What constitutes a successful strength test of concrete?

A successful strength test of concrete is determined by evaluating the compressive strength of concrete cylinders taken from a batch mix. This test is typically conducted at a designated curing period, such as 28 days, which is recognized as the standard time frame for the concrete to achieve its intended strength.

The correct choice focuses on the average strength of two cylinders at the 28-day mark. This is significant because relying on an average helps mitigate the variability inherent in concrete due to factors like mixing, placement, and curing conditions. Ensuring that the average strength of at least two cylinders meets or exceeds the specified strength allows for a more reliable assessment of the concrete's performance. This average serves as a key indicator of the overall quality and consistency of the batch.

Other potential answers may focus on maximum strength or individual test results, which do not provide the same level of reliability or assurance as the average. Variability in concrete strength is common, and using multiple cylinders to calculate an average ensures that outliers do not unduly influence the assessment of concrete quality.