When must CLSM achieve its specified compressive strength?

The specified compressive strength of Controlled Low Strength Material (CLSM) must be achieved prior to loading. This is critical because CLSM is often used in applications where it serves as backfill, a subbase material, or a structural fill, meaning it will be subjected to loads from the structures being built or those that are otherwise applied on top of it.

Achieving the specified compressive strength before loading ensures that the material can adequately support the weight and stress it will experience, which is vital for structural integrity and safety. It also adheres to engineering standards and codes that require that materials reach their specified strengths to perform properly in their intended applications.

In contrast, other answer choices do not align with this requirement. For example, achieving compressive strength after initial curing does not guarantee that the CLSM will be suitable for loads placed upon it, as there may be insufficient strength during that time. Similarly, requiring strength before mixing or at any time during placement does not ensure that the material has the stability and performance characteristics necessary when actual loads are applied.