What method is commonly used to obtain concrete samples for testing?

The method of random sampling from the truck or discharge point is the standard practice for obtaining concrete samples for testing. This approach helps ensure that the samples are representative of the concrete being supplied at that moment, capturing any variations in the mix that could occur due to factors such as material consistency or mixing efficiency.

When concrete is actively being discharged from a truck, taking samples at this point makes the most sense because it reflects the concrete's immediate condition and properties. Sampling during this stage allows for the assessment of factors such as temperature, workability, and actual mix proportions, which are critical for quality control and assurance.

Other methods mentioned, such as continuous mixing from a batch plant, sampling after 24 hours of curing, or pre-mixed sampling, do not accurately represent the in-situ conditions of the concrete being placed. Continuous mixing may not capture the variability in individual batches, while sampling after 24 hours can miss crucial early-age characteristics. Pre-mixed sampling may not reflect how the concrete behaves once it is mixed and placed. Thus, random sampling at the discharge point ensures that the testing is relevant and useful for maintaining quality standards in concrete construction.