By: Edited by David Banasiak
State Highway 89 originated as an old
logging road and consisted of approximately 0.57 ft of asphaltic
concrete (AC) over a granular base of volcanic origin. It
receives a high volume of logging trucks due to the timber
harvesting in the area. The sections of the highway in northern
California--through the Sierra Nevada Mountains--are located in
an area of high tensile stress where the primary thermal fatigue
factor is cold temperatures.
Don Wion, maintenance
superintendent in Alturas, Calif., recommended a section on
Highway 89 to be scheduled for a maintenance overlay. Thermal
expansion and contraction has caused high tensile stress and
distress consisting of pavement bleeding, rutting and alligator
and transverse cracks. Previous maintenance treatments consisted
of digouts and grader blankets to repair broken pavement base
failures.
From Caltrans' past experience, pavement
reinforcing fabrics (PRF) have been minimally effective in areas
of high tensile stress caused by thermal- and moisture-related
expansion and contraction. It was decided to evaluate Glasgrid
to determine its effectiveness in retarding reflective cracking.
Glasgrid is a high-strength, high-modulus fiberglass mesh
capable of turning crack stresses horizontally and dissipating
these stresses within the new overlay, its maker says.
The
site's average annual daily traffic for 1992 was 1,600 with 32%
trucks and an equivalent single axle load count of 107,000. The
proposed maintenance strategy for this area consisted of a 1
1/2-in. AC overlay.
Paving begins
On the day of
construction the road was swept clean with a leaf blower to
ensure that the surface was not contaminated. Cracks wider than
1/4-in. were filled with an asphalt mix prior to the placement
of the fiberglass mesh. The mesh was installed from the back of
a 1-ton dual tired Caltrans maintenance truck and then rolled by
the same truck. The mesh was unrolled without any problems and
adhered well to the pavement without wrinkles. It was placed in
the northbound lane full width for a 330-ft long section.
The asphalt was delivered to the site in belly-dump trailer
trucks and dumped directly onto the Glasgrid in a windrow at a
temperature of approximately 190 deg. The tires of the asphalt
trucks did not stick to the mesh or cause separation from the
pavement or wrinkle. A Blaw-Knox PR200B pneumatic tired paver
with a pick-up machine was used to pave over the grid. No
sagging problems occurred between the paver and the grid. A
control section was set up in the southbound lane opposite the
mesh section. The control also was one lane wide and 330 ft
long.
The test site was visited in 1994 and 1995. In 1994,
one partial transverse crack was observed on the outer edge of
the pavement over a culvert at the north end of the mesh
section, but no other pavement distress was present. There were
no changes in 1995 from the 1994 evaluation.
During the 1994
evaluation of the control section, three partial transverse
cracks and approximately 15 ft of longitudinal cracks were
observed. In 1995, alligator cracking with associated pumping
appeared in the control section.
It appears that the mesh
has reinforced the AC overlay and retarded reflective cracking
in an area of high tensile stress. Due to the heavy traffic on
this highway, consideration is being given to an extensive
rehabilitation within the next three years. Glasgrid will be
considered as an option in the overlay strategy.
More
information on products and/or services mentioned is available
by writing in the appropriate number on the reader service card
found in this issue.
