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, 1984[1]; Hughes, 1989[2]). Generally, temperature differentials greater than about 14°C (25°F) can potentially cause compaction problems (Willoughby et al., 2001[3]). Figure 3 describes the general sequence of events leading to temperature differentials.

Figure 1. Damage from Construction-Related HMA Temperature Differentials.

Figure 2. Probable temperature differential damage on a state route in the Cascade Mountains.

Figure 3: Temperature differential sequence of events.

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.

Footnotes    (↵ returns to text)
  1. Hughes, C.S.  (October 1984).  “Importance of Asphalt Compaction.”  Better Roads, Vol. 54, No. 10.  pp. 22-24.
  2. Hughes, C.S.  (1989).  National Cooperative Highway Research Program Synthesis of Highway Practice 152: Compaction of Asphalt Pavement.  Transportation Research Board, National Research Council.  Washington, D.C.
  3. Willoughby, K.A.; Mahoney, J.P.; Pierce, L.M.; Uhlmeyer, J.S.; Anderson, K.W.; Read, S.A.; Muench, S.T.; Thompson, T.R. and Moore, R.  (2001).  Construction-Related Asphalt Concrete Pavement Temperature Differentials and the Corresponding Density Differentials.  Washington State Department of Trasportation.  Olympia, WA.  http://www.wsdot.wa.gov/research/reports/fullreports/476.1.pdf.