Explain The Cbr Method Of Flexible Pavement Design Principles
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An asphalt pavement is made up of multiple layers, namely subgrade, sub-base, base, surfacing and wearing course. While there are design considerations involved in a pavement from the geometric, functional and drainage aspects, the structural design indicates estimation of appropriate thicknesses of the pavement layers.
Environmental parameters used in pavement design primarily include variation of temperature and moisture conditions (including freezing and thawing situations) during the service period of the pavement. In the design guidelines such variations are accounted either (i) considering expected/equilibrium value of the temperature and the moisture content during the design period [4, 8, 10, 16] or, (ii) by dividing the total design period into certain time intervals and considering the effects of temperature and moisture content (in terms of incremental damage) for these time intervals [1, 15]. For design purpose, it is generally assumed that the temperature affects the stiffness of the asphalt layer and the moisture content affects the stiffness of the unbound granular layer and the subgrade. Further, it may be noted that there always exists a temperature profile and moisture content profile (that is, variation along the depth of the pavement). For design purpose, a representative values of temperature and moisture content are chosen which carry the equivalent effects of such variations along the depth [17, 18].
Figure 1 presents a simplified thought process involved in the design of asphalt pavement using mechanistic-empirical method. Elaborate flow-charts are available in the literature [13, 14, 22, 36] and in various guidelines [1, 4, 6, 15, 16, 20, 37, 38]. In this approach, trial thicknesses of the pavement layers are assumed and the critical stress/strain values (typically strain is considered for design of asphalt pavements) at the critical locations are computed by structural analysis of the pavement. The computed strain values are compared with the allowable values and the design thicknesses are finalized through iterations. This process is repeated for all the types of structural distresses which are dependent on the layer thicknesses.
Although the principle is the same (as explained in Fig. 2), some guidelines [1, 15] do not specify allowable strain. In this case, first a trial pavement section is chosen and the expected distress level is estimated at the end of the design period. If the distress level is within the permissible limit, the design is accepted, else the thicknesses are modified in the next iteration [15, 50].
Figure 3 shows a conceptual diagram of a pavement design chart considering fatigue (bottom up) and rutting distresses. A three-layer asphalt pavement structure is assumed in this case, constituted with an asphalt layer, an unbound granular layer and a subgrade. Two design variables (that is, asphalt and granular layer thicknesses) and two structural distresses (that is, fatigue and rutting) are considered.
The paragraph above has discussed one of the extreme ends of the design curve (refer to Fig. 3); an interesting observation can be made on the other extreme end of the design curve. It is seen that when the granular layer thickness is quite high, the design curve turns in the reverse direction. That is, referring to Fig. 3, for a given granular thickness EH, two design asphalt layer thicknesses are possible, these are HB and HA (a pavement designer will obviously choose the lower thickness). This happens because, at a higher thickness of granular layer, when the thickness of asphalt layer is lowered further during the process of design iterations, the strain (horizontal tensile) at the bottom of the asphalt layer (below some threshold value of the thickness) starts decreasing; further reduction in asphalt layer thickness even reverses the strain from tensile to compressive [4, 38]. Thus, in principle, it is also possible to design a thin asphalt layer over a thick granular layer [20], but sometimes there may be issues especially for heavy-trafficked urban roads [4].
When it is decided to use a new material in pavement design, the design needs to be re-worked [62]. For example, when a cementicious material is decided to be used as a base layer, it may provide benefits in terms of (i) possible utilization of locally available marginal aggregates and (ii) possible reduction of the asphalt layer design thickness, because of the reduction of the strain value due to higher stiffness of cementicious layer and its contribution to additional fatigue life [4, 48, 49]. However, additional cost due to the usage of cemented material, shrinkage potential and long term durability of the cementicious material, special construction requirements etc. need to be considered while finalizing the design [4, 48, 49].
A pavement design problem (for new pavement as well as overlay), for a given set of input parameters, is expected to have multiple design solutions. Economic analysis is generally used to choose the best design [2, 4, 15, 19, 20, 69]. Further, a pavement in its entire service period may undergo many rounds of rehabilitation and maintenance activities. Planning (at the initial design stage) for appropriate maintenance measures and their application timings for a given road stretch (or for a road network) is an interesting but complex problem [70].
A number of old and new pavement design guidelines have been referred in this paper to discuss various principles related to asphalt pavement design. The pavement design guidelines referred in this article are only representative, and in no way provides an exhaustive account of the different pavement design practices followed across the world.
