Effect of Fly Ash Content and Aggregate Gradation on the Durability of Concrete Pavements

In the regions where temperature fluctuations are large ranging from below freezing in winter to high temperatures in summer, concrete highway pavements fail because of poor durability rather than lack of strength. The effect of fly ash replacement of Portland cement and the use of dense aggregate gradation on the durability of concrete mixtures is studied in this research. Eight concrete mixtures were cast, four of them using the aggregates as per current specifications and the other four mixtures using dense graded aggregates. For each of these two gradations, different fly ash replacement amounts of 30, 35, 40, and 45% for Portland cement were used. All mixtures were designed for 6% air content, and $25–38\mathrm{mm}$ $(1\text{to}11∕2\mathrm{in.})$ slump for the freshly mixed concrete. The study shows that the increase of fly ash content from 30 to 45% increased the durability of concrete mixtures without the loss of compressive and flexural strengths. The concrete mixtures containing dense graded aggregates, and higher percentage of fly ash required less water content indicating the beneficial effects of fly ash and dense graded aggregates. When the fly ash content was increased from 30 to 45% in the concrete mixtures, the permeable pore space decreased in the specific gravity, absorption, and voids in hardened concrete tests; the electric charge passing through the layers reduced in the rapid chloride ion permeability tests; and the spacing factors were smaller in the microscopical determination of air-void tests indicating greater durability. These experimental results proved the benefit of using higher content of fly ash in concrete. The use of dense graded aggregates also proved beneficial.