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The main objective of this study was to evaluate the effect of aggregate particle packing optimization and cement reduction on Nebraska slip-form pavement concrete performance. A literature review was conducted to examine different aggregate optimization tools, quality control tests, and historical data of Nebraska Department of Transportation (NDOT) mixtures. It was found that the Modified Toufar Model has good potential in optimizing particle packing and predicting packing degrees. The combined void content test was found to be useful to experimentally justify optimized aggregate gradations. Two specific pavement concrete workability tests, i.e., the Box Test and the VKelly Test, were used to evaluate the effect of cement reduction and optimized aggregate gradation on pavement concrete workability. The Box Test ranking was modified to provide a more detailed and objective evaluation. Analysis of different aggregate combinations has shown that experimental packing from the combined void content test has a high correlation with estimated packing from the Modified Toufar Model. Results showed that when the optimized aggregate gradation is used, cement content can be effectively reduced by up to 1.0 sack (94 lb/yd3) without compromising the fresh properties, mechanical properties, and permeability. Based on the results of free and restrained shrinkage, it was justified that shrinkage and cracking potential can be reduced in optimized concrete mixtures. Freeze/thaw resistance can be slightly improved with optimized mixtures. A mix design improvement procedure considering both theoretical and experimental packing and the minimum excess paste-to-aggregate ratio can be used to design concrete with an optimum cement content.The main objective of this study was to evaluate the effect of aggregate particle packing optimization and cement reduction on Nebraska slip-form pavement concrete performance. A literature review was conducted to examine different aggregate optimization tools, quality control tests, and historical data of Nebraska Department of Transportation (NDOT) mixtures. It was found that the Modified Toufar Model has good potential in optimizing particle packing and predicting packing degrees. The combined void content test was found to be useful to experimentally justify optimized aggregate gradations. Two specific pavement concrete workability tests, i.e., the Box Test and the VKelly Test, were used to evaluate the effect of cement reduction and optimized aggregate gradation on pavement concrete workability. The Box Test ranking was modified to provide a more detailed and objective evaluation. Considering one of the goals of the study was to maximize the use of local materials, locally available cementitious materials and aggregates from East and West Nebraska were selected. Analysis of different aggregate combinations has shown that experimental packing from the combined void content test has a high correlation with estimated packing from the Modified Toufar Model. Results also demonstrated that the current aggregate combination is not the optimum gradation and can be improved. The experimental program included in this study consisted of three Phases. Phase 1 focused on obtaining promising aggregate blends by maintaining the standard cement content (564 lb/yd3, 335 kg/m3). Fresh concrete properties were the main criteria to select promising blends. Phase 2 included an evaluation of performance of pavement concrete with cement content reduced by 0.5 sack (47 lb/yd3, 28 kg/m3) steps for other reference and optimized aggregate blends. Results justified that when optimum gradation is used, cement could be reduced up to 94 lb/yd3 (56 kg/m3). Phase 3 is the performance evaluation phase, which included evaluating the reference mix and selected promising mixes for setting time, modulus of elasticity, free shrinkage, restrained shrinkage, and freeze/thaw resistance. Mixtures with reduced cement and optimized aggregate gradation have shown improved freeze/thaw resistance and lower shrinkage rate. Finally, a mix design improvement procedure incorporating theoretical and experimental particle packing and using excess paste-to-aggregates ratio as the control parameter was proposed. To sum up, the study has justified that the Modified Toufar Model and the combined void content test can be useful tools in aggregate gradation optimization. In order to evaluate workability of pavement concrete more accurately, the Box Test ranking was modified based on image analysis of surface and edge quality. It was also proved that mixtures with reduced cement content and optimized aggregate gradation perform better in terms of freeze/thaw resistance and shrinkage.
The ultimate goals of this study included investigation of the optimal ranges for paste content, amount of cementations materials and aggregate gradation for concrete paving mixtures. In general, the optimum concrete mixtures developed in this study contained low paste content (below 23%), and were characterized by low scaling and sorptivity. In addition, it was also possible to achieve high cement replacement levels for these mixtures. Finally, for optimized fly ash mixtures, the selection of well graded aggregate gradation with high packing density increased the most desired paste content for those mixtures, thus indicating that combined aggregate gradation has strong influence on concrete performance. Lastly, concrete mixtures developed with optimum ranges of variables studied in this research contained low cement content. The overall scope of the research was divided into three distinctive phases, each of which is described briefly below: PHASE I: This phase consisted of statistical optimization of the proportions of concrete binder. The Central Composite Design methodology (CCD) was used to design the experiment for the optimization of binder in three types of concrete mixtures: a) cement + fly ash, b) cement + GGBFS, and c) cement + fly ash + GGBFS. The variables studied in each of these systems included: paste content (from 21 to 25 % by mixture volume) and total content of supplementary cementitious material (SCM) in the mixture. This was expressed as weight percent of total binder, and varied depending on the binder system used. PHASE II: The main goal of this phase was to investigate the effect of different aggregate gradations on the fresh and hardened properties of optimized concrete mixtures developed in PHASE I, as well as to identify the most desired aggregate gradations for paving mixtures. Different aggregate gradations were prepared by blending of 2, 3 or 4 different sizes of aggregates based on concept of Shilstone's Coarseness Factor Chart. PHASE III: The concept of air-free paste-aggregate void saturation ratio (k") introduced in PHASE II seemed to fairly accurate link the properties of concrete mixtures with their paste content. Thus, it was decided to further investigate this concept in connection with aggregate packing density ( ). In addition, it was believed that defining optimum values of "k" will allow for revising the paste content ranges developed in PHASE I for different systems, and thus define more general optimum paste ranges for paving mixtures.
By optimizing the packing of the combined aggregate gradations, the cement paste content needed to make concrete can be reduced, improving sustainability, cost, performance, durability, and workability. Optimization can be achieved using theoretical and empirical techniques, or waste concrete material as an intermediate size fraction. However, the potential for improvement is currently limited by prescriptive grading specifications that require meeting individual requirements for fine and coarse aggregates.From this study, using various optimization techniques, it was found that by inclusion of an intermediate sized aggregate material, a reduction in cement paste up to 16% is possible for 35 MPa and 50 MPa mix designs typically used in Ontario bridge decks. The aggregate materials used were a natural sand, and two crushed limestones of 19.0 mm and 6.7 mm maximum size. From these findings, recommendations are made for improving the current Ontario Provincial Standard Specification (OPSS) 1002 used for concrete aggregates.
The design of concrete mixes is becoming increasingly complex, with the addition of new materials in the compounds, such as organic admixtures, fibres and supplementary cementitious materials. Moreover, the list of properties which concretes are required to possess for certain applications has increased, and interest is developing in rheology, durability, deformability and whole-life behaviour. This book presents a number of simple models for the understanding of a concrete system, and provides the techniques for developing more sophisticated models for the practical design of concrete mixes.
Present computer techniques for determining aggregate blends to meet a specified gradation for bituminous pavement mix design are not entirely satisfactory. Often aggregate blends must still be optimized by trial-and-error hand calculations to approximate the desired gradation. In addition to gradation limits and tolerances, there are limitations for the amounts of particular aggregates to be used in a mix design or for the total amount of fines that can be present in the mix. This study was undertaken to develop the logic for a computer optimization techniques that would blend the given aggregates to conform as closely as possible to the desired gradation and to fall within the specified limits. As a result, two computer programs have been written using two different optimization techniques and are presently being used for bituminous pavement mix design at the U.S. Army Engineer Waterways Experiment Station (WES). Both programs satisfy the requirements of this investigation. (Author).