TOP DOWN CRACKING IN BITUMINOUS PAVEMENT by MD.IMTHIYAZ

TOP- DOWN CRACKING IN FLEXIBLE PAVEMENT

VISVESVARAYA TECHNOLOGICAL UNIVERSITY

BELGUAM-590014

A SEMINAR REPORT ON

TOP DOWN CRACKING

SUBMITTED BY

MOHAMMED IMTHIYAZ M.A

UNDER THE GUIDANCE OF

Mr. B.V. Kiran Kumar

ASST.PROFESSOR

DAYANANDA SAGAR COLLEGE OF ENGINEERING

DEPARTMENT OF CIVIL ENGINEERING

DEPARTMENT OF CIVIL ENGINEERING, D.S.C.E 1


(HIGHWAY TECHNOLOGY)

SHAVIGE MALLESHWARA HILLS, KUMARASWAMY LAYOUT

BANGALORE-560078

DAYANANDA SAGAR COLLEGE OF ENGINEERING

SHAVIGE MALLESHWARA HILLS, KUMARSWAMY LAYOUT

BANGALORE-560078

DEPARTMENT OF CIVIL ENGINEERING

CERTIFICATE

This is to certify that seminar work entitled “TOP DOWN CRACKING” was presented by

MOHAMMED IMTHIYAZ M.A, bearing USN 1DS08CHT01 student of 2nd semester, M.Tech

Highway Technology, Department of Civil Engineering, in the partial fulfillment for the award of

M.Tech in Highway Technology under the Visvesvaraya Technological University (VTU),

Belgaum, during the year 2008-09. The report is approved as it satisfies the academic

requirements in respect of SEMINAR WORK prescribed for the Post Graduation degree.

Seminar Guide Head of Department

Mr. B.V. Kiran Kumar Dr. B.S Thandaveswara

Asst. Professor Professor and Head of Department

Department of Civil Engineering Department of Civil Engineering

DSCE, Bangalore-560 078 DSCE, Bangalore-560 078



Examiners:

Chapter 1

INTRODUCTION

1.1 General

Top down cracks (TDC) in pavements initiate at the pavement surface and propagate

downward. Top down cracking appears to be a common mode of Flexible pavement

distress in at least several states and countries. Traditionally, pavement cracking is

thought to initiate at the bottom of the pavement layer where the tensile bending

stresses are the greatest and then progress up to the surface (a bottom-up crack).

Most traditional transfer functions used in mechanistic-empirical structural design are

based on this concept . However, the late 1990s saw a substantial focus on a second

mode of crack initiation and propagation, top-down cracking.

Although not fully understood at this time, there are three basic views on the of top-

down cracking mechanism

High surface horizontal tensile stresses due to truck tyres (wide-based tyres and

high inflation pressures are cited as causing the highest tensile stresses).

Age hardening of the bitumen binder resulting in high thermal stresses in the

bituminous surface (most likely a cause of the observed transverse cracks).

A low stiffness upper layer caused by high surface temperatures.

Likely, the mechanism is some combination of the above. The pavement top-down

cracking is not thoroughly understood and, at this time, is generally not considered as a

causative factor for pavement cracking although it probably should be. Further, for two

states that recently studied cracking origins (Florida and Washington State), both

reported that top-down cracking is far more common than assumed. In fact, the Florida

DOT reports that top-down cracking is dominant for their flexible pavements due for



rehabilitation. Currently, the National Cooperative Highway Research Program

(NCHRP) is addressing the issue with Identification of the Design Conditions and

Critical Factors That Are Related to the Top Down Cracking of flexible pavements.

Two simple suggestions may help in the identification of top-down cracking. First, in

thick bituminous pavements, consider top-down cracking as a possible cracking

mechanism. Generally, previous research has found that in pavements thicker than

about 160 mm (6.3 inches) top-down cracks can be and often are the dominant form of

cracking. We cannot assume pavement cracks are bottom-up. Second, before deciding

on a maintenance or rehabilitation strategy, take a pavement core on a suspect crack.

Usually, a pavement core will show whether a crack is top-down or bottom-up. It will

also show the extent to which the crack has propagated, thus defining the extent of

needed milling prior to overlay.

Top down cracking has become an bitumen surface course distress of growing concern

that must also be dealt with during the design, construction, maintenance, and

resurfacing of long-life bitumen pavements. The surface course is designed for heavy

vehicle loadings and general traffic conditions in terms of rutting resistance, durability,

noise levels, smoothness, and frictional characteristics. The surface course must be

properly maintained and should be renewable on an 18 to 22 year cycle. A pavement

management and maintenance system is very important to achieving this objective. It is

very important that top- up cracking, which is a rather complex surface distress mode

related to tensile and shear stresses associated with non-uniform tyre stresses,

interlayer slippage, thermal stresses, stiffness gradients, construction problems such as

segregation, and premature bitumen binder age hardening, is mitigated in order to

achieve satisfactory overall pavement performance.

Pavement maintenance is the key to pavement preservation. Which includes all the

methods and techniques used to retire and reinstate or maintain a specified level of

service as well as to prolong pavement life by slowing its detorietion rate.

Generally neglecting or delaying the road maintenance activities may increase the

overall cost of repair as well as increase in vehicle operating costs for road users. For a

proper perspective of maintenance problems, it is useful to review the link of activities



leading from the design stage through the construction stage before maintenance takes

over.

Right from the very beginning, the structural design of flexible pavement is facing with

uncertainties such as traffic prediction and assumptions of pavement layer strength in

the design. During construction, quality of road will also depend on work site and

supervisory staff. Inclement weather also affects quality control by increasing chances

of pavement layer contamination, which requires special attention by the supervisors.

Finally after the road construction, both environmental and traffic stress will contribute to

possible of the road to deteriorate. The rates of deterioration will much depend on the

severity of traffic loads and variability of the road materials as well as environment

effects.

To ensure the smooth operation the road pavement has to be constantly maintained

and upgraded.

.



Figure 1.1 Comparison of Top-Down Cracking at the Surface of Bitumen Concrete Pavements With Cracks in Drying Mud and Cracks Associated With Flexible

Pavement Base Failure.

1.2 Aim and objective

The aim of this study is to assess the overall flexible pavement maintenance activities.

The study is carried out for following objective:

1. To Study the properties and characteristics that most strongly influence surface

cracking performance.

2. To study pavement maintenance activities and rehabilitation works carried out in

the flexible pavement.

3. To study the design specification for bitumen mixtures that would mitigate

surface cracking in pavements.

1.3 TOP DOWN CRACKING

The bitumen concrete surface course of long-life pavement is a wearing surface that is

custom designed for specific heavy vehicle loadings and general traffic operating

conditions (rutting resistance, durability, noise levels, smoothness and frictional

characteristics, for instance). This bitumen concrete surface must also be renewable

(systematic maintenance with appropriate periodic resurfacings or recyclings) on about

an 18 to 22 year cycle. It is imperative that surface distresses, such as top down

cracking, do not require more frequent resurfacings and, most importantly, that any top

down cracking does not extend below the surface course and impair the overall

structural integrity of the pavement. Top down cracking does not significantly affect the

structural capacity of the bitumen pavement during its early stages of mainly longitudinal

surface cracking. However, with time, secondary multiple, interconnecting cracks,

moisture damage and raveling accelerate the surface distresses (potholing for instance)

and impact severely on the functional serviceability of the pavement. Unfortunately, this



can occur rather quickly, particularly with poorly constructed surface courses (materials,

mix designs and practices) subjected to overloaded trucks. Eventually, the top down

cracking and associated distresses, if not mitigated, will impair the structural integrity of

the long-life bitumen pavement.

