summarizing a report about " Thermal cracking of pavement ", I couldn't fine Civil Eng. Field so I chose applied science!

Thermal cracking analysis

· Thermal cracking definition

Thermal cracking on flexible pavements poses serious concerns in cold-regions of the world,

especially in Canada. Influenced by temperature changes, thermal cracks are known to initiate

other forms of pavement deterioration. Thermal cracking is attributed to the development of high tensile stresses under repetitive exposures to extremely low temperatures or moderate daily temperature cycles otherwise referred to as thermal fatigue cracking. Furthermore, the mechanisms of low temperature cracking develop as As the temperature drops to an extremely cold level, tensile stresses induce the formation of low temperature cracking due to the pavement’s tendency to contract. The development of tensile stresses in AC pavements occurs as a result of the friction between the pavement and the base course layer that resists the contraction. A microcrack would develop at the edge and surface of the pavement if the tensile stress induced in the pavement equals the strength of the AC mix. The crack would eventually penetrate the full depth and across the AC layer under repeated temperature cycles or the occurrence of colder temperatures. According to field observations, it has been shown that the cracks start at the surface and progress down through the entire pavement. Provided that the coefficient of contraction of the stabilized layer is greater than that of the AC layer, it may be possible for a thermal crack to reflect up through the AC layer from an underlying stabilized layer (p.6-7).

2.2 Factors influencing temperature cracking

(1) material, (2) environmental, and (3) pavement structure geometry

1. Material Factors

Several material factors can affect the thermal behavior of asphalt-aggregate mixtures. These factors include asphalt cement, aggregate type and gradation, asphalt cement content, and air-void content:

a- Asphalt cement

The single most important factor affecting the severity of low-temperature cracking in an AC mix is the temperature-stiffness relationship of the asphalt. The most important considerations are the stiffness or consistency (i.e., viscosity or penetration) at a cold temperatures and the temperature susceptibility (i.e., the range in consistency with temperature). Lower viscosity (or penetration) grades or lower temperature performance graded materials will have a reduced rate of increasing stiffness with decreasing temperature. This results in a lower potential for low-temperature cracking [16]. It has been found that the addition of polymer to liquid or heated asphalt generally improves field performance because it imparts flexibility to the asphalt (p.7)

b- Aggregate type and gradation

Aggregates that have high abrasion resistance, low freeze-thaw loss, and low absorption show improved resistance to transverse cracking. Little variation in low-temperature strength is associated with aggregates that possess these characteristics. The low-temperature strength is reduced through absorptive aggregates because the asphalt cement remaining in the mix for bonding is less than it would be in a mix with a non-absorptive aggregate. Little influence on low-temperature strength can be related to the gradation of the aggregate used in the mix, provided that the mix is designed to provide reasonable resistance to rutting (p.7)

c- Asphalt cement content

No significant influence on a mix's low-temperature cracking performance has been reported when changes in asphalt cement content occur within a reasonable range of optimum. Increasing asphalt content increases the coefficient of thermal contraction and decreases the stiffness. This leads to equilibrium between the thermal stress developing and the stress that developed before the asphalt cement content was changed (p.7)

d- Air-void content

The degree of compaction and related air void content and permeability do not significantly influence the low-temperature cracking characteristics of the mix (p.7)

e- Reclaimed Asphalt Pavement (RAP)

(20% RAP, PG 58-34) are comparable, as evident from Table 2. The fracture temperatures of

mixture M2 (control mixture: 0% RAP, PG 58-28) and mixture M4 (40% RAP, PG 58-28) are

also comparable. Both M2 and M4 mixtures fractured at around -30ºC, but the later exhibited

sudden failure.

The binder grade and RAP content affected the fracture stress of the asphalt concrete mixtures.

However, in terms of the fracture stress, the control HMA mixtures (M1: 0% RAP, PG 52-34

and M2: 0% RAP, PG 58-28) could be considered to be the least resistant to thermal cracks in

comparison with the HMA mixtures incorporating RAP (M3: 20% RAP, PG 58-34 and M4: 40%

RAP, PG 58-28). The mean fracture stress values for all tested mixtures are illustrated in Figure

4. The highest fracture stress was obtained for mixture M3 (20% RAP, PG 58-34), followed by

mixture M4 (40% RAP, PG 58-28). However, the fracture stress values for the HMA mixtures

incorporating RAP are not significantly high to suggest that they will be prone to thermal

cracking at low temperatures.

The TSRST results of each asphalt mixture showed specimen-to-specimen variations, as can be

seen from Table 2. The standard deviation and coefficient of variation for the test results of all

asphalt mixtures are summarized in Table 3. As expected, the coefficients of variation for

fracture stress were significantly higher compared to those for fracture temperature. This is why

fracture temperature is used to rank low-temperature cracking resistance of asphalt concrete

mixtures [11].

