Temperature Differentials

Construction-related HMA temperature differentials are large mat temperature differences resulting from placement of a significantly cooler portion of HMA mass into the mat. This cooler mass comes from the surface layer (or crust) typically developed during HMA transport from the mixing plant to the job site. These cooler areas will reach cessation temperature more quickly than the surrounding mat. Roller patterns developed based on general mat temperatures may not be adequate to compact these cooler areas before they cool to cessation temperature resulting in isolated spots of inadequate compaction. Thus, temperature differentials can cause isolated areas of inadequate compaction resulting in decreased strength, reduced fatigue life, accelerated aging/decreased durability, rutting, raveling, and moisture damage (Hughes, 1984¹; Hughes, 1989²). Generally, temperature differentials greater than about 14°C (25°F) can potentially cause compaction problems (Willoughby et al., 2001³). Figure 3 describes the general sequence of events leading to temperature differentials.

Construction-related temperature differentials and aggregate segregation display the same symptoms and result in the same types of damage, which can cause them to be confused with one another. However, the ultimate damage mechanism, excessive air voids (often expressed as "inadequate density") is the same in both cases.

How Temperature Differentials Form

Until the mid-1990s it was generally assumed that the paver blended the cooler crust back into the rest of the load. Infrared temperature measurements and thermal imaging showed otherwise: the cooler material passes through the paver relatively intact and is placed in concentrated areas of the mat at regular intervals, one for each truckload (Willoughby et al., 2001³). The result is a pattern of distressed spots, often every couple of hundred feet, in an otherwise sound mat.

To the eye these spots can look like aggregate segregation, but tests on such areas often show no segregation at all. Because the spots are small, the random sampling in most quality control and acceptance plans will often miss them. They can be patched for a time, but the eventual remedy is resurfacing earlier than planned, on the order of 20 to 80 percent earlier depending on severity.

Minimizing Temperature Differentials

There are three main ways to limit temperature differentials:

1. Prevent the crust from forming. Reducing heat loss between the plant and the job site helps, for example with insulated truck beds and short hauls. Differentials can still occur with these precautions in place.
2. Remix the HMA on site. The paver itself does not remix adequately, but many material transfer vehicles do. Remixing claims are best checked with a thermal camera behind the paver: differentials that appear without the device and disappear when it is added are good evidence that it works.
3. Compact cooler areas in time. Even spots placed much colder than the surrounding mat have some time before they reach cessation temperature. With rollers close behind the paver and compacting aggressively, differentials approaching 39°C (70°F) can be rendered harmless. Density checks are needed to confirm this is happening.

Detecting Temperature Differentials

A hand-held infrared thermometer can be used to check the mat behind the paver for cold spots. A spot about 14°C (25°F) colder than the surrounding mat may be a problem, and its density should be checked after compaction. Hand-held thermal imaging cameras, and thermal imaging bars mounted on the screed that image the full mat width in real time, give more complete coverage.

@misc{pavint-temperature-differentials,
  author = {{Pavement Interactive}},
  title = {Temperature Differentials},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/construction/compaction/temperature-differentials/}},
  note = {Accessed [date]}
}