Figure 1.2 Typical Severe Transverse Thermal Cracking and Top-Down Cracking. The TDC and Associated Distresses are Most Severe in the Outer Wheel Path.



CHAPTER 2

TYPES AND CAUSES OF TOP DOWN CRACKING

2.1 Types of distress

A variety of structural distress is considered in flexible pavement Design and analysis.

These include:

Bottom up fatigue (or alligator) cracking

Surface down fatigue or longitudinal cracking

Fatigue in chemically stabilized layers (only considered in semi rigid pavement)

Permanent deformation or rutting

Thermal cracking

Rutting distress is predicted in absolute terms. therefore the incremental distress

computed for each analysis period can be directly accumulated over the entyre target

design life for the pavement.

2.1.1 Cracking

Cracking distress (Bottom up/surface down fatigue cracking, thermal cracking) is

predicted in terms of a damage index, which is a mechanistic parameter representing

the load associated damage within the pavement structure. When damage is very small

(eg.0.0001) the pavement structure would not be expected to exhibit significant

cracking. As computed damage increases, visible cracking can be expected to develop

in few locations along the pavement surface.

2.1.2 Bottom up fatigue (or alligator) cracking



This type of fatigue cracking first shows as short longitudinal cracks in the wheel path

that quickly spread and become interconnected to form a chicken wire/alligator cracking

pattern. These cracks initiate at the bottom of the bituminous layer and propagate to the

surface under repeated load applications.

This type of fatigue cracking is a result of repeated bending of bituminous layer under

traffic. Basically, the pavement and bituminous layer deflects under wheel loads that

results in tensile strains and stresses at the bottom of the layers. With continued

bending, the tensile stresses and strains cause cracks to initiate at the bottom of the

layer and then propagate to the surface .This mechanism is illustrated in figure below.

The following briefly lists some of the reason for the higher tensile strains and stresses

to occur at the bottom of the bituminous layer

Relatively thin and weak bituminous layers for the magnitude and repetitions of

the wheel load.

Higher wheel loads and higher tyre pressures

Soft spots or areas in unbound aggregates base materials or in the subgrade

soil.

Weak aggregate base/Sub base layers caused by inadequate compaction or

increases in moisture contents and or extremely high ground water table.

Figure2.1 Bottom up Fatigue cracking



Figure 2.2 Line diagram of Fatigue Cracking



Figure 2.3 Close up view of Fatigue cracking in pavement surface

2.1.3 Surface – down fatigue cracking or longitudinal cracking

Most fatigue crack initiated at the bottom of bituminous layer and propagates upward to

the surface of the pavement. However, there is increasing evidence that suggest load

related cracks do initiate at the surface and propagate downwards. There are various

opinions on the mechanisms that cause these types of cracks, but there are no

conclusive data to suggest that one is more applicable than other. Some of the

suggested mechanisms are

Wheel load induced tensile stresses and strains and strains that occur at the

surface and cause cracks to initiate and propagate in tension. Aging of the

bituminous surface mixture accelerates this crack initiation-propagation process.

Shearing of the bituminous surface mixture caused from radial tyres with high

contact pressures near the edge of the tyre. This leads to cracks to initiate and

propagate both in shear and tension.

Severe aging of the bituminous mixture near the surfacing resulting in high

stiffness and when combined with high contact pressures, adjacent to the tyre

loads cause the cracks to initiate and propagate.

The downward fatigue cracking mechanism is illustrated in figure shown below.



.Figure 2.4 Top down fatigue cracking

Figure2.5 Top down fatigue cracking

Figure 2.6 Top down Longitudinal Cracks in Pavement Surface

2.1.4 Permanent deformation or rutting

Rutting is a surface depression in the wheel path caused by inelastic or plastic

deformation in any or all of the pavement layer and subgrade. Pavement uplift



(shearing) may occur along the sides of the rut. Ruts are particularly evident after a rain

when they are filled with water. There are two basic types of rutting: mix rutting or

instability rutting and subgrade rutting or consolidation rutting. Mix rutting occurs when

the subgrade does not rut yet the pavement surface exhibits wheel path depressions as

a result of compaction/mix design problems or it can also be defined as Failure is

attributed strictly to the bitumen mixture properties and usually occurs within the top 2

inches of the bitumen concrete layer. Subgrade rutting occurs when the subgrade

exhibits wheelpath depressions due to loading. In this case, the pavement settles into

the subgrade ruts causing surface depressions in the wheelpath. Or it can also be

defined as The result of excessive consolidation of the pavement along the wheel path

due to either reduction of the air voids in the bitumen concrete layer, or the permanent

deformation of the base or subgrade.

The possible problems due to rutting can be the Ruts filled with water can cause vehicle

hydroplaning, can be hazardous because ruts tend to pull a vehicle towards the rut path

as it is steered across the rut.

Possible causes: Permanent deformation in any of a pavement's layers or subgrade

usually caused by consolidation or lateral movement of the materials due to traffic

loading. Specific causes of rutting can be:

Insufficient compaction of bituminous layers during construction. If it is not

compacted enough initially, bituminous pavement may continue to densify under

traffic loads.

Subgrade rutting (e.g., as a result of inadequate pavement structure)

Improper mix design or manufacture (e.g., excessively high bitumen content,

excessive mineral filler, insufficient amount of angular aggregate particles)

Ruts caused by studded tyre wear present the same problem as the ruts described

here, but they are actually a result of mechanical dislodging due to wear and not

pavement deformation.


http://training.ce.washington.edu/WSDOT/Modules/09_pavement_evaluation/studded_tires.htm


Figure 2.7 Consolidated rutting

Figure 2.8 Consolidated rutting in pavement surface

Figure 2.9 Consolidated rutting in pavement surface



Figure 2.10 Measure of consolidated rutting in Pavement

Figure 2.11 Instability rutting

2.1.5 Thermal Cracking

Cracking in flexible pavements due to cold temperature or temperature cyclic is

commonly referred to as thermal cracks. Thermal cracks typically appear as transverse

cracks on the pavement surface roughly perpendicular to the pavement centerline.

These cracks can be caused by the shrinkage of the bituminous surface due to low

temperature, hardening of the bitumen and daily temperature cycles.



Cracks that result from the coldest in temperature are referred to as low temperature

cracking. Cracking that result from thermal cycling is generally referred to as thermal

fatigue cracking. low temperature cracking as associated with regions of extreme cold

whereas thermal fatigue cracking is associated with regions that experience large

extremes in daily and seasonal temperatures.

There are 2 types of non load related thermal cracks: Transverse cracking and block

cracking. Tranverse cracking usually occur first and are followed by occurrence of block

cracking as the bitumen ages and becomes more brittle with time. Tranverse cracking is

the type that is predicted by models. While block cracking is handled by material and

construction variables.

Figure 2.12 Thermal Cracks or Block Cracks on Pavement



Figure 2.13 line diagram of Thermal Cracks

2.2 CAUSES OF TOP-DOWN CRACKING

Mechanistic pavement design has historically relied upon engineering assumptions that

include the use of a wheel load modeled by a uniformly loaded contact patch (or

multiple patches) and a single modulus value assigned to a bituminous layer in

pavement. These assumptions are considered reasonable when determining stresses

and strains at the underside of the pavement layers away from the loading points.