2. Environmental Factors

Several environmental factors can affect low-temperature cracking. These factors

a. Temperature

It was reported that, for a given mix, as the pavement surface temperature is reduced, the incidence of thermal cracking is increased. The ambient air temperature and wind speed both affect the pavement surface temperature. The majority of low-temperature cracking occurs when the temperature decreases to a level below the glass transition temperature and is maintained at this level

b. Pavement age

The incidence of thermal cracking is associated with older pavement. This occurs as a result of the increasing stiffness of aging asphalt cements. The aging characteristics of a mix may be affected by the air void content. In addition, as the pavement's service life increases, the probability of more extreme low-temperatures occurring will increases

3. Pavement Structure Geometry

Several pavement structure geometry factors can affect thermal cracking response. These factors include pavement width, pavement thickness, coefficient of friction between the AC layer and base course, subgrade types, and construction flaws :

a. Pavement width

It has been suggested through field investigations that thermal cracks are more closely spaced in narrow pavements than in wide pavements. Initial crack spacing for secondary roads of 24 feet (7.3m) width is approximately 98.4 feet (30m). As the pavement ages, secondary and tertiary cracks develop and the differences in crack spacing are not apparent.

b. Pavement thickness

In general, lower incidence of thermal cracking has been recorded for thicker AC layer pavements. In a study made Burgess et al., it was fond that increasing the thickness of the AC from 3.9 inch to 12 inch (10cm to 25cm) resulted in one-half the cracking frequency when all other variables were consistent .

c. Coefficient of friction between the AC layer and base course

It was found that the existence of a prime coat on an untreated aggregate base course layer reduces the incidence of low-temperature cracking. This result was attributed to the fact that an AC layer is "perfectly" bonded to the underlying granular base with a reduced thermal contraction coefficient (because the granular base has a lower thermal contraction coefficient than the AC). The gradation of the base course, particularly the percentage of material finer than the No. 200 sieve, may have a minor influence on the incidence of low-temperature cracking.

d. Subgrade type

The frequency of low-temperature cracking is usually greater for pavements on sand subgrades than on cohesive subgrades

e. Construction flaws

Steel roller compaction of asphalt layers at high temperatures and low mix stiffness creates transverse flaws. As the pavement cools, cracks may be initiated at these flaws, often spaced closer than the width of a lane

2.3 General Maintenance Practices Overview

Increasing budget constraints require that states and local agencies perform more work with less money. Historically, the emphasis of local highway departments has been on building new roads, but the new focus is on maintaining and preserving existing pavement surfaces. This shift has resulted in three types of pavement maintenance operations [3]:

a. Preventative Maintenance

A preventive maintenance program is a systematic approach to using a series of preventive maintenance treatments over time. A single treatment will improve the quality of the pavement surface and extend the pavement life, but the true benefits of pavement maintenance are realized when there is a consistent schedule for performing the preventive maintenance. 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 strategy [3]. Benefits of pavement preservation include improved customer service and substantial life cycle cost savings; treatments are especially cost-effective when applied early in the life of a pavement. In addition, by extending the life of a pavement section until it can be rehabilitated, preventive maintenance allows an agency to even out its maintenance budget from year to year, which otherwise can vary greatly. Preventive maintenance activities can include conventional treatments such as crack sealing, chip sealing, fog sealing, rut filling, and thin overlays. They can also include emerging technologies; such as ultra-thin wearing courses, very thin overlays, and microsurfacing applications. Aside from crack treatments, all of these treatments leave the pavement with a new wearing surface. Preventive maintenance is generally planned and cyclical in nature. Its intent is to repair early pavement deterioration, delay pavement failures, and reduce the need for corrective maintenance and service activities. Although this type of maintenance is not performed to improve the load-carrying capacity of a pavement, it extends the pavement useful life and level of service [3].

b. Corrective Maintenance

Corrective maintenance is performed to improve or extend the functional life of a pavement. It is a strategy of surface treatments and operations intended to retard progressive failures, and reduce the need for routine maintenance and service activities [3]. Corrective maintenance differs from preventive maintenance primarily in cost and timing. While preventive maintenance is performed when the pavement is still in good condition, corrective maintenance is performed when the pavement is in need of repair, and is therefore more costly. Delaying maintenance allows increased occurrence of pavement defects and increased severity, resulting in more extensive and expensive work. Consequently, the life cycle costs of the pavement will be considerably increased when corrective maintenance is performed. Corrective maintenance is much more reactive than preventive maintenance, and is performed to correct a specific pavement or area of distress. Activities include structural overlays, mill and overlays, pothole repair, patching, and crack repair [3]

