As prematurely as 1993, the Environmental Protection Agency and Federal Highway Administration singled out asphalt pavement as “America’s No. 1 recycled product” in a statement to Congress; asphalt pavement maintains its leading status in recycled materials as it is reclaimed and reused at a higher rate every successive year than any other product within the nation (Ross, 2018). This paper will analyze the use of recycled asphalt pavements within new pavements by first providing an introduction of recycled asphalt pavements that will include a brief background, and historical perspective, and an examination of the state of the practice for recycled asphalt pavement use, and last but not least, there will be an exploration to see what are the best practices for the increasing recycled asphalt pavement use which will provide insight into the myriad of ways that recycled asphalt pavement gets used.
The majority of the freeways within the nation are built with hot mix asphalt, and as this substructure weathers through time, it needs to be preserved and reconditioned in adaptation to the changing times.
(Amirkhanian, Herndon & Corley, 2014). Hot mix asphalt that consists of stone, sand, or gravel bound together by asphalt cement provides a cost-effective and convenient way for roads to paved out. A classic asphalt mixture would normally consist of 95% asphalt aggregate while the rest is usually an asphalt binder. Although the asphalt aggregate makes up most of the mixture, it routinely is the one that makes up a small portion of the cost compared to the asphalt binder.
Every day there are alterations being done to sidewalks, roadways, and highways, and the use of recycled asphalt has considerably increased, generating a huge market for those involved in the business of recycled asphalt. The component of installing brand new asphalt for roadways and sidewalks are components of scarcity and high-value. By utilizing the sidewalks that get destroyed in order to make new ones, one can avoid drying out the already low supply of asphalt binders and aggregates. With a growing market for recycled asphalt pavement and with a substantially restricted aggregate and binder stock, hot mix asphalt manufacturers have started utilizing reclaimed asphalt pavement as an extremely critical integral part of hot mix asphalt. (Stephanos & Pagan-Ortiz, 2011). Although there happen to be a diverse set of elements that explain the overall interest in recycled asphalt use in new asphalts, the two primary elements that explain this phenomenon are how it will benefit the economy and how it will benefit the environment.
Using recycled asphalt is a practical and utilitarian option to utilizing untouched asphalt aggregates and binders since it minimizes the use of these untainted materials in the generation of hot mix asphalt (Izaks, Haritonovs, Klasa & Zaumanis, 2015). However, recycled asphalt doesn’t only help in reducing the use of scarce asphalt materials, but also assists in conserving energy. The aggregates of a recycled asphalt pavement, such as sand, rocks, or gravel, get conserved as well when recycling asphalt, and the asphalt that was used decades ago can be used today, saving the economy lots of funds. The recycling of asphalt also recycles the fine mineral particles within, which helps to conserve natural resources; additionally, using asphalt compared to other materials such as concrete “require about 20 percent less energy to produce and construct” (“Recycling & Energy Reduction”, n.d.). At a time where natural resources are being used at a such at a high rate, it’s critical to try to recycle their use whenever possible, and recycled asphalt accomplishes this task successfully by paving a pathway to preserve energy and natural resources.
