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 (HIGHWAY TECHNOLOGY) DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 1

TOP- DOWN CRACKING IN FLEXIBLE PAVEMENT 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

Mr. B.V. Kiran Kumar

Head of Department

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:

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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 topdown 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. DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 3

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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.

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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. DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 5

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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. DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 6

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 7

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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.

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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.

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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

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Figure 2.2 Line diagram of Fatigue Cracking

Figure 2.3 Close up view of Fatigue cracking in pavement surface DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 10

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 11

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 12

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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.

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Figure 2.7 Consolidated rutting

Figure 2.8 Consolidated rutting in pavement surface

Figure 2.9 Consolidated rutting in pavement surface

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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.

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 16

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 17

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 18

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 19

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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. DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 20

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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 (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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 21

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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 bottomup 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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 22

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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. DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 23

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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).

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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%.

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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 tyrepavement interface. These stresses may be intensified by thermal stresses at the surface.

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 27

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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-

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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 macrocracking. 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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 29

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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 (DCSEf) 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.

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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. DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 31

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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: DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 32

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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

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 34

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 35

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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 topdown 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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 36

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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 DEPA

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•

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. DEPA

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•

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 noncompressible 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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 39

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

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 41

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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.

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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

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•

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 5 CONCLUSION AND REFERENCES DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 44

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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 DEPA RTMENT OF CIVIL ENGINEERING, D.S.C.E 45

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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.

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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

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