Sustained environmental focus in recent years has triggered a renewed consciousness of recycling practice and its extensive application to an increasing number of materials. The recycling of asphalt shingles in particular is sought out for the benefits resulting from commercial applications potential and the conservation of landfill space. Currently, the practice is partially limited by environmental and health concerns regarding the content of harmful materials embedded in aged shingles. Another aspect is the behavior that can be expected in regards to the performance of the asphalt used in road pavement. Furthermore, the technology necessary for recovering and reusing asphalt shingles has been improving for more than three decades and gaining trust from regulatory authorities and road pavement contractors alike.

1.1.Scope and Objectives

The current estimations show that a quantity of over 11 million tons of waste is generated by roof demolition, reconstruction, and refurbishing on a yearly basis in the United States alone [1], as tear-off roofing material. Additional waste is generated during the production and installation processes in the form of scrap material. The various byproducts of roof renovation and demolition projects, such as tear-off shingles and scrap cutouts are, in majority, disposed of by means of landfill tipping, an increasingly costly procedure in the recent years, as the landfill space is reduced by accelerated exploitation. The tests presented here show that good road performance can be obtained through the incorporation of reclaimed asphalt shingle into hot mix asphalt (HMA). This is especially important in the context of bituminous roofing products containing up to 36% asphalt cement on a fiberglass or organic substrate [2].Reclaimed or recycled asphalt shingles (RAS) have a great recovery potential that can support the propensity of the construction and roads industry to become more environmentally friendly and economically feasible. The main motivation of this study is to examine various aspects of the lifecycle of asphalt shingles. The report is formulated as a white paper aimed to provide comprehensive information regarding the technical, ethical, and commercial aspect of recycling asphalt originating from industrial waste and post-consumer bituminous roof tiles.

1.2.Significance of the Study

The technical requirements of and the road plan alternatives for the utilization of post-manufacturing and post-consumer RAS are presented here, along with the economic and environmental considerations that drive the process forward. Although the RAS recycling industry has been around for decades, it has yet to see significant results and widespread implementation. The main challenges in this field regard the separation of materials however this is a straightforward process for most of the component parts, while regulations allow for a certain percent of deleterious materials to be incorporated in the final product without a significant impact on the performance grading (PG).

1.3.Report Structure

The present report is organized into 7 main sections. The second section presents a background of the issue of RAS and focuses on the manufacturing and composition of asphalt roof tiles, the presently available options for waste management, and the economic considerations that should be taken into account during the recycling process. The third section is dedicated to discussing the currently available varieties of asphalt shingles, presenting technical information about asphalt roof tiling composition and other factors such as the amount of scrap material resulted from various manufacturing and installation processes, and provide an overview of the general roof waste. The fourth section goes into an in-depth presentation of the potential usage of RAS as an alternative to landfill disposal. To evaluate the performance of the RSA incorporated in HMA, the fifth section presents test results for various mixtures and draws a comparison between recycled asphalt shingles (RAS) and reclaimed asphalt pavement (RAP), the latter being currently accepted by the American Association of State Highway and Transportation Officials (AASHTO) to be present for up to 20% in the asphalt binder without grading modifications.The next section is focused on environmental and health issues, such as asbestos contamination and its importance in recycling, polycyclic aromatic hydrocarbon (PAH) emissions, and the regulatory framework of utilization of RAS in HMA. Finally, the references can be found in the last section.

2.Background

The residential roofing market is dominated by bituminous roof tiles, which due to feasibility and economic considerations account for over 65% of the residential sector [3][4]. The United States produce 11 million tons of tear-off waste on a yearly basis. Almost 70 manufacturers of asphalt shingles function across the United States and create another 1 million tons of scrap resulting from the production process [1], whereas Canada accounts for an additional 140 million square meters (or 1.5 billion square feet)of asphalt shingles installed during residential projects every year, predominantly in Central Canada region [3]. Asphalt shingles dominate the United States market with almost 80% of the residential building being covered in bituminous tiling. The manufacturers produce 11.6 billion square meters (or 12.5 billion square feet) of tiles on a yearly basis, the equivalent of 5 million roofs.

The average waste expected for 1 square foot of roof envelope is between 1-2 kilograms (or 2-5 pounds), and varies as a function of the bituminous tile type and layers [4]. In terms of composition, shingles are manufactured from asphalt cement in proportions between 19 to 36%, depending on the base of fabrication. Thus the ratio on fiber glass substrate ranges between 19 to 22%, whereas on cellulose substrate the asphalt it varies between 30 to 36%. Other components are the mineral granules in ratios between 20 and 38%, the mineral stabilizer, present in quantities between 10 and almost 40%, and the cellulose of fiber glass substrate ranging between 2 and 15%.

Table 1. Bituminous tile composition in the residential sector

Due to the large proportion of asphalt and mineral aggregate and the amount of material available as waste, bituminous tiling is an important target for reintegration in usable mixes and the reintegration is facilitated by the fact that the materials are in general found isolated from other scrap. Asphalt shingles are in general isolated from other waste during demolition projects and are technologically facile to recycle, which contributes to cost cutting. Most of the recovered materials are used for road pavement as a component of new HMA.

Fig. 1. Asphalt Cement Price Index Per US Ton, New Jersey Department of Transportation.

Although the asphalt cement price has been decreasing and is now at the same level as 2009, recycling of material from asphalt shingles can further contribute to savings for road contractors. The graphic shows the price evolution per US ton of asphalt cement as an average of the prices at north and south of Route 195 [5][6].

2.1.Bituminous Tiling Product Composition and Manufacturing

Asphalt shingles were first delivered to the market in 1901 [7], being preceded by bulk roofing solutions such as asphalt prepared rolls [8]. Bituminous tiles rapidly gained market share due to many advantages over other types of envelopes, such as price competitiveness, lightness, and ease of installation and maintenance. Typically, the manufacturing process of asphalt shingles can be described by six stages: base saturation, coating, applying surfacing aggregate, drying, applying a finish and packaging. The felt, cellulose, or glass fiber backing is saturated by being sunken into a hot asphalt basin until the desired thickness is obtained, and then mineral aggregate is applied to one of the sides for the purpose of aging and damage prevention. The other side is covered in finely grained sand, mica, or talc to prevent the tiles from gluing to each other during transportation. During the final manufacturing stages, the bituminous tiles are finished, cut into shape, and packaged [9].

Fig.2. Six successive steps of asphalt shingle manufacturing.

Asphalt is obtained through petroleum refining and is a high-viscosity, dark semi-liquid [10] which needs to be oxidized prior to utilization in roof tiles production. The oxidation process requires oxygen gas to be introduced into the semi-liquid mix to increase the viscosity and is stopped when the product reaches the required viscosity threshold [9].

Table 2. Tile composition as percentage of the total weight [2]

Fig.3. Asphalt shingle cross-section schematic

Bituminous roofing products come in a great variety of shapes, thicknesses, and weights. The net product density is between 57 kg (125 lbs.) and 173 kg (380 lbs.) for one square foot of roofing envelope area [3]. The variation stems from the multilayered nature of some products, along with the thickness of the asphalt layers, and the mineral aggregate coating. Most often, shingles are shaped in strips or in the regular arrangement commonly known as 3-Tab. The standard sizes of the 3-Tabs are 1000mmx336mm, which is the metric standard, and 12inx36in, the English standard [3].Architectural or laminated shingles are a third variety that offers multiple types of tiles of different shapes and sizes which can be combined for a better visual aspect and an embossed appearance. They are approximately one and a half times heavier than other varieties, have extended lifetime warranties and are designed to withstand greater lateral loads such as wind or snow.

