When should a tremie pipe be used to pour concrete?

A tremie pipe is a specialized tool used in the process of pouring concrete underwater or in situations where it's essential to avoid contamination or disruption of the concrete mix. Its primary function is to allow the concrete to flow directly to the bottom of a void or cavity, which is particularly important in the case of structures like drilled piers.

When pouring a drilled pier, the use of a tremie pipe ensures that the concrete is delivered precisely to the intended depth, minimizing the risk of segregation or contamination from water or soil. This method aids in maintaining the integrity and quality of the concrete being placed in challenging environments.

In contrast, the other options outline scenarios that are less applicable to the specific uses of a tremie pipe. Pouring quick-dry concrete, dealing with a high rock content, or pouring concrete in sandy soil does not inherently necessitate the controlled delivery method provided by a tremie. Instead, tremies are particularly essential where the vertical delivery of concrete through water or unstable soils is critical, making their use in drilled pier scenarios optimal.