At least 17 streets in Baltimore were paved with recycled glass, creating road surfaces that could sparkle when sunlight or streetlights hit them. Baltimore’s approach is part of a broader practice of using crushed waste glass in asphalt mixes. Properly processed glass can replace some of the conventional aggregate used in road construction.Wearing surfaces containing 10 to 15% crushed glass have performed well. According to the Federal Highway Administration, recycled glass has also been used in road projects elsewhere in the United States. New York City used about 225,000 metric tonnes of glass in resurfacing applications between 1990 and 1995.However, earlier projects found problems when glass was used in high percentages or as large pieces. The guidance recommends using properly processed glass in suitable proportions for asphalt paving.
How did Baltimore’s recycled glass roads create a sparkling effect
Waste glass was tested for use in hot mix asphalt during field demonstrations in the United States in the late 1960s and early 1970s. Test paving strips were placed at about 33 locations across the United States and Canada. Baltimore later used glass in at least 17 streets, producing a sparkle effect from reflected sunlight or street lamp light.Early projects often used more than 25% glass and coarse particles. The guidance says high glass percentages and large particles probably contributed to stripping and raveling problems reported during those demonstrations. Glass does not absorb asphalt cement binder and bonds poorly to it. Moisture damage can become a concern when high percentages and large gradations are used in surface mixes.
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How is crushed glass turned into a paving material
According to the FHWA guidance, turning waste glass into a suitable paving material requires several processing steps before it can be mixed into the asphalt. Waste glass is first cleaned to remove materials such as metal, plastic, and paper. It is then crushed and screened to produce smaller, more suitable pieces while removing flat, elongated, and sharp-edged particles.For surface pavements, the guidance recommends using glass smaller than 4.75 mm, or the No. 4 sieve, as a fine aggregate mixed with conventional materials. Asphalt wearing surfaces containing 10 to 15% crushed glass have shown satisfactory performance. Higher proportions, potentially up to 25%, may be suitable for base or binder courses, where the glass is not exposed directly at the road surface.
Crushed glass can affect the strength and performance of asphalt
Well-crushed glass can also affect the performance of asphalt mixes because of its angular shape and high friction angle. The guidance says these properties can contribute to good lateral stability, with tests reporting stability comparable to, and in many cases better than, conventional mixes. Waste glass also has a specific gravity about 10 to 15% below that of conventional aggregates, which can increase the yield of an asphalt mix.Its low thermal conductivity may also help mixes containing glass retain heat for longer. Skid resistance tests on pavements containing waste glass have produced results within recommended limits. However, larger glass particles measuring more than 19 mm can become slippery when wet and should not be used in surface mixes. The guidance also notes that using more than 15% glass can increase pavement reflectivity and glare, particularly when the surface is wet.
What makes recycled glass suitable for asphalt paving
Asphalt mixes containing crushed glass can be designed using standard laboratory procedures. The guidance recommends testing the mix for stripping potential and says conventional pavement structural design methods are appropriate.Most highway departments allow 5 to 10% glass. The guidance says performance is generally best when crushed glass is used as a sand or fine aggregate substitute.The same methods and equipment can be used to store, mix, place, and compact asphalt containing waste glass. Field testing should follow the procedures used for conventional hot mix asphalt. The guidance identifies a need for uniform specifications covering sizing, debris levels, and mix limitations. It says a lack of adequate and consistent glass supply remains a major constraint on wider use.
