This Guide provides basic information about the design and use of asphalt-rubber. The information in this guide represents the Rubber Pavements Association (RPA) suggested best practices for the making of asphalt-rubber, and the use of asphalt-rubber as a seal coat material or in hot mix asphalt and other related uses. By the use of this guide, the RPA does not in any way warrant the performance of asphalt-rubber, but rather provides advice and suggestions that should be helpful in producing a quality product to meet the needs of the designer or user of the product.
Asphalt-rubber is a mixture of hot asphalt binder and crumb rubber manufactured from scrap or waste tires. Asphalt-rubber contains visible particles of scrap tire rubber. This guide focuses on asphalt-rubber as defined by the American Society of Testing and Materials (ASTM) D-8. The ASTM D-8 defines asphalt-rubber as “a blend of asphalt cement, reclaimed tire rubber, and certain additives in which the rubber component is at least 15 percent by weight of the total blend and has reacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles”. This ASTM standard definition was developed in the early 1990’s soon after the patents ended for asphalt-rubber. Asphalt-rubber as described in this guide is a product in the public domain. Asphalt rubber is a sustainable product because of its many environmental benefits and excellent engineering performance.
This guide also contains a historical collection of research studies by individuals, numerous companies and many government agencies, that helped to develop this very unique asphalt binder. This collective effort spans a period from approximately 1965 to the present, where asphalt-rubber application grew and expanded from seal coats to asphalt binders used for hot mix asphalt paving and warm mix asphalt.
The authors cannot list all the people that have contributed to this guide, but certainly the late Charles McDonald should be singled out for his vision to create such a unique asphalt binder. His early work in Phoenix, Arizona, to research, develop and patent asphalt-rubber primarily as a seal coat material initiated all the work that followed. McDonald was helped by Gene Morris who, at that time, was the research director for the Arizona Department of Transportation. Morris advanced McDonald’s early work by sponsoring research studies and test projects in the state of Arizona. These two individuals were two of the early pioneers and champions of asphalt-rubber and deserve much credit for all that followed.
Since that early work in the 1960’s and 1970’s, much additional research and development continued and was sponsored by the City of Phoenix, Arizona Department of Transportation, California Department of Transportation, Florida Department of Transportation, Texas Department of Transportation, and the Federal Highway Administration. The University of Arizona, Arizona State University and University of California at Berkeley also contributed to the early research on asphalt-rubber. Private companies including Sahuaro Asphalt and Petroleum (no longer in business), Arizona Refining Company (no longer in business), International Surfacing Inc. (now International Surfacing Systems) and Crafco also contributed to the early development of asphalt-rubber as a binder and seal coat material. Later FNF Construction Inc., Cox Paving and Granite Construction contributed to the development of the use of asphalt-rubber binder in hot asphalt mixes. These agencies, universities and businesses
collectively sponsored considerable research, technical and practical developments that is included in this guide. Much of this early work was reported on in workshops and summary reports including: the 1980 Scottsdale Workshop [First, 1980], the 1989, Kansas City Seminar [National, 1989], the 1993 FHWA Crumb Rubber Modifier Workshop [Crumb, 1993] and the 1996 FHWA Summary of Practice [Hicks, 1996].
Later on, several very successful international conferences on asphalt-rubber were held, where research studies were reported on by authors from countries around the world. These international conferences included the first conference in Tempe, Arizona in 1998 (no published report), followed by AR2000 held in Portugal [AR2000, 2000], AR2003 held in Brazil [AR2003, 2003], AR2006 held in Palm Springs, California [AR2006, 2006] and AR2009 held in Nanjing, China [AR2009, 2009]. These international conferences have also been a source of background material for this guide. Type your paragraph here.
Asphalt-Rubber and “the 5 Ws”
Who – The RPA is an international non-profit organization that promotes the beneficial use of asphalt-rubber. RPA members represent contractors, scrap tire recyclers, equipment suppliers, engineers, government agencies, academe, environmentalists and other interested persons.
What – Asphalt-rubber is a binder used in various types of flexible pavement construction including surface treatments and hot mixes. According to the ASTM definition (ASTM D8, Vol. 4.03, “Road and Paving Materials” of the Annual Book of ASTM Standards 2001) asphalt-rubber is “a blend of asphalt cement, reclaimed tire rubber, and certain additives in which the rubber component is at least 15 percent by weight of the total blend and has reacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles”. In addition asphalt- rubber physical properties fall within the ranges listed in ASTM D 6114, “Standard Specification for Asphalt-rubber Binder,” [ASTM D 6114, 2009]. Recycled tire rubber or scrap tire crumb rubber is used for the reclaimed tire rubber portion of asphalt-rubber binder. The asphalt-rubber is formulated and reacted at elevated temperatures and under high agitation to promote the physical interaction of the asphalt cement and scrap tire crumb rubber constituents, and to keep the scrap tire crumb rubber particles suspended in the blend. Asphalt-rubber is typically used as either a Type 1 or Type 2 formulation. For purposes of this guide both Type 1 and 2 are considered as equal. Asphalt-rubber is a combination of scrap tires and asphalt used as an asphalt binder. It is a material defined and specified by ASTM, as well as various state agencies and large city communities. It can be used as a seal coat or as a hot mix binder. Asphalt-rubber contains visible particles of scrap tire rubber.
When – asphalt-rubber was developed in the late 1960’s and has been used in above mentioned states since that early development. It has gone from a seal coat type material to a hot mix or warm mix asphalt binder and can be used with modern paving equipment.
Why – asphalt-rubber was initially developed as a maintenance seal coat material to hold older cracked pavements together until an overlay or reconstruction could be accomplished. Over the years asphalt-rubber has been shown to reduce the degree and severity of cracking while being applied in thin applications. Also, it reduces maintenance and provides a smooth riding, good skid resistant and quiet surface. It has also been demonstrated to be environmentally beneficial in terms of reducing energy and CO2 emissions. In addition to this it is a good sustainable engineering use of waste tires, thus reducing or eliminating the potential negative liabilities of scrap tire piles, such as burning and the breeding ground of life threatening insects, namely mosquitoes and undesirable vermin. Given asphalt-rubber many overall benefits there is every reason to know why it should be considered as a pavement surfacing material.
Where – asphalt-rubber is successfully used in many parts of the world, however, the greatest and most continuous use has been primarily in Arizona, California, Texas and Florida. Admittedly, these are in warmer climate areas of the United States; however, both Arizona and especially California have colder climate areas and have had good success with the use of asphalt-rubber in such colder climates.
Frequently Asked Questions (FAQs)
What is asphalt-rubber? - Asphalt-rubber as described in this manual of practice is a mixture of hot asphalt (bitumen) with ground tire rubber from waste tires as defined in ASTM D- 8, [ASTM D8, 2009], where asphalt-rubber is “a blend of asphalt cement, reclaimed tire rubber, and certain additives in which the rubber component is at least 15 percent by weight of the total blend and has reacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles”. In addition to this asphalt-rubber conforms to the specification requirements of ASTM Standard D 6114, [ASTM D 6114, 2009]. Asphalt-rubber contains visible particles of scrap tire rubber, Illustration 1.
Illustration 1 – Asphalt-rubber with rubber particles compared to other forms of asphalt binder
How is asphalt-rubber different from typical unmodified asphalt? – Asphalt-rubber is a form of modified asphalt. The major difference from unmodified asphalt is that asphalt- rubber contains a minimum 15 percent ground scrap tire rubber. The scrap tire rubber imparts resilience to the asphalt-rubber (AR) binder. The addition of the ground scrap tire rubber means that a greater amount of AR can be applied in a seal coat. Typical unmodified asphalt binder seal coats use an asphalt emulsion and the resultant asphalt residue left on the pavement before the chips are applied is approximately 0.20 gal/sq. yd. (0.9 liters/square meter). An AR application rate for a seal coat is in the range of 0.55 gal/sq. yd. (2.5 liters/square meter) to 0.75 gal/sq. yd. (3.4 liters/square meter). This greater degree of binder application improves the crack resistance
capability of the seal coat and reduces the rate of aging of the asphalt-rubber binder compared to a typical seal coat binder.
AR is also used in gap graded or open graded hot mixes. AR gap graded hot mixes contain from 7 to 9 percent of binder by weight of the mix whereas dense mixes with unmodified asphalt contain typically 4.5 to 5.5 percent asphalt by weight of mix. Open graded asphalt mixes generally contain typically 6 percent asphalt by weight of mix, whereas AR open graded mixes contain 9 to 10 percent binder by weight of mix. The greater degree of AR binder in the mixes affords a greater film thickness which imparts greater cracking resistance and slower aging of the binder. The base asphalt for the AR is generally one to two grades less viscous then the routine unmodified asphalt used in a dense graded hot mix in the same climatic zone. Also, since the AR contains the rubber it improves the fatigue cracking resistance. The AR mixes are generally placed in less thickness. AR gap graded mixes are generally placed in the range of 1 to 2 inches (25 to 50 mm) in thickness and the open graded 0.5 to 1.0 inch (12.5 to 25 mm) in thickness.
How easy is it to make? – Asphalt-rubber is typically made either at the project paving site or near to it as practical. However, with proper care and planning asphalt-rubber can be transported up to several hundred miles from a blending operation to a paving project via a specially equipped transport (nurse) truck equipped with heating and agitation. Some special equipment is needed to make the asphalt-rubber binder, keep the asphalt-rubber heated and agitated; and pump the asphalt-rubber into the hot plant, or to spray apply it as a seal coat material. This equipment is referred to as blender or spray applied equipment and is available from various venders. Spray applied equipment has been manufactured since the late 1970’s and many units are still in use. Blending equipment has been manufactured since the mid 1990’s and many units are still in operation. Presently, all equipment is computerized and has been successfully used in the USA as well as many other countries including but not limited to Portugal, Spain, Italy, Brazil, Russia and China. Generally speaking asphalt-rubber is relatively easy to make with the proper equipment and training.
How much does asphalt-rubber cost? – Costs of any material moves up and down over time. Just before the asphalt-rubber patents ended in approximately 1994 asphalt-rubber cost about twice as much as normal asphalt. Since then the cost have come down and the most recent cost comparisons at the time of this printing indicate that asphalt-rubber cost about 10-20 percent more than regular asphalt. However, many asphalt-rubber paving materials are placed thinner than regular asphalt pavements and last longer than regular asphalt. Oftentimes when the final design is completed asphalt-rubber pavements may actually cost slightly less than other pavement surfaces.
Why don’t more agencies use asphalt-rubber? – Asphalt-rubber needs a ready supply of scrap tire crumb rubber and in some places supply, or at least, an economical supply source is not readily available. In addition as stated above some special blending or spray application equipment is needed to manufacture asphalt-rubber. This equipment like all equipment is a capital expense and contractors need some assurance that there will be enough future asphalt- rubber projects to warrant making such a capital investment. Some equipment suppliers have been able to rent or lease the equipment which helps to mitigate this obstacle to asphalt-rubber use. Adoption of any new technology or product takes time, even in the case of Arizona and California routine use of asphalt-rubber took several decades to accomplish.
Chapter 1 – Brief History
Asphalt-rubber is a mixture of hot asphalt and crumb rubber derived from waste or scrap tires. It is used extensively in the highway paving industry, particularly in the states of Arizona, California, Texas and Florida. It is a material that can be used to seal cracks and joints, be applied as a chip seal coat and added to hot mineral aggregate to make a unique asphalt paving material. The American Society of Testing and Materials (ASTM) defines asphalt-rubber as “a blend of asphalt cement, reclaimed tire rubber and certain additives, in which the rubber component is at least 15% by weight of the total blend and has reacted in the hot asphalt cement sufficiently to cause swelling of the rubber particles,” [ASTM, 2009] This definition was developed in the late 1990’s, however the story of how asphalt-rubber was originally invented, patented, how it has been and how it is presently used, how it is made, and its benefits which have increased with time, that story begins in the 1960’s. The initial development of asphalt- rubber started in the mid 1960’s when Charles McDonald, then City of Phoenix Materials Engineer, began searching for a method of maintaining pavements that were in a failed pavement condition as a result of primarily cracking [Morris, 1993]. McDonald’s early efforts resulted in the development of small, prefabricated asphalt-rubber patches that he called “Band-Aids”, Figure 1. Others had investigated the use of rubber in asphalt as noted in a Federal Highway Administration (FHWA) report in 1971, [Rostler, 1971]. As noted in this report most of the reviewed studies involved the incorporation of unvulcanized natural rubber or latex rubber in asphalt. Some research did involve the testing of vulcanized rubber in asphalt but results were somewhat inconclusive.
