Colin Lobo, senior vice president of engineering at the National Ready Mixed Concrete Association has a response to comments from Pat Harrison and Jerry Holland on the question of prescriptive versus performance specifications for concrete:

Just want to clarify what we saw in specs that we reviewed (see Five Most Onerous Mix Requirements) . What we identified were prescriptive requirements stated as general requirements and not related to any specific application. For example in ACI 318, w/cm limits are specifically stated for some durability exposure. We did not see any stated exposure class to the stated limits on w/cm (This has been in 318 since 2008). Other requirements were also not clearly identified as being related to slab mixes. So I don’t think this review can be taken as a dialogue on what is being discussed in your blog. I think we are bemoaning the inclusion of prescriptive provisions with no clear stated or assumed performance objective as identified to a specific application – this causes inherent conflicts within a spec with no apparent benefit or ability to optimize mixtures.

We saw limits on SCMs for ACI 318 exposure class applied to all concrete, minimum cement content much higher than those in ACI 301 for slabs, etc. Restrictions on aggregate grading were not clearly evident as applicable to slab mixes. I think our frustration was that most of these specs are even consistent with ACI 301.

We have worked with ASCC through Ward Malisch and Bruce Suprenant and developed a checklist for discussion between producers and contractors. This is on the P2P section of our website. These were not developed as “specification” items but as a means to start the conversation. Clearly what is needed for exposed slabs in terms of performance measurements are more subjective than quantifiable. We teach some of the ACI 302 info in our courses – so it's not trying to ignore what is needed for these applications. Realize that many of these items are not conducive to enforceable specification requirements.

I think many producers are likely fine with prescriptive requirements if the responsibility for resulting problems are not attributed to them.