Therefore, the current mechanistic-empirical pavement methods are based on the

tensile strains at the bottom of bitumen layers to prevent bottom-up fatigue cracking and

compressive strains at the top of the subgrade to prevent subgrade rutting. However,

when trying to determine the pavement response close to the wheel loads this type of

analysis will be incapable of capturing the effects of temperature depth gradients within

the pavement structure and the effect of complex tyre-pavement interactions. The

analysis of these last two aspects is considered to be a key component to the

understanding of the surface cracking phenomena similar to the surface rutting

mechanisms in the bitumen pavements.

However, there are different views among researchers whether the surface cracking

phenomena is caused only by the pavement surface stresses, or whether the pavement



structure plays some role in the top-down cracking formation. Nevertheless, items that

have been associated to the surface cracking phenomena include

1) Pavement tyre loading such as load magnitude and tyre type effects

2) Pavement temperature and temperature gradients

3) Bitumen binder and mix aging

4) Pavement structure

5) Mix properties and raw materials used

6) Issues related to the construction such as segregation of mix.

2.2.1 Top-down Cracking Phenomenon

Top-down cracking in bitumen pavements initiates from the top and propagate

downwards through the bitumen concrete layer over time. Svasidisant, Schorsch and

Baladi have defined three categorizers for the top-down cracking. In the first stage

single short longitudinal cracks appear just outside the wheel path in the pavement

surface. Over time the cracking reaches a second stage where the short longitudinal

cracks grow longer and sister cracks develop parallel to and within 0.3 to 1.0 meters

from the original cracks. At the third stage the parallel longitudinal cracks are connected

via short transverse cracks.

Also, Myers, Roque and Ruth (1997) reported the location of surface cracks being just

outside the wheel path and the cracks penetrate to depths ranging from just under

pavement surface to the entire depth of bitumen layer.

The Federal Highway Administration (FHWA) Accelerated Loading Facility (ALF) study

(Stuart, Mogawer & Romero, 2000) for bottom-up fatigue cracking showed that the

transverse bottom-up cracking started in the wheel path area. Longitudinal top-down

cracks occurred at the outer edges of the wheel paths where the surface of the

pavement has a high curvature. Also fatigue cracks were smaller at 28°C than at 19 and

10°C, indicating how crack propagation changes with temperature.

The time interval for the cracks to appear seems to be very versatile ranging from one

year to five years. The study by Svasidisant et al. (2002) shows that surface cracks had



propagated through all bitumen layers in a 15 year old pavement with rubblized base. In

pavements with the same base structure but only 9 to 10 years old, surface cracks had

propagated 100% through the surface layers but only about 50% and 20% through the

intermediate and base layers, respectively.

2.2.2 Pavement Loading and Tyre Effects

Rutting in bitumen pavement includes densification and shear flow of hot-mix bitumen,

but the majority of severe instable rutting results from shear flow within the bitumen

mixtures. Top-Down Cracking (TDC), which is usually found in longitudinal path, is also

considered as a shear-related failure. As a result, shear stress is believed to be one of

the critical factors affecting pavements performance, and it is necessary to well

understand shear stress in bitumen pavements. One possible cause of the difficulty in

explaining these distresses is that the effective contact stresses between the tyre and

the road/pavement surface are not known and are not used effectively in design and

analysis procedures. However, traditional methods of pavement analysis assumed that

contact pressure is the same to tyre inflation pressure and that it is uniformly distributed

over a circular contact area and acts in the vertical direction. In fact, it has been

recognized from some research that the tyre-pavement contact area is not circular and

that contact stress is neither uniform nor equal to tyre inflation pressure. One of the

main factors influencing the contact stress is the type of tyre and its associated inflation

pressure and load.

For the analysis of surface cracking, it is believed that lateral stresses initiate cracking

at the pavement surface which somehow propagates downwards. These cracks are

neither of the traditional fatigue nor reflective nature. Hugo and Kennedy attributed

cracks to the presence of horizontal shear stresses induced on the pavement surface.

Analytical work by Kunst (1996) illustrated how inward radial horizontal stresses could

lead to tension at the edges of circular load.

Jacobs described the occurrence of maximum tensile stresses at the surface of the

pavement through analytical evaluation and predicted tensile stresses at the edge of a



truck tyre on the pavement surface, which were sufficient to cause fracture. The tensile

stresses were found to dissipate rapidly with increasing depth; i.e., they existed in the

top 10 mm of the bitumen layer. Tensile stresses were generated at the edge of tyre

load because measurements were obtained from a bias ply truck tyre.

Myers stated that longitudinal surface cracking appears to be initiated by significant

tensile stresses (Mode I tensile failure) that are induced under radial truck tyres.

Thermal stresses contribute to the initiation mechanism as a secondary factor.

Research stated that cracks advance only in critical conditions. The mechanism of crack

development is highly dependent of load spectra (magnitude and position) and

differential pavement temperature gradients and pavement structure. Tensile stresses

were found to be more significant in thicker and stiffer bitumen concrete pavements.

Therefore the mill and fill rehabilitation technique may be more suitable to prevent

surface cracking than overlay. However, use of a linear elastic layer analysis did not

allow for analysis of crack growth or discontinuities in the pavement.

Myers(2000) also explained that the tyre structure has significant influence on contact

stresses. The stress state induced by radial or wide base radial tyres was determined to

be potentially more detrimental to pavement surface than the stress state induced by

bias ply tyres.

There are distinct differences in the fabrication of radial and bias-ply tyres. In biasply

tyres, the air container is made from crisscrossing layers of rubberized fabric and in

radial tyres it is formed by radially running plies of rubberized cord or steel cord on

commercial vehicle tyres.

2.2.3 Temperature Depth Gradients

Temperature effects in bituminous materials have very significant effects on the

stiffness of the bitumen layers. The pavement structure will experience a wide range of

temperatures as a function of the daily and annual variation of temperature/climate.

Climatic effects models can be used to predict the in-situ pavement temperatures.

These models have been calibrated against real pavements and can be considered

reasonably accurate.



In work conducted by Rowe, Sauber, Fee and Soliman(1999), it has been shown that by

using layered elastic analysis and a uniform distributed load it is possible to compute

significant tensile stress at the surface of the pavement adjacent to the wheel loading

when temperature depth gradients are considered. Consequently, the use of proper

temperature depth.information is also considered of prime importance as the correct

definition of tyre loading.

A paper by Svasidisant et al. (2002) reports 30°C diurnal temperature difference

between the bitumen surface and base course during daytime and 10°C temperature

difference during nighttime. These temperature differences cause differential stiffness

values in the bitumen pavement.

Schorsch et al. report that negative temperature differences which are consistent of

evening and nighttime temperatures produce the highest surface tensile stresses in the

pavement. They also recommend that to prevent the effects of nighttime temperatures,

the bitumen base course should be designed at higher stiffness than the bitumen

surface course.

Usually it is expected for bottom-up cracking that thin pavements (<150 mm) are in

strain control thus requiring softer binder and mix to prevent fatigue, and thick

pavements (>150mm) are in stress control requiring stiffer binder and mix to prevent

cracking.

However, the FHWA-ALF study (Stuart et al. 2001) concluded that mixtures were most

of the time in stress control regardless of the depth of the pavement structure and most

of the cracking happened in the intermediate 19°C temperature and not in 28° or 10°C.

Also, the model of loading changed from strain to stress for 100 mm pavement with a

change in temperature from 28 to 19°C.

