What happens if the concrete level is lower than the top of the mold during rodding?

When conducting a test that involves rodding concrete, it is important that the concrete reaches the appropriate level within the mold. If the concrete level is lower than the top of the mold during rodding, it indicates that there is insufficient concrete to fill the mold properly. This situation necessitates the addition of more concrete to ensure that the mold is filled to the required level. This is important because an accurate representation of the concrete's properties depends on having the proper volume and density within the mold. Filling the mold completely allows for an accurate measurement and testing of the compacted concrete, which is crucial for obtaining valid results.

In the context of the other options, canceling the test or adjusting only one layer would not address the fundamental issue of insufficient concrete volume in the mold, and additional layers being unnecessary contradict the requirement for adequate material coverage in the mold for proper testing.