Installation is facilitated by the self-sealing adhesive bands applied on the back of the tile strips by the manufacturer. The sealant is subsequently polymerized in the sunlight and each tile becomes locked with the inferior one, thus creating increased adverse weather resistance. An alternative to glue polymerization is the T-lock or tie lock interlocking system, based on heavier tiles and aimed at windy climate regions. The T-lock system has been discontinued in some parts of the world, such as the United States, as the asphalt cement prices increased and the manufacturers were unable to provide the necessary thickness of the product for the same cost – however it can still be found covering many roofs today. Estimations show that the asphalt shingle market is dominated by 3-Tab tiles, with 65% of the market, followed by architectural shingles with 15% and interlocking tiles with about 5%.

Typically the weight of asphalt shingle packs is in the range between 27 kg (60 lbs.) and 39kg (85 lbs.), with an average at 34 kg (75 lbs.). Under the assumption that every 9 square meters (or 100 sq. feet) of roof require three packs of shingle, the average corresponding area unit will weight approximately 100 kg (or 220 lbs.). Given the variation between manufacturer standards and the products, the exact type of installed shingle will influence the exact weight of the roofing envelope and the asphalt waste resulted after the removal of the tiles.

The bituminous tiles production depends on the substrate materials used for asphalt impregnation. The materials used as backing can be either organic, such as cellulose of felt, or inorganic, such as glass fiber. Organic materials have been available historically for much longer than glass mats and they are preferred for their larger suppleness and malleability in cold weather, dominating countries such as Canada, where they account for 90% of the roofs. The more modern alternative of glass fiber is preferred in the United States and is slowly making its way into Canada however the recyclable materials in the near future will be mostly based on organic substrate given the current distribution of the roofing envelopes market.

2.2. Asphalt Roofing Product Varieties

In addition to asphalt shingles, organic substrate roof envelopes include roofing membranes (roll roofing) and saturated felt. The membranes are composed of a heavy duty substrate sheet, typically asphalt on felt, and coated with mineral aggregate. This membrane type can be used for valley sealing and insulation (flashing), or as the starting band at the eaves of bituminous tiling roofs.

Fig. 4. Installation of starter strip on asphalt shingle roofs

Fig. 5. Roof valley insulation

It is common for asphalt shingle envelopes and various other building envelopes to be underlined with a layer of asphalt-saturated felt for better waterproofing and thermal insulation. The felt thus prepared has markings for partial overlaying. In most cases, only two weight standards are used for such purposes. The #15-graded felt weighs 6.8kb (or 15 lbs.) per 1 sq. ft., whereas the #30-graded felt weighs 13.6 kg (or 30 lbs.).

All types of asphalt roofing products incorporate three raw materials: a substrate or backing membrane, asphalt, and mineral coating aggregates.

The substrate sheeting constitutes the backbone of the asphalt hydrothermal proofing and is the product with the greatest influence on the final flexibility and strength of the membrane. Cellulose felt contains both new and reused fibers originating from wood, paper, and cardboard. Historically, felts made from asbestos have been utilized by manufacturers, however these have long been made obsolete due to the discovery of the correlation between asbestos and various grave respiratory illnesses. Fiber glass substrate sheets appeared on the market during the last twenty years. The sheets currently used in roofs are manufactured by fusing fiberglass threads with urea formaldehyde resin or phenol formaldehyde.

Asphalt, a product of petroleum refining, is utilized to impregnate the substrate base because of its excellent hydrophobic, conserving and strengthening properties.Asphalt is a refinery byproduct which account for less than 3% of the production of the refineries in the United States. It is composed of the bituminous remnants which are left over after the distillation of all the lighter fractions present in crude oil. Such remnants are further refined into multiple products besides asphalt. The refining process is a multi-stage operation of distillation, liquid liquid extraction and partitioning, and resin and asphaltenes extraction (or deasphalting). The output of petrol-extracting facilities is not a single type of asphalt but a range with different quality grades and climate suitability.The bituminous semi-liquid is used to saturate and cover the base mat, thus ensuring the necessary insulation and waterproofing performance. Organic fibrous bases are typically impregnated with two bituminous layers of different types. The primary coating is meant to be absorbed by the cellulosic fiber, cement them, and fill in the undesired spacing, whereas the secondary layer is composed of denser asphalt with higher viscosity. The fiber glass bases do not necessitate to first saturation layer, being thin and nonabsorbent. Small orifices are drilled through such fiberglass bases, contributing to the resistance of the bonding between the encapsulating asphalt and the mat. This layer of asphalt accounts for the protection against erosion of the exposed surface and serves to embed the fine aggregate coating.

Mineral aggregate coating is composed of mineral granules, natural or industrial ceramic chips, and natural stone particles covering the unprotected exterior of the bituminous roofing product. Mineral aggregate coating serves multiple purposes, such as protection against UV radiation, partial fireproofing of the roof, lifetime extension through protection against physical damage, and the creation of architectural visual appeal by incorporating and blending multiple types of natural-looking stone types and nuances. Finer grained silica coats the back side of roofing membrane and shingles for practical purposes. Other raw materials are utilized in the membrane and shingles manufacturing process, such as fillers, stabilizers, mineral dust or limestone.

The three main residential roof envelope solutions are defalcated for comparison in terms of primary material distribution in the table below. The quantities are shown in hundreds of square feet.

Table 3. Percentage defalcation and comparison of roofing solutions

Asphalt shingles dominate the North American continent markets, with the United States having a preference toward glass fiber substrate and a Canadian preference for organic substrate [1][3]. The industrial and commercial roofing market is dominated by low-slope envelopes, while asphalt membranes are often preferred to asphalt shingles.

3.Waste Management

The current industrial and commercial roofing market observes multiple types of roofing systems, out of which three main types depend on asphalt utilization. These are the rubberized asphalt envelopes, 2-ply modified bituminous envelopes, and 4-ply BUR membranes (built-up roofs) [3]. The 2 and 4-ply roofs dominate the market with up to 80% of the low slope roofs in industry, commercial buildings and the institutional public sector.

Table 4. Waste stream generation as % of the total in the industrial & commercial sector

The estimated volume of waste and the percentage of raw materials resulted from manufacturing and installation scrap, and roof demolition and reconstruction as a percentage of the total show that almost all the waste is created by re-roofing.

The industrial and commercial sector is estimated to account for only 20% of the total, whereas the residential sector produces the remainder. The primary components that contribute to the waste stream are the mineral coating aggregate (57%), asphalt (35%), and saturated substrate (9%) in terms of weight.

Table 5. Waste type segmentation per type of waste as % of the total

3.1.Waste Sources and Landfill Practice

The waste chain generators of bituminous shingles can be categorized as tear-off (or post-consumer waste) and industrial scrap (or post-production waste). The first variety is resulted from residential and commercial roof demolition and maintenance, whereas the second variety is defined as the waste generated in each manufacturing stage, such as cutouts and non-conformal material. Tear-offs and industrial scrap are occasionally found mixed with deleterious materials, in which cases they require being isolated prior to re-processing. In the cases where they are found mixed with certain materials such as paint, chemicals, pesticides, and other household hazardous waste, the EPA regulations require separation before landfill tipping. Typically, the undesirable materials included in asphalt shingles are wood, plastic packaging, nails, and cellulose.

