Final answer:
The usual cause for returning a concrete mix and truck back to the plant is related to quality issues with the mix or estimating errors that lead to excess concrete.
Step-by-step explanation:
The usual cause for returning a concrete mix and truck back to the plant is quality issues with the concrete mix, such as improper mix proportions, contamination, or if the mix has started to set before it has been applied (which would mean the concrete is no longer workable). Another reason might be the overestimation of the required volume of concrete, leading to an excess that cannot be used. Ensuring that the concrete mix meets the specifications is critical for the integrity of the construction project.