Controlled low strength material, abbreviated CLSM, also known as flowable fill, is a type of weak, runny concrete mix used in construction for non-structural purposes such as backfill or road bases.
CLSM consists of a mixture of Portland cement, water, aggregate and sometimes fly ash. Unlike ordinary concrete, CLSM has much lower strength. The strength of CLSM is less than 1,200 pounds per square inch (8.3 MPa), while ordinary concrete has strengths exceeding 3,000 pounds per square inch (21 MPa). As a result, CLSM is not suitable for supporting buildings, bridges, or other structures. Instead, it is primarily used as a replacement for compacted backfill. It also flows much better than ordinary concrete, having the consistency of a milkshake. The first known use of CLSM was in 1964. CLSM is typically a ready mix concrete rather than soil cement which is a low strength cement made using local soil, and is similar to a slurry.