Laboratory vs. Field Conditions

This table (taken largely from TRB, 2000[1]) gives a brief outline of some of the major differences between mix design laboratory conditions and actual field conditions.  NAPA’s The Effect of Testing and Production Procedures on Mix Design Results, Information Series 112 is an excellent resource.

Laboratory Conditions Field Conditions
Asphalt Binder
Aging is simulated using the TFO, RTFO or PAV.  All of these methods are only rough simulations of actual asphalt binder aging. Aging is much more complex – especially after construction when it is highly dependent upon construction quality and the environment.
After mixing, the loose mix is generally aged to allow for asphalt binder absorption and an increase in viscosity. After mixing the loose mix can be immediately transported to the construction site or can be placed in storage silos for up to a week.
Gradation is carefully measured and controlled. During the manufacturing process aggregate gradation will change slightly as it passes through the cold feed bins, aggregate dryer and drum mixer/pugmill.
Aggregate used is completely dry. Even after drying, aggregates typically contains between 0.1 – 0.5 percent by weight moisture.
Oven heating of the aggregate usually results in uniform heating of the coarse and fine aggregate. In a drum plant there is often a distinct temperature difference between the coarse and fine aggregate.
Fines are retained during the mixing process. Some fines are collected in the mix plant baghouse.  If all of these fines are not put back into the mix (practically, they cannot be because baghouse efficiencies are less than 100%) the aggregate gradation will change slightly.
If RAP is used, it is heated to the same uniform temperature as the virgin aggregate. If RAP is used its degree of heating may be different than the virgin aggregate.
Mixing Process
The mixing process occurs on essentially unaged asphalt binder for the Hveem and Marshall methods.  The Superpave method roughly simulates short-term aging using the RTFO. The mixing process can substantially age the asphalt binder.  A mixing time of 45 seconds can increase asphalt binder viscosity by up to 4 times.
Compaction uses a laboratory device and a small cylindrical sample of HMA.  This combination attempts to simulate the particle orientation achieved by field compaction with rollers. Particle orientation and compactive effort can vary widely depending upon roller variables and the environment (e.g., temperature, wind speed).
Compaction is relatively quick (< 5 minutes) and thus occurs at an almost constant temperature. Compaction can take a significant amount of time (30 minutes or more in some cases) and thus occurs over a wide range of mix temperatures.
Compaction occurs against a solid foundation. Foundation rigidity will affect compaction.  Compaction can occur against a range of foundations: some can be quite stiff (like old pavement) while some can be quite soft (like a clay subgrade).

Footnotes    (↵ returns to text)
  1. Hot-Mix Asphalt Paving Handbook 2000.  Transportation Research Board, National Research Council.  Washington, D.C.