The early experiments that McDonald attempted were unique in that fairly high percentages of vulcanized crumb rubber were mixed with hot asphalt. McDonald’s early experiments used the product of asphalt and a fairly high amount of rubber in the form of premade patches. These patches were generally 24 in. x 24 in. (0.6 m x 0.6 m) and consisted of asphalt-rubber placed on paraffin coated paper with 3/8 in. (9.5mm) chips embedded.
Figure 1 - Charles McDonald Asphalt-rubber Band-aid
Recognizing that fatigue cracking generally occurred in larger areas rather that small patches
couldn’t handle, the concept was extended to full pavement sections by spreading the asphalt-
rubber with slurry seal equipment, Figure 2, followed by aggregate application with standard chip spreaders [McDonald, 1981]. This process had two distinct construction problems. First, in order to achieve the desired reaction of the asphalt and crumb rubber in the limited time available in the slurry equipment, it was necessary to employ asphalt temperatures of 450°F (232°C) and higher. Second, the thickness of the membrane varied directly with the irregularity of the pavement surface. This resulted in excessive materials in areas such as wheel ruts and insufficient membrane thickness in between.
Figure 2 - Asphalt-rubber applied as a slurry seal
Although these early experiments with asphalt-rubber involved much trial and error they ultimately led to the present day asphalt-rubber binder and its many uses. The following Chapters present a guide to the manufacture and use of asphalt-rubber.
Asphalt-rubber is composed of asphalt (bitumen) and vulcanized rubber derived and recycled from scrap tires. Asphalt is the binder that is used for pavement seal coating and paving. Almost all of the asphalt binders produced in the United States of America (USA) and the World today is obtained by the processing of crude oil [Hobson, 1975], [Ekholm, 2002]. Many refineries in the USA and other parts of the World are located near water transport or are supplied by pipelines from the crude field or marine terminals. Figure 3 shows a listing of many of the sources of asphalt-bearing crude oils.
Figure 3 - Sources of Asphalt
The first step in the processing of all crude petroleum is the straight reduction by distillation. The distillation principle is based on the concept that various crude fractions which have different boiling point temperature ranges. Because asphalt binder is made up of the highest temperature boiling fractions, it becomes the residuum from the refinery tower. The crude oil is introduced into a distillation tower where the lightest components vaporize, rise to the top, cool, condense, and are drawn off for further processing. The bottom fraction from this unit is called
vacuum processed, steam refined asphalt binder. The grade of asphalt binder is controlled by the amount of heavy gas oil removed. Figure 4 is a typical refinery installation.
Figure 4 - Typical Petroleum Refinery
Vacuum residuum is subjected to solvent de-asphalting to extract additional amounts of high boiling point temperature fractions for applications such as lube manufacture. A high temperature softening point, hard asphalt binder is obtained by this process. This hard asphalt binder can be used as a blending component for producing paving grade asphalt binders. The resultant asphalt binder contains asphaltenes, resins and oils. The asphaltenes are of a solid like nature, resins which are of glue like nature and the oils are of a lubricating nature. All three components are blended to produce an asphalt binder. To this asphalt binder other additives or modifiers or fillers may be added to make a particular type or grade of asphalt material. Considerably more information about the nature of petroleum asphalt can be obtained from the Asphalt Institute [Asphalt, 1989]. Typically the oil component of petroleum asphalt is the component that interacts with crumb rubber derived from scrap tires and causes the swelling of the scrap crumb rubber particles.
Rubber has become over time a rather generic term. It is used to describe anything that is of a rubber-like nature. Originally rubber referred to natural rubber derived from plants like the rubber tree. However, now rubber refers to a whole family of synthetic rubbers, including chloroprene rubber, neoprene and styrene butadiene rubber to name a few. Generally anything that has bounce, stretch, elongation and memory (meaning it returns to its original shape after the force of deformation is removed) is referred to as rubber or rubber-like. For purposes of this guide the focus is on the vulcanized rubber used in tires. Tire rubber is a solid or dense rubber. The solid rubber used in tires is basically of a recipe type of material. The recipe ingredients are mixed by machines called internal mixers and/or mills. Once the formula of ingredients is thoroughly mixed, small amounts of special ingredients is added. These additives are known
collectively as curatives, these generally include accelerators, activators, and sulfur. These ingredients are typically very chemically active. They are the ingredients in a rubber formula that cause the rubber to become vulcanized (cross-linked). Vulcanization is an irreversible chemical process which transforms the chewing gum like rubber into a product with tensile, elongation, and memory similar to a rubber band. This amazing transformation from one physical state to another is what makes vulcanized rubber different from plastic [Rubber, 1973]. Plastics are typically thought of as thermal plastic materials. That is as plastics get hot soften and when they get cold they harden much like asphalt cement. Rubber, however, over a much higher and lower temperature range maintains its original room temperature properties such as elongation, memory, tensile and hardness. The many rubber properties and components are ultimately formed into tires to meet the needs of the traveling public.
Tires, Scrap Tires and Crumb Rubber
Millions of new tires are produced annually around the world to meet the needs of vehicle owners. In the US it is estimated that one scrap tire is produced annually for each person, which is approximately 300 million tires. New tires are manufactured worldwide by several tire companies as shown in Figure 5, [Peralta, 2009].
Judging from the Tire Business statistics the worldwide tire market is upwards of 900 million tires produced annually. Tires are composed of many layers as shown in Figure 6. Furthermore tires contain many different types of material.
Figure 6 - Tire Components, Michelin Fact-Book 2003, (Michelin, 2003)
The most important component of a tire is the elastomer (natural and synthetic rubber). The second most important component of a tire is carbon black. Carbon black is mainly used to enhance rigidity in tire treads (to improve traction, control abrasion and reduce aquaplaning) and to add flexibility and reduce heat buildup in sidewalls [Shulman, 2000]. The proportion of natural and synthetic rubber varies according to the size and use of the tire. The generally accepted rule of thumb is that the larger the tire and the more rugged its intended use, the greater will be the ratio of natural to synthetic rubber [Rahman, 2004]. In general, truck tire rubber contains larger percentages of natural rubber compared to that from car tires (Artamendi, 2006). The third largest component is steel, mainly high grade steel. This provides rigidity, and strength as well as flexibility to the casing. The most common traditional textiles used in rubber are nylon, rayon and polyester. In recent years, a range of new textiles, primarily aramid, which is an ultra-light weight material, have been substituted for more traditional materials, primarily in the more expensive tires [Rahman, 2004].
Scrap tires and Crumb Rubber
As the millions of new passenger tires wear out they become available for reprocessing into crumb rubber which ultimately can be used in asphalt-rubber. The passenger tires are composed of in total about 70 % rubber. The rubber is composed of synthetic rubber (27%), natural rubber (15%) and carbon black (28%). Other components include 15 % steel and 16 % fabric, Figure 7.
Figure 7 - Composition of Passenger Tires
To create crumb rubber the scrap tires are commutated, by shredding and grinding or cryogenic reduction in size. Crumb rubber used in asphalt-rubber asphalt normally has 100 percent of the particles finer than No. 4 (4.75 mm) sieve. The majority of the particle sizes range within No. 20 (1.2 mm) to No. 40 (0.42 mm). Some crumb rubber particles may be as fine as No. 200 (0.075 mm). The specific gravity of the crumb rubber varies from 1.10 to 1.20 (depending on the type of production) and the product must be free from any fabric, wire and/or other contaminants [Rahman, 2004]. The initial step in the production of crumb rubber is shredding the scrap tires. Scrap tire rubber is delivered to rubber processing plants either as whole tires, cut tires (treads or sidewalls), or shredded tires.
The shredded tires are further reduced in size and the steel belting and fiber reinforcing are separated and removed. Crumb rubber is produced by one of three processes. The granulator process produces cubical, uniformly shaped particles ranging in size from 3/8 in. (9.5 mm) down to No. 40 (0.42 mm) sieve, which is called granulated crumb rubber.
The crackermill process, which is the most commonly used, produces irregularly shaped torn particles sized from No. 4 (4.75 mm) sieve to No. 40 (0.42 mm) sieve.
The micro-mill process produces a very fine ground crumb rubber, usually ranging from No. 40 (0.42 mm) sieve down to as small as No. 200 (0.075 mm) sieve [Epps, 1994]. In practice there are two methods of producing crumb rubber ambient and cryogenic.
Ambient grinding can be classified in two ways: granulation and crackermill. Typically, the material enters the crackermill or granulator at “ambient” or room temperature. The temperatures rise significantly during the grinding process due to the friction generated as the material is being “torn apart”. The granulator reduces the rubber size by means of a cutting and shearing action. A screen within the machine controls product size. Screens can be changed according to end product size.
Rubber particles produced in these methods normally have a cut surface shape and are rough in texture, with similar dimensions on the cut edges. Crackermills are low speed machines and the rubber is usually passed through two to three mills to achieve various particle size reductions and further liberate the steel and fiber components. The crumb rubber produced in the crackermill process is typically long and narrow in shape and has high surface area [Rahman, 2004].
The schematic in Figure 8 is an example of a typical ambient scrap tire recycling plant. The process is called ambient, because all size reduction steps take place at or near ambient air temperatures, i.e. no cooling is applied to make the rubber brittle [Reschner, 2006].
In this plant layout, the tires are first processed into chips of 2 in. (50 mm) in size in a preliminary shredder (A). The tire chips then enter a granulator (B). In this processing step the chips are reduced to a size smaller than 3/8 in. (9.5 mm), while liberating most of the steel and fiber from the rubber granules. After exiting the granulator, steel is removed magnetically and the fiber fraction is removed by a combination of shaking screens and wind sifters (C) [Reschner, 2006]. While there is some demand for 3/8 in. (9.5 mm) rubber granules, most applications call for finer mesh material, mostly in the range of No. 10 (2.5 mm) to No. 30 (0.85 mm) mesh. For this reason, most ambient grinding plants have a number of consecutive grinding steps (D). The machines most commonly used for fine grinding in ambient plants are [Reschner, 2006]:
High speed rotary mills;
Extruders or screw presses;
Ambient grinding can be operated safely and economically if the bulk of the rubber output needs to be relatively coarse material, i.e., down to approximately No. 20 mesh (1.2 mm) material [Reschner, 2006].
A related form of scrap tire grinder is the powderizer [Granutech, 2011]. It appears that the powderizer produces a rubber particle that is similar cryogenic particle. The high output powderizer is coupled to a 50HP electric motor and supported on a cast-steel base. The powderizer feed should be wire-free -1⁄4” tire chips. The chips must be fed into the powderizer via a controlled feed device (i.e., auger). Depending on screen selection, the feed rate could be as high as 30 pounds per minute, or as low as 12 pounds per minute. Output sizes range from 5/32 to 30 mesh, Figure 9.
Cryogenic Tire Grinding
This process is called “cryogenic” because whole tires or tire chips are cooled down to a temperature of below -112oF (-80oC). Below this “glass transition temperature” rubber becomes nearly as brittle as glass and size reduction can be accomplished by crushing and breaking [Reschner, 2006]. The use of cryogenic temperatures can be applied at any stage of size reduction of scrap tire. Typically, the size of the feed material is a nominal 2 in. (50 mm) chip or smaller. The material is cooled in a tunnel-style chamber or immersed in a “bath” of liquid nitrogen to reduce the temperature of the rubber or tire chip. The cooled rubber is ground in an impact type reduction unit, usually a hammer mill. This process reduces the rubber to particles ranging from 1⁄4 in. (6 mm) to less than No.30 (0.85 mm) sieve. Steel from the scrap tire is normally separated out of the product by using magnets. The fiber is removed by aspiration and screening. The resulting material appears shiny, clean, with fractured surfaces and low steel and fiber content due to the clean breaks between fiber, steel and rubber [Rahman, 2004]. This type of size reduction requires less energy and fewer pieces of machinery when compared to ambient size reduction. Another advantage of the cryogenic process is that steel and fiber liberation is much easier, leading to a cleaner end product. The drawback, of course, is the cost for liquid nitrogen [Reschner, 2006].
The crumb rubber industrial process takes place in three stages [Recipneu, 2006]: 1. Shredding raw material;
2. Cryogenic processing;
3. Bagging and storage.
Shredding raw material consists of fragmenting light and heavy tires into small, homogeneously cut pieces – the “chips” [Recipneu, 2006)].
Cryogenic processing performs the complete and individualized separation of rubber, steel and textiles without noticeable waste or losses in material, Figure 10. This is a continuous, automatically controlled process which takes place under an inert nitrogen atmosphere [Recipneu, 2006].
