To avoid contamination of aggregate samples, one must ensure that containers are:

The primary requirement for using containers for aggregate samples is that they must be clean and suitable for the specific material being tested. Cleanliness is crucial because any residual material or contaminants from previous samples can alter the test results and compromise the accuracy of the testing process. Additionally, the container must be appropriate for the type of aggregate being contained, considering factors like moisture retention, permeability, and possible reactions with the material. Using a clean and appropriate container helps maintain the integrity of the sample, ensuring that the aggregate can be accurately tested for its properties.

Other considerations, such as whether the container is new and unused, while potentially helpful, are not as critical as ensuring it is clean and suitable for the aggregate. Strength and capacity of the container may also be important in practical terms, but they do not directly address the issue of avoiding contamination, which is the main focus of the question. Therefore, cleanliness and appropriateness take precedence in ensuring the quality and integrity of aggregate sample testing.