When working with a compactor on a road, you face two dangers regarding the asphalt. First hazard is to compact the asphalt while it is still too hot. This can lead to an unwanted spooning of the asphalt to the side of the street. The second and more common (at least in Asia) hazard is to use the compactor on asphalt that is too cold already which may lead to “cracks” in the road and handicaps the road from the start. With a suitable compaction system that is able to indicate underlying temperature via surface temperature, the two addressed hazards can be controlled and monitored.

However, the temperature measured on the surface of asphalt does not reflect the actual temperature of the asphalt core. That could mean that a temperature measured on the surface of 100 degrees can well stand for a core temperature of 130 degrees. This may be the case especially at the start of the so called ""cool down"" process. The asphalt thickness is one of the main influencer on the “cool down“ process. Please find below a picture of a “cooling down “model based on different asphalt thickness and corresponding core temperature.

(vertical scale shows material temperature in C° of the core, horizontal scale shows time in minutes passed, the 3 different curve show material thickness in 2,5 cm, 4 cm and 7,4 cm)

The graph shows that the thickness of the asphalt laid determines the cool down curve of its core temperature. The thicker the asphalt the less exponential is the trend of the temperature curve. The different shaded area shows the temperature range where compacting is recommended. Please note, according to the material, the cool down process may vary and the temperature range may shift up or down.

Below, please find an example of a deviation of core temperature from surface temperature without the influence of external factors to demonstrate the temperature difference between core and surface over a time period of 24 minutes.

The graph shows, that the initial curve of temperature difference between core and surface is most critical at the start when asphalt is freshly laid but smoothens out after some time of cooling down. Please Note, the graph shows the temperature of a specific material mixture and cannot be taken as "default" cool down process for other material. The values may vary when choosing different material.

Unfortunately, there is no accurate method yet of how to translate surface temperature to core temperature due to many factors that constantly vary, accelerate or slow down the cooling process. The change in temperature depends on time passed, the thickness and the asphalt mixture itself, its density and various external factors e.g.

• Outside temperature
• Wind-speed
• Wind-temperature
• Clouds, Sun, Shadow
• (underlying)- Ground temperature

I would like to add, that the measurement of the asphalt surface temperature already gives the normal compactor operator a very good indication of how the core temperature ranges. Experienced compactor operator may even deviate a value of core temperature that is very close to the actual value when operating their system in their familiar environment. Still, compactor operator have to be aware that the temperature measured on the surface may vary from the actual core temperature during compaction process.