Mix Design for Flowable Fly‐Ash Backfill Material

This three‐phase study examines whether flowable fly ash is an economically and technically feasible material for use as backfill around municipal water, sewer, and storm‐drainage pipelines. The scope of this paper is limited to the mix design methodology of the high‐ash slurry. The proportions of fly ash and cement are determined to yield the properties sought. Properties of interest are strength characteristics, durability, workability, and set time for construction considerations. Phases II and III involve testing of flowable fly‐ash design mix on selected instrumented pipe sections under field‐simulated static and dynamic conditions. A mix of 90% fly ash, 10% cement, 35% water/fly‐ash ratio, 6% super‐plasticizer/fly‐ash ratio, and 0.75% accelerator/fly‐ash ratio is found to serve well, even under severe seasonal cycles. The mix ingredients are also seen as environmentally safe to use as a backfill material.