TACA Conveyor - Spring 2008 - (Page 19)

INDUSTRY NEWS Position Statement 7 The Misconception of W/CMT Ratio Design How to Spend Owners’ Money with No Return on Investment F rom a historical perspective, most engineers are taught that the use of a water to cement ratio (w/cmt) can be a design tool that is an implicit indicator of concrete performance. Unfortunately, the practical reality of using w/cmt for design is much less successful, is no guarantee of any actual performance, and in fact quite costly. The w/cmt ratio is a simple mathematical relationship between the weight of water to the weight of cementitious material in a concrete mix design, not a measure of performance. The cementitious material includes the cement and any pozzolanic materials that are used as a replacement or in addition to the cement. A speciﬁcation that makes use of a w/cmt will state a maximum w/cmt not to exceed within the mix design. The idea is that by limiting the w/cmt, some performance characteristics such as compressive strength, shrinkage, or creep can be controlled. But since it is only a mathematical relationship between the weights of materials, it is only through assumption that a speciﬁcation can hope to achieve certain levels of performance. As an example, a standard commercial concrete speciﬁcation may include the following requirements: • fc’ = 3000 psi at 28 days • Maximum w/cmt = 0.45 • Minimum cement content = 5.0 sacks (470 lbs.) • Maximum Slump = 4 in. • Maximum 1.5 in. aggregate Based on the max w/cmt of 0.45, and the 470 lb minimum cementitious content, the maximum water content for the design can only be 212 lbs (0.45 x 470 lb = 212 lb). However, the question becomes, is 212 lb of water enough to generate a 4.0 in max slump? According to ACI 211 Chp. 6, Table 6.3.3 the water content for a 1.5 in aggregate, with a 3 to 4 in slump is 300 lb of water. Assuming that a standard ASTM C494 Type A water reducer is used (8% water reduction) the required water content becomes 276 lb (300 lb x 92% = 276 lb). If the speciﬁed design only allows 212 lb and ACI states we need 276 lb, there is a 64 lb difference in necessary water content. A standard rule of thumb used in the ﬁeld is that 1 gal/yd3 of water will increase the slump by 1.0 in. In this case the 64 lbs is about 7 ½ gallons of water, thereby reducing the slump by about 7.5 inches. Essentially, it makes the slump of the concrete 0 inches, if we followed the speciﬁed w/cm and used the minimum cementitious content. Since this is not a practical concrete, then the question becomes, what choices are left to make it a concrete that can be produced, delivered, placed, and ﬁnished? If we use the water content of 276 lb, and keep the max w/cmt in place, the new cementitious content becomes 613 lb (276 lb ÷ 0.45 = 613.) The original 470 lbs has now been increased by 1.5 sk/yd3 to satisfy the w/cmt requirement. So, in an effort to satisfy a w/cmt ratio that had no real basis in the performance of the structure, to meet the design requirements and practical delivery and placement issues, the following consequences result: • The cost per yard will increase accordingly with a 1.5 sk/yd3 increase. • The concrete will be prone to higher shrinkage and creep with the higher paste content. • The higher paste content will accelerate hydration and heat generation during warm and hot weather conditions, decrease ﬁnishing time, and intensify the potential for plastic shrinkage cracking. • There is no guarantee of durability and more paste is subject to deterioration and chemical attack. The attempt to use w/cmt to create durable and cost-effective concrete can actually result in just the opposite. A more sensible approach is for the speciﬁcations to explicitly detail the performance characteristics, rather than assume they can be accomplished with a mathematical ratio. This position statement from the Texas Aggregates and Concrete Association is presented for reader interest by the editors. The opinions expressed are not necessarily those of the magazine. Reader comment is invited. The attempt to use w/cmt to create durable and cost-effective concrete can actually result in just the opposite. A more sensible approach is for the specifications to explicitly detail the performance characteristics, rather than assume they can be accomplished with a mathematical ratio. TACA Conveyor • Spring 2008 19

Table of Contents Feed for the Digital Edition of TACA Conveyor - Spring 2008

TACA Conveyor - Spring 2008 Contents Transforming Vision Into Action TACA Members Tee Off 2007 Environmental Seminar Get Ready for Los Cabos TACA Position Paper Solomon Colors Expansion J. D. Power Award CHRYSO and Promix J. Irwin Miller Winner TXI Concrete Group Developing Effective Concrete TACA Members Index of Advertisers

TACA Conveyor - Spring 2008

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