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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Waste glass that is crushed and screened can be used as a portion of fine aggregate in asphalt paving mixes. Satisfactory performance has been obtained from hot mix asphalt pavements incorporating 10 to 15 percent crushed glass in wearing surface mixes. The term "glasphalt" has at times been used to describe these pavements. Higher blends, incorporating perhaps up to 25 percent, could potentially be used in base or binder course mixes. Hot mix asphalt surface course pavements with more than 15 percent waste glass may experience deterioration due to stripping of the asphalt cement binder from the waste glass.
At the present time, the commercial use of waste glass in asphalt paving applications has been limited to communities such as the City of New York, where the quantity of waste glass produced and collected provides sufficient incentive to recycle it in pavement applications. Most of the earlier applications of glass use have been limited to test pavements or specialty applications.
In the late 1960's and early 1970's a number of studies and field demonstrations were undertaken in the United States. to examine the potential for using waste glass as an aggregate substitute material in hot mix asphalt. (See references 1,2,3,4,5.) During this period test paving strips were placed at approximately 33 locations throughout the United States and Canada.(6)
From the mid-1970's through the mid-1980's, the City of Baltimore made use of glass in its street pavement program. At least 17 streets were paved with glass to produce a "sparkle" effect, resulting from the reflection of sunlight or street lamp light off the glass pavement. In the mid-1980's research activities were undertaken on Long Island and a glass processing plant was designed and began operations, processing over 12,600 metric tons (14,000 tons) of mixed waste glass for use as an aggregate substitute in paving applications.(7)
More recently, numerous paving projects using waste glass have been undertaken around the country. However, by far the most aggressive program has been undertaken by the City of New York's Department of Transportation, where from 1990 through 1995 approximately 225,000 metric tons (250,000 tons) of glass has been used in resurfacing applications.(8)
Flat and elongated particles that could contribute to pavement raveling, stripping, poor skid resistance, abnormally high tire wear, and excessive glare were all identified by early researchers as potential problems. Since glass does not absorb any of the asphalt cement binder, and since glass is also "hydrophilic," moisture damage (stripping) is a particular concern that has been identified, especially when high percentages and large gradations are introduced into a surface course mix.(9) Many of the early investigators recommended the addition of lime as an antistripping agent to reduce potential stripping problems. Early glasphalt projects used high percentages of glass (greater than 25 percent by weight of the mix) with coarse glass gradations (greater than 12.7 mm (1/2 in)). Current data suggest that the use of high glass percentages and large particles of glass probably contributed to most of the stripping and raveling problems that were reported during the early test pavement demonstrations of the 1960's and 1970's.
The high angularity of cullet, compared with rounded sand, can enhance the stability of asphalt mixes where properly sized cullet is used. Stabilities comparable and, in many cases, better than conventional mixes have been reported.(5,7,10) Other beneficial characteristics include low absorption and specific gravity and low thermal conductivity, which reportedly offers enhanced heat retention in mixes with glass.(10)
MATERIAL PROCESSING REQUIREMENTS
When used in asphalt concrete, glass processing must include the removal of ferrous and nonferrous metal, plastic, and paper. In most waste glass processing plants this requires screening, magnetic and eddy metal current (non-ferrous metal) separation, air classification, and/or handpicking operations. Although 100-percent removal of all paper, plastic, and debris from postconsumer glass streams is unlikely, an acceptable glass product can be achieved in most instances, particularly if mix designs limit glass to 10 to 15 percent of the mix.
Crushing and Screening
Crushing and screening are required to achieve proper sizing and to eliminate flat and/or elongated and sharp-edged glass particles.
Some of the glass properties that are of particular interest when glass is used as fine aggregate in asphalt paving include gradation, specific gravity, and durability.
Gradation: Waste glass used in asphalt surface pavements should be processed to a fine aggregate size (less than 4.75 mm (No. 4 sieve) and blended with conventional aggregates to conform to gradation requirements in accordance with AASHTO T27.(11) Larger top sizes ranging from 9.5 mm to 15.3 mm (3/8 to 5/8 in) should be suitable for use in base course mixes.
Specific Gravity: Due to a specific gravity approximately 10 to 15 percent below conventional aggregates, waste glass can be expected to provide a greater yield (more volume of asphalt concrete per ton).
Durability: Glass is a brittle material and coarse particles greater than 4.75 mm (3/8 in) in size can be expected to break down during handling. Consequently, it is preferable to process (crush and screen) waste glass into a fine aggregate size, which is minus 4.75 mm (No. 4 sieve), prior to its use in surface course asphalt paving mixes.
Some of the properties of an asphalt mix containing glass that are of particular interest include frictional properties, mix stability, stripping resistance, and reflectivity.