Figure 10 - Schematic of a cryogenic scrap tire processing plant (Reschner, 2006)
In the cryogenic cooling the 2 in. (50 mm) tire chips are dropped into a long continuously operating freezing tunnel (B), and are cooled down by the action of liquid nitrogen at around - 321oF (-196oC), resulting in a cold exchange between the chips at room temperature and the liquid nitrogen. When the chips are cooled to a temperature of -112oF (-80oC), the glass transition point is reached for all the rubber polymers of the tire, and then the “rubber” behaves like glass [Recipneu, 2006]. The tire chips are then dropped into a high RPM hammer mill (C). In the hammer mill, the chips are shattered into a wide range of particle sizes, while, at the same time, liberating fiber and steel. Because the rubber granules may still be very cold upon exiting the hammer mill, the material is dried (E) before classification into different, well defined particle sizes (F) [Reschner, 2006]. The separation, drying, sorting and purifying of the various materials follows the next steps and lead to the completion of the process [Recipneu, 2006]:
Separation of textiles using a shaker screen and different suction profiles;
Magnetic separation of steel;
Drying the granulated rubber;
Sieving the rubber into different standard sizes;
Elimination of dust and steel contaminations.
From the cryogenic line, the granulate that is obtained moves on to various silos, where it is put
into big-bags over palettes, made of synthetic raffia fiber, which can carry up to 1.2 tons. The
packaged product is then stored until delivery [Recipneu, 2006]. Generally speaking, cryogenic scrap tire processing is more economical if clean, fine mesh rubber powder is required [Reschner, 2006]. In the cryogenic-grinding process the equipment cost is less, operating costs are lower, productivity is increased, and the product has better flow characteristics than ambient ground rubber [Adhikari, 2000]. The cryogenic technology allows the efficient production of rubber powders and very small rubber granules, with negligible steel or textile contaminations, and minimizes the wastes obtained in the recycling operation. The cryogenic products obtained maintain the molecular structures of the initial rubber polymers, which are not degraded in this process by side reactions of oxidation, devulcanization, or scission/reduction of molecular weights [Recipneu, 2006]. Crumb rubber used in asphalt-rubber normally has 100 percent of the particles finer than No. 4 (4.75 mm) sieve. The specific gravity of crumb rubber is approximately 1.15, and the product must be free of fabric, wire, or other contaminants [Chesner, 1998].
Crumb rubber properties have been reported to affect conventional binder properties [Oliver, 1981]. Natural rubber tends to be superior to synthetic rubber for elastic properties and that synthetic rubber is more stable than natural rubber with regard to the interaction conditions of time and temperature. Earlier studies reported that truck tires are considered rich in natural rubber, while passenger tires are rich in synthetic rubber. Recent studies and reports show the difference between truck tire rubber and passenger tires has been reduced [Jensen, 2006].
Whether by ambient or cryogenic means the crumb rubber process can be summarized as a series of interrelated steps wherein the whole scrap tire is reduced in size to crumb rubber suitable for use in the production of asphalt-rubber, Figure 11.
Figure 11 - From whole tires to crumb rubber suitable for asphalt-rubber
By using the ingredients of asphalt, scrap tire crumb rubber and other components or additives if specified asphalt-rubber can be produced.
Rubber-modified Hot-mix Asphalt Laboratory Performance Study
Three common types of failure in asphalt pavement are moisture damage, rutting distress, and fatigue cracking. Moisture damage of an asphalt pavement results in
the separation and removal of asphalt binder from the aggregate surface. This process is known as “stripping,” and it can cause a decrease in both the strength and durability of the asphalt pavement.
Rutting is a permanent deformation of the pavement that is usually caused by consolidation or lateral movement of the materials in any of a pavement’s layers or subgrade due to traffic loading. Rutting can result in vehicle hydroplaning and a tendency of a vehicle to move towards the rut path as it is steered across the rut. Aggregate shape and texture as well as asphalt binder stiffness play key roles in the rutting behavior of pavements.
Fatigue cracking, associated with repetitive traffic loading over time, is considered to be one of the most significant distress modes in flexible pavements. The fatigue life of an asphalt pavement is directly related to various engineering properties of typical hot-mix asphalt (HMA). These properties of the mix are affected by the properties of the materials in the mix plus the complicated microstructure of asphalt concrete, which is related to the aggregate gradation, the aggregate-binder interface, the void-size distribution, and the interconnectivity of voids. As a result, the fatigue properties of asphalt mixtures are very complicated and sometimes difficult to predict.
A project was completed through the Asphalt Rubber Technology Service (ARTS) to explore the effects of several different variables on several different mix performance properties of hot-mix asphalt (HMA) made with crumb rubber modified (CRM) asphalt binders and recycled asphalt pavement (RAP).
The experimental design detailed in this study included the use of two rubber types (ambient and cryogenic), four rubber contents (0 percent, 5 percent 10 percent, and 15 percent by weight of virgin binder), three crumb rubber sizes (minus #14 mesh, minus #30 mesh, and minus #40 mesh), and four RAP contents (0 percent, 15 percent,
25 percent, and 30 percent by weight of the modified mixture). Two granite aggregate sources and two binder grades (PG 64-22 and PG 52-28) from the same source were used for this project. The binder source and both aggregate sources were all commonly used sources in South Carolina.
The following findings were obtained based upon the experimental results:
l Effect of crumb rubber type:
l The two types of crumb rubber (ambient and cryogenic) generally showed similar effects on both the rheological properties of asphalt binder and the engineering performance properties of HMA mixes. This implies that either ambient or cryogenic crumb rubber can be utilized in HMA.
l At the 15 percent crumb rubber level, ambient crumb rubber resulted in higher viscosities than cryogenic crumb rubber. When using 15 percent or higher crumb-rubber content, the use of cryogenic crumb rubber yields better binder pumpability and mix workability.
l Effect of crumb rubber content:
l Increasing rubber content caused increasing
viscosities of asphalt binders.
l Increasing rubber content caused decreasing wet and dry ITS values. This shortfall can be offset by the inclusion of RAP in the mix.
l Increasing rubber content caused decreasing creep stiffness values and G*sinδ values in long-term aging conditions. This implies that crumb rubber can help reduce the effects of aging and can increase resistance to fatigue cracking.
l Increasing rubber content caused an increase in fatigue life of mixtures.
l Rubber content did not affect the initial stiffness of the fatigue beams.
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l Increasing rubber content caused increasing rut resistance.
l Increasing rubber content caused decreasing resilient modulus values. This shortfall can be offset by the inclusion of RAP in the mix.
l Increasing rubber content caused increasing optimum mixing and compaction temperatures for HMA mixes (similar to the effect of Styrene- Butadiene-Styrene (SBS) modification of binders).
l Effect of crumb rubber size:
l Rubber particle size did not affect ITS values or
rut resistance. Particle size was not used as a variable for any additional properties in this study; however, it is recommended to do further research using this as a variable.
l Effect of RAP content:
l Increasing RAP content caused increasing
viscosities of asphalt binders.
l Increasing RAP content caused increasing wet and dry ITS values.
l Increasing RAP content caused increasing creep stiffness values and G*sinδ values in long-term aging conditions. This shortfall can be offset by the inclusion of crumb rubber in the mix.
l Increasing RAP content caused a decrease in fatigue life of mixtures. This shortfall can be offset by the inclusion of crumb rubber in the mix.
l RAP content did not affect the initial stiffness of the fatigue beams.
l Increasing RAP content caused increasing rut resistance.
l Increasing RAP content caused increasing resilient modulus values.
l Increasing RAP content caused increasing optimum mixing and compaction temperatures for HMA mixes (similar to the effect of SBS modification of binders).
This fact sheet was produced through a partnership of the S.C. Department of Health and Environmental Control’s Office of Solid Waste Reduction and Recycling and Clemson University’s Asphalt Rubber Technology Service. For more information, visit www.ces.clemson.edu/arts/.
Printed on RECYCLED Paper DHEC OR-0941 10/10
Quick Facts: Overall Conclusions
l Either ambient or cryogenic crumb rubber can be used in HMA.
l Minus #16 mesh, minus #30 mesh, and minus #40 mesh crumb rubber all exhibit the same performance with respect to ITS values and rut resistance. Other properties should be examined with respect to these mesh sizes in future research.
l Crumb rubber can help reduce the effects of aging and help increase resistance to fatigue cracking.
l RAP can help improve ITS values (moisture susceptibility) and resilient modulus values.
l Both crumb rubber and RAP can help increase rut resistance.
l The use of crumb rubber can offset the shortfalls of RAP with respect to aging and fatigue cracking.
l The use of RAP can offset the shortfalls of crumb rubber with respect to ITS and resilient modulus values.
Xiao F. and Amirkhanian S.N., “Laboratory Investigation of Utilizing High Percentage of RAP in Rubberized Asphalt Mixtures”, Materials and Structures, Vol. 43, No1-2, pp. 223-233, 2010.
Xiao F., Amirkhanian S.N. and Juang C.H., “Prediction of Fatigue Life of Rubberized Asphalt Concrete Mixtures Containing Reclaimed Asphalt Pavement Using Artificial Neural Networks”, Journal of Materials in Civil Engineering (ASCE), Vol.21, No.6, pp.253-261, June 2009.
Xiao F. and Amirkhanian S.N., “Laboratory Investigation
of Moisture Damage in Rubberized Asphalt Mixtures Containing Reclaimed Asphalt Pavement”, The International Journal of Pavement Engineering, Vol.10, No.5, pp.319-328, 2009.
Xiao F., Amirkhanian S.N., and Juang C.H., “Rutting Resistance of Rubberized Asphalt Concrete Pavements Containing Reclaimed Asphalt Pavement Mixtures”, Journal of Materials in Civil Engineering (ASCE), Vol. 19, No. 6, pp. 475-483, June 2007.
Pavement Sealers have been used successfully for around 60 years. Recently, environmental activists have claimed that pavement sealers are only used for cosmetic reasons, but this trivializes the important role that sealers play in pavement maintenance.
The benefits of sealers are best illustrated through examining the life cycle of asphalt pavement and the important role that asphalt sealers and refined tar sealers play in pavement protection programs.
Asphalt Pavement Basics
Any asset requires maintenance in order to maintain its value and to extend the useful life of the asset. An asphalt pavement driveway adds value to any home. An asphalt pavement parking lot adds value to a commercial lot. In the case of an airport, the asphalt pavement runway is an essential asset.
Asphalt hardening (the binder in the pavement) is an oxidation process and is a function of its exposure to air on the surface and within the pavement. If circulation of air through the interconnected void spaces in the pavement can be prevented or reduced, the rate of hardening of the asphalt will be slowed and the life of the pavement extended. Pavement sealers are used to do just that - close the surface pores to lengthen the life of the pavement1.
Asphalt Pavement Preventative Maintenance
There are three components of a pavement preservation system (Figure 1): preventive maintenance, minor rehabilitation and routine maintenance.
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Figure 1-Pavement Preservation System
2308 Mount Vernon Avenue, Suite 134 Alexandria Virginia 22301
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One method of preventative maintenance is the use of pavement sealers, which in turn makes minor rehabilitation and routine maintenance easier. Pavement sealers are applied to protect the surface of Hot Mix Asphalt (HMA) pavement from the corrosive effects of gasoline, diesel oil, motor oil, and grease that drip or are spilled onto an asphalt-based surfaces. Sealers also prolong the life of asphalt based pavements by decreasing oxidation (that occurs via exposure to air) and ultraviolet light bleaching (that occurs via exposure to sunlight) as well as preventing moisture from entering the pavement. The net effect is an extension of the performance life of new or existing asphalt-based pavement2.
A study by the Washington State Department of Transportation (DOT) contains the following conclusion:
An effective pavement preservation program integrates many preventive maintenance strategies and rehabilitation treatments. The goal of such a program is to extend pavement life and enhance system-wide performance in a cost-effective and efficient way. Studies show that preventive maintenance is six to ten times more cost-effective than a “do nothing” maintenance strategy3
Source: http://www.slurry.com/index.php/property-management/142-home-owners-and- commercial-pavement-preservation.html
A study by the University of Minnesota concluded the following:
Historical cost data show that routing and sealing cracks on a 75-foot-wide runway costs anywhere from $2,000 to $5,000 per 1,000 feet. So maintaining a 5,000-foot runway could cost up to $25,000 for each crack repair project alone. This type of maintenance is done periodically over the life of the pavement, and may be repeated several times. Routing and sealing cracks prevents water from infiltrating the underlying pavement
2 http://www.wsdot.wa.gov/biz/mats/pavement/Technotes/PavementSealer2007.pdf3 http://www.mnltap.umn.edu/pdf/asphalt.pdf
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layers and helps preserve the structural integrity as well as maintain a smooth ride. After several years, a thin overlay may be needed to address weathering or drying out of the pavement surface. Costs for this type of maintenance range from $40,000 to $50,000 per 1,000 feet. If the preventive maintenance program over a 20-year period included three crack-sealing projects and a 2-inch overlay, the airport owner would have spent approximately $325,000 for maintenance; at the end of the 20 years, however, the pavement would still be in good condition. Reconstruction might not be needed for many years. Without maintenance over a 20-year period, total reconstruction may be needed, at an estimated cost of about $140,000 to $315,000 per 1,000 feet. That same runway would cost anywhere from $750,000 to $1,375,000 to replace. The costs of a no maintenance strategy are at least two times higher in this simple case study4.