Design procedures For flexible pavements, structural design is mainly concerned with determiningappropriate layer thickness and composition. The main design factors are stresses due to traffic load and temperaturevariations. Two methods of flexible pavement structural design are common today:Empirical design and mechanistic empirical design.Empirical designAn empirical approach is one which is based on the results of experimentationor experience.Some of them are either based on physical properties or strength parameters ofsoil subgrade.An empirical approach is one which is based on the results of experimentationor experience.An empirical analysis of flexible pavement design can be done with or with outa soil strength test.An example of design without soil strength test is by usingHRB soil classification system, in which soils are grouped from A-1 to A-7and a group index is added to differentiate soils within each group.Example with soil strength test usesMcLeod, Stabilometer, California Bearing Ratio (CBR) test.CBR test is widely known and will be discussed.Mechanistic-Empirical DesignEmpirical-Mechanistic method of design is based on the mechanics of materialsthat relates input, such as wheel load, to an output or pavement response.In pavement design, the responses are the stresses, strains, and deflectionswithin a pavement structure and the physical causes are the loads and materialproperties of the pavement structure.The relationship between these phenomena and their physical causes aretypically described using some mathematical models.Along with this mechanistic approach, empirical elements are used when definingwhat value of the calculated stresses, strains, and deflections result inpavement failure. The relationship between physical phenomena and pavement failure is describedby empirically derived equations that compute the number of loading cycles tofailure. Traffic and LoadingThere are three different approaches for considering vehicular and trafficcharacteristics, which affects pavement design.Fixed traffic: Thickness of pavement is governed by single load andnumber of load repetitions is not considered.The heaviest wheel load anticipated is used for design purpose.This is an old method and is rarely used today for pavement design.Fixed vehicle: In the fixed vehicle procedure, the thickness is governedby the number of repetitions of a standard axle load.If the axle load is not a standard one, then it must be converted to anequivalent axle load by number of repetitions of given axle load and itsequivalent axle load factor.Variable traffic and vehicle: In this approach, both traffic and vehicleare considered individually, so there is no need to assign an equivalent factorfor each axle load.The loads can be divided into a number of groups and the stresses, strains,and deflections under each load group can be determined separately; and usedfor design purposes.The traffic and loading factors to be considered include axle loads, loadrepetitions, and tyre contact area.Equivalent single wheel loadTo carry maximum load with in the specified limit and to carry greater load,dual wheel, or dual tandem assembly is often used.Equivalent single wheel load (ESWL) is the single wheel load having the samecontact pressure, which produces same value of maximum stress, deflection,tensile stress or contact pressure at the desired depth.The procedure of finding the ESWL for equal stress criteria is provided below.This is a semi-rational method, known as Boyd and Foster method, based on thefollowing assumptions:equalancy concept is based on equal stress;contact area is circular;influence angle is 45; and soil medium is elastic, homogeneous, and isotropic half space. The ESWL is given by: (1)
Figure:ESWL-Equal stress concept Example 1Find ESWL at depths of 5cm, 20cm and 40cm for a dual wheel carrying 2044kg each.The center to center tyre spacing is 20cm and distance between the walls of thetwo tyres is 10cm.SolutionFor desired depth z=40cm, which is twice the tyre spacing, ESWL = 2P=22044= 4088 kN.For z=5cm, which is half the distance between the walls of the tyre, ESWL = P = 2044kN.For z=20cm,==3.511.Therefore, ESWL = antilog(3.511)= 3244.49 kNEquivalent single axle load Vehicles can have many axles which will distribute the load into differentaxles, and in turn to the pavement through the wheels.A standard truck has two axles, front axle with two wheels and rear axle withfour wheels. But to carry large loads multiple axles are provided. Since the design of flexible pavements is by layered theory, only the wheels onone side needed to be considered. On the other hand, the design of rigidpavement is by plate theory and hence the wheel load on both sides of axle needto be considered.Legal axle load: The maximum allowed axle load on the roads is calledlegal axle load. For highways the maximum legal axle load in India, specified by IRC, is 10tonnes.Standard axle load: It is a single axle load with dual wheel carrying80 KN load and the design of pavement is based on the standard axle load.Repetition of axle loads: The deformation of pavement due to a single application of axle load may besmall but due to repeated application of load there would be accumulation ofunrecovered or permanent deformation which results in failure of pavement.If the pavement structure fails with number of repetition of load and for the same failure criteria if it requires number of repetition ofload , then and are considered equivalent.Note that, and equivalency depends on the failure criterionemployed.Equivalent axle load factor: An equivalent axle load factor (EALF)defines the damage per pass to a pavement by the type of axle relativeto the damage per pass of a standard axle load.While finding the EALF, the failure criterion is important.Two types of failure criterias are commonly adopted: fatigue cracking andruttings.The fatigue cracking model has the following form:(2) 2b1af7f3a8