c. Emergency Maintenance

This maintenance activity may be performed during an emergency situation, such as when a blowout or severe pothole must be repaired immediately, generally for safety reasons or to allow for traffic to use the roadway. Emergency maintenance also describes those treatments that hold the surface together until a more extensive rehabilitation or reconstruction treatment can be accomplished. When emergency maintenance is needed, some of the typical considerations for choosing a treatment method are no longer important. Cost may be the least important consideration after safety and time of application are considered. Materials that may not be acceptable when used in preventive or corrective maintenance activities, for cost or long-term performance reasons, may be highly acceptable when used in an emergency situation [3].

2.4 Thermal Cracking Construction Processes

a. Crack Evaluation and Assessment for Crack Maintenance

The formation of cracks in asphalt pavements will occur with normal traffic and time, but the severity and occurrence is not predictable by any single common method. Generally and historically, crack treatments are used as a means to slow the rate of deterioration of pavements and prevent water from entering the lower pavement layers [3, 24, 25]. Decisions for the correct maintenance measures and crack treatments are determined by predictive measures based on previous experience or the current state of the pavement. The severity and occurrence of cracking is done through evaluation of the pavement.

b. . Maintenance Method Determination

Maintenance planning and design may involve advance scheduling of maintenance techniques according to pavement management systems (PMS). A PMS is “an established, documented procedure treating many or all of the pavement management activities listed in a systematic and coordinated manner” [26, 27]. Determination of the proper techniques for a PMS or repairing a specific distress may require experience or rational decision making. Hicks et al. lists cost, reliability, availability of contractors, environment, and other factors such as availability of materials and time of year placement as the key components for PMS decision making [27]. Many studies have isolated cost and cost-effectiveness as the main concern, where a balance between cost and performance will often determine the final material or method selected [24, 25, 27].

Hicks et al. investigated the cost compared to the expected life of several treatments used for preventative maintenance. Using present serviceability index (PSI) as the performance gauge, the researchers concluded that the cost-effectiveness of the treatments varied based on the specific distresses in the road. The performance of rout and seal treatments and rout and fill treatments were effective in longitudinal and transverse cracking distresses, but mixed results were seen with other treatments [27]. Hand et al. presented a literature review and a limited survey regarding cost-effectiveness of crack and joint sealing after the Wisconsin Department of Transportation implemented a “no-seal” policy in 1990 [25]. The basis of this research was that crack sealing is historically accepted as a maintenance practice, but may not be cost-effective due to research findings.

The result of the study concluded that crack sealing practices will retard the deterioration of asphalt pavements, but treatments may only be cost-effective in certain climates or when there is little structural damage to the pavement. Selection of the material will vary with the specific road, but identifying the proper material or technique for cracking will often be determined by the severity and density of cracking [28].

After assessing maintenance needs and determining the different materials that may be used, the selection of the specific material or product should be based on the desired properties needed for the maintenance. Short preparation time, quick and easy application, short cure time, adhesiveness, cohesiveness, resistance to softening and flow, flexibility, elasticity, resistance to aging and weathering, and abrasion resistance are the most desirable properties to consider according to the FHWA [15].

3 The selection process for Mn/DOT relies on cost effectiveness. Decisions are made based on three questions; 1) Does the treatment enhance pavement performance? 2) Is the treatment cost-beneficial? 3) What is the best treatment method used? Determination of the best treatment is generally based on the pavement type, crack conditions, extent of distress, roadway use and level of traffic, climate and environmental factors, traffic loading, cost of treatment, expected life, availability of qualified staff and contractors, availability of quality materials, time of year of placement, facility downtime, pavement noise, and surface friction. Cracking treatments depend on the severity, edge deterioration, and frequency of cracks.

3. COMPONENTS OF BEST MANAGEMENT PRACTICES OF CRACK MAINTENANCE

Best Management Practices (BMPs) can be seen as effective and practical maintenance methods which can prevent or reduce the deterioration of asphalt pavement as a result of existence of thermal cracking. Typically, for a BMP to be successful in crack maintenance and repair it should address the following four phases [15]:

1. Determining the need for crack treatment

2. Planning and designing the crack treatment project

3. Construction

4. Evaluating and assessing the performance of the crack treatment

3.1 Determining the Need for Crack Treatment

Relatively quick assessments can be utilized to determine whether an asphalt pavement is in need for crack treatment or not. Based on such assessments, the appropriate actions can then be determined. Evaluation of the existing pavement conditions and knowledge of future rehabilitation plans are required to obtain such assessments [15].

a. Pavement/Crack Evaluation

A review of construction, maintenance, and other records; the following can be determined [15]:

