Flowable fill, called controlled low-strength material (CLSM) by ACI Committee 229, is a fluid mixture of portland cement, water, fine aggregate, and fly ash with a 28-day compressive strength of less than 1200 psi. It is used as a replacement for compacted granular backfill. CLSM is faster and less labor intensive to place than granular fill, it can flow directly into trenches and other voids, it self-levels, compacts without equipment, and hardens in a few hours.
CLSM can replace compacted soil as structural fill or backfill in many applications. Utility companies often specify CLSM instead of soil for backfilling around pipes or conduit. It serves as strong, stable sub-base under roads. Some state DOTS use CLSM to repair old bridges by converting them into culverts.