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Sagar BhattaraiDefinition and Types of PavementsIt can be said as a relatively stable layer constructed over the natural soil for the purpose of supporting and distributing the wheel loads and providing an adequate surface for the movements of vehicles with certain speed safely, comfortably and economically. One of the objectives of a well-designed pavement is to keep temporary deformation of the pavement with in the permissible limit so that the pavement can sustain a large number of repeated load applications during design.There are three major types of pavements: flexible or asphalt pavements, rigid or concrete pavements, and composite pavements.? Flexible pavement (Asphalt Pavement)The pavements which have very low flexural strength and are flexible in their structural behavior under the load are called flexible pavements. The flexible pavement layer reflects the deformation of the lower layers on the surface of the layers. Thus if lower layer somehow gets deformed the surface of the pavements also gets deformed.The flexible pavement layers transmit the vertical loads to the lower layers by grain to grain transfer through their point of contact in the granular structure. The load spreading capacity of the flexible pavement depends on the type of material and the mix design factors. The basic principle of flexible pavement design is based on the layered systems with better materials on the top where the intensity of stress is high and inferior materials at the bottom where the intensity is low. This design principle makes possible the use of local materials and usually results in a most economical design.? Rigid Pavements (Concrete pavement)Rigid pavement are those which possess considerable flexural strength The rigid pavements are made of cement concrete which may either plain, reinforced or pre-stressed. The rigid pavements have a slab action and are capable of transmitting the wheel loads stresses through a wider area below. The main difference between rigid and flexible pavements in the structural behavior is that the critical condition of stress in the rigid pavement is the maximum flexural strength occurring in the slab due to the wheel load and the temperature changes where as in flexible pavement it is the distribution of compressive stress to the lower layers and lastly over the soil subgrade.? Composite pavementA composite pavement comprises of multiple, structurally significant layers of different composition. In its widely used form, composite pavement consists of PCC as a bottom layer and bituminous layer as a top layer resulting in an ideal pavement with the most desirable characteristics. The bottom layer (PCC) provides a strong base and the bituminous layer (top) provides a smooth and non-reflective surface. But this type of pavement is very expensive and is rarely used as a new construction. In its economical form brick sand witched concrete pavement can be used which is widely under research in India and this, the top and bottom layers of cement concrete with a brick sand witched in the neutral axis zone are provided. But this technique is still infancy.Flexible pavement layers? Surfacing course and binder is the top course and is provided to provide a smooth, abrasion resistant, dust free, reasonably water proof and strong layer.? Base course is the medium through the stresses imposed are distributed evenly to the underlying layers.? Sub base layer provides additional help in distributing the loads.? Subgrade course is the compacted natural earth and the top of the sub grade level is also known as the formation level.Fig. Different layers of asphalt pavementFlexible pavements are layered systems with better materials on the top and cannot be represented by homogeneous mass, so the use of Burmister layered theory is more appropriate. Burmister (1943) first developed solutions for two-layered systems and then extended them to a two-layered system (Burmister 1945). With the advent of computer the theory can be applied to a multilayer system with any number of layers.The exact case of a two-layer system is the full-depth construction in which a thick layer of HMA is placed directly on the subgrade. If the pavement is composed of three layer (e.g. an asphalt surface course, a granular base course and a subgrade) it is necessary to combine the base course and the subgrade into a single layer for computing stresses and strain in the asphalt layer or to combine the asphalt surface course and base course for computing the stresses and strains in the subgrade.Strains considered for asphalt pavement designThe following two critical strains are considered for the asphalt pavement design.? Horizontal tensile strain on the underside of the lowest asphalt bound layer should be within the permissible value. If the strain is excessive, fatigue of the layer will result. The limit up to which the asphalt pavement can withstand the horizontal tensile strain without fatigue is said to be the Asphalt Pavement Fatigue Endurance Limit.? Vertical compressive strain at the surface of the subgrade layer should be such that excessive permanent deformation (rutting) should not take place due to overloading.Fig. Tensile and compressive strain in asphalt pavementThe two forms of pavement structural failure most commonly considered are permanent deformation (rutting) in the wheel path and fatigue cracking. The greatest proportion of rutting occurs in Summer when temperature are high and it is often visible on the approaches to road junctions, traffic lights and on inclines where heavy vehicles move more slowly. The development of a rut takes place in two stages. The first stage is the densification where the aggregate skeleton becomes more closely packed. The second stage maybe called a “shear phase” where the material moves laterally causing the development of shoulders whether one or both of these stages occurs depends largely onthe mix type. Probably the most commonly used indicator of pavement rutting is the magnitude of vertical compressive stress or strain at the top of the subgrade. An empirical power law equation is developed to relate this key stress (or strain) to surface rutting.Critical Tensile Strain: The tensile strains at the bottom of asphalt layer have been used as a design criterion to prevent fatigue cracking. Two types of principal strains could be considered. One is the overall principal strain based on all six components of normal and shear stresses. The other, which is more popular and was used in KENLAYER, is the horizontal principal strain based on the horizontal normal and shear stresses only. The overall principal strain is slightly greater than the horizontal principal strain, so the use of overall principal strain is on the safe side.Huang (1973a) developed charts for determining the critical tensile strain at the bottom of layer 1 for a two-layer system. The critical tensile strain is the overall strain and can be determined frome= (q/E1) Fein which e is the critical tensile strain and Fe is the strain factor, which can be determined from the chartsFatigue