When pricing concrete, our salespeople have a problem with projects for which there are conflicting specification requirements. Some engineers and architects in our area specify both compressive strength and maximum water-cement ratio, but the water-cement ratios are ridiculously low for the strength given. For instance, for a 4000-psi 28-day strength, the maximum water-cement ratio is 0.35 for air-entrained concrete and 0.44 for non-air-entrained concrete. What is the basis for these extremely low water-cement-ratio values? And is there any way that I can convince the engineer or architect that they don't have to be that low?
The requirements appear in the August 1992 version of the American Institute of Architect's (AIA) MASTERSPEC. They're probably based on Table 3.10 of Specifications for Structural Concrete for Buildings (ACI 301-89) and the similar Table 5.4 in Building Code Requirements for Reinforced Concrete (ACI 318-89). However, the AIA values are a misuse of the tables cited. The tables give values of maximum permissible water-cement ratios to be used when acceptable field test records or trial mixture data are not available. The tables do not apply when acceptable field test records or trial mixture data are available. Nor do the tables apply to concrete containing admixtures other than those used exclusively for the purpose of entraining air. American Concrete Institute committees 318 and 301 recognized the problems caused by misuse of these tables and deleted them from the current versions of the respective documents ACI 318-95 and ACI 301-96. However, the requirements are still included in many specifications. Unless these low water-cement-ratio values are needed to ensure adequate durability, you could tell the architect or engineer that such specification requirements can actually produce lower-quality concrete. That's because the increased cement contents contribute to higher drying shrinkage and heat generation. This, combined with a higher modulus of elasticity (stiffer concrete) increases the possibility of random cracking caused either by restrained drying shrinkage or thermal contraction.