Economic and environmental asphalt recycling
First publishedin World Highways
Proper sizing of recycled materials allows different aggregate sizes to be graded accordingly to suit different asphalt grades
Recycling materials offers the road ahead for highway construction - * Don Brock writes
Recycling has been used in the US for over 50 years in various industries. Today, steel is 100% recycled, and many other products that we have can be recycled.
Environmental groups have aggressively pushed industries to recycle more, but it is either economically driven or legislatively driven.
In the highway industry it has predominately been economically driven and discouraged by stakeholders such as aggregate producers and liquid bitumen producers who see a reduction in their sales as more recycle is used. Salesmen for aggregate, liquid asphalt or polymers often plant seeds of doubt in the minds of State Agencies. This results in agencies being very cautious and limiting the amount of recycled material due to concerns about viability.
Researchers also present a deterrent to using more recycling as more research is inevitably required.
In the late 70s and early 80s when recycling was first used, certain states required the use of a softer grade of binder in order to obtain compaction. This led to the perception that the artificially softened binders extended the pavement life and should be used with high percentages of recycled materials. This deterred the use of recycled material since softer liquids were not available. States required the use of softer liquids when more than 15-20% recycled material was used.
All of these together have combined to restrict the amount of recycled road materials used in the US. Through considerable effort in the last five years the amount of recycling has increased to approximately 20-25% and in some cases as high as 50%. Most of the roads featuring 50% recycled materials are for private work and have performed quite well.
But producers of hot mix asphalt must make money and are economically driven.
Milling of existing roads is a very good construction technique
Today in the US, aggregate cost averages from US$9-$13.53/tonne, depending upon the location of the aggregate source. Liquid asphalt is in the $451-$541/tonne range. Using 5.5% asphalt at $541/tonne, the virgin asphalt or glue costs $29.80/tonne, using $10.82/tonne aggregate cost, the total virgin material cost is $40.60/tonne.
Milling of existing roads allows the correction of the profile, does not require raising shoulders, guardrails, bridge clearances and is a very good construction technique in itself. As asphalt roads are milled they generate recycled material that can be used in the new mix in percentages of up to 50-60% with current plant and mix technology. However, this is not being widely used in public works projects in the US.
While the industry tries to make asphalt pavement complicated, it basically uses just two ingredients to make the pavement, the glue (liquid asphalt) and aggregate. Generally the aggregate is produced from local quarries and has not travelled more than 48-64km.
The aggregate on the road is at least the same quality as the aggregate being produced in quarries and in some cases may be better since reserves of the higher quality materials have been depleted. In the early days of recycling in the United States, contractors often haphazardly milled the material off the road and placed it into stockpiles without seriously managing it to prevent segregation, then used as much as 50% in the mix. While many successful projects were carried out, the transportation community considered recycled materials as an inferior product.
High percentages of recycled materials were produced in the late 70s and early 80s when liquid asphalt prices were as high as $180.40/ tonne. However after oil prices dropped in the mid 80s and Superpave mix designs were introduced, the amount of recycled material was lowered to approximately 10% in most mixes. The Superpave mix design required controlling the voids in the mix. Since recycled material was treated as crusher run type material, and care was not taken to prevent segregation, asphalt contents and voids could not be controlled with much over 20% recycled material.
RAP stockpile before screening
With the increase of liquid asphalt costs in early 2000, it became necessary to review how recycled material could be used to produce a high quality product while controlling asphalt content and air voids. In asphalt mixes, the surface of an aggregate is glued together with liquid asphalt. The amount of liquid asphalt required depends on the surface area of the aggregate. Using smaller aggregates increases the surface area and therefore increases the quantities of bitumen required and significantly so. If the material segregates, either less or more liquid asphalt, and -74 micron material can be placed in the new mix. When trying to hold the air voids at 4% and control the asphalt content within 0.2%, this is impossible without controlling the gradation of the material. This has led to the practice of reprocessing the recycled material. By treating the recycled material like virgin aggregates by taking it apart and separating it by sizes, it can be placed in multiple feeder bins and fed to the plant in a conventional manner. This allows the production of a uniform, high-quality mix. By fractionating or separating the RAP to the same sizes of the virgin mix, the air voids and asphalt content in Superpave mixes can be controlled to meet all requirements of this mix design.
Today in high traffic areas in the United States skid resistant aggregates are required to prevent early degradation of the surface. If the recycled material was produced originally with skid resistant content it can be taken apart and reused while meeting the skid resistance requirements. However, if the recycled material was produced from a softer aggregate it has generally been exempted from use in the surface mix. If the recycled material is fractionated, the -6mm material does not affect the skid resistance in a 12mm pavement. The -6mm has a higher percentage of liquid asphalt and can reduce the cost significantly.
