No Dirt or Dust: Cleanliness is
Extremely Critical
Just as a lack of fines can precipitate raveling, dirt or dust
in the aggregate can cause raveling, Granite Rock's Munson
says.
"The cleanliness of the aggregate is very critical in mitigating raveling. Dirty aggregates coated with dust or fines
create a barrier to direct contact (bonding) between the
asphalt cement and the aggregate.
"The weakest portion of the resulting aggregate-dustasphalt bond is the aggregate-dust adhesion.
"After several repetitions of the physical forces of traffic
loading, the forces of adhesion holding the dust to the aggregate will break, and dislodging of the aggregate from the
pavement mass will occur."
An excess amount of dust or fines also has the potential
to absorb a high percentage of asphalt cement, lowering
the effective asphalt content of the mix, leading to binder
content problems outlined earlier, Munson adds.
Pavement distresses related to moisture damage improve
with the use of anti-stripping additives. Antistrip additives
promote adhesion between the mineral aggregate and the
asphalt in hot mix asphalt mixtures. They can be as complex
as manufactured chemicals or as simple as lime.
Antistrips work, but their higher cost must be balanced
against lower maintenance expenditures over time.
In Wisconsin, moisture damage in HMA pavements – specifically raveling, stripping, rutting and alligator cracking –
occurs occasionally as it does nationwide, and tests of a mixture's potential to suffer such damage is a standard element of
highway construction specifications, the state reported.
In the early 1990s, the Wisconsin DOT adopted the
Tensile Strength Ratio test, using the ASTM D4867 standard
method, to predict mix susceptibility to moisture damage.
WisDOT required that antistrip agents be added when
the TSR of a mixture at optimal asphalt content fell below
70 percent. The enhanced mixture then had to obtain a TSR
value of at least 75 percent.v
Smooth as glass.
It's time.
topconpositioning.com/mmgps-paving
Text INFO to 205-289-3789 or visit www.betterroads.com/info
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