Pavement age

Pavement geometric design

Pavement section boundaries

Traffic

Climate

Type and extent of previous maintenance treatments Condition rating

Following the review, the performance of a shoulder survey should then be done on a small representative sample of about 419.1 ft (150 m) of the pavement section. This will determine the amount, type, and condition or severity of cracks, as well as the condition or effectiveness of any previously applied crack treatments [15].

b. Determining the Type of Maintenance

The density and general condition of the cracks will determine the appropriate type of maintenance for cracked pavements. If the cracks are abundant and not exhibiting a high degree of edge deterioration, then either chip seals, slurry seals, or similar means can be used to treat them. On the other hand, crack repair strategies, such as partial-depth patching or spot patching, can be utilized if cracks are low to moderate in density and have typically progressed to a point of high edge deterioration. Sealing or filling operations can be utilized effectively if cracks are moderate in density and show moderate to no deterioration at the edges [15].

c. Maintenance Approaches

Although little distinction has been made in the past between crack sealing and crack filling, the purposes and functions of each must be clearly understood so that the most cost-effective and long-lasting treatment is applied [15].

Crack Filling: The placement of materials into non-working cracks to substantially reduce the infiltration of water and to reinforce the adjacent pavement. Non-working refers to horizontal and/or vertical movements less than 0.1 inch (2.5 mm). Non-working cracks typically include mostly longitudinal, diagonal cracks and some block cracks. Such cracks have do not move much due to the close spacing between the edges. Since there is minimal flexibility expected, the materials used for crack filling are non-rubberized products, e.g. crumb rubber, AC-3, and asphalt emulsion. Crack filling is therefore simply filling the cracks that do not show significant movement [4, 5, 6, 7]. Simple overbands are usually used with filling operations [6, 7]. Crack filling involves placing the filling material and spreading it out over and into the crack(s) with a squeegee. Squeegees are typically U or V shaped to push the material and concentrate it over the crack [6, 7]. Crack filling materials may be hot applied rubber or polymer asphalts, or cold applied emulsion-based products. The emulsion products assist with forming a good adhesive bond with the crack wall and additives such as Styrene Butadiene Rubber (SBR) latex ensure that the material can endure some degree of movement. In some cases, hot applied fiber modified asphalt binders may be used [8, 9]. For short-term crack-fill performance (1 to 3 years) in pavements with nonworking cracks (less than 0.1 inch (2.5 mm) of horizontal crack movement) and low to moderate traffic levels, asphalt cement should be placed in flush-fill configuration [10, 11, 12, 13]. For long-term crack-fill performance (between 5 and 8 years) under the above conditions, an asphalt rubber or rubberized asphalt may be placed in either a flush-fill or overband configuration, or a fiberized asphalt may be placed in an overband configuration [10, 11, 12, 13].

Crack Sealing: The placement of specialized materials either above or into working cracks using unique configurations to prevent the intrusion of water and other incompressibles into the pavement cracks. Working cracks refers to horizontal and/or vertical crack movements greater than 0.1 inch (2.5 mm). Transverse cracks are a good example of working cracks; however, some longitudinal cracks may also meet the movement criterion [4]. Crack sealing involves thorough crack preparation followed by the placement of a high-quality material in a specific configuration [5, 6, 7]. Crack sealant materials are rubberized products that have the ability to seal the crack and flex with the pavement’s movement. They are used for active cracks that continue to extend both in size and severity with time and the ravages of the traffic and weather. Crack sealants have excellent adhesive and cohesive properties. In other words they firmly adhere to the walls of the cracks and do not tear or split when the cracks widen [4]. Reservoirs are generally associated with sealing operations [6, 7]. In a sealing operation, sealant is placed either flush with the surface or slightly recessed within a cut reservoir. The purpose of the reservoir is to create room for enough material to be applied, create a desirable sealant shape, and provide a uniform surface for the sealant to adhere to. The sealant also may be recessed to prevent plow and traffic damage [6, 7]. For sealing working cracks, the preferred sealant is usually elastomeric. This means the sealant has a low modulus of elasticity and will stretch easily and to high elongations (usually around 10 times its non-strained dimensions) without fracture. Such sealants also recover over time to close to their original dimensions [8, 9]. For short-term crack-seal performance (between 1 and 3 years) in pavements with ordinary working cracks 0.1- 0.2 inch (2.5-5 mm) of horizontal crack movement) and moderate traffic levels, a standard rubberized

asphalt should be placed in a simple Band-Aid configuration [10, 11, 12, 13]. For medium-term crack-seal performance (between 3 and 5 years) under the above conditions, either a standard rubberized asphalt may be placed in a recessed Band-Aid configuration or a modified rubberized asphalt may be placed in a simple Band-Aid configuration [10, 11, 12, 13]. For long-term crack-seal performance (between 5 and 8 years) under the above conditions, a modified rubberized asphalt sealant should be installed in either a standard or shallow recessed Band-Aid configuration [10, 11, 12, 13].

