Considerations FOR more durable hma pavements Ryan Barborak, P.E. Texas Department of Transportation

Bonding Bonding is critical for the performance of the pavement Design assumes all layers are bonded Bonding is achieved through the use of a tack coat Keys to successful bonding include Clean surface Uniform tack coat application Proper amount of tack coat Residual asphalt

Bonding Demonstration 11 sheets of plywood: 48”x8”x11/32” Span length 36” Weight of Person ~60 lbs Measured deflection over span Unbounded Fully bonded 21 times difference = 2 5/8” = 1/8” Courtesy of Wayne Felix Source: http://www.asphaltpavement.org/PDFs/Engineering_ETGs/Mix_201409/Blow_Tack%20Coat%20Best%20Practices.pdf

Bonding Studies have shown that: 10% bond loss reduces the fatigue life by 50% 30% bond loss reduces the life by 70% No bond reduces the life by 60% to 75% Distresses include alligator (fatigue cracking), shoving, potholes, and rutting Failure may occur due to bonding even though mix design is acceptable “When in doubt, tack” - Asphalt Institute Source: http://www.asphaltpavement.org/PDFs/Engineering_ETGs/Mix_201409/Blow_Tack%20Coat%20Best%20Practices.pdf

Tack, Bond, Seal Conventional Tack Spray Paver Tack only Tracking Resistant Asphalt Interlayer (TRAIL) Hot Applied Asphalt Tack, bond, seal Emulsified Asphalt Tack, bond Spray Paver Tack, bond, seal Underseal Seal Seal coat binders/aggregates Apply just enough rock to keep binder from tracking Should it be tacked?

Tack Tracking in Wheel Paths – Consider Less Tracking Tack

Interlayer Material Guide

Thermal Segregation Thermal segregation is defined as temperature differences in the hot mix asphalt (HMA) mat as it is placed. It is a potential cause of premature failure in asphalt roads. It can cause a significant reduction in pavement life due to low density from cold spots in the HMA mat.

Rules of Thumb – Why Does it Matter For dense graded HMA, 1% increase in air voids (above a baseline value of 7%) will result in a minimum 10% decrease in pavement life 25°F temperature differential = 1-2% more air voids No signification compaction occurs below cessation temperature (about 175°F).

Thermal Camera Examples of Thermal Segregation

Minimizing Hand Work

Thank You QUESTIONS