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.