Post-production waste accounts for 5 to 10% of the yearly total 11 million tons of waste from the United States and is relatively homogeneous for each type of bituminous product [11][12][13]. Post-consumer tear-offs compose the other 90 to 95% of the total waste. The expected lifetime of the asphalt tiles varies with the manufacturing technology employed for production; however, general product lifetime estimations pinpoint the average roof age between 12 and 15 years [7]. Shingles generated as tear-off scrap are expected to vary widely in terms of asphalt quality and mineral coating composition due to aging through long time exposure to UV radiation, frosting and thawing cycles, physical damage, and variations in temperature that have been altering their properties. The common practice of refurbishing roof envelopes by installing new shingles over the aged ones already present results in multi-layered sheets presents in post-consumer waste. This type of waste is typically not packaged and usually requires the recycler to provide transportation services.

The scrap originating from post-industrial processing is usually new, dirt-free, and sometimes packaged. It is composed of a mix of manufacturing residue, damaged tiles, and non-conformal products. This type of waste is preferred in the recycling process by road contractors and regulatory authorities because it is not contaminated with deleterious materials that have to be removed, other than the packaging which is more facile to separate than other material, e.g. nails. The collection procedure requires either the recycler or the manufacturer to provide transportation to the recycling facility. At this point, most of the bituminous product discarding process is executed through landfill tipping. The tipping cost is influenced by the possibility to repurpose the scrap material. The landfills that isolate recyclable asphalt derivatives for reutilization in automotive and roadway applications will charge less per load. This procedure spares space, is environmentally beneficial, and reduces discarding costs.

3.2.Waste Recycling Process

The waste supply originating in both post-production and post-consumer processing necessitate a few recycling steps. The primary stage of the recycling process involves the separation from the supply of all deleterious materials, after which the bituminous tiles undergo dimensional reduction through several grinding techniques.The machineries used in this process are crushers, screening sieves, and shredders. Dust reduction and material cooling are managed during the grinding and shredding process by adding water in the machineries. After the size reduction and grinding, the parts containing iron, such as nails and other fixtures are removed by powerful magnets. Cellulose-based impurities and other light foreign matter are reduced by blowing and vacuuming, whereas wood remnants and roof flashing are discarded at the picking stations. A second sieve screening separates the bituminous tile remains based on size and send the larger fragments back into the shredding process.The final uses of recycled asphalt tiles are discussed in more detail in the fourth section.

The recycling process of asphalt shingles encompasses the following stages: material separation, tile shredding, sifting or size screening, blending, water cooling, grinding, sizing, and a secondary stage of sieving and screening [3].

During the first stage, the shingles undergo separation from deleterious materials. Iron-containing parts such as nails are isolated with a rotating magnet. Sometimes wood is found among the shingles, however it cannot be extracted by a magnet or melted in the hot mix, and therefore it has to be extracted manually or be removed in a flotation tank. In the shredding phase, the tiles are grinded into particles <13mm by a rotary shredding machine typically followed by a hammer mill. After undergoing shredding, the particles are sifted, graded according to size and deposited. The HMA mixes place a stringent size restriction on the grinded particles to ensure the asphalt uniformity. Deposited particles resulting from shingle shredding can re-fuse together and have to be processed and sifted again prior to utilization in HMA. For this reason, recyclers blend the aggregate with sand to prevent agglutination. Because the grinding and hammering process may heat the asphalt particles and cause them to fuse together, the shingles undergo shredding while being watered and kept cool. This part of the process is also in place to control dust and follow specific regulations. Watering is strictly limited due to the natural tendency of the scrap to become humid and the necessity of having dry material for hot mixing. The equipment low performance can sometimes trigger a secondary run through the shredding and sizing process until the particles reach the desired size.

3.3.Reclaimed Bituminous Product

The great recycling potential of asphalt shingles is given by seven possible methods to reutilize the recovered product, as follows: re-mix into hot mix asphalt, cold patch integration, countryside and temporary roadways, parking spaces, integration into various mineral aggregates, as energy or fuel source, and reintegration into new bituminous product [3]. Asphalt shingle recycling has the potential to curb discarding costs, spare landfill, and contribute to more economical production processes as opposed to virgin asphalt utilization. However, launching recycling facilities is not free of risks associated with cost-related uncertainty, market volatility, obtaining necessary licenses and permits, and a potentially high variability in the supply chain in terms of material quality.

For the North American continent, the largest market for reutilization of bituminous tiles is constituted by road contractors and the HMA industry. The legislation in most states manifests a propensity towards utilizing shingles pertaining to manufacturer scrap in up to 5% HMA mixes due to the contamination and aging degradation that are assumed to impact tear-off material and increase supply variability. Since more than 550 million tons of HMA are produced yearly in the United States alone [14] by approximately 4,000 facilities, a replacement as small as 5% of the virgin asphalt amount is equivalent to 27.5 million tons. This has the potential to reduce the greenhouse gas footprint of the continent by 1.5 million tons on a yearly basis.

3.4.Economic Considerations

After almost a decade of accelerated increase in the asphalt price due to the underlying tendency of oil prices to grow, the current market is seeing a drop in petrol prices, accompanied by a subsequent drop in asphalt market value. In 2016, the barrel price hit a new low of 27 USD, a level unseen since 2004 [6]. If between 2000 and 2008 the petrol price grew from 25 USD to almost 125 USD for a barrel, in the second half of 2014 it started decreasing again and lost approximately 70% of its market value. The main drivers for this drop are the domestic United States production, which pushes other major petrol extractors to seek out new markets, and the necessity of the oil producing countries to compete in Asia.

Fig 6.Crude oil barrel price between 2006 and 2016.

The highly volatile oil market saw extractors closing their gates and a slow-down of investment in the industry. Worldwide, a number of 68 projects exploiting petrol and gas have been postponed or suspended as a consequence of the market evolution. The total value of these exploitations is estimated at 380 billion USD, and their extracting capacity is around the figure of 87 million barrels per month. Countries belong to the Organization of the Petroleum Exporting Countries (OPEC), an organization with 13 members [15], have postponed, abandoned, or suspended programs with a total output of over 500,000 barrels per day, or 15 million barrels per month.

The current environment is displaying a fall in the overall production capacity, which will ultimately drive prices to grow in the future. However, the slow-down is not occurring fast enough due to petrol exploitations in the Gulf of Mexico. Gas and diesel prices are low at this point, providing large supply sources for the refineries in the United States and Canada. These facilities made significant investments in cocker units for the conversion of crude oil. Although the current oil price situation is not immediately encouraging recycling, the process can provide a strategic advantage later on. Sparing natural resources and reducing the amount of petrol used for production of virgin asphalt is economically feasible and will trigger a delay in the future oil price increases. Furthermore, the landfill tipping fees continue to rise as the landfill space is occupied by demolition and construction waste. The United States Environmental Protection Agency (EPA) is currently promoting hundreds of programs dedicated to curbing the environmental impact by sparing landfill space, producing electricity, and curbing greenhouse gas emissions.

