When should a compressive strength test be halted?

The correct answer is based on the fact that the integrity of the compressive strength test heavily relies on the functionality of the testing equipment. If any malfunction is indicated by the equipment, such as an error message, inconsistent readings, or mechanical failure, continuing the test could lead to inaccurate results that do not reflect the true strength of the concrete specimen. It's crucial to ensure that all testing equipment is operating correctly to maintain the reliability and accuracy of the test outcomes.

In this context, the other choices do not universally warrant halting a compressive strength test. While the size of the specimen can affect the test procedure, it does not automatically mean that the test should be stopped if the specimen fits the specifications. Similarly, while elevated temperatures may influence the setting and curing of concrete, a limit of 30 degrees Celsius is not a standard threshold for halting a test. Lastly, uncertainty in procedure can indeed cause concern, but it does not necessitate stopping the test unless that uncertainty directly affects the testing equipment's operation or consistency of results. Thus, prioritizing the functionality of testing equipment is essential in maintaining the validity of compressive strength testing in concrete.