2.2.4 Pavement Structure

Structural issues affecting pavement age are more controversial. The study done by

Matsuno and Nishizawa (1991) concluded that pavement cross section had little effect

on high tensile strains that developed to the pavement surface due to the soft bitumen



mix.They attributed to the top-down cracking caused by the mix properties at pavement

surface. They concluded that in one to five year old pavements high tensile strains in

hot pavement surface were causing cracking because at shadowy areas the cracking

was absent. Also, Myers et al. have concluded that the pavement structure had little to

do with the surface tensile stresses initiation, and surface cracking was caused by high

tensile stresses generated at pavement surface by the radial truck tyres. However,

Myers (2000) concludes the pavement structure affects crack propagation.

In a study by Uhlmeyer et al (2000) three to eight year old pavements which were more

than 160 mm (6.3 in) thick exhibited top-down cracking in and around the wheel paths.

They concluded that the pavement thickness has an effect on the surface cracking

initiation which contradicts the previous findings.

Svasidisant et al. (2002) studied bitumen overlays on top of rubblized concrete slabs.

They concluded that differential stiffness differences in the bitumen pavement surface

and base layers could result in significant tensile stresses at the pavement surface. The

Magnitude of the surface tensile stresses increases as:

• Ratio of bitumen surface course to the base course moduli increases

• Base layer moduli increases such as stabilized or rubblized base

• Thickness of the bitumen layer increases in pavements with conventional aggregate

base

They also found that the quality of the rubblization process has a direct impact on the

modulus of the rubblized layer which can vary from 200 to 13,000 MPa. The

mechanistic analysis results also suggest that the rubblized layer underneath the

bitumen layer may cause top-down cracking, although it reduces the rutting and bottom-

up cracking potential.

2.2.5 Aging

The aging of bitumen binder has been attributed to be the major cause of top-down

cracking in many studies such as Hugo & Kennedy (1985) in South Africa, Wambura et

al. (1990) in Kenya, and Gerritsen (1987) in Netherlands. In South Africa and Kenya,

severe age hardening occurred in two year old pavements that had high air void



content, in this case around 8%. In Kenya the severe age hardening happened in the

top few millimeters of the bitumen pavement surface. Studies in Netherlands (Gerritsen,

1987) also report severe age hardening of newly constructed pavement surface that

was not properly compacted and also had low binder content.

2.2.6 Mix Composition and Raw Materials

Mix Composition and Raw Materials Harvey and Tsai studied the effects of bitumen and

air void content on mix fatigue and stiffness. The variables in the fatigue study were:

one aggregate and bitumen source, five bitumen contents ranging from 4 to 6%, and

three air void contents ranging from 1 to 3%, 4 to 6% and 7 to 9%. The test used was

third-point controlled strain flexural beam test developed under the SHRP research

program. A 10 Hz haversine wave was used and testing was carried out at 19°C (66°F)

temperature. Two strain levels were used (300 and 150 micro-strains) with average

fatigue life of 50,000 and 500,000 repetitions, respectively. They concluded that the

results clearly indicate that the low air void content increased fatigue life and mixture

stiffness. Increased bitumen content increased fatigue life and decreased stiffness.

Micromechanics study of top-down cracking by Myers, Mohammad and Fu (2003) state

that rutting and cracking may be related and bottom-up and top-down cracking may not

be the only patterns of cracking. They predicted tensile stresses inside the pavement

below surface which is consistent to top-down cracking predicted by FEM analysis.

Cracking took place at a higher temperature where rutting is usually assumed to be

dominant.

Who concluded that surface cracking took place at higher pavement temperatures. The

WesTrack experiment (Tsai, Harvey & Monismith, 2001) indicated that fine and fine-plus

mixtures were less prone to bottom-up cracking than coarse graded mixtures. A study

by Pellinen, Christensen, Rowe, and Scharrok (2004) suggests that the mix volumetric

property that best correlated to the cracking in the WesTrack experiment was Voids

Filled with Bitumen, although the correlation was at best moderate. Mixtures that had

VFA above 53% had less cracking than mixtures with Voids Filled with Bitumen below

the average.



The other volumetrics for crack resistant mixtures were Vbeff > 9%, air void content <

6%, and Voids in mineral Aggregate (VMA) < 14%. Based on the report by the

independent WesTrack evaluation group, “Performance of Coarse Graded Mixes at

WesTrack - Premature Rutting” (FHWA Final Report, 1998), the mixture performance at

WesTrack was different than typically seen on other high truck traffic pavements.

Coarse mixtures cracked during the winter months and then rutted during the summer

months. Evaluators noted that usually pavements that exhibit fatigue cracking do not

exhibit significant plastic deformation. Also, the fatigue cracks developed first in the

transverse direction and then in the longitudinal direction. They noted that usually,

longitudinal cracks are the first sign of fatigue, followed by the transverse cracks (which

indicates top-down cracking pattern).

2.2.7 Construction Issues

The construction issues have been reported to affect the formation of surface cracks.

Surface defects can cause surface cracking based by Uhlmeyer et al.. A study by

Schorsch et al.found that surface cracks initiated from the segregate pavement areas.

They conducted field and laboratory tests to quantify the segregation using nuclear

gauge measurements to identify the air void differences in the segregated and

nonsegregated areas. Laboratory measurements included indirect tensile strength,

gradation, and binder content measurements to verify segregation. Unfortunately

loading time or test temperature was not reported for comparisons. Segregated spots

had lower tensile strength than non-segregated areas.

A poor pavement compaction has been cited as a source of surface crack initiation and

propagation in pavements in several studies discussed above. Based on the research

conducted in Africa, air void content around 8% was considered poor, while this is the

typical required in-situ air void content in the U.S. The European mix design and

construction specifications tend to require lower design and in-situ air void contents. For

instance, in Finland the required in-situ air void content of the mix (measured using dry

method) is less than 5% to prevent aging and moisture damage in the mix (PANK).



Schorsch et al. found that in segregated pavements the air void content varied between

1.8 to 12%. The average air void content of the segregated pavements was 6.1% with

standard deviation of 2.8%. The non-segregated control sections had an average of

3.8% air void with standard deviation of 2%. The highest measured air void content in

the control cores was 8.1%. This suggests that the low air void content provides better

resistance against cracking.

Based on the literature it can be concluded that the air void threshold for better

performing mixtures seems to be less than 6%.



Chapter 3

TECHNICAL DETAILS OF TOP-DOWN CRACKING

3.1 Literature review to understand the mixture characteristics and properties:

A literature review was undertaken in order to understand the mixture characteristics

and properties that affect crack development and propagation. Several different fatigue

approaches were reviewed and their significance was determined when discussing

longitudinal surface-initiated top down cracking. It was also important to review previous

studies that investigated surface cracking in the field.

3.1.1 Fractures in Bitumen Pavements

Among all the types of failure in pavement, cracking is one of the most predominant.

Many factors influence cracking in pavement such as the pavement structure and the

mixture characteristics.

There are two main types of cracking in bitumen pavements. These are thermal

cracking and fatigue cracking. Thermal cracking is caused by the stresses that are

induced when low ambient temperatures cool the surface of the road. Fatigue cracking

is associated with traffic loading and is generated through repeated stresses.

Myers(1997) found that a probable cause of longitudinal surface initiated wheel path

cracking is the high tensile stresses caused by modern radial truck tyres at the tyre-

pavement interface. These stresses may be intensified by thermal stresses at the

surface.



