Question: I’m bidding on a project that has a 4000 psi design strength specification and a maximum 0.45 water/cementitious materials ratio (w/cm) requirement. I can produce 4000 psi at a much higher w/cm than 0.45, so why is this impractical value being specified?
Answer: You’ve likely encountered an attempt by the engineer to include requirements for structural design and durability. Only 4000 psi is required for structural purposes. However, if the specifier is concerned about durability – because of, say, exposure to sulfates or deicing salts – the concrete must reach 5000 psi to 5500 psi. Your specifier didn’t word the specifications in accordance with ACI 318 “Building Code Requirements for Structural Concrete and Commentary” (2014).
Before 2008, it was common to specify design strength as well as a maximum water/cementitious materials ratio (w/cm) and/or a minimum cementitious factor. To keep specifiers from arbitrarily selecting these values, ACI modified 318 in 2008 to follow eurocode performance-based exposure classes, which reference maximum w/cm or minimum cementitious factor depending on the conditions the concrete is expected to face during its lifetime. The maximum w/cm values are designed to produce concrete with lower permeability or higher abrasion resistance than mixes with a normal structural strength requirement.
However, almost 10 years after ACI codified exposure classes, many specifiers don’t use them. Of course, project owners and designers can specify whatever they want, but deviating from the code should not be undertaken lightly.
Another problem is with the code’s wording. According to Chapter 19 (19.2.1.1):
The value of design strength shall be specified in construction documents and shall be in accordance with (a) through (c):
(a) Limits in Table 19.2.1.1(b) Durability requirements in Table 19.3.2.1 [author’s note – these are the exposure classes]
(c) Structural strength requirements.
Therefore, design strength specifications should reflect the maximum w/cm or minimum cementitious factor in the exposure classes. But since relationship between w/cm and strength depends on the materials used, how can the specifier determine the correct correlation before a supplier is selected?
In addition to durability concerns, there may also be other requirements – such as those affecting the finishing of a slab – for the mixture. Section 8.4 of ACI 302.1R-15 “Guide to Concrete Floor and Slab Construction” (2015) includes design requirements for maximum w/cm and minimum cementitious factor. ACI 302.1 is a guide, though, not mandatory. Any information in it the specifier wishes to include in contract documents must be explicitly entered.
P.S. Beginning in February, the American Concrete Institute will provide guides, recommended practices, and other non-mandatory documents free of charge to members.
