How is the test for compressive strength of concrete typically performed in the laboratory?

The test for compressive strength of concrete is performed by placing concrete specimens in a compression testing machine. In this process, the specimens are precisely aligned within the machine to ensure that the applied force is uniformly distributed. The load is then gradually increased until the specimen fails, allowing for the measurement of the maximum load the concrete can withstand before breaking. This procedure is essential for determining the compressive strength of the concrete, which is a critical property for evaluating its performance in structural applications.

While the other options touch on different aspects of concrete testing or quality evaluation, they do not directly address the methodology for assessing compressive strength. Weighing specimens and comparing them to known values or checking for surface defects does not provide direct insight into the material's strength. Similarly, while allowing concrete to cure for 28 days is a standard practice to ensure that the concrete reaches its intended strength before testing, it does not describe the testing method itself. The focus on alignment and loading to failure is what makes the selected answer the most accurate description of how compressive strength testing is carried out in the laboratory.