3.2 Mechanisms of Fracture in Bituminous Pavements

3.2.1 Traditional Fatigue Approach

The traditional fatigue approach is based on the assumption that the maximum tensile

strains are located at the bottom of the bitumen concrete layer. These strains develop

cracks and propagate from the bottom upward into the bituminous layer. Several fatigue

models have been developed to explain this phenomenon.

One of the first fatigue models was presented by Monismith et al...(1985). The following

relationship defines the fatigue behavior of a particular mixture:

Nf = A(1/ et )^b ( 1/ Smix )^c

where, Nf is the number of load applications to failure, A is a factor based on bitumen

content and degree of compaction, et is the tensile strain, Smix is the mixture stiffness and

b and c are constants determined from beam fatigue tests.

The Bitumen Institute developed the following empirical relationship in 1982 for a

standard mix with an bitumen volume of 11% and an air void volume of 5%:

Nf =0.0796 (εt )^-3.291 (E)^-0.854

where, Nf is the number of load applications to cause fatigue cracking in 20% of the

pavement area, εt is the tensile strain at the bottom of the surface layer, and E* is the

dynamic modulus of the bitumen mixture.

Another equation used to calculate the fatigue life of a mixture was developed under the

SHRP program (Sousa et al., 1996). As in the previous two equations for fatigue life, it

is a function of the mixture stiffness and bitumen content.

Nf = Sf (2.738 x10^5) (ε0 ^0.0771/FB) ( S0 ^-2.720)



where, Nf is the number of load cycles to failure, VFB is the voids filled with bitumen, ε t is

the tensile strain, S0 is the loss of stiffness, and Sf is a factor that converts laboratory

measurements to anticipated field results. The value of S f is 10 for a pavement that is

10% cracked. All of these models show that there are many variables that affect the

fatigue cracking performance of bitumen mixtures including mixture stiffness,

bituminous concrete content and air voids. Also, this shows that there is no simple or

reliable way to predict the fatigue life of an bitumen mixture.

Myers (2000) found that the addition of a stiffness gradient in cracked bitumen concrete

significantly increased the tensile stresses in the surface of the bituminous concrete

layer. None of the traditional fatigue approaches considers discontinuities (i.e. the

presence of a crack) in the bitumen layer or stiffness gradients in the bitumen layer that

may be caused by temperature or aging. The position of the load was also found to be a

contributing factor. Traditional approaches also do not allow for the possibility of

changes in the load positioning (wander) in the field. She concluded that current

methods for the design and evaluation are inadequate for longitudinal top-down

cracking because they consider only average conditions and this mechanism occurs

primarily under critical conditions.

3.2.2 Fracture Mechanics Method

Another method to explain fracture in bitumen mixtures is the fracture mechanics

method, which introduces the concept of crack propagation. The rate of crack

propagation can be predicted using the following relationship known as “Paris Law”.

Da/dN = A (ΔK) ^n

where a is the crack length, N is the number of load repetitions, A and n are parameters

depending on the mixture and KΔis the difference between maximum and minimum

stress intensity factors during repeated loading. According to Ewalds and Wanhill



(1986), the fracture mechanics approach identifies three different stages. These are the

initiation phase where micro-cracks develop, the propagation phase where the micro-

cracks develop into macro-cracks and where crack growth becomes stable, and the

disintegration phase where the material fails, and crack growth is unstable.

Graph 1 Fatigue Crack Growth Behavior

3.2.3 Dissipated Creep Strain Energy

Roque et al.(1997) found that the Dissipated Creep Strain Energy (DCSE) limit is one of

the most important factors that control crack performance in bitumen concrete mixtures.

The DCSE limit is the difference between the fracture energy (FE) and the elastic

energy (EE) at the instant of failure. The fracture energy is obtained from a strength test

as the area under the stress strain curve up to the point where the specimen begins to

fracture. The elastic energy can be obtained from the resilient modulus (MR).



Zhang(2000) introduced the concept of a threshold between micro-damage and macro-

cracking. Micro-damage was defined to be damage that was determined to be healable.

Macro-cracking was determined to be non-healable damage, even over long rest

periods and temperature increases. Zhang (2000) found that if the threshold was not

reached, cracks would not initiate and the mixture would be able to heal. Conversely, if

the threshold was reached the crack would grow and the mixture would not be able to

heal. She determined that the dissipated creep strain energy limit (DCSE f) was a

suitable threshold.

3.3 Mixture Properties Related to Fatigue Resistance

Many different material properties influence the fatigue resistance of bitumen concrete

mixtures. Therefore, it is necessary to review each of these properties to obtain a clear

understanding of fatigue resistance in bitumen pavements.

3.3.1 Mixture Stiffness

The mixture stiffness is defined as the ratio of the stress to the strain. For bitumen

mixtures, the stiffness is a function of time, temperature, and loading. The stiffness of an

bitumen mixture is affected by the binder stiffness, gradation, air void content, and

bitumen content. As a mixture ages the stiffness increases due to oxidation of the

binder. This increases the stiffness of the mixture and produces a mix that is more brittle

and less crack resistant.

3.3.2 Air Void Content

The amount of permeable air voids in a mix is related to the degree that the binder is

exposed to air and water. The exposure of binder to air and water results in the

oxidation of the binder and an increase in the rate of age hardening. The increase in

age hardening increases the stiffness and brittleness of a mixture.



The air void content is a function of aggregate gradation and degree of compaction.

Monismith et al.(1985) found that by increasing the air void content excessively resulted

in a decreased fatigue life.

3.3.3 Voids in the Mineral Aggregate (VMA)

VMA is the volume of the inter-granular void space between the aggregate particles of a

compacted pavement mixture. This void space includes the air voids and the bitumen

not absorbed into the aggregate. VMA is a function of degree of compaction, aggregate

gradation, aggregate shape, and air voids. It is an important factor in the durability of

bitumen mixtures. Generally, increased VMA values will increase the durability of a

mixture. Excessive VMA with high bitumen content will affect the durability adversely

because the high binder content tends to allow the aggregate particles to be pushed

apart.

3.3.4 Bitumen Content and Theoretical Film Thickness

Bitumen content is a very important factor in the cracking resistance of a mixture.

Bitumen content affects many material properties including air void content and film

thickness. Lower bitumen content has been generally associated with inadequate

amounts of bitumen in a mixture. Monismith(1981) found that there is an upper limit to

the amount of bitumen that can be incorporated in a mixture, but that this limit should be

approached in order to increase the fatigue resistance. Pell and Taylor found that once

the optimum bitumen content is exceeded; there will be a decrease in fatigue

resistance. Valkering and Van Gooswilligen found that an approximate 1% decrease in

the binder content was found roughly to halve the traffic-related fatigue life.

The theoretical bitumen film thickness is a function of the effective bitumen content and

the surface area of the aggregate particles. For any given bitumen content, as the

surface area of the aggregate particles increases the theoretical bitumen film thickness

decreases. Very thin bitumen films contribute to excessive aging of the binder and in



turn, more brittle mixes and decreased cracking resistance. Thicker bitumen films

contribute to a more flexible and durable mixture. Kandhal and Chakraborty (1996)

suggested a minimum bitumen film thickness to produce durable mixtures. They

concluded that an optimum film thickness for bituminous, compacted to 4 to 5% air void

content, should be higher than 9 to 10 microns.

