What should the thickness probe be set higher for and by how much?

When determining how to set the thickness probe regarding the roll down, it's important to account for the expected percentage of compaction. The correct answer indicates that the thickness probe should be set higher for a roll down of 20-25% by ¼ inch. This adjustment ensures that when the asphalt is initially placed, it accounts for the anticipated compaction that will occur during the rolling process.

Setting the probe higher allows for a more accurate measurement of the thickness after compaction, ensuring that the final surface meets specifications. In this case, a ¼ inch increase is a standard practice for the expected roll down of 20-25%, based on industry standards and best practices in asphalt paving.

Without the appropriate adjustment in probe height, the risk of underestimating the required thickness could lead to inadequate pavement strength and durability, potentially resulting in premature failures or the need for repairs. Thus, understanding the relationship between roll down percentages and necessary probe settings is crucial for achieving optimal paving results.