4.Utilization of Reclaimed Asphalt Shingles

Tear-off and industrial scrap bituminous tiles are prepared for reutilization either by mix construction and demolition recyclers or by asphalt-only recyclers. For tear-off asphalt tiles, the recycling operation starts at the demolition sites, where unused tiles are gathered in debris recipients together with the scraps and cutouts resulted from the roof enveloping procedure. Sometimes these remainders may be mixed with other construction and demolition scrap, whereas other times the contractors may provide separate containers for the various waste types, separation which is aimed at preparing the debris for recycling from the earliest stages. The benefits of separating [3] the construction and demolition debris directly on site are the reduced costs of recycling and higher quality of the products that incorporate the recycled material. The majority of tear-off shingles are obtained in roof renovation procedures, where an old shingle envelope is detached and new shingles are installed. The waste thus obtained is expected to include deleterious components (nails, wood pieces, metallic flashing, plastic and paper wrapping, tarpaper, etc) [1] [16]. An alternative to on site separation of bituminous products is their isolation from mixed construction and demolition scrap at the recycling facility.An important point of the activity of the recycling facilities during the separation of debris is to detect the potential presence of asbestos in salvaged scrap. For this purpose, each load is subject to a sampling and analysis procedure. The exact nature of this procedure depends on the amount of shingles in the load and the type of debris found in the mix, however it is common practice for such facilities to possess a staging point where the bituminous waste is deposited until the test results are obtained, to prevent asbestos contamination of the materials that were previously analyzed and declared safe [16]. As soon as the sample testing has returned negative for asbestos, the waste is transported into the processing area for the start of the recycling process, which begins with the material separation.

4.1.Hot Mix Asphalt and Aggregate Production

The largest recycling potential for post-industrial and post-consumer bituminous tile waste lies in the direction of producing hot mix asphalt (HMA) incorporating a fraction of RAS. There are two major pathways to use recovered shingles in new HMA, which are the incorporation as mineral aggregate or as asphalt binder [11][17][18]. Asphalt has great agglutinant properties, durability and flexibility, and the potential to create cementing mixes from granular compounds, for which reason is it extensively utilized by hot mix asphalt manufacturers in the processing of pavement cements. The composition of bituminous shingles, respectively up to 36% asphalt and up to 40% mineral aggregate, is a valuable supply source for the production of HMA. Both laboratory research and field studies have been sponsored by governmental authorities of various states to create a better understanding of the consequences and impact of the utilization of recycled asphalt shingles in HMA. Two major studies are presented in the next section [30][33]. Comparisons have been made between RAS utilization and RAP (recycled asphalt pavement), a recycled material which is commonly accepted by many road and transportation authorities as a viable replacement for 20% of the HMA virgin material. The existing literature presents some of the beneficial aspects of incorporating reclaimed bituminous product as a fraction of HMA [4][11][18][19][20][21][22][23]. Asphalt rutting and cracking can be significantly reduced by the presence of base mat fibers used in bituminous tiles, whereas the cost of the hot mix asphalt is reduced proportionally with the fraction of recycled material. The recycling procedure has a positive impact on the market demand of virgin binder and aggregate, which can trigger a slight price decrease. At the same time, the environmental impact is reduced.

The bitumen concentration in HMA mixes usually ranges between 5-7% [3]. The exact composition of the mixes varies according to climacteric (precipitation and temperature range) and load conditions (expected traffic, number of vehicles and their type, road type).Due to the high variability of conditions between states, each Department of Transportation has to independently confirm the suitability and performance of the pavements obtained from RAS fraction combinations with virgin asphalt. However, the high-density asphalt found in shingles is expected to impact all pavements when the percentage goes over a certain threshold. Further research needs to establish whether a softer grade HMA mixture can compensate for a larger amount of RAS present in the composition.

4.2.Cold Patch

Besides the constituents of hot or warm pavement, cold patch asphalt contains a solvent that keeps the mixture usable for extended periods of time. RAS can be utilized as part of the mixture along with mineral aggregate and the emulsion resulting in the patching amalgam. This type of patching is widely used in the United States and it is mainly used for covering potholes and temporarily refurbishing damaged roads [24]. Besides the environmental benefits and the increased pavement resistance, cold patch mixed with RAS sees an increased serving life and cost savings.

The field testing demonstrated that cold patch based on a RAS mixture performed extremely well, and was more durable than HMA and cold patches without RAS in composition [3]. This was put to the presence of glass and organic fibers incorporated in the mixtures, which improve the structural integrity in case of heavy loads, working to maintain the patch in one piece.

4.3.Other Miscellaneous Utilizations

Besides the widely spread utilization as fraction of new asphalt, RAS can be utilized to manufacture new shingles, for energy recovery, and as mineral aggregate in roadway development.

New shingles. Tests initiated by the Asphalt Roofing Manufacturers Association have been trying to shed light on the economic and technical aspects or the RAS utilization as a supply source for new asphalt shingles. Another aspect of the recycling process is the reprocessing of factory scrap into the high-viscosity asphalt necessary for covering glass fiber base tiles. The factory-scale testing procedures involved the evaluation of the behavior of the equipment used in the recycling process. The tests demonstrated that technical difficulties are met by producers when attempting to deliver the required product features in the cases where recycled waste had been used [25].

Energy recovery. Asphalt has an energy content equivalent to 20,000 BTU for each pound [26] and therefore recycling practice can be extended to applications such as energy recovery in the form of fuel extraction. On the European continent, the practice of fuel recovery from bituminous tile waste has been a standard for decades, whereas in the United States this practice only started in the recent years [1]. Given the asphalt content present in shingles, the organic substrate tiles with 30 and 36% asphalt content can take energy values between 6,000-7,000 BTU per pound, whereas glass fiber substrate tiles can take energy values between 3,800-4,000 BTU per pound. The existing literature [1] documents energy values up to 8,500 BTU per pound. Although the utilization of bituminous waste as a combustion supplement can trigger the release of asbestos into the atmosphere at temperatures under 980o C (or 1,800o F) [27], the validity of this concern depends on whether the asbestos was to be found in the shingles in the first place. The utilization of bituminous tiles as a fuel source finds applications in cement kilns. The organic substrate material is combustible and maintains the working temperature, while the mineral coating and the asphalt present in tiles contributes to the clinker a globular, lumpy type of cement typically 3-25 mm in diameter, fused at a temperature below the melting point (process called sintering) [16][28].

Roadway development. Shingles undergoing a recycling process are grinded into smaller parts of mineral aggregate, and these fractions can be further incorporated into the aggregates used in the development and repair of roads. After grinding to suitable size, the shingles are introduced into a gravel amalgam. Mud rural roadways can be covered with the resulting mixture. The beneficial aspects of the practice are dust control, and a reduction in the road maintenance necessity and the noise caused by vehicles [2]. Such recycling initiatives have been implemented with good results in North Carolina, Raleigh [29], and in Iowa, where a study initiated by the Department of Transportation confirmed the improvement in dust control for two years on the portions where the recycled mixture has been applied.

5.Testing of RAS in HMA

Recycled asphalt shingles (RAS) are disused roof shingles made from asphalt cement and aggregate which are recycled and re-used. RAS is obtained from old roofs as post-consumer tear-offs, new construction scrap, and from building demolitions. Reclaimed asphalt pavement (RAP) is produced mainly in the new construction phase, and is recycled with RAS, where it is crushed and integrated into asphalt mixes for new construction. RAS and RAP once extracted can be incorporated into the hot-mix asphalt (HMA) process used to manufacture asphalt pavements and surfaces.

The impetus for using recycled asphalt in RAS and RAP is to re-use valuable materials that would otherwise be wasted when sent to landfills, to curb environmental impact, and to cut down on initial manufacturing costs. These asphalt recycling programs have been ongoing for many years in the US and globally.