Approximately 10 years ago polymers began to be added to asphalt mixes and many states required a reduction in the amount of recycled material when polymers were added. It was perceived that the recycled material would dilute the effect of polymers and therefore would be detrimental to the performance of the pavement. Limiting RAP percentages when polymers were used and requiring softer liquid asphalt limited the amount of recycled material used. Around five years ago seven test sections were placed at the National Center for Asphalt Technology (NCAT) Test Track in Auburn, Alabama. Section One was an all virgin mix using a PG 67-22 grade of asphalt. This would be similar to an AC-30 or a 60/70 pen asphalt.
Section Two used 20% RAP with a PG 67-22.
Section Three used 20% RAP with PG 76-22.
Sections Four, Five, Six and Seven used 45% RAP; Section Four using a softer binder, Section Five used a standard grade of asphalt, and Section Six used a polymer modified asphalt giving a PG 76-22. Section Seven used Sasobit, a Warm Mix additive in order to achieve compaction. It was found that all sections achieved compaction without any additives.
The screening process is a crucial part of the use of recycled materials in asphalt production
Most researchers in the United States have been concerned about cold weather cracking where high percentages of RAP were used. But during the test, it was observed that the low temperature properties did not vary considerably. The high temperature properties produced a stiffer mix. This actually made a better surface mix since it would be more resistant to rutting. The 45% RAP sections all rutted less than the controlled virgin section.
The two sections with 20% RAP were produced with very low air voids and as a result the 20% RAP with the standard grade rutted more.
After two years and 20 million cycles the only cracking occurred in the high RAP with polymers. However, it was minor and further research showed that this was reflective cracking from the lower levels. The original track was milled to a depth of 51mm and 51mm of new material was placed in the inlay. In two years reflective cracking came through as would be expected and it was noted that the polymers did not slow propagation any more than virgin materials. After the completion of the two year cycle of 10 million Equivalent Single Axle Loads (ESALs), researchers continue to remain cautious and said that another round of tests should be carried out to see how the sections held up over a longer period. Approximately 20 million ESALs have now been placed on the sections and all of the sections continue to perform with additional cracking in the sections with polymers. The 45% RAP with the standard grade of liquid asphalt cement continues to show very little additional rutting and no cracking. With the 45% RAP and standard grade of liquid (PG 67-22) the material cost would be approximately 53% less than the 20% RAP with polymers and it would have better performance. The softer asphalt, which actually rutted slightly more, offered no value and no improvement in performance.
With the introduction of Warm Mix technologies that use foaming of the asphalt, the viscosity of the original liquid asphalt remains the same throughout the production process. The foaming process creates a low viscosity liquid and is ideal when using a high percentage of recycled material. Bonuses of the Warm Mix technology, other than the ease of compaction, include the elimination of smoke and smell, a reduction in the drying cost, and a longer life pavement.
Sorting recycled material effectively will help boost asphalt quality
The use of high RAP mixes reduces the cost of the mix. The addition of Warm Mix asphalt using foam processes further reduces the cost and can produce a better, longer lasting mix, which is surprisingly more resistant to stripping. The virgin aggregate as it is processed through the plant is super-heated to a temperature above the final mix temperature.
As it progresses from the drying process to the mixing process it meets the recycled material.
The hot super-heated rock evaporates the moisture from the recycled material and begins to melt its older, harder binder. This transfers a portion of the recycled liquid to the virgin aggregate, producing a film of harder asphalt around the rock. As the new binder is added it forms a second layer on top of the first layer which is more flexible and durable.
The combination of Warm Mix and high RAP achieves the greatest potential to reduce stripping.
The combination of foamed Warm Mix with high RAP produces an excellent product at the lowest conceivable cost, stretching highway investment, reusing resources, leading to sustainability and a greener environment.
This represents the future of the asphalt paving industry.
Tests show that HMA is 100% recyclable.
Milling corrects road profile, corrects drainage, eliminates raising shoulders and guardrails, and maintains bridge clearance and generates RAP. By fractionating RAP and using Warm Mix (hot foam) with 50% RAP, it will produce a rut resistant, longer life pavement. It can be produced with a standard grade of AC.
Density can be achieved with one less roller and centreline joint density is substantially improved, while more road can be paved at substantially less cost and greenhouse emissions and imported oil use are greatly reduced.