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Figure 1 (above): Damage from Construction-Related HMA Temperature Differentials. The two adjacent raveled spots are surrounded by relatively undamaged pavement. |
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, 1984; Hughes, 1989). Generally, temperature differentials greater than about 14°C (25°F) can potentially cause compaction problems (Willoughby et al., 2001). Below is a slideshow of temperature differential infrared images from around Washington State. Figure 5 describes the general sequence of events leading to temperature differentials.
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Figure 1 (above): Damage from Construction-Related HMA Temperature Differentials. The two adjacent raveled spots are surrounded by relatively undamaged pavement. |
| Transport truck is loaded with HMA at the mixing plant. |  |
HMA at the extremities (crust) cools faster than the internal HMA. | |
Truck dumps into the paver hopper and crust stays intact through the paver augers. | |
Paver places the crust as cool spots or cool streaks in the mat. |
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Figure 5: Temperature Differential Sequence of Events |
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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.
Video: Temperature Differentials