5.1.Joint Study of PG Grading and Performance Tests

Incorporating RAS in HMA is a technology that has been developed for more than thirty years and in this time has become more accepted by construction entities and government. One reason for a push in the direction of recycling used materials is the increasing prices of standard, virgin materials. Minnesota state sponsored many research studies on RAS in HMA in recent decades. Here we look at the results of a recent study investigating the use of post-consumer tear-off RAS, manufacturer waste RAS, and RAP. Later we look at the results of a Missouri research study.

AASHTO has a national standard of requirements for recycled shingles in hot-mix asphalt, which is a set of guidelines for state highway departments and for manufacturers to inform design and production. These standards are only for HMA, not for other uses such as hot in place, cold in place, or cold recycled.

The majority of research studies on the performance of recycled shingles in hot-mix asphalt showed that there is no compromise in pavement and surface quality, given that suitable quality assurance and control methods are adhered to.

5.1.1.PG Grading and Performance Tests

The Minnesota Office of Environmental Assistance, now named the Minnesota Pollution Control Agency, funded a research study into the use of RAS in HMA mixes. US states with warmer weather have observed the benefit of adding RAS to improve rutting resistance.

Bituminous Roadways Inc. provided gyratory samples of the three mix samples for indirect tensile strength testing (IDST) and performance-grade (PG) testing and the three mixes were made with the same virgin asphalt binder PG 58-28. See Table 6 for details on the samples and mixes used in this study. The HMA mix with 20% RAP was used as a control.

 

Table 6. Composition of HMA mix samples

Ten randomly selected samples of RAS were used in the HMA mixes for experimentation. All RAS were examined for percentage content of paper and fiberglass in the mix, asphalt binder, performance grading on reclaimed binder, and gradation. Paper and fiberglass are harmful materials which were removed beforehand. Glass fibers were removed using sieves and paper was incinerated.

Asphalt binder extractions were done according to AASHTO T-164 Method A, which is a centrifuge method, with toluene used as an extraction solvent. This procedure removed fines and small particulate matter. The method used for binder recovery was ASTM D 5404 Standard Practice for Recovery of Asphalt from Solution using Rotary Evaporator. The PG of the asphalt binders was ascertained with AAHSTO R-29 Standard Practice for Grading and Verifying the Performance Grade of an Asphalt Binder. The results for the asphalt binder extraction from two shingle sources had a similar standard deviation of approximately 2% for all the ten samples that were tested. See Fig. 7, Tables 7 and 8.

Figure 7. Composition of RAS binder

 

The average binder waste from the manufacturer was 19.6% and the average tear-off waste was 36.4%. This percentage difference in the asphalt was compensated when used in the mixes. Concerning the allowable 5% RAS, the inclusion of RAS by the manufacturer would be approximately 1% and tear-off RAS approximately 1.8%. So tear-off RAS mixes had less virgin binder and will therefore be stiffer. The RAP used was very uniform as can be seen in Table 9.

Table 7. Manufacturer waste RAS PG Grades

Table 8. Tear-off RAS PG Grades

Table 9. Bituminous Roadways Inc. RAP PG Grades

 

Figs. 8, 9, 10 and 11, and Tables 10, 11, 12 and 13 show that the contractor exercised excellent quality control, by the values of the asphalt concrete content, RAS gradation and mix gradations. It was observed using finer sieves than number 10 that the tear-off gradation was finer for manufacturer waste. This is possibly due to the removal of all metal during processing.

Figure 8. Tear-off scrap asphalt shingles gradation

Figure 9. Manufacturer waste RAS Gradation

Figure 10. RAP Gradation

Figure 11. RAS HMA mix gradation

Table 10. Tear-off RAS gradation

Table 11. Manufacturer waste RAS gradation

Table 12. RAP gradation

Table 13. RAS HMA mix gradation

Manufacturer waste shingles had an average of 1.74% of glass fibers, with a standard deviation of 1.15, and an average of 1.43% of paper waste, with a standard deviation of 0.48, which indicates a variable amount of harmful materials. The limitation for deleterious materials according to the Provisional AASHTO is 0.5%. The effects that these unwanted materials have in the binders and mix, have to be investigated in more detail.

Figure 12. Extracted harmful materials

 

Rolling thin-film oven (RTFO) tests were conducted on the extracted binder as the shingles went through a hot mix plant. Virgin binders have an average loss due to RTFO of approximately 0.5%, see Fig. 12. The average mass loss is 0.8% with a high standard deviation, which shows that the loss due to the heating process is variable. The loss could be due to roof felting or more speculatively from paper and other harmful materials in the RAS, calling for further investigation.

Figure 13. The change in mass of rolling thin-film oven RAS binder

 

Fig. 13 and Table 14 show that the RAS binder content met requirements at nearly 99.95%. The RAP binder had a large ash content.

Table 14. Ash in binder

 

Recovered binder from all three mixes was PG in accordance with AASHTO R-29. The PG grading for the high temperature properties of the manufactured waste RAS mix was one full grade higher than the 20% RAP mix, and the tear-off RAS mix was 1.5 grades higher than the 20% RAS mix. If 5% of the RAP is substituted with shingles it does have a notable effect on the binder stiffness. PG grading data are graphed in Figs. 14 and 15, and displayed in Tables 15, 16 and 17.

Figure 14. High temperature PG grade of RAS binder

Figure 15. Low temperature PG grade for RAS HMA mix

 

The data for the low temperature PG grading is M-value controlled. Since the RAP binder demonstrated good properties at low temperatures the incorporation of tear-off into the mix did not change the low temperature properties. However the manufacture waste mix had a PG grade of 0.5 for low temperatures, and the binder for this mix would be have a grade of -22.

Table 15. Tear-off RAS HMA mix PG grades

Table 16. Manufacturer waste RAS HMA mix PG grades

Table 17. PG grades of RAP HMA mix

 

The BBR data for the binders extracted from the three mixes is summarized in Table 18 below.

Table 18. Bending beam rheometer results

 

From the data in Table 18 for -18°C, it can be seen that the inclusion of shingles into the mix marginally increases stiffness and significantly decreases the M-value. This shows a large change in the relaxation behavior of the mix. For mixes with shingles, the m-values of 0.300 are low. As the m-values correspond to stiffness, it is necessary to compute the thermal stresses for the three mixes. Direct tension tests were also conducted in order to measure the fracture resistance differences between the three binder mixes.

In Fig. 16 the master curves for creep stiffness are presented, and it can first be seen that the two binder mixes with shingles are not as stiff on shorter time intervals and lower temperatures. These mixes also have flatter curves, corresponding to lower M-values, than the binder containing only RAP, which shows that they are stiffer at higher temperatures and longer loading times. It can also be seen from Figs. 16 and 17 that binders with tear-off RAS are marginally stiffer than binders with manufacturer RAS.

Fig. 16 includes graphs of pure shingle binders and for comparison PG 58-28 binders in a PAV solution are included. The binder mix with tear-off RAS is stiffer than the manufacturer waste RAS, and RAS in general decreases the stiffness of the binders at lower temperatures. As the temperature increases the stiffness of the RAS mixes do not increase at as much, and at higher temperatures are stiffer than RAP and PG 58-28. Again these trends can be seen in Fig. 18. Binder mixes with manufacturer RAS have less stress at higher temperatures compared to the RAP binder and tear-off RAS. All three mixes have less thermal stress than the control PG 58-28.

