Top Down Cracking : Seminar Report and PPT

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.

Introduction

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.

Objectives

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.