Controlled low-strength material (CLSM) is a backfill product that flows as easily as thick pancake batter and is self-leveling. Its consistency is like that of a slurry or lean grout, yet several hours after placement the material is hard enough to support traffic loads without settling. Typical 28-day compressive strengths range from 50 to 200 psi. Because CLSM flows and needs no compacting, it is ideal for use in tight or restricted-access areas where placing and compacting soil or granular fill is difficult or impossible. Although CLSM costs more per cubic yard than most soil or granular backfill materials, its many advantages result in lower in-place costs.
A typical CLSM mix contains cement, water, fly ash, and fine aggregate. Ready mix producers can combine these components in varying proportions to meet specific performance requirements and to take advantage of locally available material. Most chemical admixtures can be used in CLSM including water reducers, superplasticizers, and accelerators. It is usually batched and mixed at a central mixing plant in accordance with ASTM C 94 procedures for concrete, then delivered to the jobsite by ready mix truck. For most applications, CLSM can be placed continuously. It needs no compacting and little or no spreading and finishing.