Figure 16. Master curves of creep stiffness for binders

Figure 17. Master curves

Figure 18. Thermal stress

 

Direction tension measurements were conducted at three difference temperatures to ascertain mix critical temperatures, see Figs. 19 and 20.

Figure 19. DT results

Figure 20. Critical temperatures

 

These graphs show only small differences between the 20% RAP and manufacturer RAS mixes. The tear-off RAS mix is brittle at higher temperatures. Strength master curves were derived from the measured strength values, and compared with thermal stress curves to look for points of intersection. The plot of critical temperature comparisons shows that the manufacturer RAS mix does not affect the critical temperature TC of the mix. The incorporation of tear-off RAS however, does increase TC by several degrees Celsius.

IDT tests were conducted on eight HMA mixes under the guidelines of AASHTO TP 9, which is the Standard Test Method for Determining the Creep Compliance and Strength of HMA. BR Inc. gathered four random, loose HMA samples from the control, manufacturer and tear-off test mix types. The results for the IDT stiffness and strength are in Tables 19 and 20, respectively, and Figs. 21, 22 and 23. The three mixes made with the virgin asphalt binder PG 58-28.

Table 19. Creep stiffness in gigapascals (GPa)

 

The incorporation of tear-off RAS significantly increases the stiffness of the mixes across all temperatures, with the largest increases exhibited at -20°C. Incorporation of RAS also caused an increase in stiffness but only at the temperatures 0°C and -10°C. The stiffness for the RAS mix was the lowest of all mixes at -20°C. This corroborates the trend seen in the asphalt binder BBR data.

It can be concluded that the RAS mixes were not compromised in a significant way by the inclusion of shingles. However, this does stand in contradiction with the binder strength results for the tear-off binders.

Figure 21. Mix stiffness at 100 seconds

Figure 22. Mix stiffness at 500 seconds

Figure 23. Tensile strengths

Table 20. Tensile strength

5.1.2.Indirect Tensile Tests Study

A research study conducted in the state of Missouri [30], by a quality control team from the Pace Construction Company (PCC), prepared three different HMA mixes in accord with the state Department of Transportation (DOT) SP190C. Mix 1 was composed of all virgin material, Mix 2 had 20% RAP included, and Mix 3 had 15% RAP and 5% ground RAS included.

The RAS are known to have been regularly tested for asbestos by St. Louis Country Department. RAS were ground and screened, and passed 3/4 opening screen. PG 64-22 and PG 58-28 were used in each mix.

Approval by the local authority was given to lay about 500 tons of each mix as pavement in St. Louis County. Standard quality assurance and quality control tests were performed on the mixes and were determined to be close to virgin Superpave performance. The local DOT relaxed its harmful materials limit for this trial from 0.5% of aggregate to 3% of the RAS.

Mixes 2 and 3 containing: 20% RAP; and 15% RAP and 5% RAS, respectively, were taken as samples and sent to the University of Minnesota Civil Engineering Dept. for indirect tensile tests. For these tests the samples were compacted down to 5% void volume by the PCC.

The results of the indirect tensile (IDT) tests is summarized in Table 21 below. The tests were conducted in adherence to AASHTO TP9-96 [31]. Table 22 shows the IDT stiffness results at 100 and 500 seconds, and further results are shown in Figs. 24, 25, 26 and 27. The IDT strength results are displayed in Table 23 and Figs. 28 and 29.

IDT stiffness test results indicate that the inclusion of RAS significantly increases the stiffness at the lower temperatures of -20°C and -30°C. The largest increase in stiffness was seen with the stiffer binder grade mix PG-22 was used, where the stiffness was three times higher for the -20°C test and two times higher for the -30°C test. Using the softer PG-28 mix the stiffness was predictably less. With the PG-22 mix at temperatures below -10°C, the inclusion of RAS greatly improved the stiffness. This increase however, will most likely have the undesirable effect of increasing thermal stress, which will result in more thermal cracking. Thermal cracking will similarly be expected in the PG-28 mix but to a lesser extent due to the lower stiffness values.

Figure 24. Stiffness at 100 seconds

Table 21. Overall tests

Figure 25. Stiffness at 500 seconds

Table 22. Creep stiffness GPa

Table 23. Tensile strength

Figure 26. Stiffness at 100 seconds

Figure 27. Stiffness at 500 seconds

Figure 28. Tensile strength by PG 64-22

Figure 29. Tensile strength by PG 58-28

5.1.3.Study Comparison

We can compare these IDT test results directly with the previous results, obtained from the study in Minnesota, for the 20% RAP and 15% RAP, 5% RAS mixes, which were both prepared with PG58-28 binder. The comparisons are featured in Figs. 30 and 31. We can see that the study done in Minnesota shows that those mixes containing RAP, and containing both RAP and RAS, are not as stiff. This must be due to differences in the tear-off RAS materials.

Differences between the RAP and RAS used in the studies can also be understood from the different PG limitations of the extracted binders, where for the Minnesota study the PG limits for the 20% RAP were 64.2-29.2, consult Table 17, and for the Missouri study the limits were 79.7-28.8. For the 15% RAP/5% RAS in the Minnesota study, the PG limits were 73.2-28.8, as can be seen in Table 15, and for the Missouri tests the PG limits were 99.5-4.

Figure 30. Creep stiffness at 100 seconds

Figure 31. Creep stiffness at 500 seconds

 

5.1.4.Joint Conclusions

Joint conclusions and recommendations were considered for both the Minnesota and Missouri studies contained in the section. Both studies had the goal of testing the performance of RAP and RAS in HMA mixes at low temperatures.

The Minnesota study used the same binder to prepare all three mixes that were tested: 20% RAP, 15% RAP and 5% tear-off RAS, and 15% RAP and 5% manufacturer waste RAS. The results of that study showed that the tear-off RAS and the manufacturer RAS behaved differently in the HMA mixes. Manufacturer RAS increases the stiffness and did not compromise the tensile strength. The critical temperature of the binder increased by an inconsequential amount, which is also promising. Tear-off RAS incorporation generally had deleterious effects to the mixes by significantly lowering the strength of the binder in the high temperature tests and increased its critical temperature. Tear-off RAS did however increase the stiffness marginally. The rheology tests on the extracted binder suggested that RAS incorporation lowered the M-values by a large margin, while only increasing the stiffness by a small amount. This says that RAS inclusion decreases the temperature susceptibility of the binders, which makes them stiffer than standard binders and RAP binders at middle range temperatures.

By contrast, the Missouri study used two binders, PG 58-28 and PG 64-22, for RAP and RAS mixes. For PG 22 mixes below -10°C, RAS incorporation increased stiffness significantly, which would result in larger thermal stresses. In the PG 28 mix this effect would be less pronounced. It could not be concluded if a softer grade would remedy this problem, as the cost would increase and make RAS inclusion less economical overall.

5.2.HMA Resistance and Modulus Testing

Newcomb et al. [30] tested dense-graded and stone mastic asphalt with incorporated RAS. The mixes they studied contained 5% and 7.5% RAS with two asphalt cements mixes with different penetration grades. The resilient moduli they measured for mixes with 5% RAS had lower indirect tensile strengths (IDTS) in comparison to virgin mixes. Mixes with 7.5% RAS had equal or lower IDTS to the 5% RAS mixes and for colder temperatures exhibited higher IDTS. These results indicate that for larger percentages of RAS in HMA, stiffer mixes are produced giving brittle behavior for low temperature loading.

