Evaluation of Reducing Cement Content in NDOR Class R Combined Aggregate Gradations

The main objective of this research project was to achieve a cement reduction on Nebraska slip-form pavement concrete through aggregate particle packing optimization. A literature review was conducted to examine different aggregate optimization tools, quality control tests, and historical data of Nebraska Department of Transportation (NDOT) pavement mixtures. It was found that the Modified Toufar Model has good potential in optimizing particle packing and predicting packing degrees. The combined aggregate void content test was found to be useful to experimentally justify optimized aggregate gradations. The Box Test with a modified index and image analysis tool for surface void estimation was used to evaluate the effect of cement reduction and optimized aggregate gradation on pavement concrete workability. Considering one of the goals of the study was to maximize the use of local materials, locally available aggregates from both 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 NDOT standard 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). Phase 2 included an evaluation of the 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) with satisfactory key fresh and hardened concrete properties. Phase 3 is the performance evaluation phase, which included evaluating the reference mix and selected promising mixes for slump, air content, setting time, compressive strength, modulus of rupture, modulus of elasticity, surface and bulk resistivity, free shrinkage, restrained shrinkage, and freeze/thaw resistance. 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. Results from the study demonstrated that with the optimized aggregate gradation, cement content can be reduced without compromising key fresh, mechanical, and durability properties