3.3.5 Binder Viscosity

Pell and Taylor concluded that an increase in binder viscosity resulted in an increase in

fatigue resistance. Malan et al. concluded that higher viscosity bitumen’s proved to be

more crack resistant on lightly trafficked roads, while lower viscosity bitumen’s resulted

in better crack resistant mixtures on highly trafficked roads. This can be explained by

the constant kneading effect of the moving loads on high traffic pavements. This

kneading effect brings the volatiles to the surface of the pavement and prevents

excessive viscosity gradients. The viscosity of a bituminous binder is influenced by

aging and maybe more importantly, by temperature. To prevent premature cracking, the

binder viscosity is chosen based on the climate of the region where the mixture will be

placed. In low temperature climates, unusually low viscosity binders should not be used

because of the risk of extreme temperature shrinkage.

3.3.6 Aggregate Gradation

Aggregate gradation plays a very important role in the structure of a mixture. The quality

of aggregate interlock is primarily responsible for the mixture’s response to load. The

aggregate gradation affects VMA and bitumen film thickness.

The opinions on the effect of gradation on fatigue resistance are divided. Monismith et

al. found there is an insignificant effect on fatigue resistance that is not explained by air

void content and bitumen content. Malan et al. concluded that continuously graded

bitumen mixture designs are less susceptible to surface cracking than gap graded and



semi-gap-graded designs. Continuously-graded mixtures tend to have higher bitumen

film thickness and are more able to dissipate the shrinkage stresses.

3.4 World-wide literature Review

In general, world-wide literature can summarize the problem in the following list:

France: Top-down cracks form within 3-5 years of paving.

United Kingdom: Top-down cracks form within 10 years of paving on AC thicknesses of

180 mm or more.

Netherlands: Top-down cracks common for AC thicknesses of 160 mm or greater.

Japan: Top-down cracks commonly observed and occur within 1-5 years of paving.

California: Analysis showed that top-down cracks could form due to truck tyre edge

stresses that produce high surface tensile strains.

Washington State: Top-down cracks form within 3-8 years of paving on AC thicknesses

of 160 mm or greater.

Florida: Top-down cracks form within 5-10 years of paving on a wide range of AC

thicknesses.

Gerritsen et al. (1987) reported that pavements in the Netherlands were experiencing

premature cracking in the wearing courses. Further, the cracks did not extend into the

intermediate/binder course. These surface cracks occurred both inside and outside the

wheelpath areas, and, in some cases, soon after paving. This caused Gerritsen et al. to

conclude that there was likely more than one causative effect. The surface cracking

outside of the wheelpath had low mix strength characteristics at low temperatures.

Further, they noted low binder penetration values could be related to higher thermal

stresses. The surface cracks in the wheelpath areas were largely attributed to radial

shear forces under truck tyres near the tyre edges. Their conclusion was that both

thermal and load related effects caused the observed surface cracking. They



recommended that the binder film thicknesses be increased to reduce early age

hardening of the mixes.

Dauzats et al. (1987) also published results that described surface initiated cracking on

pavements in France. They noted that the cracks could be either longitudinal or

transverse and occurred typically three to five years following construction. They

estimated that these types of cracks were initially caused by thermal stresses and then

further propagated by traffic loads. It was noted that a rapid hardening of the bitumen

binder likely contributed to this type of pavement distress.

Work reported by Matsuno and Nishizawa (1992) noted that longitudinal surface

initiated cracking of the bituminous wearing course was commonly observed in Japan

about one to five years following construction. Their observations and analyses are of

special interest. First, they observed that the longitudinal cracks did not extend under

overpasses (shaded areas). Second, analysis of FEM results showed that very high

tensile strains occur at the edge of truck tyres at or near the surface of the bituminous

wearing course. These high strains occur when the upper portion of the bituminous is

at a low stiffness due to high surface temperatures. They also noted that if the

bituminous is not hardened due to aging effects, the small cracks that form are

eliminated by the kneading action of tyres. This change as the bituminous ages. They

analyzed two thicknesses of bituminous: 200 mm (8 inches) on heavy traffic routes and

l00 mm (4 inches) on light traffic routes. For both thickness cases using a peak surface

temperature of 60°C (140°F) (decreasing with depth) and associated stiffness of about

200 MPa (29,000 psi) at 60ºC (140°F), they reported similar tensile strains of over

1400x10-6 mm/mm (inch/inch) near the pavement surface. Thus, they concluded that

bituminous thickness is not a major factor with this type of cracking.

A study on large transport vehicles and their effects on pavements were reported by

Craus et al. in (1994) work done for the California Department of Transportation. Their

analyses showed that large tensile strains occur at the top of the bituminous wearing

courses. Specifically, these strains are due to high tyre edge stresses for conditions



where the upper bituminous is at a low stiffness due to high surface temperatures

(stiffness ratios of less than 0.5 produced the largest tensile strains). It is of special

interest that the California and Japanese studies drew the same conclusions concerning

the cause of surface initiated cracking.

Nunn (1998) reported that surface initiated cracking was common on UK motorways.

Typically, these surface cracks were observed about 10 years after paving. Nunn noted

that for pavements with bituminous thicknesses exceeding 180 mm (7 inches), there

was no evidence of fatigue cracking in the lower intermediate/binder course—only the

wearing courses. Additionally, he showed that there was a discontinuous relationship

between the rate of rutting and the thickness of the bituminous layers. For combined

bituminous thicknesses greater than 170 mm (6.7 inches), the rutting rates on about 50

pavement sections were about 200 times less than for bituminous layers with

thicknesses less than 170 mm (6.7 inches). For sections with less than 170 mm (6.7

inches) of bituminous the rutting rates were about 100 mm (1 inch) per million ESALs

and 0.4 mm (0.016 inches) per million ESALs for greater than 170 mm (6.7 inches).

Such dramatic measurements suggest that a very different distress mechanism occurs

at the “breakpoint” thickness. Nunn also summarized recent work performed in the

Netherlands that showed for bituminous thicknesses exceeding 160 mm (6.3 inches),

cracks initiated at the pavement surface and eventually penetrated to a depth of about

100 mm (1 inch). He also noted that the Netherlands work indicated for full depth

cracks in thinner pavements that the cracks propagated from the top of the pavement

surface downward. Nunn showed that the surface initiated cracking in the UK could be

either longitudinal or transverse. The transverse cracks were related to low binder

penetration values (typically about 15). He also stated that the pavement sections with

and without surface cracking had no significant difference in measured deflections. He

concluded the cause of the surface initiated cracking was due to horizontal tensile

stresses generated by truck tyres at the bituminous surface. Wide based tyres

generated the highest tensile stresses.

Myers et al. (1998) reported that surface initiated cracking in Florida was found to

represent 90 percent of the observed cracking in pavements scheduled for



rehabilitation. Thus, this type of cracking predominates in Florida. They noted that this

type of cracking is generally observed on pavements five to ten years following

construction. The bituminous thicknesses in their study ranged from 50 to 200 mm (2 to

8 inches). The cracks were most often longitudinal with surface crack widths of about 3

to 4 mm (0.12 to 0.16 inches) decreasing with depth. The total crack depths ranged

from about 25 mm (1 inch) to the full depth of the bituminous layer. Based on computer

modeling, it was concluded that tensile stresses under the treads of the tyre (not the

tyre edges) were the primarily cause of the cracks. Further, wide base tyres caused the

highest tensile stresses. They noted that the tensile stresses dissipate quickly with

depth suggesting that this might be the reason the cracks essentially stop growing;

however, they felt this needed further investigation. They concluded that surface

initiated cracking is not a structural design issue but more related to mixture

composition. Specifically, they concluded that more fracture resistant bitumen mixes

are needed.