Another University of Minnesota study states,
Typically, pavements perform well under loads until a particular point in their life spans, at which time they deteriorate precipitously and rapidly to failure. Experience shows that spending $1 on pavement preservation before that point eliminates or delays spending $6 to $10 dollars on future rehabilitation or reconstruction costs5.
A Federal Highway Administration report states,
Conclusive history shows that a properly formulated asphalt rejuvenator meets stipulated requirements and is a proven method to extend pavement life at a low cost6.
As a final example, a study focused on evaluating the effectiveness of preservation methods concluded,
The cost analyses show that many preventative maintenance treatments are cost-effective compared to HMA (Hot Mix Asphalt Pavement) overlays, particularly if performed when the pavement condition prior to treat is fair (or better)7.
Choice of Pavement Maintenance Sealers
TYPES OF SEALERS
The two basic types of sealers are (1) refined tar based sealer (2) asphalt based sealer. Both are emulsions which mean they are formulated to be applied as water-based liquid. Aggregates added to the emulsions include sand, mineral filler, or a blend of these. The addition of aggregate to the emulsion increases the density of the mixture and provides a friction component to the sealed pavement surface8. The two types of emulsions are described as follows:
4 http://www.airtap.umn.edu/Publications/Briefings/2005/Briefings-2005-Summer.pdf5 http://www.fhwa.dot.gov/pavement/preservation/ppc0605.cfm
8 http://www.wsdot.wa.gov/biz/mats/pavement/Technotes/PavementSealer2007.pdf3[page3image5816672] [page3image5819584] [page3image3775888] [page3image5816880] [page3image5817920] [page3image5813552] [page3image5814176] [page3image5782768] [page3image5814384] [page3image5010416] [page3image3798144] [page3image5814592] [page3image5815008] [page3image5779440] [page3image5815216]
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Refined Tar-Based Sealer
Refined tar is a by-product of the distillation of crude coal tar. Refined tar differs from crude coal tar in that during the distillation process, the lighter end oils are removed from the refined tar. Refined tar based sealer is resistant to attack from petroleum based products. This property makes it ideally suited for parking lots or other locations where concentrations of oil and grease leaks are common. Refined tar-based sealer is also highly resistant to ultraviolet light bleaching. It is stable, homogeneous, easy to apply, and has been handled safely by professionals and do-it-yourselfers for decades. Refined tar based sealer has traditionally been used at gas stations, truck and bus terminals, airport aprons and taxiways9 as well as on residential driveways and commercial parking lots.
Asphalt based emulsions
Asphalt-based emulsions have many of the same beneficial properties as refined tar- based emulsions, but they are less resistant to corrosion by petroleum-based products, ultraviolet bleaching, and salts. An asphalt emulsion is a mixture of liquid asphalt and water. Manufacturers add special chemicals and pigments to the asphalt emulsions to improve performance but they remain susceptible to the damage caused by petroleum products10.
Service Life of Preventative Maintenance Sealers
Many factors determine the service life of a protective sealer. First of all, use of the proper product specification is critical. An example of a poor product specification would be using an asphalt based emulsion at a gas station, where gasoline spillage would be virtually unavoidable.
Just to name a few other factors that affect the service life of a sealer are:
-amount of UV light exposure -traffic volume and loads -exposure to petroleum products -coating age
-pavement condition at time of sealing
Generally the pavement sealer industry uses general service lives of 1-3 years for asphalt-based sealer and 3-5 years for refined tar-based sealer.
9 http://www.wsdot.wa.gov/biz/mats/pavement/Technotes/PavementSealer2007.pdf10 http://www.wsdot.wa.gov/biz/mats/pavement/Technotes/PavementSealer2007.pdf
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A study conducted by the Washington State DOT suggests that the service life of both sealers used in rest areas in Washington State were as follows:
Refined Tar Based Sealer 8-10 years Asphalt Based Sealer 4-6 years11
In the Texas Engineering Extension Service publication Lone Star Roads suggests that
Seal coats are the best tool for preventing or arresting pavement deterioration. As the name implies, a seal coat “seals” the pavement. No different than putting a new roof on a house, the seal coat fills voids and cracks, keeping water out of the pavement structure. It is said that the only reason for the rock [that is, aggregate added to the emulsion] is to keep tires out of the asphalt! This is not totally true, because it also creates a great skid resistant surface for traffic12.
A study conducted by the U.S. Army Corps of Engineers concluded that
Tar-based [Refined tar] materials have been used for many years as seal coats to prevent damage to pavement from fuel spillage. Tar [Refined tar], unlike asphalt is not a petroleum product and is not greatly affected by petroleum fuel spillage13.... Military installations have all of these problems, with fuel spillage plus the possibility of a sabotage scenario. Such a scenario could involve fuel being intentionally dumped on an airfield in order to interfere with airplane operations14.
Contrary to the claims of environmental activists, pavement sealers allow homeowners; businesses and other infrastructure facilities such as airports to maintain their asphalt pavements safely and economically. Research has shown that shown that when asphalt pavement is maintained properly, the owner can extend the life of their pavement and delay costly reconstruction.
13 http://dx.doi.org/10.1061/(ASCE)0899-1561(1994)6:1(137)14 http://dx.doi.org/10.1061/(ASCE)0899-1561(1994)6:1(137)
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With the interstate highway in place and due to expensive costs of building new pavements, preserving existing pavement structures has become the focus of transportation agencies. Preventive maintenance is one of the main techniques in preserving pavement structures, and crack sealing is one of the most important procedures of preventive maintenance. A lot of different materials are used today for crack sealing purposes. Hot rubber asphalt is a very commonly used material for sealing purposes. However, this material can be hazardous due to high operating temperatures. This can put construction crews or the public at risk when a hose carrying very hot sealing material bursts. Also, hot rubber asphalt may stick to vehicles’ tires due to lack of adherence to the pavement. Thus, alternative sealing materials, such as cold pour sealants, have often been the subject of research studies. This study comes as an attempt to determine the feasibility of using hot pour and cold pour sealants. This will be achieved by comparing the long-term performance of both hot and cold pour sealing materials. For the purpose of the study, seven sealing materials were selected: four hot pour sealants designated as H1, H2, H3, and H4 and three cold pour sealants designated as C1, C2, and C3. These materials were applied on eight pavement maintenance sections for testing purposes in five districts in Texas. These districts are Atlanta, El Paso, Amarillo, San Antonio, and Lufkin. A total of thirty-three test sections were constructed between January and April 2001. The main criteria in determining the best sealing material will be the cost-effectiveness. Hence, a cost analysis will be done in two stages for this study. The first one is an initial cost analysis, which was already performed at this point of the research study. This analysis was prepared using the initial costs required in constructing each procedure treatment. The second cost analysis, which is the life-cycle cost analysis, will be performed at the end of the monitoring period of the study. In this analysis, cost of the treatment procedures with regard to their service life will be compared. So far, the initial cost analysis has been completed using two different approaches; both approaches showed that treatments using hot pour sealants cost less than those using cold pour sealants. To evaluate the performance of different sealing materials, the test sections were visited and the treatment jobs were
evaluated according to American Association of State Highway and Transportation Officials (AASHTO) procedures (Ref 1). Three investigation visits were conducted: the first one about three months after the construction (Summer 2001), the second one about one year after the construction (Winter 2002), and the third one approximately 18 months after the construction (Summer 2002.) The visits indicated relatively excellent performance for the hot pour sealants in the majority of the test sections. On the other hand, cold pour sealants showed drastic decline in their performance with time.
State transportation agencies utilize crack sealing as one of the most common procedures of preventive maintenance. The main purpose behind crack sealing is to create a watertight barrier that hinders moisture from reaching the under-layers of the pavement structure. Pavement cracks can be either longitudinal or transverse, and sealing such cracks would have a remarkable effect on prolonging the service life of the pavement. In general, rubberized materials are used as crack sealing agents due to their ductile properties.
In the Texas Department of Transportation (TxDOT), as is the case in many other transportation agencies, hot rubber asphalt has been the most commonly used material for sealing purposes. It is relatively inexpensive and has been proven to perform well after years of usage in pavement preventive maintenance. However, hot rubber asphalt requires being heated at elevated temperatures during the application process. Hot rubber asphalt creates a big hazard for the workers and the public at these very high temperatures. Furthermore, the heating process takes time and this causes a considerable amount of loss of time.
Due to the negative attributes of hot pour sealants, cold pour sealants have come into consideration. The most commonly used cold pour sealants are asphalt emulsions. As opposed to hot pour sealants, cold pour sealants do not need to be heated prior to application. They can be used directly in the ambient temperature. Therefore, they are considered to be safer and more time efficient. Also due to their relatively low viscosity, cold pour sealants can penetrate and fill cracks more effectively. However, they require more time to cure and set, which adds to the time needed to complete the sealing job. Cold pour sealant application is more susceptible to environmental conditions. Therefore, curing
time for the cold pour sealants may vary remarkably due to different environmental conditions.
Another difference between the cold and hot pour sealants is the format in which they are commercially stocked and provided. Usually, cold pour sealants are supplied in gallons and hot pour sealants are, on the other hand, supplied in solid blocks. This difference was considered during the initial cost analysis.
1.3 Past Research Experience
It is well understood that applying appropriate preventive maintenance treatments at the right time extends the service life of pavements. Lin et al. (Ref 2) stated that each dollar invested in preventive maintenance at the appropriate time in the life of a pavement might save $3 to $4 in future rehabilitation costs. However, the cost-effectiveness of preventive maintenance is usually derived from observational experience. Even if it is based on observational experience, transportation agencies can still apply the knowledge and take advantage of the cost-effectiveness of preventive maintenance. In FY2001, TxDOT allocated at least $324 million to preventive maintenance treatments. Because of these huge amounts of investment, TxDOT has a great interest in the effectiveness of preventive maintenance treatments. In their study, in which TxDOT participated, the researchers investigated 14 test sites that were subjected to four different preventive treatment procedures (thin overlay, slurry seal, crack seal, and chip seal). TxDOT’s distress score concept was adopted to evaluate the effectiveness of preventive maintenance treatments on these sections. The investigated section is given a score from 1 to 100 (very good to very poor). The distress score is a product of what are so-called utility factors, which reflect the contribution of different kinds of pavement stresses including: rutting, patching, and different kinds of cracking. It is seen that although crack seal treatment improved pavement performance, the distress score remained almost the same as computed in this study. There was no improvement in the distress score after the crack seal treatment. This is due to the current TxDOT distress evaluation system making no distinction between a sealed and an unsealed crack. Lin et al. (Ref 2) concluded that when the initial cost was considered, crack seal treatment provides the best alternative for a low traffic route with sound underlying pavement structure.
The emphasis of the Interstate Highway program is shifting from capital investment to maintenance and operation. Senior executives, legislators, and the public believe that maintenance is the key not only to protecting the multibillion dollar highway infrastructure but also continuing to provide a safe and efficient transportation system. The Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991 placed major emphasis on preservation of the system and environment. ISTEA established the Interstate Maintenance Program, which called on states to implement pavement, bridge, and other management systems to extend their life and maximize their efficiency. One of the major methods in pavement preservation is crack sealing. Like any other engineering procedure, crack sealing faces challenges. These challenges can be financial or technical. Because crack sealing is a tedious and labor intensive operation, most of the cost is due to labor expenses. Sims (Ref 3) reported that the associated costs are approximately $1800 per mile with 66% attributed to labor, 22% to equipment, and 12% to materials. However, the procedure of crack sealing is not standardized in practice yet. Hence, construction procedures that minimize road closure and increase laborers’ safety must be adopted, and training for better skills and material selection must be improved regularly. It is the role of research to determine the proper procedure for repairing cracks and improving field performance of the sealants.
