From which part of the batch should the composite sample from a truck mixer be obtained?

The correct answer is the middle portion because the composite sample obtained from a truck mixer is intended to represent the entire contents of the batch uniformly. By taking the sample from the middle portion, one is likely to achieve a more accurate representation of the overall mixture, as this section minimizes the impact of variations that may occur at the front or back of the mixer where material can be affected by factors such as segregation or settling.

Sampling only from the front portion may yield a sample that contains different aggregate sizes or a higher concentration of water and cement paste, while drawing from the back may result in a sample that is not homogeneous. The top portion may also suffer from segregation due to the weight of the materials above it. By focusing on the middle portion, the sampler captures a cross-section that is more likely to reflect the overall characteristics of the concrete being produced.