As these definitions indicate, the objectives of crack sealing are significantly more difficult to accomplish than those of crack filling. Sealing requires considerably more forethought, greater costs, and the use of more specially formulated materials and more sophisticated equipment [15].

Determining whether to seal or fill

Frequently, the first cracks to appear in an asphalt pavement are transverse cracks. However, several different types of cracks may appear at one time. In these cases, one treatment, using a material appropriate for the most demanding crack type, is desirable. Though crack width may be a factor in determining whether to seal or fill, the amount of annual horizontal movement of the targeted crack type should be the principal basis for this decision. Normally, working cracks with limited edge deterioration should be sealed, whereas non-working cracks with moderate to no edge deterioration should be filled. Whether a crack is working or non-working can generally be determined by its type. Working cracks are usually transverse in orientation with an annual horizontal movement that is equal to or more than 0.11 inch (3 mm); however, some longitudinal and diagonal cracks may meet the 0.11 inch (3 mm) movement criteria. Materials placed in working cracks must adhere to the crack sidewalls and flex as the crack opens and closes. Non-working cracks typically include diagonal cracks, most longitudinal cracks, and some block cracks. Because of the relatively close spacing or free edges between non-working cracks, little movement occurs. Minimal movement permits the use of less expensive, less specialized crack-filler materials. Table 3.1 provides recommended criteria for determining which cracks to seal and which to fill [15].

Table 3. 1 Recommended criteria for determining whether to seal or fill

When to seal and when to fill

Crack sealing is a preventive maintenance activity. Ideally, it is conducted shortly after working cracks have developed to an adequate extent and when temperatures are moderately cool (7 to 18°C), such as in the spring or fall. When newly developed cracks are sealed, the deteriorated crack segments (i.e., secondary cracks, spalls) that adversely affect seal performance are minimized. Typically, transverse thermal cracks in AC flexible pavements appear 2 to 7 years after construction; whereas transverse reflection cracks in AC overlaid concrete pavements often develop 1 to 3 years after resurfacing [15].

Sealing in moderately cool temperatures is beneficial from two standpoints. First, cracks are partly opened so that a sufficient amount of material can be placed in the crack if cutting is not to be performed. Second, the width of the crack channel, whether cut or uncut, is nearly at the middle of its working range. This is important to the performance of the sealant material because it will not have to undergo excessive extension or contraction. Most crack filling operations can be conducted year-round; however, they often take place during cool or moderately cool weather (2 to 13°C) [15].

Crack filling operations can be preventive or routine in nature, depending on the highway agency's approach to treating the cracks. Like sealing operations, preventive crack filling maintenance should be conducted shortly after non-working cracks have adequately developed. Depending on the type of cracks to be filled, this may occur between 4 and 8 years after construction or resurfacing. Durable filler materials should be used to reduce the number of repeat applications. By filling cracks shortly after they are fully developed, further crack growth is delayed. Historically, most crack filling has been performed on a routine basis with inappropriate materials that provide less than desirable performance. This approach to crack filling is rarely cost-effective because treatment performance is generally poor and maintenance costs are high. In addition, the safety of the workers and traveling public is compromised, since the filling operation must be repeated frequently [15].

3.2 Planning and Design

a. Primary Considerations

The following factors should be addressed when planning crack sealing or crack filling operations [15]:

Climatic conditions (general conditions and at the time of installation)

Highway classification

Traffic level and percent trucks

Crack characteristics and density

Materials

Material placement configurations

Procedures and equipment

Safety

The choice of an appropriate material, placement configuration, and determination of procedures and equipment to be used, based on existing and future roadway conditions, are mainly the core of the planning process. The choice of given procedures or materials to be used can be influenced by the site-specific climatic conditions during treatment operations. For example, the use of a heat lance may expedite operations in areas where moisture or cold temperatures present scheduling problems. In deciding which materials and procedures are to be

used, the overall climatic conditions must also be considered. Materials that will not significantly soften and track at high temperatures is a requirement for hot climates, while very cold climates will generally require materials that retain good flexibility at low temperatures [15].

b. Selecting a Sealant or Filler Material

There are many crack treatment material products available, each with distinct characteristics. The products essentially comprise three material families and are often grouped by material type, and according to their composition and manufacturing process. The principal material families and types are cold pour sealants, hot pour sealant, and chemically cured processes [15].