Smith et al (Ref 4) developed a checklist with the desirable properties of sealing material. Some examples are the ability to be easily placed over the crack, adequate adhesion to remain bonded with the crack faces, resistance to weathering, and resistance to abrasion. Sealing and filling materials are categorized as thermoplastic materials (hot applied and cold applied) and thermosetting chemically cured materials. In this study, both types of thermoplastic sealing materials will be used. Hot applied thermoplastic materials are those that are heated and harden when cooled, usually without chemical change. They possess temperature dependent properties and experience hardening with age. They are the most commonly used crack sealing materials. To enhance their performance, modifiers such as polymers, rubber, or fibers are usually added to hot applied materials. On the other hand, cold applied materials are those that set by releasing of solvents or breaking of emulsions. Emulsified and cutback asphalt are typical cold applied thermoplastics. Cold applied materials are usually modified as well. According to Smith et al.’s questionnaire
survey, asphalt rubber as a hot applied material is mainly used in dry climates. They stated that the life expectancy of rubber asphalt is 4.3 years in warm conditions and 2.2 years in cold conditions. Thirty-one agencies that used hot asphalt rubber rated its average effectiveness as good to very good. Emulsified asphalt (cold applied thermoplastic) had a mean life expectancy of 2.3 years in warm dry conditions. However, for wet conditions slightly over one year average life expectancy is found for emulsified asphalt. An average effectiveness rating of fair was determined from a response of 20 agencies that used this material.
In a study to compare performance of various materials and procedures in treating cracks in asphalt concrete pavements, Smith and Romine (Ref 5) conducted research on a total of four transverse crack seal sites and one longitudinal crack fill site. These treatments were installed in locations in the US and Canada in 1991. At each site several experimental treatments were applied. Each treatment consisted of a material, a placement configuration, and a crack preparation procedure. Comparison was basically based on the percentage of failure that occurred on the treatment after installation. Failure in this study was signified by distresses like full-depth pullouts and full-depth adhesion and cohesion loss. The percentage of failure was calculated as the ratio between the length of failed section and the original length of the treatment. In the study, all materials used, except for proprietary emulsion and fiberized asphalt, showed percentage of failure less than 10%. In addition, simple band-aid sealant configuration experienced between four and twenty times more failure than the reservoir-and-flush and the recessed band-aid sealant configurations.
Masson et al (Ref 6) states that hot pour crack sealants are generally composed of four basic ingredients, which are bitumen, oil, polymer, and filler (usually recycled rubber). They conducted a study to investigate and quantify the proportions of these ingredients in four typical sealant samples in a performance-based four-year study. After physico- chemical analysis of the four sealant samples, they tried to examine the correlation between the composition of the sealant and its performance in low and medium temperatures. To determine the composition and properties of the sealants, a series of physico-chemical test methods were performed on each sealant. These methods were viscometry, fluorescence microscopy, infrared spectroscopy, thermogravimetry, and modulated differential scanning calorimetry (MDSC). In addition to that, low temperature tensile testing was performed on
the sealant samples. It was found that the physico-chemical properties of crack sealants were related to crack sealant performance. Viscosity and filler content affect adhesion, which controls short-term performance. In other words, low viscosity and low filler contents enhance the bonding of sealant to asphalt concrete (AC), whereas high viscosity and high filler contents introduce interfacial defects that can become failure at the sealant- AC interface. Furthermore, the short-term performance predicted from viscometry and filler content as obtained from microscopy correlated well with the 1-year field performance of the sealants in a wet-freeze climate. A reasonable correlation was also found between the outcome MDSC test and 4-year performance in wet-freeze climate.
1.4 Objectives of this Study
This study is a continuation of an ongoing process of monitoring performance of treatment procedures using two types of crack sealants. The main objective of the analysis in this report is to compare the long-term performance of hot pour sealants to that of cold pour sealants. For the purpose of this comparison four types of materials are used. These materials are: hot pour crack sealant, hot pour joint seal, cold pour crack sealant, and cold pour joint seal. Hot pour crack sealant is basically composed of rubber asphalt and cold pour sealant is composed of different asphalt emulsions.
Crack sealant refers to the sealing materials that are used to seal the cracks generated in asphalt pavements, while joint seals are used to seal the joints of concrete pavements. Joint sealants were included in this study because they must pass a bonding test, and it was thought that the bonding test might be useful for crack sealant specification requirements.
2. First Year Summary
In this ongoing research, hot pour sealants and cold pour sealants were compared in terms of performance, ease and safety of installation, and cost effectiveness. The project will be completed in three years.
During the first year, surveys on crack sealing techniques and materials have been completed. Nine states and twenty-five districts in Texas have participated in the survey. Also, thirty-three test sections were constructed on eight roads in five districts in Texas. Both hot and cold pour sealants were applied on the cracks in the test sections. Construction cost analysis was determined after the construction work was completed. This analysis did not take long-term performance of the pavement into consideration, which may influence the cost effectiveness. More comprehensive cost analysis would be the life- cycle cost analysis. At this stage of the project, life-cycle cost analysis could not be performed, because the service life of the treatment procedures is required to calculate the life cycle cost. Test sections have been inspected regularly during the first year of the project. During the first year, every test section was investigated twice.
2.2 Survey Results
Surveys were conducted in twenty-five districts in Texas, and in nine states in the USA. Twenty-one out of twenty-five districts in Texas responded to the survey. Hot pour sealants were commonly used sealing materials in all districts, while cold pour sealants were used only by some of the districts. The survey included ten questions; each was answered in the form of a ranking such as: poor, fair, good, and excellent. Overall performance of hot pour sealants seemed to be better than that of cold pour sealants, while resistance of hot pour sealants to flushing and bleeding appeared to be poor. Effective life of hot pour sealants also was much higher than effective life of cold pour sealants.
Nine other states also responded to the survey. All of the states used hot pour sealants, and five of them also used cold pour sealants. Ten questions that are the same ones used in the Texas districts were utilized in the states’ surveys. According to the states’ survey, hot pour sealants perform well except for resistance to flushing and bleeding, while
cold pour sealant was ranked poor in most of the cases. Effective service life of cold pour sealants was never higher than three years, while effective service life of hot pour sealants went up to five years. Both districts’ and states’ survey results clearly showed that hot pour sealants performed better than cold pour sealants.
2.3 Material Properties of Sealants
Of each type, hot pour and cold pour, both crack sealants and joint sealants were used in this study. Crack sealants are used to fill the pavement cracks, whereas joint sealants are generally used to seal concrete pavements’ joints. Two different cold pour crack sealants and one cold pour joint sealant were applied. Cold pour crack sealants were designated as C1 and C2, and they met TxDOT requirements for Item 3127 specifications. Cold pour joint seal designated as C3 satisfied TxDOT requirements of DMS-6310, Class 9 specifications. Three hot pour crack sealants (H1, H2, and H3), and one hot pour joint sealant were used. H1 and H3 satisfy TxDOT’s GSD Spec. 745-80-25, Class A, and H2 satisfies GSD Spec. 745-80-25, Class B requirements. Joint sealant H4 met DMS-6310, Class 3 specification requirements. Laboratory test results of the sealing materials used in this study are depicted in Appendix A. Specifications for GSD 745-80-25, Item 3127 and DMS-6310 are located in Appendix B.
Eight of thirty-three test sections were overlaid with a chip seal layer during the following summer in order to observe the tendency of sealants to bleed. The bleeding problem was basically expected to occur in sections treated with hot pour sealants since it was recorded earlier in the surveys.
2.4 Initial Cost Analysis
Cost analysis for construction was done for the non-covered test sections. Sealing materials, equipment for traffic control, sealing equipment, hot pour equipment, and crew labor cost were taken into consideration when calculating costs. Cost analyses were done in two ways. The first method was to determine the total amount spent to seal a crack; then, this amount was divided by the total length of the treatment to determine the cost per foot. It was found that the longer the crack, the lower the cost, because some costs are constant regardless of length of crack. Therefore, sealants applied on long sections may seem to be cheaper. The second method provides more reliable comparison. A 50,000 ft crack length
was assumed for all sealants. The production rate (feet per hour) from the test sections was used to determine the time required to seal a 50,000 ft crack. The cost for sealing 50,000 ft was calculated and the other costs such as equipment preparation, traffic control, etc. were added to calculate the total cost. Cost analyses show that using the same volume of sealants, cold pour sealants can seal more cracks than hot pour sealants. A 115 ft crack can be sealed using one gallon of cold pour sealant, while only a 75 ft crack can be sealed with one gallon of hot pour sealant. On the other hand, per gallon cost of cold pour sealant is almost twice that of hot pour sealant. However, construction cost is not the sole factor in cost effectiveness. Performance of a sealant is also another significant factor. Also, field performance allows for determining lifetime cost. However, life-cycle cost analysis can only be done when all the treatments reach failure point.
2.5 Evaluation Technique
2.5.1 Non-Covered Sections
Determining short-term and long-term performance of sealants on non-covered and covered test sections is one of the primary objectives of this project. Short-term performance of 25 non-covered sections was determined at the end of 4 months after the construction. Sections were also visited for visual observation once in the winter and once in the summer to gather information for long-term performance. Test sections were visually monitored for the following types of failure:
Open previously sealed cracks
Loss of seal in previously sealed cracks
Settlement and bleeding of sealants
Pullout of material
Spalls or secondary cracks in or near the sealed crack
A pointed tool was used to determine the strength of bonding between the sealant and pavement. Pullout tests were conducted by two individuals to eliminate bias in observation. They ranked the easiness of pulling sealant as “Easy,” “Medium,” or “Difficult.” This ranking determines adhesion and cohesion loss of the material. Settlement and bleeding of
sealants were also measured. Since settlement is common for cold pour sealants, water may accumulate in the settled areas and penetrate into the crack which leads to loss in adhesive and cohesive forces. Height of the hot poured sealant is critically important in terms of ride quality. All other failures were inspected visually and recorded.
Treatment effectiveness can be calculated using percent failure. Percent failure is calculated by dividing failed length of sealed cracks by total length of sealed cracks.
Percent Failure = 100 * (Failed length/Total length) Percent Treatment Effectiveness = 100 – Percent Failure
After that, a treatment effectiveness versus time graph is plotted. This graph will be helpful in predicting the life of treatment if the effectiveness trend can be somehow extrapolated.
2.5.2 Covered Sections
Bleeding is the main problem when a pavement is overlaid or chip-sealed after crack treatment. If excessive crack sealant is placed, sealing material fills the voids and tends to penetrate through the chip seal surface creating a shiny, glass-like, reflecting surface. Strategic Highway Research Program (SHRP) identifies three levels for bleeding:
Low: Coloring of pavement surface is visible
Moderate: Distinctive appearance with excess asphalt already free
High: Free asphalt gives the pavement surface a wet look; tire marks are evident
The SHRP Manual recommends measuring the area of bleeding surface, but in this project only length of the bleeding sections was measured.
The eight overlaid or seal-coated test sections were observed for sealant bleeding through the subsequent seal coat or overlay. These sections were designated as “Covered” sections. Sections were observed at the end of three months after construction for short- term performance. Roads will be monitored once in winter and once in summer every year to determine long-term performance. These sections were visually inspected and the rate of
bleeding for each sealant type was recorded. Bleeding amount and rate were used to determine the rate of failures, which determines the treatment effectiveness.
2.6 Performance Results for Non-Covered Sections Four Months after Crack Seal Construction
In Atlanta, no newly developed cracks were observed on sections where H1 and H2 sealants were applied. Cohesion and adhesion of these sealants were ranked as “Difficult,” and there was no sealant loss. Cold pour sealants C1, C2, and C3 on this section did not have any newly developed cracks as well. C1 and C2 showed adhesive and cohesive failures. C3 sealant had pullout problems in some parts of the treatment. C1 was ranked as “Medium” and C2 and C3 were ranked as “Easy” to pullout.
2.6.2 El Paso
Similar results were obtained in El Paso, where heavy border traffic is taking place. Failures were observed on wheel paths. All the failures were observed on cold pour sealant treated sections.
In Amarillo, failure had a scattered pattern. Failed sections of the treatment were not confined to certain parts of the pavement. This is possibly due to weather conditions in Amarillo, where freeze/thaw cycles are likely to occur. The C1 sealant section showed excessive failure, while the other sections H1, H3, and H4 exhibited very good performances.
2.6.4 San Antonio
Failures were observed in cold pour sealed sections in San Antonio. Sections sealed with C1 exhibited newly developed thin cracks. Depression of cold pour sealants was more severe in the test section in this district.
All sealants showed very good performance in Lufkin. No failure signs were observed in these test sections. As was the case in other districts, cold pour sealants were softer than hot pour sealants.
