CONSTRUCTION FABRICS By WILLIAM T. SUNLEY, Engineer of Local Roads and Streets

Less than twenty years ago, fabrics were thought of as being useful only for clothes and gunny sacks. Today, engineering fabrics are extensively and quite effectively used in highway work. Synthetic materials, because of their relative high strength and durability, are the primary reasons for this success.

There are many applications of fabric in the highway industry. Some of the most common are ground stabilization, erosion control, crack control and filtration. These fabrics are manufactured from many different materials such as polyester, polypropylene, nylon, etc. using different methods such as knitting, weaving, spun binding and hot needle punching to produce a thin, strong and light fabric typically weighing about 4 ounces per square yard.

Ground stabilization is accomplished by placing a fabric directly on the soil subgrade prior to constructing a granular subbase. The fabric prevents soil from mixing with the aggregate subbase. Too much fine material (soil) will cause the aggregate to lose its strength and thus its wheel load capacity is reduced. A square yard of fabric in place without the subbase costs about $1.10. The strength of the fabric is equivalent to adding about an inch of granular material to the subbase. It is recommended that subbase thickness not be reduced because of the fabric; instead, the added strength would provide an extended service life of the roadway.

The same fabric can be used for erosion control by building silt fences. The fabric is stretched like fencing to stop water-borne soils. For about $4.40 per foot, silt fences can be used to slow erosion in ditches and to control erosion by preventing soil from escaping around the edges of a site during construction/maintenance operations. Typical design includes a fence 3 feet high on a 6-foot posts on 5-foot centers with the fabric buried 6 inches into the ground.

Probably the largest use of fabrics is for minimizing cracks in an existing pavement from reflecting through a new resurfacing. This is known as reflective crack control. The fabric is placed over an existing pavement

December 1995 / Illinois Municipal Review / Page 11

prior to overlaying with asphaltic concrete. No fabric can stop reflective cracking, it can only slow it down.

The most popular application is on an alligator pattern of cracked asphalt (flexible) pavements. The fabric is placed over the entire surface in what is known as an area-wide treatment. Asphalt cement is used to bond the fabric to the existing pavement for a total cost of about $0.70 per square yard.

No treatment, including the use of fabrics, has been successful in slowing down transverse crack reflection in concrete pavements nor in concrete pavements overlaid with asphalt. Two feet wide strips of fabric glued down with asphalt cement have been cost effective at about $1.00 per lineal foot in controlling the reflection of longitudinal cracks and joints in concrete pavements.

A secondary but maybe as valuable a benefit is the partial waterproofing and stabilizing of the underlying pavement. The fabric glued down with asphalt cement tends to hold in place any pieces of the underlying pavement that may come loose during the life of the overlay. Increased riding surface life is the benefit. However, it is not wise to reduce the overlay thickness because of the use of fabrics.

Fabrics make excellent filters in that water can flow through while soil cannot. This attribute makes fabrics useful for envelopes around underdrains and under rip rap. Filter fabric complete and in place before rip rap stone is placed costs about $1.90 per square yard.

An aggregate-lined ditch can be constructed by placing aggregate over filter fabric. Another alternative to an aggregate lined ditch is known as a fiber mat. It is made by melt-bonding heavy monofiliment nylon fibers together where they intersect during entanglement to form an open but strong flexible mat 3/4 inches thick. After the soil is prepared and seeded, the fiber mat is placed and the voids in the mat are just filled with topsoil. The fiber mat, including the soil preparation, seeding, and topsoil filling the mat itself, costs about $9.70 per square yard. Performance is comparable to concrete ditch lining in some relatively steep grade applications.

Many other fabrics and mats made of both natural and synthetic materials exist and are used for other purposes. This discussion has been limited to a few of the uses in highways. Additional information is available through the Technology Transfer Center. Contact Willy Scheller at 217/785-5048 for video tapes and publications. •

Credits to: John Saner, Local Project Development Engineer.

Page 12 / Illinois Municipal Review / December 1995