Cold pour sealants

Cold pour sealants are those that are applied at ambient temperatures and therefore do not require heating. This type of material is more appropriate for cracks of 3/16 inch or less in width. Cold pour sealant should not be applied if the air temperature is below 50°F and falling. This temperature should be based on a reading taken in the shade and away from any form of artificial heat. Due to the low viscosity of cold pour sealant, the material will penetrate into the crack easily without any need for routing procedures. It is critical that the vertical surface of the crack be clean to insure that the cold pour will adhere to the crack. The sealant should be applied using a barrel pump or pressurizing system to provide an uninterrupted flow of cold pour sealant through the hose to the wand. Depending on the humidity and temperature, curing time can vary from 30 minutes to several hours [28]. Examples for those types of sealants are the cutbacks, emulsified asphalts, and polymer modified liquid asphalts.

Cutback Asphalt

Cutback asphalts are liquid asphalts which are manufactured by adding (cutting back) petroleum solvents (also called cutter stock or diluents) to asphalt cements. They are made to reduce the asphalt viscosity for lower application temperatures. Application to aggregate or pavement causes the solvent to escape by evaporation, thus leaving the asphalt cement residue on the surface. Based on the relative rate of evaporation, cutback asphalts are classified into three types: Rapid Cure (RC), Medium Cure (MC) and Slow Cure (SC). The type of distillate (solvent) used in their production determines the grade of the cutback asphalt. Rapid Cure grades are typically blended with light, highly volatile diluents, such as naphtha, that will evaporate quickly and leave a hard, viscous-base asphalt to function with the aggregate on the road. Medium Cure grades are using a less volatile kerosene-type of solvent which evaporates more slowly, leaving a base asphalt of medium hardness or viscosity. Slow-Curing blends contain a low-volatility fuel-oil type solvent and require the longest curing period [73].

MC-3000

MC-3000 is medium cure cutback asphalt product that is used in bituminous seal coat [73]. Cutback asphalts such as MC-3000 flow more readily than emulsified asphalts and are more likely to penetrate surface cracks. If the distributor does not apply the asphalt uniformly to the roadway surface, the cutback asphalt will flow together better than a high float emulsion will [74]. 35

Asphalt Emulsions

Asphalt emulsions are formed by the milling of raw asphalt into microscopic particles, which are dispersed in water with the aid of a chemical emulsifying agent called a “surfactant” (sometimes referred to as “soap”). In such cases, the dispersed asphalt forms discrete droplets, which are intrinsically insoluble in water. The emulsion is said to be “stabilized” if the asphalt droplets remain well-dispersed such that phase separation does not occur. Stabilization is achieved through the use of surfactants, which consist of polar molecules comprised of a hydrophilic (water loving) “head” and hydrophobic (water avoiding) “tail.” The tail of the surfactant molecule is attracted to the asphalt particles, forming a coating around each particle, consisting of the hydrophilic heads of the emulsifying agent. The hydrophilic portions of these surfactants strongly associate with water and aid in keeping the droplets dispersed and in suspension [22]. The primary purpose of emulsions is to coat the edges and partially fill cracks, but they can be used as crack filler. Emulsion are safe and easy to use, but are limited to use in warmer seasons [3].

Polymer modified liquid asphalts

These types of materials are composed of polymer modified asphalts that are emulsified. The physical and chemical characteristics of the polymer and its compatibility with the chemistry of the asphalt determine the physical property enhancements [22].

Hot pour sealants

Hot pour sealants are sealants that must be heated to high temperatures in preparation for application. As the material cools, the hot thermoplastics harden. These types of materials generally consist of asphalt cement with or without the addition of a modifier. The simplest and most common type of modifier added to asphalt cement is rubber. Modifiers give the asphalt desirable properties, such as high elasticity and high melting point. Unlike cold pour sealants, hot pour materials should not be applied when the cracks and pavement surface are damp. The hot pour sealant is heated in a double-jacketed heater using heat transfer oil so that no direct flame comes in contact with the shell of the vessel containing the sealant. To ensure that the sealant is circulated during the heating process to achieve a uniform rise in temperature and to maintain the desired temperature, the heated reservoir should be equipped with an agitator. Temperature should be monitored through accurate temperature gauges to avoid overheating the material. Ideally, the material should be maintained between 350 and 375°F. The placement of hot pour sealant can begin after the application temperature is attained. If bubbling occurs, moisture still exists in the crack and work must be postponed until the cracks are dry. In most cases, the hot pour sealant will cure in about 15 to 30 minutes [28]. Some research has shown that “high-end” hot pours can last more than six years [43]. Most new products are prepackaged. This eliminates mistakes in mixing, assures a uniform product, and ensures a more efficient operation. Not all products are suitable for every climate, as locations with extensive freeze-thaw cycles need sealants with more ductility, whereas warmer areas need sealants with less flow in hot weather [75]. The major categories of hot pour sealant types are the asphalt cement, fiberized asphalt, asphalt rubber, polymer-modified asphalts, and low modulus rubberized asphalt.

