Development of design parameters for virtual cement and concrete testing.
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Development of design parameters for virtual cement and concrete testing.

  • 2013-12-01

Filetype[PDF-3.99 MB]

  • English

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      The development, testing, and certification of new concrete mix designs is an expensive and time-consuming aspect of the concrete industry. A software package, named the Virtual Concrete and Cement Testing Laboratory (VCCTL), has been developed by the National Institute of Standards and Technology as a tool to predict the performance of concrete mixes quickly using computer simulation of the hydration behavior of concrete. This software requires thorough characterization of the raw materials of a concrete mix design in order to accurately model the hydration reactions. A two-phase testing program was implemented to evaluate the how well the VCCTL software can predict concrete performance. The techniques required to characterize portland cement were developed and implemented to provide input data for the VCCTL software. The resulting virtual materials were simulated, and a second testing program was performed on physical specimens to evaluate the accuracy of those simulations. The accuracy with which the software simulated basic properties of concrete, such as strength, elastic modulus, and time of set, were examined. The process of acquiring cement phase volume and surface area fraction data has been improved substantially through the use of automated scanning electron microscopy. This has resulted in a more efficient process to obtain cement characterization data for use in the VCCTL software. Comparison of isothermal calorimetry data and corresponding time of set data has shown that a typical Type F high-range water-reducing admixture (superplasticizer) delayed time of set and shifted the main silicate hydration peak by the same amount of time. At the dosages explored within this study, the delay was proportional to the dosage rate. The empirical predictions for compressive strength, which were based on elastic modulus and developed for concretes using coarse aggregates that were mineralogically and/or microstructurally different than typical Florida limestone aggregates, were not accurate for concretes made with Florida limestone. More work is needed to accurately predict compressive strength based on the elastic properties of concrete containing Florida limestone coarse aggregates. A more fundamental approach to the simulation of concrete strength should be investigated. Detailed characterization of the elastic properties of Florida limestones used to produce coarse aggregates for portland cement concrete should be performed. A database of properties of concrete mix designs containing Florida aggregate for use with the VCCTL software and other projects should be created. The VCCTL software was found to be an effective tool for the simulation of elastic modulus of portland cement concrete, provided the materials being simulated are properly characterized. The VCCTL software currently does not have a means to incorporate the effects of admixtures on cement hydration. An initial attempt to integrate the effects of a water-reducing admixture, using heat of hydration data, was successful for a Type F water reducer, but the software significantly underestimated the setting time for a Type D water reducer. More work is needed to reliably incorporate the effects of admixtures into the VCCTL software. There are a number of materials that can be modeled in the VCCTL software that were not considered for this research. There is support for both fly ash and blast furnace slag hydration in the VCCTL software, though the accuracy of the model in this respect is largely unknown. The techniques required to characterize these materials are also more involved due to the significant glassy (amorphous) phase contents of their compositions. The methods by which these materials can be characterized and the accuracy with which they are simulated in the VCCTL software should be explored.
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