10. Segregation
10.4 Thermal Segregation
Thermal segregation is not identifiable without infrared monitoring, and therefore it will often go unnoticed. As with physical segregation, there are many locations where thermal segregation may originate. This type of segregation occurs most often in trucking procedures and paver operation.
Thermal segregation is not identifiable without infrared monitoring, and therefore it will often go unnoticed.
In trucking, the mix cools more quickly near the edge, bottom, and top of the haul truck. This cooler material is not always remixed with the hotter material, leading to temperature segregation during laydown and compaction. The result can be more variability with in-place density and a nonuniform surface. This problem can only be detected by infrared technology. Truck beds are often insulated for longer haul distances to reduce the potential for temperature segregation.
The use of MTVs has shown benefit in reducing segregation. The MTV remixes the mix, reducing both physical and thermal segregation. Temperature segregation can be minimized with a balanced, uninterrupted paving operation.
There are various ways to detect thermal segregation. These range from using a handheld infrared “gun” that spot checks surface temperature to more automated systems. These automated systems can scan the full width of the mat, record surface temperatures and location, and send the results in real time for analysis (see Figure 159). This allows for decisions regarding the construction operation and timely corrections if needed.
Source: VÖGELE
Figure 159. Paver Equipped with an Automated Infrared Scanner
Research by Willoughby et al. (2001)1 concluded that if the mix temperature differential is less than 25 °F (~15 °C), generally consistent compaction can be expected. But if the mix temperature differential is 25 °F (~15 °C) or greater, differential compaction can occur, and the in-place air void content will fluctuate.
Highly segregated areas of coarse materials will tend to lose heat faster than surrounding areas of the freshly placed mat and appear in thermal imaging. Thermal segregation can be an indicator of the presence of physical segregation. Infrared data can identify cool spots in the mat and predict segregated areas (see Figure 160).
1K. Willoughby, J. Mahoney, L. Pierce, J. Uhlmeyer, K. Anderson, S. Read, . . . R. Moore, Construction-Related Asphalt Concrete Pavement Temperature Differentials and the Corresponding Density Differentials (Washington State Department of Transportation, 2001).
Source: Dorogi i Mosti Journal (top) and Pavement Interactive (bottom)
Figure 160. Segregated Areas (on right) Predicted by Cool Spots on Infrared