Frictional Properties: Skid resistance tests results that have been reported have shown waste glass pavements to fall within recommended skid resistance testing limits. Nonetheless, large glass particles (greater than 19 mm (3/4 in) in size) that have at times been incorporated into poorly processed surface course pavements could become slick when wet and should be avoided in all surface mixes containing glass.
Mix Stability: The angular shape and high friction angle (approximately 50°) of well-crushed glass contributes to good lateral stability. This is a positive feature, particularly where vehicular braking and acceleration are considerations.
Stripping Resistance: Glass is not absorptive and bonds poorly to asphalt binder. Antistripping agents such as hydrated lime introduced as 2 percent of the aggregate mix by weight have been used in previous demonstrations to reduce potential stripping problems. Poor immersion-compression test results (retained stability), a measure of the potential for stripping problems, can also be expected where a high percentage of oversized glass particles are introduced into a mix.(10)
Reflectivity: Large percentages of glass in a surface pavement (greater than 15 percent by weight) produce a noticeable increase in the reflectivity of the pavement. Depending on the size of the glass particles, this could produce a noticeable glare, particularly on wet pavements. Smaller glass particles and lower percentages of glass can help to reduce reflective glare problems.
Asphalt mixes containing crushed glass can be designed using standard laboratory procedures. Conventional fine hot mix aggregate gradations, as specified in AASHTO M29,(12) may be used. It is recommended that mix design testing include stripping potential evaluations as outlined in AASHTO T283.(13)
Currently most highway departments allow the use of 5 to 10 percent glass in their asphalt mixes. Although some areas use 6.4 mm to 12.7 mm (1/4 in to 1/2 in) gradations and larger, many users are taking a more conservative approach to gradation size. The City of New York has lowered its specified gradation top size in its mix design to minus 9.5 mm (3/8 in) from 15.3 mm (5/8 in). Los Angeles has specified the use of minus 9.5 mm (3/8 in) glass. Studies in Virginia and Florida also have recommended that minus 9.5 mm (3/8 in) gradation be used.(9)
Most data at the present time indicate that larger gravel-sized glass particles will reduce pavement performance, and that optimum performance can best be achieved by using crushed glass as a sand or fine aggregate substitute (less than 4.75 mm, or No. 4 size sieve). When waste glass is used as a fine aggregate substitute material, glass performance in hot mix asphalt should be comparable to conventional mixes.(14) Where larger, gravel-sized glass particles are used, raveling and stripping in particular could be a problem.
The introduction of an antistripping agent such as hydrated lime (approximately 2 percent by weight of aggregate) could be beneficial, but performance should be satisfactory if only fine-grained (minus 4.75 mm (No. 4 sieve) glass is used and substitution rates do not exceed 15 percent.
Base course applications, being less susceptible to stripping, rutting and skid resistance, could tolerate the introduction of glass particles up to 15.3 mm (5/8 in) in size, resulting in substitution of both coarse and fine aggregates in the mix. Although higher percentages could probably be introduced, a 15 percent weight limitation would provide for a conservative design. Extraneous debris (e.g., paper, dirt, etc.) sometimes associated with waste glass can be expected to adversely impact mix quality. The introduction of excessive debris from Material Recovery Facilities can be expected to increase the void content of most mixes, if the asphalt content or the mix gradation is not adjusted. Recent recommendations suggest that extraneous debris should be limited to 5 percent of the glass by weight to avoid significant impacts on glass quality.(15)
Conventional AASHTO pavement structural design methods are appropriate for asphalt pavements incorporating waste glass in the mix.
Material Handling and Storage
The same methods and equipment used to store or stockpile conventional aggregates are applicable for waste glass, particularly where glass is properly precrushed to a fine, sand-size fraction, where additional breakdown is not a concern.
Mixing, Placing, and Compacting
The same methods and equipment used for conventional pavements are applicable to asphalt pavements containing waste glass.
The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing waste glass. Mixes should be sampled in accordance with AASHTO T168,(16) and tested for specific gravity in accordance with ASTM D2726(17) and in-place density in accordance with ASTM D2950.(18)
The development of uniform specifications concerning sizing, levels of debris and mix limitations are needed to facilitate glass use. There is some uncertainty regarding the need for antistripping agents such as lime if glass is reduced to a very fine aggregate size (less than 6.35 mm (1/4 in)). The most limiting constraint to glass use is the lack of an adequate and consistent supply of the product. In only a few instances, such as in the City of New York, have provisions been made to establish a continuous market supply of glass. The elimination of hand sorting and crushing of all glass to produce a market-ready aggregate product is probably required to achieve more widespread glass use.