The use of Fiber in Asphalt

A mixture of polypropylene and aramid fibers was used in this study to evaluate the
performance characteristics of a modified asphalt mixture. In coordination with the City of
Tempe, an asphalt concrete conventional mixture was selected for paving on Evergreen Drive in
Tempe, Arizona. The designated road section within the construction project utilized two asphalt
mixtures as part of this study: a control mix with no fibers, and a mixture that contained one
pound of fibers per ton of asphalt concrete. The laboratory experimental program included:
triaxial shear strength, dynamic (complex) modulus, repeated load permanent deformation, beam
fatigue, crack propagation, and indirect diametral tensile tests. The data was used to compare the
performance of the fiber modified mixture to the control. The results showed that the fibers
improved the mixture’s performance in several unique ways against the anticipated major
pavement distresses: permanent deformation, fatigue cracking, and thermal cracking.

In this study, a construction project to utilize fibers in conventional asphalt concrete
mixture was coordinated with the City of Tempe, Arizona. An asphalt concrete mixture
designated as Type C-3/4 base and surface course was selected for paving on Evergreen Drive
located east of the Loop 101 and north of University Drive in Tempe, Arizona. The designated
road section had two asphalt mixtures: a control mix with no fibers, and a mixture that contained
one pound of fibers per ton of mix. The fibers were a blend of polypropylene and aramid. The
addition of fibers was done at a batch asphalt plant in Phoenix..Basically, no repair work was done and the 2-inch overlay was placed on a much-deteriorated section of Evergreen Drive. Only the edge of the pavement was milled off to match the final overlay grade of the curb. Test sections with and without fibers were staggered on the road to allow for direct field performance comparisons considering traffic flow
and loading types (e.g., bus lanes). About 1500 lbs of each mixture were brought back to Arizona
State University (ASU) laboratories. Sample preparation included compaction of 150 mm
diameter gyratory specimens for triaxial testing. In addition, beam specimens were prepared and
compacted according to AASHTO TP8 test protocols . The performance of both mixtures
was assessed using the advanced material characterization tests that included: triaxial shear
strength, dynamic modulus, repeated load for permanent deformation characterization, flexural
beam tests for fatigue, C* line integral for fracture energy and crack propagation, and indirect
diametral tensile test for thermal cracking evaluation.


TEST RESULTS AND ANALYSES
Triaxial Shear Strength Tests
The triaxial shear strength test has been recognized as the standard test for determining
the strength of materials for over 50 years. The results from these tests provide a fundamental
basis which can be used in analyzing the stability of asphalt mixtures. This is because the
stresses acting on the laboratory specimen during the test simulate the state of stresses existing
 in the pavement, provided certain specimen boundary and geometry
conditions are met.