What does the term "pneumatically placed concrete" refer to?

The term "pneumatically placed concrete" specifically refers to the technique of placing concrete through a high-pressure hose. This method involves using compressed air to propel the concrete mixture through the hose, allowing for efficient and precise placement in hard-to-reach areas or for creating specific structural elements. This technique is particularly advantageous in construction projects where standard pouring methods may be impractical due to height, distance, or the complexity of the formwork.

The other options reference different methods of transporting or delivering concrete, which do not encompass the unique characteristics that define pneumatic placement. For instance, concrete transported by truck involves traditional delivery methods that do not utilize air pressure, while on-site mixing refers to preparing the concrete aggregate and water at the location rather than the placement method itself. Similarly, concrete delivered in bags pertains to pre-packaged concrete that is mixed with water at the site, which contrasts completely with the pneumatic process. Thus, the correct answer captures the essence of what pneumatically placed concrete is and how it is used in construction.