United States Department of Transportation
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Edition:Final report; Mar. 2009-June 2011.
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Abstract:Portland cement concrete (PCC) is the world’s most versatile and most used construction materials. Global demand for PCC sustainability has risen as of late. To meet that need, engineers have looked to alternative binders such as fly ash, silica fume, ground granulated blast furnace slag (GGBFS), and other supplementary cementitous materials (SCMs) to increase pavement durability while lowering the initial and life-cycle cost.
Ternary mixtures were produced and the fresh and hardened characteristics were determined. Fresh concrete properties of air content, slump, unit weight, and set time were determined. Hardened concrete properties measured included: compressive strength, flexural strength, length change, coefficient of thermal expansion, modulus of elasticity, Poisson’s ratio, rapid chloride permeability, and freeze-thaw durability.
Compressive strength results showed equal to or greater compressive strengths especially at later ages of 56 and 90 days. The compressive strengths of all mixtures with SCM replacements up to 80 percent met LADOTD specifications of 4000 psi. The ratios of the seven to 28 day compressive strengths showed that they are more resistant to early age cracking due to the lower modulus at early ages allowing for more creep. Flexural strengths of the ternary mixtures were generally greater than 650 psi with some reaching 1000 psi. These results show that the mixtures will prove adequate for most concrete paving applications, including interstate applications. The results also indicate that the pavement thickness may be reduced in some instances for certain traffic loading conditions. The length change, or shrinkage, results showed that the ternary mixtures performed the same of better than the control mixtures. This ensures that the risk of shrinkage cracking of properly mixed, placed, and cured ternary concrete mixtures is no greater than that of currently mixed, placed, and cured concrete mixtures. Additional curing may be required to prevent plastic shrinkage cracking. The rapid chloride permeability results show that the ternary mixtures will easily meet the new permeability specifications for all structural class concrete requiring less than 1500 Coulombs at 56 days or 27 kOms-cm at 28 days of age. The CTE results show that addition of SCMs increase the CTE for certain mixtures, and decrease the CTE values for mixtures containing both class C and class F fly ash. The freeze-thaw results showed adequate freeze-thaw durability when the entrained air content was sufficient to prevent frost damage. The results point to an inadequacy in the ASTM standard for high SCM replacements in that the resulting concrete is usually not of sufficient strength to resist freeze-thaw damage at 14 days of age when the test is started. A change may need to be instituted for states where freeze-thaw damage is of concern where the concrete being tested is allowed to cure for a greater numbers of days before the onset of testing.
A portland cement replacement level with SCMs of about 70 percent for LADOTD concrete projects was determined to be reasonable. Care should be taken when interpreting these results and the results apply only to the materials used and tested through the course of this study. Producers and contractors wanting to implement these results are strongly encouraged to produce trail batches with their locally available materials to ensure the mixture’s ability to meet and exceed the standards and specifications. The cost benefit ratio for implementation of the results may be as high as 21 depending upon the mixture used for construction and the number of cubic yards of concrete constructed in the state on any given year. Implementation of ternary mixtures will result in an estimated 62,000 tons of CO2 saved for PCC pavements only and the number will be increased when accounting for structural concrete.
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