course is placed prior to the overlay, then the fabric should be between the
leveling and overlay courses.
c. Tack Coat. The fabrics are bonded to the existing asphalt surface or
the surface of a leveling course by means of a tack coat of asphalt cement.
The selection of a tack coat should be a function of the temperature of
the asphalt pavement surface at the time of construction. As with the tack
coat quantities, the type of tack coat is somewhat a variable with fabric
type. For most fabrics, asphalt cement grades from AC-5 (AR-2000) through
AC-20 (AR-8000) cover the range of applicability. When selecting a final tack
coat material and the temperature is known, the fabric manufacturer's
requirements should be consulted.
The amount of tack coat required is dependent on the condition and
texture of the asphaltic surface on which the fabric is to be placed and on
the type of fabric. Most common fabrics require about 0.20 - 0.30 gal/sq. yd.
of residual asphalt. Figure 12 relates surface texture to tack coat quantity.
The designer may use the word description of the existing pavement surface or
perform texture tests. The texture measure in Figure 12 is based on the putty
impression test. The test equipment consists of 1) a 6-inch (150 mm)
diameter by 1-inch (25mm) thick metal plate with a 4-inch (1OOmm) diameter,
1/16-inch (1.56mm) deep recess machined into one side, and 2) a 15.90-gram
ball of silicone putty. When placed on a smooth surface, 15.90 grams of
putty will smooth out to a 4-inch (1OOmm) diameter circle, 1/16-inch
(1.56mm) deep, thus completely filling the recess.
The silicone putty is formed into an approximate sphere and placed on
the pavement surface. The recess in the plate is centered over the putty, and
the plate is pressed down in firm contact with the road surface. The more
irregular the surface texture (the higher the macro-texture) the smaller the
resulting putty diameter because more material is required to fill the
surface texture. Average texture depth, based on volume per unit area, is
calculated from an average of four diameter measurements.
d. Overlay Reinforcement. Fabrics that are suitable for use over
distressed asphalt pavement surfaces shall meet the specifications noted in
Table 20. Generally, all the fabrics available and in use that meet these
specifications are of the nonwoven type. Some products have had more
widespread use than others. The designer should check the data furnished
herein against any other manufacturers' literature, information or data that
may be more current.
e. Layer Separation. Fabrics may be used as layer separators to prevent
contamination of base materials in flexible pavements. Layer separation also
permits expeditious construction in soft, yielding subgrades. Layer
separation may be a valuable treatment to consider in frost potential areas.
The fabric will prevent the contamination of nonfrost susceptible materials
by minimizing subgrade intrusion into base
Change 1, September 1985
41
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