In other study Gryzbowski et al. [22] looked at rutting for a range of mixes incorporating RAS and compared them to standard mixes in accelerated laboratory testing, using a Georgia Department of Transportation (DOT) Loaded Wheel Tester. The test was done for 8,000 cycles and the 10% RAS mix showed less rutting than the standard mix, with a depth difference of 3.0 mm. This constitutes a large improvement in resistance, however they noted that harder mixes are more susceptible to thermal and fatigue cracking.

The following research [33] comes from laboratory tests of stress, strain, and deformation of HMA, determined with measurements of the dynamic modulus, resilient modulus, and resistance to rutting in the Asphalt Pavement Analyzer (APA). Six asphalt mix types were investigated. These include: Mix 1, a standard binder course mix as a control; Mix 2, with 15.0% RAP included; Mix 3, with 15.0% RAP, 3.0% RAS and a rejuvenator included; Mix 4, with 5.0% RAS and a rejuvenator included; Mix 5, with 15.0% RAP, 3.0% RAS and a rejuvenator included; and finally Mix 6, with 15.0% RAP, 5.0% RAS and a rejuvenator included. See Table 1 for a clear display of composition of the six mixes.

 

Table 24. The types of HMA mix used in this study

5.2.1.Rutting Resistance Testing

The first of the tests conducted was asphalt pavement analyzer (APA) testing, which is used to test rutting resistance of the six HMAs. The HMA samples were tested in adherence to AASHTO TP 63-09 [34] for APA testing. The instrumentation used to simulate loaded vehicle tires over an extended period of time, was a wheel loaded with a force of 100 kN on top of a rubber hose inflated to a pressure of 750 kPa. The HMA samples were kept in constant air conditions and with a constant temperature of 58°C to represent normal environmental conditions. The wheel was run for a total of 8,000 cycles where one cycle constitutes two complete passes over the HMA sample. The average speed of load is approximately 0.6 m/s or 2.2 km/h. The loading frequency is approximately 0.5 Hz.

Figure 32. Superpave cylindrical samples after rutting resistance testing

Three samples of each HMA mix were tested, and from each sample two cylinders were prepared by Superpave gyratory. The cylindrical samples had dimensions of 150 mm in diameter and 75 mm height. The rut depths were measured over the course of each test once every cycle. Fig. 32 shows two cylinder samples with ruts after APA testing.

Figure 33. Schematic drawing of APA equipment [35]

5.2.2.Dynamic Modulus Testing

The dynamic modulus (DM) is the ratio of stress to strain of a viscoelastic substrate subjected to an oscillatory force. In these tests the DM was tested by a range of temperatures and loads in adherence to AASHTO TP 62-07 [1]. Samples of the six aforementioned mixes were tested in a range of six dynamic stress frequencies: 25 Hz, 10 Hz, 5 Hz, 1 Hz, 0.5 Hz, and 0.1 Hz, each at five different temperatures:

-10°C, 4°C, 21°C, 37°C, and 54°C. The frequencies used were designed to simulate impacts from faster to slower moving traffic, in respective order. The impacts were imparted in sinusoidal cycles and the load amplitude changed with sample temperature but remained constant at each frequency.

In general the results show a higher strain and lower DM at lower frequencies, which can be further understood by the fact that at lower frequencies the stress is imparted for longer periods of time. These results are consistent with the expected behavior of a viscoelastic material. See Table 26 for complete results.

Three cylindrical samples were prepared from each mix by Superpave gyratory and then were cored for testing. The cylindrical samples had dimensions of 100 mm in diameter and 150 mm height.

5.2.3.Resilient Modulus Testing

In the resilient modulus MR testing, the cylindrical samples of the HMA mixes, prepared by Superpave gyratory, were compressed repeatedly in a vertical direction that is aligned with the diameter of the sample, as can be seen in Fig. 34.

Figure 34. IDT apparatus and testing

The vertical displacement was recorded using linear displacement transducers and the horizontal displacement was recorded by extensometers. The samples were tested twice and for the second test the sample was rotated by 90° along an orthogonal radial axis. An environmental temperature of 21°C was maintained.

The measurements of MR were carried out in adherence with ASTM D 7369-09 [37]. The sample loading was based on the results of IDTS testing in adherence with ASTM D 6931-07 [38]. The IDTS of all mixes were compared with Mix 1, the control mix. Fig. 5 features IDTS equipment. One mix sample was tested for the six mix types. The cylindrical samples were 150 mm in diameter and 50 mm in thickness.

5.2.4.Results and Discussion

Table 25 shows the results of the APA testing for all six HMA mixes. Fig. 35 is a graph showing the average rates of deformation against the number of cycles. These results show that Mix 1, 2B, 4, and 6, have similar rutting resistances. Mix 1 and 2B were expected to have similar rutting resistances as they have previously been used. Mix 4 includes 5.0% RAS, and with a rejuvenator is less stiff without being soft. In comparison, Mix 3 has the same percentage of rejuvenator as Mix 4, but with 3.0% RAS becomes softer with a deformation of 6.0 mm. The results of Mix 2 and 5 show how significantly the rejuvenator changes the properties of the mix, both exhibiting the largest deformations, with Mix 2 having the largest. From Mix 6 we can see that including RAS with RAP and a rejuvenator, a high performance mix with similar properties to the control mix and other RAP mixes can be produced. Mix 6 had the lowest deformation.

Table 25. Asphalt analyzer test result

In Table 26 the average dynamic modulus is listed for the six mixes at particular environmental temperatures and load frequency. Fig. 36 is a graph of the master curves which are the dynamic modulus against reduced times for all mixes. Mix 1, 2B and 6 had the highest values for the dynamic modulus, while Mix 2, 3 and 5 had the lowest. It can be seen by comparing Mixes 1 and 2B, which were standard mixes with a rejuvenator, with Mixes 2, 3 and 5, that the action of the rejuvenator was to decrease stiffness significantly. However, despite this action, when the RAS was included at 5%, as in Mixes 4 and 6, the stiffness increased back to the level of the control mix.

Table 26. Full DT results

Figure 35. Permanent deformation of HMA mixes in rutting resistance tests

The measurement results for the resilient modulus MR and peak IDTS are contained in Table 27. In the graph of the average MR, Fig. 37, the highest values are shown to be Mixes 1, 2B and 6, above 3,000 MPa, and the lowest MR are Mixes 2, 3 and 5, at approximately equal to or below 2,000 MPa. In addition Mixes 1, 2B and 6 also had better rutting resistance according to the APA testing.

The testing done here showed that the standard Mixes 1 and 2B had the best overall performance in rutting resistance, dynamic modulus and resilient modulus testing. Mixes 4 and 6 with 5% RAS and rejuvenator, had high performance comparable to the standard mixes. Mixes 3 and 5, with rejuvenator, 3% RAS, and with and without RAP, respectively, were shown to have larger APA rutting depths and worse results for the dynamic and resilient modulus tests. In general it was observed that when rejuvenator was incorporated into the mix, the resistance and stiffness were compromised. In the mixes where 5% RAS was incorporated in addition, the mix resistance and stiffness improved again.

It was generally concluded that incorporating RAS into asphalt mixes makes use of valuable material that would otherwise be wasted by being sent to landfills, which in turn causes environmental and societal problems.

