Study on Partial Replacement of Sand with Fly Ash in Concrete Mixes Based on Pozzolanic Cementing Efficiency to the Strength-Weight and Cost-Strength Ratio

The utilization of fly ash as sand partial replacement in concrete mix is an alternative solution for the depletion of sand resources in the Special Region of Yogyakarta. This research evaluates the effect of sand partial replacement with fly ash to the strength-weight and cost-strength ratio of concrete mixes. The strength-weight ratio was used for mechanical evaluation with higher value considered as better, while cost-strength ratio indicates its economic efficiency with lower value means more efficient. Four groups of test specimens with different amounts of fly ash were evaluated for its mechanical performance and economic efficiency. Various percentages of fly ash, sequentially 0%, 20%, 35%, and 50% were implemented in the mixes based on partial weight replacement of its components and the water-binder ratios were calculated based pozzolanic cementing efficiency method. The compressive strength was evaluated on 14 concrete cylinders with 150mm diameter and 300mm length for each variant that cured in 28 days water immersion. Test results shows that fly ash addition tend to reduce material cost and density of concretes, yet the compressive strength was not lower compared to normal concrete. Therefore, fly ash added concrete can be justified as better in mechanical performance and economically more efficiency.