What is Thermal Segregation in Asphalt Pavements?

Thermal segregation is a large temperature difference across an uncompacted hot-mix asphalt (HMA) mat, caused by cooler mix that goes into the mat. It is also called temperature segregation.

How it forms

The cooler mix is a crust that forms while the load travels from the plant to the site. The crust reaches the cessation temperature sooner than the rest of the mat. Cessation temperature is the point where rollers can no longer compact the mix. It is commonly taken as 175°F (79°C). A difference of more than about 25°F (14°C) can cause compaction problems. The cooler areas stay less dense and have more air voids.

Aggregate segregation gives the same symptoms and the same damage mechanism, so the two are easy to confuse.

In a TxDOT demonstration on Dyer Street in El Paso, severe segregation came from the ends of truck loads. Cold spots recurred about 200 ft (61 m) apart.

How crews measure it

TxDOT requires one thermal profile of the mat for each sublot, over about 150 ft (46 m) directly behind the paver. TxDOT calls a difference of more than 25°F and up to 50°F moderate segregation. It calls a difference of more than 50°F severe. For severe segregation, the crew must suspend paving and remove the cause.

A paver-mounted thermal profiler records a continuous infrared map of the new mat. A hand-held camera is the alternative, but it is harder to use to locate problem areas.

Airport specifications

FAA AC 150/5370-10H (2018) has no temperature-differential limit. It controls the cause instead. Item P-401 requires insulated or heated truck beds and covers when needed to hold delivery temperature. Its engineer’s note says contractors must use an MTV with remixing for runways and taxiways designed for aircraft of 100,000 lb (45,360 kg) or more. The note recommends an MTV for other pavements when its weight will not damage them. The resident project representative (RPR) may reject mix on the truck or in the mat after a visual or temperature check. The contractor must remove and replace segregated areas of the surface course.

Link to pavement condition

Thermal segregation is a construction-quality problem. This page does not claim that a survey can find it, and the PAVER manual has no distress type for it. A survey records the later damage. One example is raveling, the loss of coarse aggregate from the surface.