layers. The fabrics available and used for layer separation in flexible
pavements are more numerous than those for overlay reinforcement. These
fabrics should meet the requirements identified in Table 21. Generally these
are much stronger fabrics than those used in overlays and are of the woven
and nonwoven type. When considering the use of fabrics between layers the
designer should refer also to local experience.
4. ECONOMICS OF FABRIC USE. Fabric uSe must not only be an engineering
improvement, it must be a cost-effective improvement. The cost of fabric and
its installation as a layer separator will vary with the type of fabric,
experience of the contractor and size of the project. The cost of fabric has
been associated with the cost of an additional 1 (25mm) or 2 (50mm) inches of
base layer thickness or about the same as lime treated subgrade. The designer
should select thicknesses of layers as indicated in Section 7 Flexible
Pavement Thickness design. Base or subbase thickness may be reduced by 2
inches (50mm) when layer separation fabrics are specified but should not be
less than minimum values set forth in Section 7. Furthermore, on subgrades
with a CBR less than 5 no reduction in base should be applied. In cases where
designs with fabric still cost more than conventional designs, such designs
should be recommended only when there is demonstrated experience of longer
pavement life.
The use of fabric in conjunction with an asphalt overlay of an existing
pavement must be a cost-effective application. There are no demonstrated
data that reliably indicate the relative load carrying value of fabric in
terms of asphalt overlay thickness. There is evidence that with the use of
fabrics over "alligator" or fatigued cracked asphalt pavement, reflection
cracking is minimized and longer pavement life can be expected. The type of
considerations that might warrant the extra expenditure for fabrics in
asphalt overlays include: 1) areas with vertical clearance problems, and 2)
high traffic areas which are difficult to close for repairs and/or
rehabilitation.
Change 1, September 1985
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