Although there a variety of reports claiming that the first uses of recycled asphalt pavement began in 1915, it was during the wild oil embargo of the 1970s that caused oil prices to inflate considerably within the United States that caused for the use of recycled asphalt pavements to become popular. In the mid-1970s, the use of recycled asphalt pavements became routine in the majority of states “as a result of the shortage of asphalt caused by the oil embargo as well as the continuing decline in the availability of quality aggregates” (Wood, White & Nelson, n.d.) Along with the increasing use of recycled asphalt pavements there were worries that the recycled pavement mixtures wouldn’t be as durable and as of good quality as virgin asphalt mixtures, necessitating a complex and costly blending chart whenever over 25% of recycled asphalt is used within a mixture. A blending chart can be visually represented by a graph that includes the elements of the temperature of the performance grade as well as the percent of recycled asphalt pavement content by weight of mix. The recommended virgin asphalt binder grade also states that whenever a mixture contains less than 15% of a recycled asphalt mixture, there is no change in binder selection. Whenever there the recycled asphalt is between 15% and 25% within a mixture, one should “select virgin binder one grade softer than normal (e.g., such as PG 58-28 being selected if PG 64-22 would normally be used)” (Ross, 2018). The recommended virgin asphalt binder grade, as its name suggests, is only recommended; contractors who keep using high amounts of recycled asphalt within their mixtures haven’t been dismayed by the results they have been getting. However, as companies have had extensive experience by now with recycled asphalt, they report using it at their desired degrees whenever they want. “The blending chart process is time-consuming, involves hazardous solvents, and creates disposal issues” (Ross, 2018). The blending chart supposes the degree of blending between the virgin and the recycled asphalt aggregate. Roman Bonaquist is the Chief Operating Officer in Advanced Asphalt Technologies, LLC and he developed currently one of the most effective blending charts that minimizes harm. Bonaquist’s PG Blending Chart (AASTO M323) dispenses an excellent blending estimate for recycled asphalt pavements, but this estimation can’t be applied to recycled asphalt shingles. He determines the blending estimate through a variety of elements such as a measure of how well the recycled asphalt mixes with the virgin asphalt, the plant type, the sort of operations within the plant, the specific method of recycling that was used, and a variety of performance tests.
There are a variety of procedures of recycling asphalt and using the recycled asphalt to restore pavements and roads, such as hot recycling, cold recycling and full-depth recycling (“Asphalt Recycling: How is Asphalt Paving Recycled”, 2018). In hot recycling and cold recycling, only the upper pavement layers are pulverized which are then repaved with a mixture of the pulverized pavement with a new binder and necessary additives. On the other hand, full-depth recycling removes all of the pavement through multiple stages, and completely restores the pavement. However, there are specifics to each of these procedures where each has its certain benefits and disadvantages. For example, in hot in-place recycling of asphalt, there are three fundamental steps to the process, including heating scarification, repaving, and remixing. In the process of heater scarification of asphalt, a truck goes over the asphalt while it is heating and softening the asphalt which it then can easily penetrate at the depth it desires with its scarifiers. The asphalt that’s removed gets a rejuvenating agent added to it that enhances its binder viscosity, and gets compacted again so it can be easily repaved about 25 to 50 mm as an overlay to the original pavement, and remixing is occasionally used when the repaving process of hot-in-place recycling of asphalt consists of new asphalt aggregates rather than only recycled asphalt (Gallagher Asphalt Corporation, n.d.). The disadvantage of this method is that it’s restricted in its aptness to restore badly damaged and rough roads, that are more comfortably restored with a traditional hot mix asphalt covering.
Contrary to hot in-place recycling of asphalt, cold in-place recycling of asphalt restores pavements by adding on a cold plant mix covering that utilizes the chemical additives of previous hot mix asphalt pavements without the need of any heating. Cold in-place recycling begins with the grinding of the asphalt pavement where it is further pulverized through the gradation control of the machine that helps to achieve the wanted size of particles to have an easier and more convenient time when repaving. After the pulverization, the aggregates are blended with a binding additive where it’s then utilizing to repave the previously pulverized road. As the last step, the mix is compacted to the wanted density and seal the asphalt to create a road of high caliber (“Cold In-Place Recycling”, 2012). The disadvantage of this method is that it’s not suited for roads or pavements that are wet, that are frosted, or rutting.
Full-depth recycling is yet another way to recycle asphalt and restore pavements by using that recycled asphalt. It is an effective method that demolishes the crevices and fissures in an aged pavement and reimposes the pavement cross-section, during which the aged pavement edifice is transformed into a secure foundation. Full-depth recycling consists of “analyzing the existing materials and road conditions, performing the pavement design, pulverizing the in-place pavement, introducing the additive and mixing, shaping the mixed material, compacting, and applying the surface course” (Walker, 2018). In full-depth recycling, the pulverization step consists of a process that utilizes a rotating cutting head, typically within a reclaimer, which usually passes over the asphalt multiple times while stabilizing additives are utilized. Asphalt emulsion and foamed asphalt are common stabilizing additives used within this process. The stabilizing agents provide a terrific way to catalyze the restoration of pavements. For example, through administering approximately 2% of cold water into hot 320 ºF asphalt cement one fulfills foaming which helps in expanding the surface area and volume of the asphalt while reducing its viscosity, allowing for an easier application of coating. (Walker, 2018) The disadvantages of using this method include being confronted by an elevated starting expense due to necessary use of stabilizing agents and multiple runs on asphalt for effectiveness, and the fact that the length of disturbance that full-depth recycling causes compared to other paving projects is more extensive. As can be seen, what is common in all recycling methods of asphalt is the pulverization of the asphalt and the repaving of the asphalt.
