Two weeks ago we ran an item in the CC newsletter describing the five most onerous prescriptive requirements for a concrete mix, according to NRMCA’s Research and Engineering Standards Committee—things like maximum water-cementitious materials ratio, minimum cement content, and aggregate grading. Producers object to these requirements, preferring to provide concrete that meets performance requirements.
Pat Harrison and Jerry Holland with Structural Services Inc. objected, asking that mix designs for exposed slabs-on-ground and other important exposed slabs be excluded from this conversation until viable performance testing is identified, particularly for a competitively bid environment. “In our experience, adjusting the type and amount of SCMs allowed (if any), or reasonable minimum cement contents required, are necessary to achieve repeatable results for varying aggregates and placing environments. The quality of the mix paste has an enormous impact on the finisher’s ability to produce a uniformly densified, durable surface with a sustainable aesthetic that is easily maintained under a variety of conditions in service. The specification of a proper w/cm ratio (ACI 302 suggests a range from 0.47 to 0.55 according to the location and application) may allow the supplier to furnish a proper minimum amount of water required for the production of consistent quality concrete without arbitrarily increasing the amount of cementitious materials or chemical admixtures. Until NRMCA can verify viable testing for long-term shrinkage and curling (warping), material set, and placing and finishing characteristics for delivered concrete, please stop bemoaning the burden placed on the ready-mixed concrete industry by prescriptive specifications for exposed slabs-on-ground and other important exposed slabs. We have been asking since 2004 for these applicable performance-based measurements, other than 28-day compressive strength and slump, with no response. Without valid measurements, would it not be better to work together to improve prescriptive specifications that the engineering, contracting, and supplier communities could agree upon?”
What’s your feeling about performance-based specifications, especially for slabs?