SMA, or open, gap-graded mixes, do not accept as much impact force as dense-graded mixes.
The test strip helps confirm that the selected amplitude and pattern are creating the desired density in the bituminous layer.
Higher amplitudes are generally required when compacting asphalt laid on granular base.
Less force is normally selected when compacting thin lifts, especially when compacting a tender, street mix design.
When referring to vibratory asphalt compactors, we define amplitude as the distance the drum moves into the mat. The weight of the drum, combined with drum movement, delivers impact force into the asphalt layer in order to reduce air voids and accelerate the densification process. The goal of the compactor operator, supervisor or quality control person is to select the maximum amplitude that the mat will accept without creating drum bouncing.
Most modern asphalt compactors have two or more amplitudes. The compactor operator must know equipment capabilities and how to set up the machine for each amplitude setting. No matter how many amplitudes are available on a compactor, they are classed in the following manner.
Low Amplitude: 0.010-0.019”
Medium Amplitude: 0.020-0.029”
High Amplitude: 0.030” or higher
Selecting the correct amplitude should be approached as methodically as possible for each phase of compaction, especially the breakdown phase that creates the majority of the target density. If the amplitude (impact force) is too low during the breakdown phase, the common results are insufficient density, the need for too many repeat passes and the inability to keep up with the paver’s production. If the amplitude is too high, there is a risk of fracturing aggregates, losing density and leaving impact marks in the fresh mat.
Questions to Ask Yourself
As a guide to help you select the correct amplitude during the breakdown phase, here are eight questions to answer when setting up the test strip or before beginning to work on any project. The answers to the questions are given a numerical value, based on each answer’s potential influence on amplitude selection.
1 What is the lift thickness? In general, the thicker the asphalt layer, the more force you can apply without damaging the layer or creating drum bouncing. A basic way to relate amplitude to layer thickness is to use the classifications shown:
Thickness Amplitude Value
Less than 2” Low -1
2-3” Medium 0
More than 3” High 1
2 What type of mix is being used? The type of mix affects the amount of impact force that can be accepted without damage to the mat. Dense-graded mixes tend to accept more force because there are a variety of aggregate sizes that help absorb the compaction energy. So, as you are creating your checklist of factors, you can think about using higher amplitude for dense-graded mixes.
Open, or gap-graded, mixes tend to be more fragile and will be damaged more easily than dense-graded mixes if too much impact force is applied. Likewise, stone mastic asphalt (SMA) designs generally will not tolerate high force and you should be thinking about lower amplitude selection.
Mix Design Numeric Value
3 What is the maximum aggregate size? Mixes with nominal aggregate size of 19 mm (3/4 in.) or larger are classed as coarse or harsh mixes. Usually, it takes more force to move the large aggregate into close contact and drive out most of the air voids. Select medium or high amplitude when compacting large stone mixes.
Mixes with nominal aggregate size 13 mm (1/2 in.) or smaller are classed as fine or tender mixes. The amplitude setting should be in the low or medium range when compacting fine mixes.
Aggregate Size Numeric Value
4 What is the ratio of aggregate size to layer thickness? The ratio of nominal aggregate size to layer thickness has a major influence on amplitude selection. The minimum design standard calls for a 3:1 ratio.
In other words, if the nominal aggregate size is 25 mm (1.0 in.), the minimum layer thickness should be 75 mm (3.0 in.). A 3:1 ratio means that this factor is basically “neutral”. A 4:1 ratio means there is more room for aggregate to move within the layer and that layer will accept more impact force without damage, so you can consider using higher amplitudes. A 2:1 ratio means there is not as much room for the aggregate to move within the layer. You will have to select lower amplitudes or operate in the static mode to avoid fracturing aggregates and creating drum bounce.
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