Ratio Numeric Value
5 What shape is the aggregate? The shape of the aggregates in a mix affects how easily the aggregates will re-orient, move into close contact and interlock for strength. Some mix designs, most notably Superpave, require that aggregates have fractured, elongated faces. Those fractured faces create high internal friction within the asphalt layer, and it takes more force to create stone-on-stone interlock.
As long as there is an adequate ratio of layer thickness to aggregate size, set up for higher amplitudes when compacting mixes with angular-shaped aggregates. Some mixes have aggregates with rounded faces. Those aggregates have low internal friction and move into close contact easily. Select lower amplitudes when there are aggregates with rounded faces.
6 What is the asphalt cement viscosity? The viscosity of the asphalt cement used in a mix affects the stiffness of the mix. In general, asphalt cement that contains modifiers such as polymers or latex have high viscosity and contribute to stiffness in the asphalt layers. More force is needed to compact mixes with modified asphalt cement.
Unmodified asphalt cement, also called “neat” oil, has lower viscosity. A mix with unmodified asphalt cement, all other factors being equal, will compact under less force, or lower amplitude.
Viscosity Numeric Value
7 What type of base is being applied? The base over which the asphalt layer is being laid influences amplitude selection. A yielding base, such as compacted granular material, tends to absorb some of the compaction energy generated by a vibratory compactor. Therefore, select higher amplitude ranges when compacting the first lift on granular material. A rigid base, such as a milled surface or compacted asphalt, can contribute to drum bouncing, especially when compacting thin mats. Be aware of the need to reduce amplitude when compacting asphalt layers on rigid bases.
Base Numeric Value
8 What is the ambient temperature? Asphalt layers compact easier at high temperature. When the ambient temperature is relatively high (say, above 70° F), the mat tends to stay hot longer and achieving density is not negatively affected. When the ambient temperature is relatively low (less than 70° F), you may need to increase the amount of force used in order to get density more quickly. Increasing amplitude is one possibility to help achieve density faster when the ambient temperature is low.
Temperature Numeric Value
<70° F 0
>70° F 1
Add It Up
After answering the eight questions, add the numeric values assigned to each answer. Select an amplitude to use as a starting point based on the following.
If 5, 6, 7, 8: High amplitude (0.030” or higher)
If 3, 4: Medium amplitude (.020-.029”)
If 0, 1, 2: Low amplitude (0.19” or lower)
If a negative number: Static or low amplitude with one drum vibratory and one drum static
Remember to use these amplitude selection guidelines only as a starting point for machine setup. Always verify that you are creating the target density without damaging the asphalt layer. Once you have selected the correct vibratory characteristics and the proper pattern, document your work and then strive for consistency. ET