Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.
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74 • March 2017 www.concreteproducts.com TECHNICAL TALK BY TOM KUENNEN The 96th annual meeting of the Transportation Research Board in January in Washington, D.C., drew over 13,000 transportation engi- neers and specialists from across the country, and around the world. More than 5,000 presentations in over 800 sessions addressed topics in multimodal transportation, including materials and design involv- ing ready-mixed concrete and precast/prestressed products. Concrete Products was there and this month presents a report on new research findings in ready-mixed and cast-in-place concrete. We'll look at precast and precast/prestressed research at TRB 2017 in an upcoming issue. For more information, visit www.trb.org. NO NEED TO SPECIFY MINIMUM CEMENTITIOUS MATERIALS CONTENT There is no need to specify minimum cementitious material contents in concrete mixes, as overspecification increases costs and a project's carbon footprint, while potentially compromising performance, say Karthik H. Obla, Ph.D, P.E., Rongjin Hong and Colin L. Lobo in their peer-reviewed paper, Should Minimum Cementitious Contents for Con- crete Be Specified? Minimum cementitious contents are commonly specified in proj- ects, the authors say. "In the U.S. many state highway agencies specify a minimum cement content between 550 and 600 lb./yd. 3 for slip-form pavement mixtures," they write. Furthermore, most state highway agencies also establish classes of concrete for various elements in transportation construction projects and these classes of concrete are typically defined by minimum content of cementitious materials among other parameters, they add. An industry survey conducted in 2014 revealed that minimum cementitious content was indicated in 46 percent of all project speci- fications. However, consensus-based industry standards in the United States—for example, from the American Concrete Institute—don't include requirements for minimum cementitious materials content for concrete mixtures for the most part, Obla, Hong and Logo state. Minimum limits on cementitious materials specified by state high- way agencies typically exceed the quantity required for intended performance, such as workability, strength and durability, they say. "This results in increased cost, a higher carbon footprint, and often negatively impacts the performance of these concrete mixtures. The performance implied or intended by the minimum cementitious mate- rial content is most often not clear or enforceable." Also, minimum cementitious materials (CM) limits restrict mixture optimization for performance, and as a result there is little incentive for concrete producers and contractors to become more knowledgeable about the ingredient materials and mixtures. "Specifications that include these requirements do not allow con- crete producers and contractors to target lower average strengths by reducing standard deviation through improved concrete quality and as a result, they do not provide any incentive for investing in improved quality management systems," Obla, Hong and Logo report, thus impeding an evolution toward performance-based specifications. On the other hand, they note three possible reasons for specifying a minimum cementitious content: • It provides assurance that a low water-to-cementitious materials ratio (w/cm) is attained, even if good control of the mixing water content is not exercised; • It ensures there is enough paste to fill the voids between the aggregates and provide adequate workability; and, • It offers corrosion protection by chemically binding the chlorides and the carbon dioxide that penetrate the concrete. They note that at any given w/cm, increasing cement contents lead to similar compressive strengths and carbonation rates, but also higher absorption and chloride penetration. A mixture with higher cement content had increased chloride threshold concentra- tion to initiate corrosion, but this benefit was offset by higher level of chloride penetration that could cause the initiation of corrosion of reinforcement. Obla, Hong and Logo examined the influence of the cementitious content on concrete performance at specific water-to-cementitious materials ratios. The experimental variables studied include w/cm ranging from 0.40 to 0.55, mixtures containing portland cement only, and mixtures containing 40 percent slag cement or 25 percent fly ash. Concrete performance was evaluated through laboratory tests on workability, strength and durability. The results show that at a given w/cm, a higher cementitious content results in higher paste volumes and poorer concrete performance. They found: • Higher CM contents increased the mixing water demand of concrete. For a given w/cm, increasing CM content resulted in similar com- pressive strengths, but increased chloride penetrability, increased initial sorptivity and greater length change due to drying shrink- age. The reduced concrete performance of the mixtures with higher CM contents is due to the higher paste volumes. "These results clearly show that at a given w/cm requiring a higher CM content is counterproductive as it leads to poorer concrete performance," the authors write. • If the strength and durability requirements for the project are defined, there is no technical basis for specifying the minimum CM content. Instead, workability can be evaluated by test pours or documentation of successful past field history. • If the main purpose of specifying a minimum CM content is to get assurance of a low w/cm, it's better to specify an appropriate compressive strength, which is a better indicator of w/cm. • In summary, moving to a performance based specification facilitates removing the minimum CM content requirement in specifications. "Based on these results the value of maintaining minimum cementi- tious content requirements in project specifications is questioned," the authors conclude. Cut costs, carbon, and boost durability by dropping cementitious minimums CONTROL VS. FLY ASH, SLAG CEMENT MIXTURES Concrete mixture variables for research on cementitious materials minimums; PC mix (bottom of table) contained only portland cement TABLE: Obla, Hong and Logo