3. Asphalt Materials and Mix Design

3.8 Laboratory Versus Plan-produced Mixes

As noted earlier, differences will likely exist between the properties of an asphalt mix designed in the laboratory and the “same” JMF produced in a batch or drum-mix plant (typically air voids and VMA). It is important to examine those differences and understand how and why the test properties or characteristics of a mix produced in a plant may vary significantly from the results predicted by tests conducted on laboratory-produced material.

These revisions are usually required for several reasons, including:

• Aggregate samples obtained for the mix design will typically have less degradation from handling compared to the aggregate that passes through the production plant.
• The asphalt binder will likely be absorbed into the aggregate particles at a different rate during mixing and storage at the asphalt plant than in the laboratory.
• The mix design was performed with oven-dry aggregate, weighed out to the nearest 0.1 g, while the plant aggregate experiences ever-changing moisture conditions that cannot be perfectly accounted for on the continuously moving plant weigh belt.
• Unless the plant aggregate feed systems are perfectly calibrated 100 percent of the time, the blend percentages may be slightly different than the percentages precisely weighed out on a scale in the lab.
• The mix design may have been conducted and certified using aggregate material produced months or sometimes years before the project production takes place and not reflect the aggregate properties of mixture being produced.

Factors such as these make it almost inevitable that the blend percentages of the mix will need to be adjusted slightly in the field to produce a mix within specifications.

Asphalt contractors, producers, and inspectors should understand the owner’s policies regarding allowable changes to the mix design.

The most common problem encountered in plant-produced mix is the failure to meet VMA and air void volumetric parameters. These properties are related, as a failing air voids test is usually the result of a changing VMA—assuming the binder content does not change. The change in VMA is most often explained by inconsistent mixture conditioning during testing or a change in the gradation and/or shape of the aggregate due to degradation during handling.

Proper mixture conditioning of samples is essential in providing accurate volumetric properties. The time and temperature of mixture conditioning can greatly affect the amount of asphalt absorbed in the aggregate, thus changing the maximum specific gravity of the mix (G_mm) and, to a lesser extent, the bulk specific gravity of the mix (G_mb). For example, G_mm samples taken at the plant and immediately tested may produce significantly lower G_mm values. This is because the mix has a higher volume due to the binder that has not had enough time to be absorbed into the aggregate. Artificially low G_mm values result in higher measured lab compacted density (lower air voids) and higher measured percent compaction on field cores.

If a sufficient conditioning time is allowed, binder will be absorbed into the mix at nearly the full absorption capacity of the aggregate, resulting in a lower mix volume because part of the binder has moved from the outside of the aggregate particles to the inside. Subsequently, the lower mix volume due to increased conditioning time will result in higher G_mm values and lower calculated densities.

For field-produced mixes to match laboratory design values, mix samples should be cured at similar temperatures for a similar length of time. Highly absorptive aggregates will magnify the importance of matching the curing time and temperature between the lab and the field.

Regarding gradation, it is important to remember that VMA is the result of the amount of aggregate packing that occurs in the mold when placed in the lab compactor. Anything that changes the void spacing in the specimen will affect the resulting VMA. A change in the percent passing on one aggregate sieve can alter the compaction characteristics of the mix and change the way the entire aggregate structure fits together.

Aggregate breakdown is common when going from mix design in the lab to plant production in the field. Each time the aggregate is handled at the quarry, loaded and unloaded into trucks, fed into cold bins, and traveled through the plant during drying and mixing, it tends to abrade the angular edges and create additional fines. This breakdown typically creates a higher percentage of minus No. 200 material (dust) relative to the aggregate samples used for the mix design, resulting in lower air voids and lower VMA (collapse) for plant-produced samples compared to the mix design, assuming the same binder content. Mix designers can take this into account when batching during the mix design process by adding more dust to minimize the difference between mix design gradation and the produced mix gradation.

When specifications only require the monitoring of air voids in the mix and not VMA, the VMA can unknowingly decrease. It is possible to simply reduce the amount of binder being added to the mix to restore the specified air voids level. Simply reducing the binder content may correct the air voids deviation but leave the mix dry with an insufficient amount of binder to provide durability. After evaluating sampling and testing procedures to ensure a standardization of curing parameters, options to restore the VMA in the mix should be explored. These may include the following, depending on the specific properties of the local aggregate:

• Make bin split/gradation changes that generate additional VMA.
• Increase the fracture content of the aggregate.
• Reduce natural sand components and increase use of washed screenings in place of regular screenings.
• Increase intermediate-sized chips (but too much will cause mix instability).
• Reduce the dust in the mixture by:
  • Washing some of the aggregates that contain high levels of dust.
  • Increasing the fine aggregates that contain smaller amounts of dust.
  • Using a dust collector surge bin to reduce the dust being returned to the mix.