Asphalt Cement

Among the thermoplastic bituminous materials, asphalt cements are characterized by little, if any, flexibility and are very temperature-susceptible. Hence, they are limited to use as 36

fillers for non-working cracks. Applicable specifications for asphalt cements are ASTM D3381, AASHTO M20 and, AASHTO M226. Asphalt cements are characterized by their quickness and ease of application, short cure time, and adhesiveness. Asphalt cement materials that are placed flush in un-routed non-working cracks can perform satisfactorily for between 2 and 4 years [15].

Fiberized Asphalt

Fiberized asphalts are most appropriate in crack filling operations since fiber particles provide minimal elasticity to asphalt and do not significantly affect temperature susceptibility. Typical to asphalt cements, fiberized asphalts are characterized by their quick and ease of application, short cure time and adhesiveness. Fiberized asphalt placed in unrouted working cracks with overband configurations can provide a maximum of 2 years of adequate service [15].

Asphalt Rubber

Asphalt rubber crack sealants are characterized by their easiness of application, short cure time, adhesiveness, cohesiveness, flexibility, and elasticity. They also show high resistance to softening, flow, and abrasion in their cured state. One applicable specification for asphalt rubber crack sealant is ASTM D5078 [15].

Low-Modulus Rubberized Asphalt

The use of low-modulus rubberized asphalt crack sealant has shown to be very effective in 3/4 inch by 3/4 inch reservoirs with less overbanding [3]. Low-modulus rubberized asphalt crack sealants are characterized by their easiness of application, short cure time, adhesiveness, and cohesiveness. They also show resistance to aging, weathering, and abrasion, as well as excellent resistance to softening and flow in their cured state. [15]. One example of a low modulus rubberized asphalt product is Elastoflex 52.

Elastoflex 52

Elastoflex 52 is formulated with a high recycled rubber content yielding a high viscosity to resist excess flow during application, and is ideal for highways and county roads. Elastoflex 52 applies and sets best in high to moderate temperatures with a recommended application temperature range of 380°F (193°C) to 400°F (204°C). Elastoflex 52 is quick melting, extremely flexible, and highly durable in cold to hot climates [76].

Polymer-Modified Asphalts

These types of materials are generally used in crack sealing applications; as crack sealing requires high performance materials. Since crack filling requires little crack preparation and leaves rough edges, these sealants do not adhere and perform well in filling applications [3]. The addition of polymer to heated asphalt generally improves field performance because it imparts flexibility to the asphalt. The degree of flexibility basically depends on the type and nature of the asphalt, the percentage of polymer used, and how the polymer is incorporated into the asphalt (i.e., mixed or melted in) [15]. An example of polymer-modified asphalt products is Elastoflex 71. 37

Elastoflex 71

Elastoflex 71, is a hot applied, polymer modified asphalt crack sealant that is highly durable in very cold to hot climates. Elastoflex 71 is self-leveling, quick-melting, and extremely flexible. Formulated with a low viscosity for maximum crack penetration, it is ideal for highways, county roads, airports, and concrete joints [76].

Chemically cured materials

Chemically cured thermosetting materials are one or two component materials that cure by chemical reaction from a liquid state to a solid state. These types of materials have been used in AC pavements only in recent years [15]. An example of chemically cured materials is self-leveling silicon.

Self-leveling Silicone

Self-leveling silicone is a one-component, cold-applied sealant that requires no tooling. ASTM D5893 can be utilized for the application of such crack sealant material. It is generally used for crack sealing practices other than crack filling. Self-leveling silicon is characterized by its short preparation time, short cure time, and good adhesiveness and cohesiveness. It exhibits excellent flexibility and elasticity, and also poses excellent resistance to softening, flow, aging, and weathering in the cured

state [15].

c. Laboratory Testing

It is highly recommended to make laboratory testing on the selected sealant or filler material to verify that the obtained material exhibits the properties for which it was selected. Material sampling is the first process in laboratory testing. A minimum sample of 2 to 4 kg should be taken from each batch, or lot, of material shipped. An agency-approved testing laboratory can then test these samples to standards or specifications prior to placement [15].

d. Selecting a Placement Configuration

The placement of sealant and filler materials in cracks can be made through numerous configurations. These placement configurations are grouped into four categories [15]:

· Flush fill: Material is dispensed into the existing, uncut crack and excess material is struck off in the flush fill configuration [15].

