Final answer:
The maximum elapsed time allowed between obtaining the final portion of the composite sample and starting the tests for slump, temperature, and air content is B. 30 minutes.
Step-by-step explanation:
The question refers to the maximum elapsed time allowed between obtaining the final portion of the composite sample and beginning the tests for slump, temperature, and air content for concrete. The correct answer to this question is B. 30 minutes. It is essential that these tests are carried out within this timeframe to ensure the accuracy and validity of the test results, which are crucial for assessing the quality and consistency of the concrete mix.
Standard practice for testing the properties of fresh concrete requires tests to commence within this duration to reflect the properties of the concrete as it would be when used in construction projects. Exceeding this time limit could lead to inaccurate test results due to the ongoing hydration and setting of the concrete mix.