At the January 2000 TRB Annual Meeting, Uhlmeyer et al. reported that top-down

cracking is common to thicker Washington State DOT BITUMINOUS surfaced

pavements (top-down cracking was typically observed when the average BITUMINOUS

thickness was about 160 mm (6.3 inches) or greater). Such cracks were generally

contained in the wearing course and averaged 46 mm (1.8 inches) in depth. The top-

down cracks generally initiated within three to eight years of paving. No hypothesis as

to cause was made.



Chapter 4

CONTROL AND MAINTENANCE OF TOP DOWN CRACKING

4.1 SOLUTIONS TO CONTROL TOP DOWN CRACKING

The two major potential solutions for top down cracking focus on the most controllable

factors

1. Improved heavy vehicle loadings control (weigh-in motion scales for instance -

difficult but imperative for developing countries) and appropriate mechanical, axle and

tyre technology implementation (suspension systems and tyres properly matched,

inflated and kept in good operating condition - very difficult, but again imperative for

developing countries);

2. Improved renewable, specialized bitumen surface courses (open graded friction

course, stone mastic bitumen and Superpave, for instance) with good permanent

deformation (rutting) resistance, and enhanced tensile and shear stress endurance.

While current applied bitumen technology activities to improve the design and

rehabilitation of flexible pavements to resist top down cracking (tensile and shear

stresses from heavy vehicle loadings) is most promising, implementation will take some

time and enhanced, available and proven, bitumen materials and construction practices

must form an integral part of any systematic approach to mitigating top down cracking of

long-life pavements, and most are being implemented now. The key aspect of the

applied bitumen technology for these durable, renewable surface courses is enhanced

cracking (tensile and shear fracture) resistance, while maintaining rutting resistance,

through improved gradations and mix volumetrics, appropriate mix design performance

monitoring and the use of bitumen binder modifiers such as polymers (crumb rubber



and styrene-butadiene-styrene (SBS), for instance). These performance requirements

are in addition to the desirable functional surface characteristics of noise level,

smoothness and frictional properties as summarized in

Functional and Structural Performance

Workable During Placement and Compaction

Contributes to Strength of Pavement Structure

Resistance to Permanent Deformation (Rutting)

Resistance to Fatigue Cracking

Resistance to Thermal Cracking

Resistance to Effects of Air and Water (Durability)

Impermeable to Protect Structure from Water

Easily and Cost-Effectively Maintained

For Surface (Wearing) Courses

Resistance to Top-Down Cracking and Associated Distress

Adequate Frictional Properties (Skid Resistance)

Acceptable Level of Tyre-Pavement Noise

Acceptable Riding Quality (Smoothness)

Aggregate Physical Characteristics and Quality

For heavy duty performance, incorporate 100 % crushed, cubical, clean coarse

and fine aggregates.

4.2 Flexible Pavement Maintenance

4.2.1 Bituminous Surface Treatments (BST)



Pavement maintenance describes all the methods and techniques used to preserve

pavement condition, safety, and ride quality, and therefore aid a pavement in achieving

its design life. The performance of a pavement is directly tied to the timing, type and

quality of the maintenance it receives. It can provide

A waterproof layer to protect the underlying pavement.

Increased skid resistance.

A fill for existing cracks or raveled surfaces.

An anti-glare surface during wet weather and an increased reflective surface for

night driving.

4.3 Crack Seals

Crack seal products are used to fill individual pavement cracks to prevent entry of water

or other non-compressible substances such as sand, dirt, rocks or weeds. Crack

sealant is typically used on early stage longitudinal cracks, transverse cracks, reflection

cracks and block cracks. Alligator cracks are most often too extensive to warrant filling

with crack sealer; they usually require an area treatment such as a patch or

reconstruction. Crack filler material is typically some form of rubberized bitumen or

sand slurry.

Purpose: Preventive maintenance. Crack filling to prevent entry of water or other non-

compressible substances into the pavement.

Materials: Heated liquid bitumen (often some form of rubberized bitumen).

Mix Design: Various, including proprietary methods.

Other Info: Before applying crack sealant, cracks need to be routed out and cleaned.



Crack sealing is best done in moderate temperatures (spring or fall) and is most

effective if performed immediately after cracks develop.

Reported average performance life ranges from about 3 - 8 years.

4.4 Fog Seals

A fog seal is a light application of a diluted slow-setting bitumen emulsion to the surface

of an aged (oxidized) pavement surface. Fog seals are low-cost and are used to

restore flexibility to an existing bituminous pavement surface. They may be able to

temporarily postpone the need for a surface treatment or non-structural overlay.

Purpose: Preventive maintenance. Fog seals are used to restore or rejuvenate

an bituminous surface. They may be able to postpone the need for a BST or non-

structural overlay for a year or two.

Materials: Slow-setting bitumen emulsion.

Mix Design: None.

A test patch may be needed to determine the proper application rate.

Other Info:Fog seals are suitable for low-volume roads which can be closed to traffic for

the 4 to 6 hours it takes for the slow-setting bitumen emulsion to break and set.

An excessive application rate may result in a thin bitumen layer on top of the original

bituminous pavement. This layer can be very smooth and cause a loss of skid

resistance. Sand should be kept in reserve to blot up areas of excess application.

4.5 Rejuvenators

Rejuvenators are products designed to restore original properties to aged (oxidized)

bitumen binders by restoring the original ratio of bitumenenes to maltenes. Many

rejuvenators are proprietary, making it difficult to offer a good generic description.



However, many rejuvenators contain maltenes because their quantity is reduced by

oxidation. Rejuvenators will retard the loss of surface fines and reduce the formation of

additional cracks, however they will also reduce pavement skid resistance for up to 1

year (Army and Air Force, 1988). Because of this, rejuvenators are generally

appropriate for low-volume, low-speed roads or parking lots.

Purpose: Preventive maintenance. Restore original properties to aged bitumen binder.

Rejuvenators may be able to postpone the need for a BST for a year or two.

Materials: Various compounds. Most rejuvenators are proprietary and thus a general

description of their constituent materials is not possible.

Mix Design: None. A test patch may be needed to determine effectiveness and the

proper application rate.

Other Info: A rejuvenator should not be applied to a pavement having an excess of

binder on the surface such as that found in slurry seal, OGFC, or BSTs. When

excessive binder is on the surface, the rejuvenator will soften the binder and cause the

surface to become tacky and slick (Army and Air Force, 1988).

The amount of air voids in the bituminous being rejuvenated should be at least 5

percent to ensure proper penetration of the rejuvenator into the pavement. If the voids

are less than 5 percent, the rejuvenator may fill the voids and thus cause an unstable

mix (Army and Air Force, 1988).

Rejuvenators should be applied in hot weather, above 20°C (70°F), so that the

rejuvenator (1) will penetrate more deeply into the bitumen pavement and (2) will cure

sooner (Army and Air Force, 1988).

4.6 Slurry Seals



A slurry seal is a homogenous mixture of emulsified bitumen, water, well-graded fine

aggregate and mineral filler that has a creamy fluid-like appearance when applied.

Slurry seals are used to fill existing pavement surface defects as either a preparatory

treatment for other maintenance treatments or as a wearing course. There are three

basic aggregate gradations used in slurry seals:

Type I (fine). This type has the finest aggregate gradation (most are smaller than the

2.36 mm and is used to fill small surface cracks and provide a thin covering on the

existing pavement. Type I aggregate slurries are sometimes used as a preparatory

treatment for bituminous overlays or surface treatments. Type I aggregate slurries are

generally limited to low traffic areas.