· Reservoir: Material is placed only within the confines of a cut crack (crack reservoir) in a - reservoir configuration, and the material is placed either flush with or slightly below the pavement surface [15].

· Overband: The material is placed into and over an uncut crack in an overband configuration. The simple Band-Aid configuration is formed if the material over the crack is shaped into a band using a squeegee. The capped configuration is created if the material over the crack is left unshaped. All configurations are based on four controlling variables [15]:

· Type of application

Direct–Material applied directly to crack channel

Bond-Breaker–Backer material placed at bottom of crack reservoir prior to material

installation in order to prevent three-sided adhesion (i.e., bonding by material to crack reservoir bottom and sidewalls)

· Type of crack channel

Uncut

Cut–Router or saw used to create uniform crack reservoir

· Strike-off or finishing characteristics

Recessed

Flush

Capped

Band-Aid

· Dimensions of crack reservoir and overband

Combination (reservoir and overband): The overband configuration is applied to a cut or routed crack reservoir.

e. Selection Procedures and Equipment

Depending on the type of treatment (sealing or filling), treatment policy, and available equipment, crack treatment consists of between two and five steps [15]. These steps are:

· Crack cutting (i.e., routing or sawing)

· Crack cleaning and drying

· Material preparation and application

· Material finishing/shaping

· Blotting

Crack cutting

Routers or saws are used to perform crack cutting. Crack cutting is often the slowest activity in sealing operations because it can inflict additional damage on the pavement if not performed properly [15].

Crack cleaning and drying

This is utilized to provide a clean, dry crack channel that is free of loosened AC fragments and other debris, in which the crack treatment material and any accessory materials can be placed. Because high percentage of treatment failures are adhesion failures that result from dirty or moist crack channels, crack cleaning and drying is perhaps the most important aspect of sealing and filling operations [15].

Material preparation and application

This involves the preparation of the crack sealing/filling materials, as well as the specific application needs that may differ for different materials [15]. Material preparation may include mixing, heating, and specific equipment or attachments required to provide desired material properties during application. Application considerations may include the specified equipment, attachments, and tools needed to properly apply the material, as well as weather restrictions at the time of application.

Material finishing

Material finishing can be accomplished in two ways. First, various sizes of dish-shaped attachments are available that can be connected to the end of the application wand for one-step application and finishing. Second, industrial rubber squeegees can be used behind the material applicator to provide the desired shape [15]. 39

Material blotting

Blotting is the application of material, including sand, dust, toilet paper, or other materials, to a freshly sealed crack to prevent the occurrence of tracking or pulling from traffic loading. Sand will generally require a truck or trailer storage, along with shovels for spreading and other tools for clearing excess sand from the roadway. Toilet paper can often be loaded on the same truck with the prepackaged sealant blocks, and can then be placed on a modified paint roller (equipped with a long handle) for easy application [15].

f. Estimating Material Requirements

Reliable estimates of material needed for a particular project is very useful in attempting to use the optimal material in each situation [15].

3.3 Evaluating Crack Treatment Performance

In order to chart the rate of failure and plan for subsequent maintenance, at least one inspection should be made each year. The treatment effectiveness during a time of near maximum pavement contraction and near maximum crack opening can be evaluated through a mid-winter evaluation. Items signifying treatment failures include the following [15]:

· Full-depth adhesion loss

· Full-depth cohesion loss

· Complete pull-out of material

· Spalls or secondary cracks extending below treatment material to crack

· Pothole

3.4 Summary of Components of Best Management Practices of Crack Maintenance

The Best Management Practices (BMPs) can be identified as effective and practical maintenance methods which can prevent or reduce the deterioration of asphalt pavement as a result of existence of thermal cracking. The first aspect of BMP is associated with the determination of the need for crack treatment. This is followed by the planning and designing of the crack treatment project as well as the construction associated with the crack treatment project. Finally the evaluation and assessment of the performance of the crack treatment should be addressed.

The determination of the need for crack treatment involves undergoing a pavement/crack evaluation to determine the pavement age, geometric design, and section boundaries. This also involves the evaluation of the traffic and climate. In addition, the type and extent of previous maintenance treatments and current condition rating of the pavement should be considered. The planning and design, on the other hand, involves the choice of an appropriate material, placement configuration, and determination of procedures and equipment to be used, based on existing and future roadway conditions. Cost effectiveness is also a crucial part of the planning and design procedures. In the construction aspect of BMP, once the most appropriate material and placement procedure are selected, proper field application must be fulfilled. This mostly involves traffic as well as safety aspects. Figure 3.1, illustrated below, summarizes the components of a BMP.

Figure 3. 1 Components of Best Management Practices