3. Asphalt Materials and Mix Design

3.9 Summary

The objective of testing plant-produced asphalt mixtures is to favorably compare the test results with the laboratory JMF. This is often difficult to accomplish because of all the variables that exist at the plant—from the type of plant used to the specific plant operating conditions. There are often differences between laboratory and plant mixes—in the gradation of the aggregates, the rounding of the aggregates as they pass through the plant, the degree of hardening of the asphalt cement, incomplete drying in the drum, and the wasting of any fines through the emission-control system.

The JMF produced in the laboratory, therefore, should serve as a good starting point. The desired properties of the mix should be checked and verified on the plant-produced, laboratory-compacted asphalt mixture. Daily tests should be run to determine the characteristics of the mix being produced (mix verification). All mix test results should be within the range of the specification requirements.

Unless they contradict the project specifications, the following guidance may be helpful when testing plant-produced mixtures. If the test results on the plant-produced mix indicate compliance with the JMF requirements, the plant should continue to operate. If one or more of the mix properties are outside the desired range, an investigation should quickly be conducted to determine the cause and extent of the deficiency. In most cases, the plant should not be shut down nor drastic changes made to the mix design based on only one set of test results. A failing sample should be followed with immediate sampling and testing of additional material, rather than waiting on the next random sample time. If the second sample results confirm the first, immediate action should be taken to bring the mix back into compliance. It is typically more desirable for the plant-produced mix to meet volumetric requirements than it is for the field gradation to exactly match the mix design.