in pavement has been defined as the phenomenon or fracture under repeated or fluctuating stress having a maximum value generally less the tensile strength of the material under the traffic loading the layers of a flexible pavement structure are subjected to continuous flexing. The magnitude of the strains is dependent on the overall stiffness and nature of the pavement. Analysis, confirmed by measurement has indicated tensile strains of the order of 30-200 microstrains under the standard wheel load. The parameters most commonly related to fatigue damage is the tensile strain at the base of the asphalt layer. Simplified laboratory tests are usually used to determine an empirical power law fatigue model based on this key strain. However application of these equations to real, pavement usually requires correction factors to account for the extremely simplified condition of the test. The relationship between allowable tensile strain in the bituminous material and the number of load application to fatigue failure has to allow to the large difference in condition between laboratory tests and the field. The research used to establish this strain criterion involve continuous cyclic loading to test specimen to failure In this pavement rest periods between load applications occur even on heavily trafficked roads and s finite time is required for a crack, once initiated to propagate through the layer. In addition traffic loading in a wheel track is not applied in precisely the same spot each time. All these factors cause much longer fatigue lives than in the laboratory tests.Careful assessment of available research has shown that insitue lives to “failure” are about 440 times those in the laboratory, at any given strain level. Failure is taken to represent the fully cracked state, in accordance with the Department of Transport definition. For “critical” conditions i.e. first appearance of wheel path cracking, the factor is 77 times. These factors have been applied to the fatigue prediction method given in the Design Manual (chart 5 and 6), which allows design value of allowable strain to be determined directly from the cumulative number of standard axles.Fig. Fatigue cracking and critical strains? The criterion for fatigue crackingThe design criterion to prevent fatigue cracking is tensile strain and the maximum value occurs at the bottom of the layer mid-way between the dual wheels in a horizontal tangential direction i.e. in the direction of trafficking except in special circumstances. The term fatigue means the failure occurs as a result of repeated applications of strain below the level which would cause rapture in single application. To prevent that in design the fatigue strength of the proposed bituminous mix needs to be known or, a alternatively mix must be designed to provide given fatigue characteristics. This characteristics is expressed as a relationship between tensile strain (€1) and the number of cycles to failure (Nf)Viz. Nf = C(1/€t)m where C & m are constant which depends on two simple mix parameters, the volume proportion of binder (VB) and its initial softening point (SP1). Note that when calculating stiffness the recovered softening point is used. This rather illogical situation simply steams from the way in which each estimation procedure was developed quite independently. The nomographs have been developed for the purpose of defining the fatigue line for a given mix to either the critical or failure condition, respectively. Having established the line, the maximum allowable tensile strains (in microstrain) for a given life (N) in terms of number of loads applications (in msa) can be read off for design purposes.Chart1. Nomograph for the determination of fatigue life of failureChart2. Maximum allowable subgrade strain to critical conditionsEndurance limit or Endurance stress?e(endurance stress)The particular value of stress for which material will operate for infinite number of cycles is called Endurance Stress. If the stress increases then the number of cycles for which the material can be used or the material will operate will be less and vice versa. In case of asphalt pavement also the case is same, i.e. if the horizontal stress in the pavement layer is increased then the number of cycles of load for which the pavement has to operate will be less due to fatigue resulting in facture. Endurance stress is completely reversed bending stress.If the horizontal tensile stress in asphalt pavement is greater than its endurance limit the fatigue cracking occurs. They are the series of interconnected cracks caused by fatigue failure of the HMA surface under repeated traffic loading. In thin pavement, cracking initiates at the bottom of the HMA layer where the tensile stress is the highestNo. of cyclesStress(?)then propagates to the surface as one or more longitudinal cracks. This is commonly referred to as “bottom-up” or “classical” fatigue cracking. In thick pavements, the cracks most likely initiate from the top areas of high localized tensile stresses resulting from tire-pavement interaction and asphalt binder aging (top-down cracking). After the repeated loading, the longitudinal cracks connect forming many-sided sharp-angled pieces that develop into a pattern resembling the back of an alligator or crocodile. It causes the problem like, infiltration, roughness of surface, and formation of pothole.The possible causes for the fatigue cracking are inadequate structural support, which can be caused by myriad of things. A few of more common are listed here:? Decrease in pavement load supporting characteristics (mainly due to poor drainage and stripping on the bottom of HMA layer)? Increase in loading (e.g. more or heavier loads than anticipated in design)? Inadequate structural design? Poor construction (e.g. inadequate compaction)The fatigue cracked pavement should be investigated to determine the root cause of failure. Any investigation should involve digging a pit or coring the pavement to determine the pavement’s structural makeup as well as determining whether or not subsurface moisture is a contributing factor. Once the characteristics alligator pattern is apparent, repair by crack sealing is generally ineffective. Fatigue crack repair generally falls into one of two categories:? Small localized, fatigue cracking indicative of a loss of subgrade support.Remove the cracked pavement area then dig out and replace the area of poor subgrade and improve the drainage of that area if necessary, patch over the repaired subgrade.? Large fatigue cracked areas indicative of general structural failurePlace an HMA overlay over the entire pavement surface. This overlay must be strong enough structurally to carry the anticipated loading because the underlying fatigue cracked pavement most likely contributes little or no strength.References:? Pavement Analysis and Design by Yang H. Huang? An Introduction to the analytical design of Bituminous Pavements(3rd Edition)S.F. Brown and Janet M. Brunton? Lecture 9&10 by A.C. Collop? Course Manual by Dinesh Kumar ShresthaAssociate ProfessorDepartment of Civil EngineeringPULCHOWK CAMPUS