2.7 Performance Results for Covered Sections
Covered test sections in Atlanta and Amarillo were constructed. Both cold and hot pour sealants were used in these test sections. After sealing, treated test sections were covered with chip seal. The chip seal applied was AC-15-5TR binder, which consisted of a minimum of 5% ground tire mixed with grade 4 aggregate.
Sections in Atlanta and Amarillo were visited two months after the chip seal was constructed. In Atlanta, C2 and H1 sealants were applied. Sections treated with H1 showed a low level of bleeding, while no bleeding was observed on the section sealed with C2. In Amarillo two sealants, C1 and H3, were applied. None of the sections showed bleeding. The chip seal proposed for Lufkin was not constructed during the first year.
3. Performance Evaluation Process
In the evaluation process, American Association of State Highway and Transportation Officials (AASHTO) procedure was adopted to calculate percentage of effectiveness. The main types of failure considered were opening of sealed cracks, full depth adhesion or cohesion loss, and spalls.
AASHTO procedure provides a standard practice for evaluating the performance of crack sealing treatment (Ref 1). This practice can be used for several types of crack sealants such as: cold applied sealants, hot applied sealants, and chemically cured sealants. It also can be used for the selection of crack sealant filler materials, placement configurations, and finishing operations. The projected life of the treatment can be determined by extrapolation of the function of treatment effectiveness versus time.
As shown in Figure 3.1, the main product of this evaluation procedure is a chart depicting effectiveness (in percentage) with respect to time of measurement. A minimum of one evaluation measurement each year is needed to provide an estimation of the performance of the crack treatment. For the most effective evaluation, measurements should be conducted during the mid- winter period when the crack is subjected to maximum opening. It is suggested that the first inspection be made during the first winter, while another can be done after winter to assess winter damage. Along with the traffic control devices, the basic apparatus needed is a distance measurement device like a measuring wheel.100 80 60 40 20 0
Predicted Life of Treatment
at 50% Effectiveness = 5 years
012345678Time After Placement (year)
Figure 3.1 Example graph of treatment effectiveness versus time
Treatment Effectiveness (%)
An unbiased sample of the pavement treated section is used for testing. The sample length must not be less than 150 m (492 ft) in length. Generally, pavement sections are grouped according to the type of treatment, sealant, or sealing procedure. When a pavement sample has been previously evaluated, it is best to use the same section for re-evaluation in the succeeding evaluation procedures. Otherwise, a minimum of 5 different pavement samples are selected each year.
To determine the effectiveness, first the length of cracks is measured and recorded to the nearest 300 mm (12 inches). Qualitative evaluation is performed by visual examination of cracks, and the type of failure is recorded. Failure can be in the form of full-depth adhesion or cohesion loss, complete pullout, spalls and secondary cracks, potholes, etc. Length of failure of all cracks is measured and recorded. The treatment effectiveness is the ratio between the length of remaining sealed crack and the length of the original treatment in percentage.
4. Field Evaluation Results
4.1 Non-Covered Test Sections
For the purpose of evaluating the long-term performance of the non-covered sections, two successive investigations were conducted after the first one. Regardless of which district the treatment was applied, the performance of hot pour sealants was better than that of cold pour sealants in general.
Five types of sealants were used in this district, two hot pour sealants (H1 and H2) and three cold pour sealants (C1, C2, and C3). C3 and H4 are joint sealants. The treatment procedures were installed on January 31, 2001 on US 290 in Morris County in the southbound, outside lane. The first investigation test for short-term performance evaluation was made on May 24, 2001. Two other investigations were conducted on February 13 and August 7 of 2002.
The pavement structure of this section was an AC overlay on Jointed Concrete Pavement (JCP), where most of the cracks were reflection cracks over the joints. These cracks were transversely spaced at 15 ft (4.5 m). The main source of the cracks was probably the heavy truck traffic that caused movements of joints, which could be seen by the naked eye in some cases.
Hot pour sealants exhibited excellent performance compared to cold pour sealants. During the winter 2002, both hot pour sealants designated as H1 and H2 scored effectiveness greater than 89%. At the summer 2002 investigation the two hot pour sealants scored an effectiveness of more than 98%.
Cold pour sealants, on the other hand, showed average effectiveness of slightly less than 70% about one year after construction when the winter 2002 investigation was conducted. By the time the third investigation was conducted in August, 2002, cold pour sealants scored an average of 66%. In contrast to C1 and C3, performance of C2 seems to continue to decrease even after the second investigation. Figure 4.1 depicts performance trends for the sections in Atlanta district.
100 80 60 40 20 0
4.1.2 El Paso
5/25 7/24 9/22 2002
C1 C2 C3 H1 H2
[page28image5876576] [page28image5876784] [page28image5876992] [page28image5877200] [page28image5877408] [page28image5877616] [page28image5877824] [page28image7086512] [page28image5878448] [page28image5878656]
[page28image5883024] [page28image5883648] [page28image5883856] [page28image5884064] [page28image5884272] [page28image5884480] [page28image5884688] [page28image5884896] [page28image5885104] [page28image5885312] [page28image5885520]
Performance trends for the sections in Atlanta district
Four types of sealants were used in this district, two hot pour sealants (H2 and H3) and two cold pour sealants (C1 and C2). The treatment procedures were constructed on May 5, 2001 on Loop 375 in El Paso County on the Border Highway in the eastbound, outside lane. The first investigation test was made on June 19, 2001 for short-term performance evaluation. The second and third investigations were made on April 10, 2002 and August 22, 2002, respectively. These test sections are located in a heavy-truck traffic area by the US-Mexico border. Therefore, most of the failures occurred on the wheel path.
The performance of hot pour sealants surpasses that of cold pour in this district as well. However, the effectiveness of the hot pour sealants used in this district (H2 and H3) dropped to slightly below 80% at the winter 2002 investigation. At the summer 2002 investigation, hot pour sealants scored an average effectiveness of 92.4%.
The performance of cold pour sealants was in general lower than that of hot pour sealants. Unlike the other sealants, the performance trend of C2 continued to drop even after the winter 2002 investigation visit, where it reached 8.4%. The performance trends of the sealing materials used in El Paso are shown in Figure 4.2.
[page29image5892176] [page29image5892800] [page29image5892384] [page29image5892592] [page29image5893008] [page29image7080464] [page29image5058576]
100 80 60 40 20 0
6/28 8/27 2002
10/26[page29image5886560] [page29image5887392] [page29image5887600] [page29image5887808]
[page29image5888432] [page29image5888640] [page29image5888848] [page29image5889056] [page29image7082144] [page29image5889888] [page29image5890096] [page29image5890304] [page29image7082928]
C1 C2 H2 H3[page29image5890928] [page29image5891136] [page29image7083152] [page29image5891760] [page29image7083040] [page29image5892384]
[page29image5892592] [page29image7083824] [page29image5893216] [page29image5893424] [page29image5893632] [page29image5893840] [page29image5894048] [page29image5894256] [page29image5894464] [page29image5894672] [page29image5894880] [page29image5895088] [page29image5895296] [page29image5895504] [page29image5895712] [page29image5895920] [page29image5896128] [page29image5896336] [page29image5896544] [page29image5897168] [page29image5897376] [page29image5897584] [page29image5897792] [page29image5898000] [page29image7864320] [page29image7864528] [page29image7864736] [page29image7865152]
Performance trends for the sections in El Paso district
In the Amarillo district, three hot pour sealants (H1, H3, and H4) and two cold pour (C1 and C3) were used. The treatment procedures were constructed on February 19, 2001 on FM 1151 in Randall County in the eastbound, outside lane. Then, three investigation visits were made on June 21, 2001, March 31, 2002, and August 15, 2002, respectively.
Except for H3, hot pour sealants showed excellent performance with an effectiveness greater than 90% even after about 13 months of installation. Hot pour sealant H3 attained only 65.8% effectiveness after the same period. However, during the summer of 2002, H3 attained an effectiveness of 85.2%.
In this district at the winter 2001 investigation, performance of cold pour sealants showed very low values. Nonetheless, at the summer 2002 investigation, the performances of C1 and C3 drastically climbed up to 84.3% and 90.8% respectively. Figure 4.3 depicts performance trends of the sealants used in test sections in Amarillo district.
Effectiveness (%)[page30image5086576] [page30image7871600] [page30image7871808]
100 80 60 40 20 0
6/14 8/13 2002
[page30image5888432] [page30image5894048] [page30image5895712]
[page30image5896128] [page30image5896336] [page30image5896544]
C1 C3 H1 H3 H4
Performance trends for the sections in Amarillo district
All the types of sealing materials were used in treatment procedures in this district. Treatment construction started on April 25, 2001 on US 87 in Bexar in the southbound, outside lane. An investigation visit was conducted on July 18, 2001. The next two investigation visits were made on March 8, 2002 and September 14, 2002.
Despite the decreasing performance of H2 at the winter 2002 investigation, which dropped to a score of only 58%, the other hot pour sealants attained an effectiveness level greater than 91%. At the summer 2002 investigation, H1, H3, and H4 reached an effectiveness level close to 100%. Similarly, the effectiveness of H2 increased approximately to 92%.
Sealant C1 failed totally at the winter 2002 visit. Unlike the other two cold pour sealants, the performance trend of C1 did not increase after the winter 2002 evaluation; whereas sealants C2 and C3 maintained effectiveness of 95% and 85% respectively. Figure 4.4 depicts the performance trends of the sealants used in test sections in San Antonio district.
[page31image5106736] [page31image7870144] [page31image7870352] [page31image7870560]
100 80 60 40 20 0
In Lufkin district two cold pour sealants (C2 and C3) and two hot pour sealants (H1 and H4) were installed on February 6, 2001 on US 59 in Polk County in the southbound, outside lane. Then, evaluation tests were conducted three times: on May 7, 2001, February 22, 2002, and August 22, 2002.
As was the case in most of the other districts, hot pour sealants attained effectiveness greater than that of cold pour sealants, scoring an average of 97% after the first investigation. The performance of H4 stayed the same. H1 exhibited an increase in effectiveness from 91% to 97% after the winter 2002 evaluation. The performances of both C2 and C3 declined after the first evaluation. At the summer 2002 evaluation, cold pour sealant C2 scored an effectiveness greater than 95%. The performance of C3 could not be measured at the summer 2002 investigation; because this test section had deteriorated significantly, it had been milled and given a new overlay. Figure 4.5 illustrates the performance trends of the sealants used in the sections in Lufkin district.
[page31image7870768] [page31image7870976] [page31image7871184] [page31image7871392] [page31image7871600] [page31image7872016] [page31image7872224] [page31image7872432] [page31image7872640] [page31image7872848] [page31image7873056] [page31image7873264] [page31image7873472] [page31image7873680]
C1 C2 C3 H1 H2 H3 H4[page31image5033040] [page31image7874304] [page31image7088640] [page31image7089200] [page31image7104544] [page31image7875760] [page31image7875968] [page31image5027776] [page31image7876592] [page31image7097264] [page31image7104656] [page31image7877632]
[page31image5097216] [page31image7883872] [page31image7884080]
4/25 6/24 8/23 2001
6/19 8/18 2002
Performance trends for the sections in San Antonio district
Effectiveness (%)[page32image7882832] [page32image7883248] [page32image7884080] [page32image7884288] [page32image7884912] [page32image7885120] [page32image7885328] [page32image7885536] [page32image7885744] [page32image7885952]
100 80 60 40 20 0
[page32image7886992] [page32image7887408] [page32image7887616] [page32image7887824] [page32image7888032] [page32image7888240] [page32image7888448] [page32image7888656] [page32image7888864] [page32image7108352]
C2 C3 H1 H4[page32image7080016] [page32image7086064] [page32image7108240]
[page32image7109808] [page32image7896352] [page32image7896560] [page32image7896768]
6/1 7/31 9/29 2002
Performance trends for the sections in Lufkin district
4.2 Covered Test Sections
As mentioned before, these test sections were installed to evaluate the tendency of the sealing materials to bleed through a chip seal or overlay. These test sections were constructed in Atlanta, Amarillo, and Lufkin districts. Results for the length of bleeding sections on covered test sections based on each visit and district are located in Table 4.1.
In Atlanta, crack seal was applied on January 31, 2001, and chip seal was applied on June 20, 2001, to Loop 281 in Harrison County in the southbound, outside lane. An evaluation was made after two months and the results are mentioned in Chapter 2. This section was evaluated again on August 8, 2002.