Figure 36. Master curves for the dynamic modulus

Table 27. IDT and resilient modulus test results

Figure 37. All resilient modulus results

6.Environmental and Health Issues

A reduced market adoption of asphalt shingles recycling can be observed across North America despite the technology available for recycling and the application potential. The major roadblocks for extensive tear-off shingle reutilization stem from environmental concerns dealing with asbestos and legal regulations [1].

Fig. 38. Emission pathways of asbestos and PAH associated with RAS

Asbestos content is known to have been historically used in roofing materials and can consequentially be present in the post-consumer shingles that are only processed in recycling facilities today. The largest issue confronting shingle recycling is to prevent the environmentally damaging emission of asbestos. The secondary emission related issue is caused by polycyclic aromatic hydrocarbon, a group of natural organic compounds that are a derivative of the petroleum refining processes and have been linked to harmful effects on the environment and the health of those exposed to high levels of the chemicals.

The workers in recycling facilities are the first ones exposed to high concentrations of the toxic substances during the recovery, separation, and grinding of the reclaimed materials. Despite a diminished impact, an equal concern can be raised regarding the health of the citizens living in the area of the recycling facility, and those involved in the exploitation of the end products originating from recycled material. Asbestos is linked to a major impact on respiratory health, and therefore potential atmospheric emissions resulted from the grinding process can seriously affect the health of the facility employees. Although PAH emissions are not airborne and their exact mechanism of propagation is somewhat less documented, they are expected to contaminate the environment through water supply and trigger illness by direct contact in those exposed.

6.1. Asbestos Contamination

Asbestos is a generic name describing a category of natural silicates with a fibrous presentation such as chrysotile, the most commonly used raw material for roofing products from the asbestos category [1]. Because of the mechanical and thermal properties of asbestos, such minerals have been used in roof and floor tiles in the late 19th and throughout the 20th century. Although the information regarding the percentage of asbestos utilized for construction purposes is not abundant, the data indicates that during the first half of the 20th century, the amount of asbestos present in asphalt shingles was in the range of 35 to 50%. Although the lifetime of a roofing envelope is limited, it is a common practice to add new shingles on top of the old ones, which contaminates the entire load put forth for recycling.

Several manufacturers are listed as using asbestos in their products such as roof substrate mats, putty cement, coating, and adhesive sealing (mastic).

In the 70™s, asbestos was forbidden from public use [4] by laws passed after the substance was showed to be carcinogenic and cause many other serious health issues. Despite the new laws, not all construction products have been made completely asbestos free. Although starting with 2002 the mineral is not extracted in the United States, it was imported and incorporated by roofing manufacturers. Thus, out of the 3.5 hundred tons imported in the United States in 2007, 55% were incorporated in roofs as mineral aggregates, coatings, and adhesives [1]. However, industrial trade groups state that asbestos has been discontinued as a component of bituminous tiles starting with 1980 [39][1].

Table 28. Asbestos containing products by manufacturer in the Unites States

6.1.1.Health concerns

The long-term contact with asbestos has been identified by the Department of Health and Human Services as causing a wide range of chronic or acute health problems, such as asbestosis, mesothelioma and lung cancer, immune system effects, and other issues [40]. Health research on employees coming into contact with asbestos showed that the main exposure pathway is inhalation [41].

Some of the health concern factors are the type of asbestos that the patient came into contact with, the exposure level and duration, as well as whether the patient was a smoker [42][43]. The first symptoms of disease can appear decades later than the exposure. The most exposed people are the workers in shipyards, manufacturing plants, mines, and construction sites [41] [43]. Due to their lightness, asbestos particles generated in grinding and shredding can persist in closed atmospheres for more than 10 years [41].

6.1.2.Case Studies

Studies have analyzed 27 thousand samples of bituminous tiles taken from multiple recycling facilities across the United States. The results have shown that under 1.5% of the shingles can be considered contaminated, the criterion being a weight percentage over 1 to be asbestos. The contamination is mostly due to materials different from the shingles themselves, e.g. adhesives. Several states have put constraints for recyclers to only use post-consumer shingles, a measure expected to avoid contaminated industrial tear-offs.

Table 29. Results of bituminous tiles tests regarding asbestos contamination

A complete discussion of the case studies can be found in [1]. The samples have been collected in 6 North American states and have been tested per load by Polarized Light Microscopy (PLM), the EPA Method 600/R-93/116. The method was preferred because it reads values in bulk measurements and is faster and more economical that Transmission Electron Microscopy.

6.2.Polycyclic Aromatic Hydrocarbon Emissions

The asphalt extraction from crude oil guarantees the presence of hydrocarbons, including varieties of polycyclic aromatic hydrocarbon in various quantities [10][44]. The PAHs are a category of more than 100 carbon and hydrogen based chemical compounds with ring structures [1]. These come into being through incomplete burning processes of organic fuels and are typically released into the atmosphere or deposited in the soil. In living organisms, they are deposited in fat and organs (liver and kidneys). Some PAHs are inoffensive however others have carcinogenic potential by ingestion, contact, and respiration. PAHs are associated with cancers, cataracts, internal organ damage, and other ailments [44]. Exposure to hot asphalt emissions is correlated with skin, stomach, and lung neoplasms. The Environmental Protection Associated isolated seven types of carcinogenic PAH [1].

The aromatic and paraffinic hydrocarbons composing asphalt can trigger the release of PAH into the environment during the processing of hot-mix asphalt [45][46][47]. Along the years, the problem of PAH emissions has been studied multiple times however not in correlation with the presence of RAS in the hot mix. No evidence point to notable differences between the case where RAS are used and the case where they are not, although the energy saving from RAS should diminish the PAH emissions when compared to virgin raw material. The conclusion of the studies is that PAH do not tend to leach, presumably a consequence of their low solubility.

6.2.1.Testing for PAH

EPA reports that polycyclic aromatic hydrocarbons, among the main polluting emissions emanated by the asphalt processing plants, are in the range of ~6kg (13 lbs.) per 100,000 tons of hot mixture [47], the characteristic annual production of a HMA factory.

Study [45] researched gas samples from hot-mix asphalt facilities in order to run a quantitative analysis of PAH atmospheric emissions. The study found 14kg (30.6 lbs/100,000 tons), equivalent to 139mg per ton of HMA product, and that the health harming emissions were reduced with 90% by the atmospheric pollution filters of the facility.

A research study [47] looking at the concentration of 29 PAH in water that was put into contact with four virgin roofing bituminous and six virgin pavement samples by the TCLP leaching process found concentrations of 4-23 mg/kg in the roofing product and 1.9-66mm/kg in the pavement, however the leached water contamination was below the detection threshold of 0.1 mg/L for all compounds.

Recycled asphalt pavement samples were tested for leaching in Florida [48]. The testing procedures were TCLP, SPLP, and DI water. The tests found no traces of the 16 measured hydrocarbons.

Bituminous tiles were TCLP tested in Maine for presence of and total polycyclic hydrocarbons [49]. Some of the harmful PAH were above the state limit for safe reutilization, however PAH were shown not to readily leach.

Static and dynamic leach tests of 10 bituminous samples [50] showed that the concentration in leached water was conformal with the European Economic Community standard for drinkable water, <0.1 μg/L.

California water streams were tested for heavy metal contamination originating from bituminous pavement and the findings showed levels under the detection threshold (<0.5 µg/L) [10].

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