ACI Concrete Field Testing Technician - Grade I Practice Exam

From what portion of the batch must composite samples from a truck mixer be obtained?

1. Top

2. Middle

3. Bottom

4. Randomly

The correct answer is: Middle

Composite samples from a truck mixer are obtained from the middle portion of the batch. This practice ensures that the sample represents the entire load of concrete as uniformly as possible. The rationale behind this approach is that the concrete can segregate during transport; materials may settle, and variations in composition could occur throughout the load. By collecting samples from the middle, testers minimize the risk of obtaining a sample that is either overly rich or too lean in certain materials, which could lead to inaccurate assessments of the concrete's quality and characteristics. Taking samples from the top could result in a mixture that is not representative, as it may contain more cement or water that has not mixed thoroughly with the aggregates. Similarly, sampling from the bottom could yield a portion that contains excess aggregates or settled materials, which again would not accurately reflect the true characteristics of the entire batch. Random sampling could lead to inconsistency and may not adhere to standards for representative sampling. Consequently, obtaining samples from the middle helps ensure compliance with testing standards and ultimately leads to more reliable quality control in the concrete mix.