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].
Explain The Cbr Method Of Flexible Pavement Design Principles
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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.
In road engineering and construction, the two main types of pavement are rigid and flexible. These two types use different materials and road construction methods, resulting in different physical properties.
This guide introduces you to the fundamentals of road design and construction. It discusses the different road construction methods and the types of pavement involved, along with key design considerations such as how to optimise pavements to reduce costs and carbon footprints.
The challenge for the pavement engineer designing the road is to select the right material and layer thicknesses so that the pavement will be serviceable for the full design life. When discussing the strength of road paving, we usually measure this in the number of vehicles they can support.
Once we know the requirements of the road paving in terms of supporting traffic and how long it should ideally last, the next step is to set out the specifications for construction. In addition to the materials and layer thickness, the type of pavement (rigid or flexible) is also a key design choice.
Much of the US Interstate network and European highway network use concrete construction, although it has been much less popular in the UK. Concrete road pavements can handle very heavy traffic flows and high axle loads. They are now more common in urban areas, ports and locations, where heavy trucks travel slowly. The design life of rigid pavement is typically 40 years, with failure usually occurring due to cracking of the slabs, or degradation at the joints.
As mentioned above, the future of pavements lies in optimisation. Pavement optimisation aims to construct high-performing road pavements quickly and economically, often utilising geogrids for mechanically stabilised aggregate layers. This approach, crucial for flexible pavements, controls aggregate displacement and delays the onset of failure in the structure.
Accelerated pavement testing (APT) is a method that assesses long-term pavement performance in a condensed time frame. By applying realistic wheel loading to a layered pavement structure, APT simulates extended in-service traffic conditions to evaluate pavement response and performance efficiently.
To gain in-depth insights into accelerated pavement testing, explore our detailed article covering the methodology, execution, and valuable findings from this crucial aspect of pavement optimisation.
APT testing in progress using a reciprocating dual wheel tandem axle
Geogrid pavement design is a strategic approach to constructing optimised road pavements, leveraging geogrids to enhance stability and longevity. Geogrids are engineered materials integrated into pavement layers, providing mechanical stabilisation to aggregate materials.
This design methodology aims to distribute loads more effectively, minimising rutting and fatigue cracking, whilst also preventing moisture and contaminants from compromising the pavement structure. Geogrid pavement design proves essential for a range of flexible pavements, ensuring durability and performance. For an in-depth exploration, refer to this insightful resource on geogrid pavement design.
Site support facilities swiftly establised by using Tensar InterAx geogrid
The terms empirical design, mechanistic design, and mechanistic-empirical design are frequently used to identify general approaches toward pavement design. The key features of these design methodologies are described in the following subsections.
An empirical design approach is one that is based solely on the results of experiments or experience. Observations are used to establish correlations between the inputs and the outcomes of a process - e.g., pavement design and performance. These relationships generally do not have a firm scientific basis, although they must meet the tests of engineering reasonableness (e.g., trends in the correct directions, correct behavior for limiting cases, etc.). Empirical approaches are often used as an expedient when it is too difficult to define theoretically the precise cause-and-effect relationships of a phenomenon.
The mechanistic design approach represents the other end of the spectrum from the empirical methods. The mechanistic design approach is based on the theories of mechanics to relate pavement structural behavior and performance to traffic loading and environmental influences. The mechanistic approach for rigid pavements has its origins in Westergaard's development during the 1920s of the slab on subgrade and thermal curling theories to compute critical stresses and deflections in a PCC slab. The mechanistic approach for flexible pavements has its roots in Burmister's development during the 1940s of multilayer elastic theory to compute stresses, strains, and deflections in pavement structures. 589ccfa754
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