Milling of pavements is currently so normalized that milling’s advantages are usually forgotten, and so is the reality that milling was one of the primary progressions that brought recycled asphalt pavement to what it is today. For the duration of many years, engineers were constantly confronted with “curbs and gutters, loss of clearances below bridges, dangerous drop-offs at drainage inlets, and/or building awkward cross sections” (West & Willis, 2014) Milling assisted in sustaining advisable pavements and freeway and furthermore, paved a pathway for a way to systematically restore damaged asphalt roads. Milling was a critical step in improving the recycling of asphalt pavements, and it is still used today as a side process in many recycling asphalt procedures. For example, cold in-place recycling extensively utilizes the process of milling through its use multi-unit trains that contain milling machines. It is through this milling machine that the cold-in place recycling process achieves in pulverizing the aggregates into the desired particle sizes.
As stated by the National Asphalt Pavement Association, “more than 79 million tons of recycled material were put to use in new asphalt pavements during 2016; more than 31% of all asphalt pavement mixture produced during the 2016 construction season was produced using energy-saving warm-mix asphalt technologies.” (“The Most Commonly Recycled Material? Asphalt”, 2018). The use of a high amount of recycled asphalt within a pavement promises splitting resistance in opposition to old pavements that are prone to easily crack. Cracked roads can prove fatal in many situations, causing car accidents as well as public disturbance and public dismay due to constant worry over how bad roads are. Also, through the restoration of a pavement recycled asphalt aggregates, one strengthens the pavement against water, and builds its resistance to water perforation. However, there are still a myriad of challenges that confronts one when trying to utilize recycled asphalt in all new pavements. To begin with, recycled asphalts only help to reduce cracks and the rutting quality of aged pavements only in a temporary manner; recycled asphalt binder is too stiff to be immune to further cracks. Therefore, even with recycled asphalt, cracking consternations nevertheless prevail.
It has been estimated that the average recycled asphalt pavement usage in United States is estimated at 12%, and this surprising statistic isn’t due to the challenge recycled asphalt poses with its stiffness, but rather due to the problem of its blending efficiency (Hossain, Musty & Sabahfer, 2012). It’s comprehended that the quantity of blending that takes place between new asphalt and recycled asphalt aggregates and binders is someplace between the extremes of full blending and absolutely zero blending. Unfortunately, a way to precisely measure the degree of blending between new asphalt and recycled asphalt hasn’t been discovered yet, and there have been ongoing studies for decades that addresses this specific issue. Although recycled asphalt pavements have repeatedly proven their durability and safety before, due to past experiences of recycled asphalts getting cracks and getting ruddy, and recycled asphalt material not surviving successfully in chilly weather, on top of the blending efficiency problem, have made governments and companies hesitant to utilize recycled asphalt and recycled asphalt aggregates at a constant rate, and they don’t wish to fully depend on it.
