirements out of the five measured. Among additional trends in the specification pool:

  • 85 percent restrict SCM dosage.
  • 73 percent cite maximum w/cm when not applicable.
  • 46 percent have minimum cementitious material content.
  • 27 percent restrict SCM type and characteristics.
  • 25 percent restrict aggregate characteristics.

NRMCA Engineering Division staff is preparing a full report on producers’ most onerous prescriptive mix design requirements, and plans surveys every three years to track progress toward performance-based specifications.