Before applying chip seal, the test sections were treated using hot pour sealant H1 and cold pour sealant C2. Sections treated with H1 showed bleeding signs of low severity level. The length of the bleeding portions was 407 ft (124 m). Figure 4.6 shows a part of the section that is treated with H1, which developed bleeding. Sections treated with C2 showed no bleeding problem. Figure 4.7 shows the C2-covered test section in Atlanta during the August 8, 2002 investigation visit.
Table 4.1 Length of Bleeding Sections on Covered Sections Based on District and Visit
First Visit Summer 2001
Second Visit Summer 2002
Figure 4.6 Atlanta, H1-covered test section during the August 8, 2002 investigation visit
Figure 4.7 Atlanta, C2-covered test section during the August 8, 2002 investigation visit
This test section, located in Randall County on FM 1541 in the Southbound, Outside Lane, was crack sealed on February 20, 2001 and was chip sealed and visited for evaluation two months after the chip seal. Cold pour sealant C1 and hot pour sealant H3 were used for crack treatment before the chip seal was applied on August 17, 2001. Then, the test sections were investigated again on August 15, 2002. Once more, the hot pour sealant seemed to engender a bleeding problem. However, the severity of the bleeding was very low. Figure 4.8 shows the H3-covered section in Amarillo during the August 15, 2002 investigation visit. On the other hand, the test section that was treated with C1 did not show any bleeding problems. Figure 4.9 shows the C1-covered section in Amarillo during the August 15, 2002 investigation visit.
Figure 4.8 Amarillo, H3-covered test section during the August 15, 2002 investigation visit
Figure 4.9 Amarillo, C1-covered test section during the August 15, 2002 investigation visit
This test section, located in Polk county on US 190 in the Westbound, Outside lan, was chip sealed on June 25, 2002, and then crack seal was applied on February 7 and 8, 2001. It was investigated once only on August 20, 2002. Two cold pour sealants (C1 and C2) and two hot pour sealants (H1 and H3) were used for crack treatment of this test section. As expected, bleeding was observed during the investigation of hot pour treated sections. However, its severity was very low. Bleeding portion lengths were 214 ft (65.2 m) and 150 ft (45.7 m) for H1 and H3 respectively. In the case of cold pour sealants, no signs of bleeding were observed. Figures 4.10 through 4.13 show the covered sections in Lufkin that are treated with C1, C3, H1, and H3 respectively.
Figure 4.10 Lufkin, C1-covered test section during the August 20, 2002 investigation visit
Figure 4.11 Lufkin, C2-covered test section during the August 20, 2002 investigation visit
Figure 4.12 Lufkin, H1-covered test section during the August 20, 2002 investigation visit
Figure 4.13 Lufkin, H3-covered test section during the August 20, 2002 investigation visit
5. Discussion of the Results
The findings of this study were obtained in two stages. The results of each stage will be discussed in order to understand the performance trend of the sealing materials. The first stage refers to the short-term performance evaluation, which was done within 3–4 months after crack sealants were placed. The overall summary of the findings of this stage is shown in Table 5.1.
Table 5.1 Effectiveness Evaluation Results for the Short-Term Performance after the First Investigation (3-4 months after crack sealing)
1st visit (3–4 months after installation)
Date of investigation
AVG. for Cold Pour
AVG. for Hot Pour
The first investigation was made shortly after the construction was done. It was found that the overall performance of hot pour sealants was slightly better than that of cold pour sealants. Regardless of the district, all hot pour sealants gave the best results, scoring an effectiveness level of approximately 100%. C1 performed the worst with 87.7% effectiveness. Except Amarillo, all the districts exhibited an overall effectiveness greater than 97%.
The second stage or the long-term performance evaluation is a long process in which several more investigations will be conducted in the following years. This stage began with the second investigation of the test sections in the winter of 2002. The overall summary of the second visit is shown in Table 5.2.
Table 5.2 Effectiveness Evaluation Results for the Long-Term Performance after the Second Investigation (Winter 2002)
2nd visit (Winter 2002)
Date of investigation
AVG. for Cold Pour
AVG. for Hot Pour
The second investigation was conducted about one year after the construction. It was found that the performance of hot pour sealants was still better than that of cold pour sealants in every district. Hot pour sealant H4 seems to have the optimum performance among other sealants. Cold pour sealant C1 has the least resistance to traffic and environmental influences with an effectiveness level of 30.3% after one year from installation. The results show a general trend of decrease in effectiveness level for all the sealants. However, the decrease is much steeper for cold pour sealants.
The third investigation was conducted about 18 months after the construction during the summer of 2002. The results of this investigation are shown in Table 5.3.
Table 5.3 Effectiveness Evaluation Results for the Long-Term Performance after the Third Investigation (Summer 2002)
3rd visit (Summer 2002)
Date of investigation
AVG. for Cold Pour
AVG. for Hot Pour
An increase in the performance of the sealants was observed during the third investigation as opposed to an expected decrease in performance with time. This can be attributed to the fact that cracks close during summer months. As is seen in Table 5.3, the investigation was made during the summer period when the temperature is expected to be at its highest. Also, at high temperatures, the viscosity of the sealing material decreases, which may cause re-filling of the generated cracks. In the case of hot pour sealants, the sealant originally plugs mainly the top part of the crack and does not penetrate all the way down to the crack root. Hence, it is more likely that the failed sections treated with hot pour sealants will recover in high temperatures due to the decrease in viscosity. Since excessive amounts of hot pour sealant are usually accumulated near the surface, when the viscosity drops, enough material will be available to seal the failed sections. On the other hand, cold pour sealants have lower viscosity than hot pour sealants. Therefore, when they are applied for the first time, they tend to penetrate the cracks more thoroughly. This leaves less surplus material and subsequently less recovery in the failed sections when the viscosity drops due to high temperatures. Figure 5.1 shows the configuration of hot and cold pour sealants after being applied in the crack.
Cold Pour Hot Pour
Figure 5.1 Sealing material configurations in the crack
The proportionality among the sealants’ effectiveness, however, remained very similar to that in the winter 2002 investigation. Again, H4 achieved the best overall effectiveness whereas C1 achieved the lowest overall effectiveness.
Since both traffic and environmental conditions vary from district to district, a comparison of sealants’ performance in each district is necessary. This kind of a comparison will provide more information about the performance of the sealants and its correlation to prevailing factors where it was installed. Weather records were extracted from www.weather.com in order to achieve a better understanding of the performance trends of sealing procedures in different districts (Ref 8). Table 5.4 includes average annual extremes, average mean temperatures, and average annual precipitation in the five districts.
Table 5.4 Weather Annual Averages for the Districts
[page42image5029120] [page42image7092448] [page42image7952304]
Max Temp. °F
Min Temp. °F
Sum Precipitation (in)
For a better understanding of the behavior of the sealing materials, they must be categorized according to their types. The first category is the hot pour sealants with H1, H2, and
H3 as crack sealants and H4 as joint sealant. The second category is the cold pour sealants with C1 and C2 as crack sealants and C3 as joint sealant.
Crack sealant H1 and joint sealant H4 performed very well, scoring approximately over 90% at the winter 2002 investigation and over 96% at the summer 2002 investigation in all the districts. Joint sealant H4 exhibited the highest performance among all other sealing materials. It showed highest values of penetration at 39.2° F and 77° F. Also, it had the maximum resilience value as is shown in Appendix A. The second best performance was attained by crack sealant H1. Although it had better performance than the other two hot pour crack sealants (H2 and H3), no significant difference in material properties could be found between H1 and the other two.
Cold pour sealants C2 and C3 showed relatively similar performance, while C1 showed the lowest performance, having an average performance of 30.3% after the winter 2002 investigation and 54.4% after the summer 2002 investigation. No significant correlation could be established between the laboratory test results and the field performance of cold pour sealants. Furthermore, annual temperature range seems to have an effect on the performance of different sealing materials. This is expected since the temperature range controls thermal movements of the cracks. This effect can be seen in the performance trends of H3 and to some extent C2 and C3. Figures 5.2 and 5.3 show performance trends of hot and cold pour sealants with respect to annual rainfall and temperature range after the winter 2002 investigation. It appears that as the temperature range decreases, the sealant effectiveness increases.
31100 80 60 40 20 0
[page44image7955840] [page44image7086064] [page44image7992656] [page44image7992864] [page44image7993072] [page44image7993280] [page44image7993488] [page44image7993696] [page44image7993904] [page44image7994112] [page44image7994320] [page44image7994528] [page44image7994736] [page44image7994944] [page44image7995152] [page44image9961472] [page44image9961680] [page44image9961888] [page44image9962096] [page44image9962304] [page44image9962512]
0.77 inches 70° F Amarillo
0.35 inches 67° F
1.40 inches 63° F Atlanta
1.22 inches 58° F San Antonio
1.67 inches 57° F Lufkin
Annual Rainfall/Temperature Range
H1 H2 H3 H4
Figure 5.2 Performance trends of hot pour sealants with respect to temperature range after the winter 2002 investigation100 80 60 40 20 0
[page44image9963760] [page44image9963968] [page44image9964176] [page44image9964384] [page44image9964592] [page44image9964800] [page44image9965008] [page44image9965216] [page44image9965424] [page44image9965632] [page44image9965840] [page44image9966048] [page44image9966256] [page44image9966464] [page44image9966672] [page44image9966880] [page44image9967088] [page44image9967296] [page44image9967504] [page44image9967712] [page44image9967920] [page44image9968128] [page44image9968336] [page44image9968544]
0.77 inches 70° F Amarillo
0.35 inches 67° F
1.40 inches 63° F Atlanta
1.22 inches 58° F San Antonio
1.67 inches 57° F Lufkin
Annual Rainfall/Temperature Range
[page44image9968960] [page44image9969168] [page44image9969376]
C1 C2 C3
Figure 5.3 Performance trends of cold pour sealants with respect to temperature range after the winter 2002 investigation
Similarly, performance trends of both hot and cold pour sealants with respect to environmental factors after the summer 2002 investigation are shown in Figures 5.4 and 5.5 respectively.
For the hot pour sealants, there is a pattern of increase in performance with the decrease of annual temperature range. This pattern can be clearly seen in the performance trend of H3 and H4 where their performance continues to increase as we go from Amarillo to Lufkin. This trend also occurs generally in the performance of H1 and H2.
For the cold pour sealants, on the other hand, two different patterns can be extracted. The first pattern is that of C1 (highest softening point, 202° F, among the cold pour sealants) where the effectiveness exhibits a continuous drop with the decrease of annual temperature range. The opposite pattern is exhibited by C2 (lowest softening point, 158° F among the cold pour sealants) in which the effectiveness increases with the decrease of annual temperature range.100 80 60 40 20 0
[page45image9973536] [page45image7955216] [page45image9973120] [page45image9973744] [page45image9973952] [page45image9974160] [page45image9974368] [page45image9974576] [page45image9974784] [page45image9974992] [page45image9975200] [page45image9975408] [page45image9975616] [page45image9975824] [page45image9976032] [page45image9976240] [page45image9976448] [page45image9976656] [page45image9976864] [page45image9977072] [page45image9977280] [page45image9977488] [page45image9977696] [page45image9977904] [page45image9978112] [page45image9978320]
0.77 inches 70° F Amarillo
0.35 inches 67° F
1.40 inches 63° F Atlanta
1.22 inches 58° F San Antonio
1.67 inches 57° F Lufkin
Annual Rainfall/Temperature Range
H1 H2 H3 H4
Figure 5.4 Performance trends of hot pour sealants with respect to temperature range after the summer 2002 investigation
99.9100 80 60 40 20 0
[page46image5072800] [page46image9986848] [page46image9987056] [page46image9987264] [page46image9987472] [page46image9987680] [page46image9987888] [page46image9988096] [page46image9988304] [page46image9988512] [page46image9988720]
0.77 inches 70° F Amarillo
C1 C2 C3
1.40 inches 63° F Atlanta
1.22 inches 58° F San Antonio
1.67 inches 57° F Lufkin
0.35 inches 67° F
Annual Rainfall/Temperature Range
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Figure 5.5 Performance trends of cold pour sealants with respect to temperature range after the summer 2002 investigation
The increase in the percent effectiveness in the summer seems to have a correlation with the maximum annual temperature. This phenomenon is largely based on the temperature range and the natural process of cracks opening in the winter and closing in the summer, plus other pavement, soil, and rain conditions. This was expected given the configuration of the hot pour sealing material in the crack. The recovery of hot pour sealants at different districts is shown
EVALUATION OF ASPHALT PAVEMENT CRACK SEALING PERFORMANCE USING IMAGE PROCESSING TECHNIQUE
Assistant Professor, Department of Civil Engineering Yonsei University
Shinchondong 134 Seoul, Korea 120-749 E-mail: email@example.com
Seung Heon Han
Associate Professor, Department of Civil Engineering Yonsei University
Shinchondong 134 Seoul, Korea 120-749 E-mail: firstname.lastname@example.org
Assistant Professor, Department of Civil and Environmental Engineering at the University of Alberta Edmonton, Alberta, Canada T6G 2W2
Master student, Department of Civil Engineering Yonsei University
Shinchondong 134 Seoul, Korea 120-749 E-mail: email@example.com
Abstract: Crack sealing is a routine and necessary operation of pavement maintenance. Manual observation of road surfaces has been the most common method for evaluating road surface cracks around the world. However it is difficult to objectively and accurately assess the road cracks based on human visual perception. The ultimate objective of this study is to evaluate crack sealing performance on highways, in order to choose the best crack sealing practice in an automated manner. As a preliminary step, this paper discusses how to define crack sealing performance and propose a research methodology to quantify the level of road surface distress using video image processing.