Type II (general). This type is coarser than a Type I aggregate slurry (it has a maximum

aggregate size of 6.4 mm (0.25 inches)) and is used to treat existing pavement that

exhibits moderate to severe raveling due to aging or to improve skid resistance. Type

II aggregate slurry is the most common type.

Type III (coarse). This type has the most coarse gradation and is used to treat severe

surface defects. Because of its aggregate size, it can be used to fill slight depressions

to prevent water ponding and reduce the probability of vehicle hydroplaning.

4.7 Micro surfacing

Microsurfacing is an advanced form of slurry seal that uses the same basic ingredients

(emulsified bitumen, water, fine aggregate and mineral filler) and combines them with

advanced polymer additives. Figures 10.1 through 10.4 show a microsurfacing slurry

seal project.

Purpose: Preventive maintenance. Repair slight to moderate pavement surface

defects, improve skid resistance.

Materials: Emulsified bitumen, water, well-graded fine aggregate and mineral filler.



Mix Design: Various, including proprietary methods.

Other Info: As opposed to a fog seal, a slurry seal contains aggregate and can thus

correct minor surface defects in a variably textured surface - filling cracks and voids,

sealing weather-tight, and providing color and texture delineation in a single pass.

4.8 Patches

Patches are a common method of treating an area of localized distress. Patches can

be either full-depth where they extend from the pavement surface to the subgrade or

partial where they do not extend through the full depth of existing pavement.

Full-depth patches are necessary where the entyre depth of pavement is distressed.

Often times, the underlying base, sub base or subgrade material is the distresses root

cause and will also need repair. Partial depth patches are used for pavement

distresses like raveling, rutting, delamination and cracking where the depth of crack

does not extend through the entyre pavement depth.

Patching material can be just about any bituminous or cold mix bitumen material as well

as certain types of slurries. Typically some form of bituminous is used for permanent

patches, while cold mix is often used for temporary emergency repairs.

One form of patching, pothole patching, probably receives the greatest amount of public

attention. Pothole patching procedures cover a wide range of methods and intentions

from permanent full-depth patches to temporary partial depth patches. Two general

patching procedures are described next.

4.8.1 Semi-Permanent Pothole Patch

Remove all water and debris from the pothole.



Square up the pothole sides so they are vertical and have in-tact pavement on all

sides.

Place the patching material into the clean squared-up hole. The material should

mound in the center and taper down to the edges so that it meets flush with the

surrounding pavement edges.

Compact the patching material starting in the center and working out toward the

edges. Compaction can be accomplished using a vibratory plate compactor or a

single-drum vibratory roller. Check the compacted patching material for a slight

crown. This is done so that subsequent traffic loading will compact it down to the

surrounding pavement height.

4.8.2 Throw-and-roll

Place the patching material into the pothole without any preparation or

water/debris removal.

Compact the patching material using the patching truck tyres (usually 4 to 8

passes).

Check the compacted patch for a slight crown. If a depression is present add

more patching material and compact.

Although it may seem that the semi-permanent technique would produce a higher

quality patch than the throw-and-roll technique, the Long Term Pavement Performance

(LTPP) Study found that the "throw-and-roll technique proved just as effective as the

semi-permanent procedure for those materials for which the two procedures were

compared directly”. Since the semi-permanent technique is more labor and material

intensive, the throw-and-roll technique will generally prove more cost effective if quality

materials are used.



CHAPTER 5CONCLUSION AND REFERENCES

CONCLUSION

1. The surface cracks in the wheelpath areas were largely attributed to radial shear

forces under truck tyres near the tyre edges.

2. Both thermal and load related effects can cause the surface cracking.

3. Longitudinal or transverse types of cracks were initially caused by thermal

stresses and then further propagated by traffic loads.

4. A rapid hardening of the bitumen binder likely contributed to propagate pavement

Cracks.

5. Cracks can form at any time of period depending on construction and type of

materials used in construction.

6. Modify current pavement design practices.

7. TDC is a major distress in segregated areas. So quality of construction should be

improved.

8. Pavement maintenance should be given more importance.



REFERENCES

1. Dauzats, M. and Rampal, A. (1987). Mechanism of Surface Cracking in Wearing

Courses. Proceedings, 6th International Conference Structural Design of Asphalt

Pavements, The University of Michigan, Ann Arbor, Michigan, July 1987, pp.

232-247

2. John J. Emery, Ph.D., P.Eng., Evaluation and Mitigation of Asphalt Pavement

Top-Down Cracking, Paper for presentation at the Assessment and

Rehabilitation of the Condition of Materials Session of the 2006 Annual

Conference of the Transportation Association of Canada.

3. Dr. Christos Drakos, Flexible Pavement Distress, from university of Florida.

4. Adam Paul Jajliardo, Development of Specification Criteria to Mitigate Top-Down

Cracking, a Thesis presented to the graduate school of the university of Florida in

partial fulfillment of the requirements for the degree of master of Engineering,

university of florida-2006.

5. Myers, L.A., “Mechanism of Wheel Path Cracking That Initiates at the Surface of

Asphalt Pavements,” Master’s Thesis, University of Florida, Gainesville, 1997.



6. Myers, L.A., “Development and Propagation of Surface-Initiated Longitudinal

Wheel Path Cracks in Flexible Highway Pavements,” Ph.D. Dissertation,

University of Florida, Gainesville, 2000.

7. Zhang, Z., “Identification of Suitable Crack Growth Law for Asphalt Mixtures

Using the Superpave Indirect Tensile Test (IDT),” Ph.D. Dissertation, University

of Florida, Gainesville, 2000.

8. Ewalds, H.L., and R.J.H. Wanhill, Fracture Mechanics, Delftse Uitgevers

Maatschappij, Delft,Netherlands, and Edward Arnold Publishers, London, 1986.

9. Garcia, O.F., “Asphalt Mixture and Loading Effects on Surface-Cracking of

Pavements,” Master’s Thesis, University of Florida, Gainesville, 2002.

10.Honeycutt, K.E., “Effect of Gradation and other Mixture Properties on the

Cracking Resistance of Asphalt Mixtures,” Master’s Thesis, University of Florida,

Gainesville, 2000

11.Huang, Y.H., Pavement Analysis and Design, Prentice Hall, Englewood Cliffs NJ,

1993

12.Jacobs, M.M.J., “Crack Growth in Asphaltic Mixes,” Ph.D. Dissertation, Delft, The

Netherlands, Nelft University of Technology, 1995.

13.Kandhal P.S. and S. Chakraborty, “Evaluation of Voids in the Mineral

Aggregates,” NCAT Report No. 96-4, National Center for Asphalt Technology,

March 1996.



14.Malan, G.W., P.J. Strauss and F. Hugo, “A Field Study of Premature Surface

Cracking in Asphalt,” Proceedings of the Association of Asphalt Paving

Technologists, Vol. 58, pp. 142-162, 1989

15.Guide for Mechanical-Empherical Design of new and rehabilitated pavement

structure by National Co-operative highway research program, Transportation

research Board, National research council-2004(Page No-3.3.8 to 3.3.14)

16.Dauzats, M. and Rampal, A. (1987). Mechanism of Surface Cracking in

Wearing Courses. Proceedings, 6th International Conference Structural Design

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