For example, this has been a problem within Minnesota. Many projects were carried out to restore old pavements and the binding within the old pavements through recycling asphalt and adding the stabilizing agents and additives to complete this task, but the cold climate of the state caused a challenge because not only were the binder and the additives stiff and brittle, but so were the recycling asphalt mix, making it inevitable to cracks in a fleeting period of time, and making the process of recycling the asphalt nearly useless (Center for Transportation Studies, 2014). Therefore, a new method must be established that takes blending efficiency into consideration so that in cold climates a lower quantity of recycled asphalt can be utilized since this problem doesn’t necessarily take place with new binder, new asphalt, and new aggregates. The lack of having such an efficient method forces governments and companies to keep on using virgin asphalt aggregates and material to carry out the construction of pavements which continues on depleting the source of aggregates that are already scarce, making this a challenge that needs to be addressed immediately. If not, the source of virgin asphalt aggregates will run out, resulting in having to use recycled asphalt aggregates. It should be addressed when there’s an opportunity to preserve the source of asphalt aggregates. By recycling asphalt, governments and companies can minimize the quantity of virgin oil necessary and might also minimize reliance on oil from other countries. The use of recycled asphalt pavements also prevents millions of dollars being spent in transportation of new asphalt, transportation of removed asphalt to landfills, and overall the excessive costs of using brand new natural resources, because the recycling consists of a straightforward process that avoids anything being transported. However, recycled asphalt is not only utilized within pavements and roads, but also is one of the highly used materials within roof shingles.
Asphalt roof shingles can also be recycled to be utilized within new asphalt pavements since they consist of nearly the same aggregates, and aim at fulfilling similar goals, such as water and crack resistance. “The composition of shingles makes them an especially appropriate material for use in asphalt pavements; shingles are typically composed of 25 to 30 percent asphalt cement, 40 to 60 percent hard aggregate contained on the 30 to 60 sieves and 3 to 12 percent fiber” which are all ingredients regularly utilized in hot mix asphalt pavements (Davis, 2018). Unlike the ease and convenience of recycling asphalt pavements to be used right away in new pavements, recycling asphalt shingles proves to be more costly and time-consuming. Roof shingles that are to be recycled are usually retrieved from old roofs that are to be torn off, and obviously not from manufacturers. The timely process of destroying a building and tearing off the roof to effectively obtain some asphalt from it is substantially extensive compared to its counterpart of simply restoring old pavements through the recoating and repaving of old pavements. Additionally, the fact that roof shingles require multiple stages of cleaning to be of beneficial use in asphalt pavements goes on to show that it is also costlier than recycling asphalt pavements. Nevertheless, there is a rise in the recycling of asphalt shingles to be used in new pavements because regardless of how costly and time-consuming this method is compared to simply recycling asphalt pavements, it still provides a way to save money, conserve energy, and avoid using scarce resources by recycling something that would otherwise hurt the environment and the economy. Also, it’s great to know that not only asphalt roads and pavements are the only source of recycling asphalt, and that there is a huge source of recyclable asphalt available for use, helping to avoid drying the source of scarce aggregates.
As can be seen, one of the most recycled products within the nation, asphalt, provides an excellent way for the community to conserve and recycle not only asphalt, but also natural and critical resources such as minerals and oils. As was explained, asphalt can be recycled in a number of diverse ways, and each method is appropriate for a certain type of pavement or road. The recycling of asphalt includes economic benefits in addition to environmental benefits, which can be seen in the millions of dollars it saves taxpayers. Despite its advantages and benefits, the use of recycled pavement in new pavements does pose its challenges and limitations, such as there still not being a method of efficiently blending recycled pavement with new pavement and recycled pavement not being too prone to cold climates. However, as the utilization of recycled asphalt increases, as is happening right now with constant constructions and alterations to infrastructures, there will be new methods of repaving new pavements most efficiently. Thus, the advantages of using recycled asphalt pavements in new pavements outweighs its advantages by far.
As of right now, hot in-place recycling and full-depth recycling provide the most cost-efficient and time-effective ways to recycle asphalt. Although asphalt recycling has come a long way since 1915, governments and companies still have a long way to go before they successfully master the science of using recycled asphalt most practically, and their lacking in this skill can be seen in their minimal use of recycled asphalt, at a shocking 12%. This paper went on to prove that even though recycling asphalt pavements to use in new pavements is a fantastic way to save millions of dollars while being considerate of the environment, the ensure that the nation doesn’t deplete their aggregate source, they need to discover a way to pinpoint the blending efficiency of a virgin asphalt and recycled asphalt mixture, which would also allow for many governments and companies to not be hesitant in using recycled asphalt. One of the most recycled materials in the nation, asphalt, needs to be used more effectively and efficiently than it already is so the durability of pavements and roads can be ensured.