Keywords: crack sealing, image processing, pavement.
Pavement crack sealing is a routine and necessary operation of pavement maintenance. This operation cost is approximately US$ 260 million every year in the USA and Canada. The crack sealing with appropriately selected material and procedure can significantly improve the serviceability and lifespan of pavement infrastructure. However, it is difficult to understand which crack sealing practice should be used as the best pavement preservation method. To the end, the main objective of this study is to evaluate the performance of crack sealing on highways. This paper proposes an image processing-based system to evaluate the crack sealing performances. This paper also addresses the important issue of how to define crack sealing performance and present a conceptual design of an image processing-based system to quantify crack sealing performance.
2. CRACK EVALUATION
2.1 Crack Mode
One of the most important aspects of this study is what
criteria need to used to measure the crack sealing performance. Although there have been some efforts to describe distresses (loss or failure) in the sealed cracks, no standardized method has been adopted to quantitatively evaluate the status of the sealed cracks.
Generally, the failure modes in sealed cracks can fall into the four categories: adhesive loss, cohesive loss, pull-outs, and secondary crack:
·Adhesive loss refers to the gap between the sealant material and the adjoining pavement edge. This failure can occur when there is insufficient bonding between the two heterogeneous materials (Figure 1).
·Cohesive loss refers to the fractures within the sealant material. It is generally a result of the internal stress caused by pavement expansion and contraction.
·Pull-outs refer to a complete removal of the sealant material in the particular portion of the sealed crack. This failure generally occurs by the combined effect of adhesive and cohesive loss (Figure 1).
·Secondary crack refers to the crack that has occurred as the continuation from the existing crack (Carter et al. 2005).
Each one of the above failure mode could also be divided into its sub-categories based on the severity of distresses. Other failure modes or distress descriptions that have been used include weathering, overband wear, tracking, stone intrusion, edge deterioration, crazing, pattering, etc. While these terms are useful in describing the particular nature of the failure, their implications are subjective and overlapping with other terms. For the accurate evaluation of the crack sealing performance, it is essential to have a well thought-out definition of the loss or failure of sealed cracks.
Figure 1. Sealed cracks showing two modes of failures: adhesive loss and pull-out
2.2 Review of Crack Evaluation Technique
Manual observation of road surfaces has been the most
common method for evaluating road surface cracks around the world. Inspectors walk on the roadways to visually understand and record where cracks exist, what types of cracks they are, and how severe they are. However, for many reasons, it is difficult to objectively and accurately assess the road cracks based on human visual perception. First, there are not enough inspection personnel who can be deployed to cover the vast area of road surfaces. Second, even the most experienced and best trained inspectors tend to produce significantly different opinions on the same road surface cracks. In many cases, the same road surface could be evaluated differently even by the same inspector if there exists some time interval between the observations. Third, the human evaluation of road surface crack is an extremely time-consuming process. Last but not least, the human inspectors are generally exposed to high-speed traffic, raising the important concern of human safety.
Video image processing is an alternative method to collect the crack data to address the disadvantages of the visual inspection (Haas et al. 2001; Lee and Lee 2004; Feng et al. 2005; Offrell et al. 2005). A vehicle is typically equipped with one ore more video cameras to capture the image of road surfaces. Once images are obtained, they can be stored in an analog device such as video cassette or a digital computer hard disk drive and processed later to indicate the locations and types of the cracks. Previous research demonstrated that the video image processing can classify road cracks into such categories as longitudinal crack, transverse crack, alligator crack, and block crack.
Although video image processing provides ample two-dimensional crack data in the form of image, it cannot capture crack depth information. Sometimes this additional information (crack depth) gives an important clue as to how severe the crack is, so there have been several attempts to use other techniques such as laser sensing to measure three-dimensional crack data (Haas and Hendrickson 1991; Offrell et al. 2005). However, the current laser technique
can scan only limited area of road surface, making it hard to produce accurate characterization of road surface cracks.
The aforementioned attempts have all been contributory to the evolution of crack evaluation techniques. Video image processing, in particular, has obtained the most attention as the major technique for automating crack evaluation process. However, their application has been limited only to the identification of crack locations and types. This paper proposes to use image processing not just to identify the location and type of cracks but also to quantify the level of distress of sealed cracks.
3. PROPOSED METHODOLOGY
In this section, a research methodology is presented. The main technology that is suggested in this paper is image processing technique. Considerable effort will be made to evaluate the feasibility of video image processing for its ability to accurately quantify the distress level of sealed cracks. To our knowledge, this is the first to propose the video image processing to evaluate the distress level of sealed cracks. To ensure the successful completion of this proposed research, visual inspection is also proposed in parallel with image processing for evaluating the crack sealing performance.
3.1 STEP1: Develop a Practical and Intuitive Definition of Crack Sealing Performance
The crack sealing performance could be defined as the failure level of the sealing on a length basis. For example, in the case of evaluating adhesive loss, the length of the adhesive failure can be compared to the total crack length, producing a certain percentage figure indicating how severe the adhesive loss is (Figure 2). In the same manner, the severity of cohesive loss or pull-outs can be quantified. The three failure modes (adhesive loss, cohesive loss, and pull-outs) could also be added up to produce their combined length, which can be compared to the total crack length, in order to produce a comprehensive distress definition of sealed cracks. Alternatively, an area based definition can be used for evaluating the crack sealing performance as long as the area of failure and the area of sealed cracks are accurately measured.
Figure 2. An example of the quantification of adhesive loss
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A = The length of the adhesive loss
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The quantification = A
of the adhesive loss
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B = The length of the sealed crack
3.2 STEP2: An Alternative Method for Crack Sealing Evaluation
The objective of this step is to develop an alternative method for a rapid assessment of crack sealing performance. Traditionally, the distress of road surfaces has been evaluated by human visual inspection. However, the visual inspection is time-consuming and subject to human errors because any field inspector can have his/her own qualitative criteria. We propose video image processing as a promising approach for evaluating crack sealing performance. Image analysis can be automated, configured to work in a continuous data acquisition and analysis mode. Furthermore, a two dimensional imaging technique (as opposed to three dimensional sensing techniques such as laser sensing) will provide the best balance between speed, accuracy, and cost for this application.
We propose a prototype analyzer of crack sealing performance, using off-the-shelf hardware and customized software. The prototype apparatus may incorporate five to six video cameras (machine vision type CCD (Charge Coupled Device) cameras) to secure the required resolution. Using the prototype assembly, a series of preliminary field experiments is proposed to discover avenues for improvement. This should be an iterative process in which results are used to modify and refine the prototype analyzer of crack sealing performance to achieve the desired system performance.
3.3 STEP3: Collect the Data of Crack Sealing Performance Test sites where old sealed cracks exist are proposed to be inspected using two different methods: visual inspection and video image processing. By correlating the result from the visual inspection with that of the video image processing, we can ensure that the analysis from the video image processing are consistent with the realities of infrastructure management. This approach is expected to enable accurate and quantitative analyses of the crack sealing performances, as well as comprehensive
documentation of the cracks in image format.
3.4 STEP4: Analyze the Crack Sealing Data
The data collected in Step 3 are then analyzed in this step.
Various parameters, such as a range of sealant products, rout profiles, pavement structures, and climatic conditions, and seasonal conditions, should be carefully evaluated to identify their impact on the crack sealing performance. It is also be interesting to see how the evaluated crack sealing performance affects the International Roughness Index (IRI).
4. CASE STUDY
In 1994, several crack sealing practices were performed on roadways in Alberta, in particular, Highway 63:00 (north of highway 28 to south of Newbrook, kilometre 13.000 to kilometre 18.000) was chosen as the main “research” site for the long term evaluation of crack sealing performance. Eight crack sealant products (Hydrotech 6160, Husky 1611, Elsro 1191, Koch 9030, Bakor 590-13A, Crafco 522,
Beram 195, and Super-Flex 100) were used with two different rout profiles (wide profile (40 x 10 mm) and narrow profile (19 x 19 mm)) Also two different compressed airs (hot-compressed-air and cold-compressed -air) were used to blow the crack to remove small debris before the sealant was poured into the crack. All the cracks on the test site were recorded along with their sealing materials and construction methods. This site is a good example of where the proposed methodology can be applied to.
Figure 3 shows an example process for identifying the distress of a sealed crack. Figure 3 (a) is the original image that shows adhesive loss and pull-outs type failures.
·First, the complement of the original figure is obtained, as shown in Figure 3 (b). In other words, each pixel value is subtracted from its possible maximum value, and the calculation result replaces the original pixel value. This is to represent the region of distress in a distinguished manner. The failure area is shown in white color.
·Second, for the effective utilization of existing image processing algorithm, the complement color image is changed into a grayscale image, as shown in Figure 3 (c). This transformation is also for efficient computation because grayscale image manipulation requires less computational power than color images requires.
·Third, binary gradient mask is calculated based on the grayscale image (Figure 3 (d)). The binary gradient mask shows lines with high contrast of light intensity of the image. This gradient mask is useful to represent a range of texture content of the image. That is, the sealed crack area with no distress shows a uniform texture which is distinguished from the texture of other regions.
·Fourth, a dilation operation is conducted on the gradient mask image, resulting in Figure 3 (e). This dilation operation is a type morphological operation which relies on a structuring element. The dilation operation has the effect of simplifying the sealed crack region and background region, respectively. In other words, those lines showing gradient information are merged together to create a region with improved consistency.
·Fifth, a hole-filling algorithm is applied. After the dilation operation is executed, there are still noisy holes in the non-crack region of the road surface. These holes are filled by the hole-filling algorithm, as shown in Figure 3 (f).
·Sixth, the original color image (Figure 3 (a)) is binarized into a black and white image (Figure 3 (g)). Based on a threshold value, each pixel value of the original image becomes either 0 or 255.
·Finally, the binary image obtained from the sixth step is compared with the binary image obtained from the fifth step. The two images are complementary to each other. Therefore, the logical “and” operation is used to find the pixel location that has white value in both images. This process produces Figure 3 (h), which displays the failure regions of the sealed cracks.
(a) Original figure
(e) Dilated gradient mask
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(b) Complement of image
(f) Binary image with holes filled
(c) Grayscale image
Binary image from the original figure
(d) Binary gradient mask
Figure 3. Crack extraction process
Final extraction of the distress of a sealed crack
The comparison between the failure region of the sealed crack in Figure 1 (a) and the white region in Figure 1 (h) indicates that the proposed method is promising in extracting the distress region of sealed cracks. The proposed method illustrates a preprocessing example of how the failure region of a sealed crack image is extracted. The future works should address the issues of how robust the proposed method is, how to label the identified failure regions, and, most importantly, and how to quantify the level of distress of the failure region.
Crack sealing is an essential maintenance strategy for pavement infrastructure, especially in North America. Various types of sealing practices have been used. However, it has been difficult to understand which sealing practice produces the best performance in terms of being able to protect road surfaces from water intrusion. It is because evaluation of the sealed cracks is generally done by manual observation, which requires significant amount of time and efforts. This paper proposed an image processing based research plan to evaluate crack sealing performance. Although the proposed method is in its conceptual stage yet, when the method is coupled with high-resolution image capturing devices installed on a vehicle, it has the potential for accurate and quantitative assessment of crack sealing performance.
 Carter, S., Ksaibati, K., and Huntington, G (2005). “Field and Laboratory Evaluations of Hot-Poured Thermoelastic Bituminous Crack Sealing of Asphalt Pavements,” Proc., 84th Annual Meeting,Transportation Research Board, Washington, D.C.
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Cameras, Laser Cameras, and a Simplified Manual Survey,” ASCE, Journal of Transportation Engineering, Vol. 131, No. 7, pp. 552-562.
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Asphalt Rubber Standard Practice Guide
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